tag:blogger.com,1999:blog-17560172255057222482024-03-19T03:21:33.595-07:00Natural VS Anthropogenic Climate ChangeWhat causes extreme climatic conditions in Europe and North America like we have been experiencing over the past decades? Is it due to natural modes of variability, or can we actually blame ourselves?
Anonymoushttp://www.blogger.com/profile/16989515393582579555noreply@blogger.comBlogger11125tag:blogger.com,1999:blog-1756017225505722248.post-86341539863603991672013-01-07T06:35:00.002-08:002013-01-07T06:39:34.335-08:00Winter extremes and natural variability<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">So far I’ve looked at patterns of precipitation and
temperature, and also related drought, determined by natural modes of
variability. Is there also a connection with snow anomalies and extreme
winters? Yes, of course there is. </span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br /></span>
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">The winter of 2009/10 had anomalously large snowfall in the
central parts of the US and in northwestern Europe, which some of you may
remember. December’s snowcover in the contiguous US was the greatest ever for
that month and Washington D.C. had to be closed down for a weak. </span><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjEJBv3SxE7jjyeMBBiDga8letonlNvDJGxwGXeOVDBn2AFi0Gy1xKWpZ8lrppXclbU_-S6MjmvEBVM0jv2tUhHYHD7KzgJGU7UPABPlf1SNbTK9VOKOY_lUbyHgQcMvpDMgrSoqvBJhyphenhyphenQ/s1600/washington+snow.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="222" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjEJBv3SxE7jjyeMBBiDga8letonlNvDJGxwGXeOVDBn2AFi0Gy1xKWpZ8lrppXclbU_-S6MjmvEBVM0jv2tUhHYHD7KzgJGU7UPABPlf1SNbTK9VOKOY_lUbyHgQcMvpDMgrSoqvBJhyphenhyphenQ/s400/washington+snow.jpg" width="400" /></a></div>
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<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Washington D.C., December 19, 2009. <a href="http://www.knowledge.allianz.com/environment/climate_change/?1575/climate-change-natural-disasters-impacts-extreme-weather">Here</a></span></div>
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<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">As mentioned in the beginning of this blog, the NAO exerts a
strong influence on wintertime climate across the North Atlantic basin, and a
negative index NAO give cold temperature anomalies in the eastern US and
northern Europe. ENSO, with its warm phase El Niño, also influences wintertime
climate over the US by a southward displaced stormtrack. A paper by <a href="http://onlinelibrary.wiley.com/doi/10.1029/2010GL043830/abstract">Seageret al. (2010)</a> takes a closer look at the winter of 2009/10 and finds that a
combination of a negative NAO and an El Niño event was causing the cold temperatures
and the anomalous amounts of snow experienced in parts of the US and Europe
that winter. The changes in storminess associated with El Niño explained snow
anomalies in western, central and southern US, while the negative NAO provided
sufficient cold air for the precipitation to fall as snow in the northeastern
US and northern Europe.</span></div>
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">In a <a href="http://www.sciencemag.org/content/293/5527/85.short">study published in Science in 2001</a>, Thomson and
Wallace find that the different phases of the NAO (<b>NAM</b>, see link) are marked by distinct differences in the frequency
distribution of significant weather events in the Northern Hemisphere that
impact human activities. They report that cold events (daily minimum
temperatures dropping below a specified threshold, frozen precipitation) occur
with much greater frequency over North America and Europe (and other places, see table below) during negative index days,
increasing the risk of frost damage and the frequency of snowfall.</span><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjso1ust1JvTtk8UjPmPj_R6t0xsVNdIgY_jmTZUDHG2jm2Y71MLxJCrtTTL_JT1jQQPKn6ei2mZhku6_6T64SXgTCbvwASRFEFIB_-u_FWA3CxocZPpMVKlu9ho8Jpmnpy5p7Yx0qM7UE/s1600/NAM+cold+events.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="500" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjso1ust1JvTtk8UjPmPj_R6t0xsVNdIgY_jmTZUDHG2jm2Y71MLxJCrtTTL_JT1jQQPKn6ei2mZhku6_6T64SXgTCbvwASRFEFIB_-u_FWA3CxocZPpMVKlu9ho8Jpmnpy5p7Yx0qM7UE/s640/NAM+cold+events.jpg" width="640" /></a></div>
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<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">Significant weather events associated with high and low NAM index days. As you can see most of the events occur with much higher frequency during NAM- (negative index). </span></div>
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<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif; font-size: x-small;"><br /></span></div>
</div>
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">So heavy snowfall events and extreme winters, like the
winter of 2009/10, are also influenced, to a big extent, by natural modes of
variability. But is there a relation to climate change? If one is thinking
about climate change in general, I think most people will relate it to warming.
The winter of 2009/10 made people really question climate change. I’ll be
discussing whether or not winter extremes are related to anthropogenic climate
change in my next post.</span>Anonymoushttp://www.blogger.com/profile/16989515393582579555noreply@blogger.com3tag:blogger.com,1999:blog-1756017225505722248.post-69936377656498443502012-12-19T06:34:00.001-08:002012-12-19T06:34:57.278-08:00Drought in the context of anthropogenic climate change
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<span style="font-family: "Trebuchet MS", sans-serif;">In the previous post I talked about European droughts
association to natural modes of variability. Since this is a blog about natural
versus anthropogenic climate change I thought it would be natural to follow up
with a question about how anthropogenic influence on climate is affecting
droughts. In particular, considering the severe drought in the US the last
summers, this question is of high relevance. </span></div>
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><span style="font-family: "Trebuchet MS", sans-serif;">A relatively <a href="http://www.nature.com/nature/journal/v491/n7424/full/nature11575.html?WT.ec_id=NATURE-20121115">newly released study</a> in Nature by
Sheffield et al. look closer at the historical record of global-scale drought
trends and actually find that they have most likely been overestimated. They report
high uncertainties in these trends over the past 60 years and little evidence
of an increase in the total area affected by drought. One might think that it
is the opposite way around in these times of a warming climate, but the climate
system is highly complex and we don’t necessarily get the results that seem
most logic. There exists a hypothesis in the scientific world saying that “wet
is getting wetter and dry is getting drier”, meaning that the areas which
normally receive a lot of precipitation will get more intense rainfall and
flooding, and that areas that are already suffering from a precipitation
deficit and drought will get more severe droughts. This will have huge
consequences for people living in such areas.</span><br />
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><span style="font-family: "Trebuchet MS", sans-serif;"><u></u></span><br />
<span style="font-family: "Trebuchet MS", sans-serif;"><a href="http://www1.ncdc.noaa.gov/pub/data/cmb/bams-sotc/2011-peterson-et-al.pdf">Another study</a> produced by a big group of scientist
for the American Meteorological Society looks at six extreme events during the
year of 2011 and tries to explain them from a climate perspective. Among these,
they look at the severe 2011 Texas drought and ask: “Was the likelihood of either
the heat wave or the drought altered by human influence on global climate?” Now
considering drought over the North American continent, it is important to point
out that ENSO with its cold phase La Niña, is considered to be a key driver of
drought conditions in the central US (Atlas et al. 1993). In the study they use
the La Niña year of 2008 as a proxy for 2011, because simulations under 2011
forcing conditions were not available, and compare to earlier decades. They find
“that extreme heat events were roughly 20 times more likely in 2008 than other
La Niña years in the 1960s and indications of an increase in frequency of low
seasonal precipitation totals.” These findings suggest that drought is more
probable now than for 40-50 years ago.</span><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgcIKdBKa5Mjtia-7uzNqLlp68kZq2go846j2HAww9P1ihGs6Iw73W61dBaMkdX8fmzfFZQzTNMsQt7CqGe_OnHI0lNh52OTC0VMGPqx0VCD9pmDv92VObPEo2rYB-CreEXPBnoPbS21q8/s1600/Texas-drought-livestock-deaths.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="308" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgcIKdBKa5Mjtia-7uzNqLlp68kZq2go846j2HAww9P1ihGs6Iw73W61dBaMkdX8fmzfFZQzTNMsQt7CqGe_OnHI0lNh52OTC0VMGPqx0VCD9pmDv92VObPEo2rYB-CreEXPBnoPbS21q8/s400/Texas-drought-livestock-deaths.jpg" width="400" /></a></div>
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Picture from Texas drought 2011, Google</div>
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</span><br />
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<span style="font-family: "Trebuchet MS", sans-serif;">This also contribute to strengthen the “dry getting drier”
hypothesis by saying that the probability of the occurrence of drought in an
already dry area like Texas, is more likely with global warming.</span></div>
Anonymoushttp://www.blogger.com/profile/16989515393582579555noreply@blogger.com0tag:blogger.com,1999:blog-1756017225505722248.post-36050678609616050302012-12-07T07:17:00.001-08:002012-12-19T06:44:47.905-08:00European drought’s relationship to global SST<span style="font-family: "Trebuchet MS", sans-serif;">So far on this blog I’ve described natural modes of
variability across the North Atlantic basin like the NAO, the AMO and AMOC and
their related climatic impacts. They are all associated with specific climatic
patterns of temperature and precipitation across large areas, and I’ve
mentioned and showed illustrations of these in my previous posts. In this post
I will only focus on the drought impact and try to give a summary of the
factors controlling droughts across Europe. </span><br />
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><br />
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<span style="font-family: Calibri;"><span style="font-family: "Trebuchet MS", sans-serif;">A study by </span><a href="http://download.springer.com/static/pdf/682/art%253A10.1007%252Fs00382-011-1028-y.pdf?auth66=1355056796_90a3d58181b5ed47a782a8ca794fff12&ext=.pdf"><span style="font-family: "Trebuchet MS", sans-serif;">Ionita et al. (2012)</span></a><span style="font-family: "Trebuchet MS", sans-serif;"> looked at variability
of European summer drought and its relation to global sea surface temperature
(SST) by using the </span><a href="http://www.drought.noaa.gov/palmer.html"><span style="font-family: "Trebuchet MS", sans-serif;">Palmer drought severity index</span></a><span style="font-family: "Trebuchet MS", sans-serif;"> averaged over the
European region (see figure below).</span> </span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgrmv64DpetWuZrYapSbJB_KEI7d-om2mB_WfHL6dWiUlrpindWjoMFPCb0CBc5uwpQ75JdvL9A296XiIPY2D3mQTVC1tcOr0PjkZXPyPHA60GoKKqt49RTdghSpKWyyZom9ETlidWL7YQ/s1600/scPDSI+time+series.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="310" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgrmv64DpetWuZrYapSbJB_KEI7d-om2mB_WfHL6dWiUlrpindWjoMFPCb0CBc5uwpQ75JdvL9A296XiIPY2D3mQTVC1tcOr0PjkZXPyPHA60GoKKqt49RTdghSpKWyyZom9ETlidWL7YQ/s400/scPDSI+time+series.jpg" width="400" /></a></div>
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<span style="font-family: "Trebuchet MS", sans-serif;">
</span><span style="font-family: "Trebuchet MS", sans-serif;">This time series show the strong interannual and decadal
variability of European drought. The authors show that winter SST has a strong
impact in determining drought variability over Europe in the upcoming summer
through different large-scale teleconnection patterns. By the use of
correlation analysis they reveal the existence of three coupled modes of summer
drought pattern and winter SST anomalies with different timescales:</span><br />
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><br />
<div class="MsoListParagraphCxSpFirst" style="margin: 0in 0in 0pt 0.5in; mso-list: l0 level1 lfo1; text-indent: -0.25in;">
<span style="font-family: "Trebuchet MS", sans-serif;"><span style="mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;"><span style="mso-list: Ignore;">1.<span style="font-size-adjust: none; font-stretch: normal; font: 7pt/normal "Times New Roman";"> </span></span></span>The first coupled mode represents the long-term
warming trend in global SST caused by anthropogenic greenhouse gasses, in
addition to a tripole-like pattern in SST resembling the positive phase of
the <b style="mso-bidi-font-weight: normal;">NAO</b>. </span></div>
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><br />
<div class="MsoListParagraphCxSpMiddle" style="margin: 0in 0in 0pt 0.5in; mso-list: l0 level1 lfo1; text-indent: -0.25in;">
<span style="font-family: "Trebuchet MS", sans-serif;"><span style="mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;"><span style="mso-list: Ignore;">2.<span style="font-size-adjust: none; font-stretch: normal; font: 7pt/normal "Times New Roman";"> </span></span></span>The second coupled mode is associated with an
inter-annual SST pattern in the Pacific which resembles the cold phase of <b style="mso-bidi-font-weight: normal;">ENSO</b> (<a href="http://www.pmel.noaa.gov/tao/elnino/la-nina-story.html">La Niña</a>) together with the
decadal fluctuation in extratropical SST resembling the <a href="http://en.wikipedia.org/wiki/Pacific_decadal_oscillation">Pacific DecadalOscillation</a> (<b style="mso-bidi-font-weight: normal;">PDO</b>). </span></div>
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><br />
<div class="MsoListParagraphCxSpLast" style="margin: 0in 0in 10pt 0.5in; mso-list: l0 level1 lfo1; text-indent: -0.25in;">
<span style="font-family: "Trebuchet MS", sans-serif;"><span style="mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;"><span style="mso-list: Ignore;">3.<span style="font-size-adjust: none; font-stretch: normal; font: 7pt/normal "Times New Roman";"> </span></span></span>The last coupled mode is associated with strong
multidecadal variability in SST across the Atlantic basin which corresponds
with the <b style="mso-bidi-font-weight: normal;">AMO</b>, also for the
interannual variability. In <a href="http://www.climatechangeatlantic.blogspot.co.uk/2012/11/a-shift-in-european-climate-in-1990_20.html">a previous post</a> I described how the AMO
exerts a strong influence on European summertime climate, including drought. As
they write in the paper: “<em><span style="color: black; mso-bidi-font-family: AdvPTimes;">According to Briffa et al. (</span><span style="mso-bidi-font-family: AdvPTimes;">2009<span style="color: black;">) the summers of 1921, 1976, and 1990 were among the driest
in the last 250 years, all these dry summers occurring during a cold North
Atlantic phase of the AMO.</span></span></em><span style="color: black; mso-bidi-font-family: AdvPTimes;">”</span></span></div>
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><br />
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<span style="font-family: "Trebuchet MS", sans-serif;">I’ve previously described the NAO- and the AMO’s impact of
heat and drought on the European climate. It now turns out that drought across
Europe is associated with four different modes of variability! (NAO, ENSO, PDO
and AMO). Drought is not an easy thing to define given the complexity of the
phenomenon, and there also exists several types of drought. So to say that
drought across Europe is determined exactly by these four modes of variability
is of course just looking at the big picture.</span></div>
Anonymoushttp://www.blogger.com/profile/16989515393582579555noreply@blogger.com0tag:blogger.com,1999:blog-1756017225505722248.post-38425599448652314692012-11-29T02:48:00.000-08:002012-11-29T02:49:26.551-08:00AMO vs global warming<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">I ended my last blog post by asking a few questions related
to the future of the North Atlantic Ocean and the AMO. What can we expect
looking forward? As I also mentioned in the end of the last post, the AMO is
very hard to model and therefore also to predict. Almost all models have a
difficulty in simulating the AMO variability. So we really don’t know what to
expect into the future…</span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br /></span>
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">But we do have knowledge and numerous studies which we can
draw theories from. We do know that the AMO is a dominant mode of variability
in the North Atlantic SST with a duration of 55-80 years (Wei & Lohmann
2012). But the question we are asking these days, and the same question I asked
in my previous post, is to what extent the North Atlantic Ocean is influenced
by global warming relative to the internal variability (AMO), and what
consequences that may have for the future. </span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
A study based on observational data by <a href="http://www.agu.org/pubs/crossref/2010/2010GL042743.shtml">Wang and Dong</a> in 2010
finds that both global warming and AMO variability make a contribution to the
recent warming in the North Atlantic basin, and that their relative
contribution is approximately equal. They also find (after removing a linear
trend and the seasonal cycle found in observational records) that atmospheric
CO<sub><span style="font-size: x-small;">2</span></sub> anomalies show a multidecadal variation approximately coinciding
with the cold and warm phases of the AMO (see figure below).</span><br />
<br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEitA3ZBtSRn2Z_G3MQD8kD91BBG55PSZS3XP9L1WY8Y5bABGRCehjBkZ5Qu8guJx7vq0tBuwJ4_MT-Ru4ujGxA2YSInE_lXM_OhxSxxc7UeuvYrf2SO1tfaHqUi_370g4Nu99BCIChC9xA/s1600/CO2+anomalies.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEitA3ZBtSRn2Z_G3MQD8kD91BBG55PSZS3XP9L1WY8Y5bABGRCehjBkZ5Qu8guJx7vq0tBuwJ4_MT-Ru4ujGxA2YSInE_lXM_OhxSxxc7UeuvYrf2SO1tfaHqUi_370g4Nu99BCIChC9xA/s1600/CO2+anomalies.jpg" /></a></div>
</div>
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">They discuss that this may be related to the ocean’s CO<sub><span style="font-size: x-small;">2</span></sub>
uptake through ocean circulation and the strength of the <a href="http://www.eoearth.org/article/Atlantic_meridional_overturning_circulation">AMOC</a>. As mentioned in
my <a href="http://climatechangeatlantic.blogspot.co.uk/2012/11/a-shift-in-european-climate-in-1990_20.html">previous post</a>, the phases of the AMO are determined by AMOC variability. There
also exists a relationship between solubility of CO<sub><span style="font-size: x-small;">2</span></sub> and ocean
temperature, which says that a warmer ocean leads to a release of CO<sub><span style="font-size: x-small;">2</span></sub>.
So a warm (cold) phase of the AMO will lead to a release (uptake) of CO<sub><span style="font-size: x-small;">2</span></sub>
to (from) the atmosphere. Summarized the
warming of the North Atlantic due to AMO variability may influence global SST
via the increase of atmospheric CO<sub><span style="font-size: x-small;">2</span></sub>.</span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br />
So based on findings
from this study it seems like there exists a two-way relationship between North
Atlantic Ocean temperatures and CO<sub><span style="font-size: x-small;">2</span></sub> concentrations in the
atmosphere. Increased amounts of atmospheric CO<sub><span style="font-size: x-small;">2</span></sub> contributes to a
warming of the North Atlantic basin. The other way around, the warm phase of
the AMO with anomalously warm SSTs contributes to an increase in atmospheric CO<sub><span style="font-size: x-small;">2</span></sub>,
which further increases global warming. With future predictions of further CO<sub><span style="font-size: x-small;">2
</span></sub>increases this does not look good. But then again the AMO is a mode of
natural variability internal to the climate system which likely will change to
a negative phase sometime in the future, which will reverse the picture. It is
pretty clear, considering the huge climatic impacts of the AMO, that further research
is needed in this field. </span>Anonymoushttp://www.blogger.com/profile/16989515393582579555noreply@blogger.com0tag:blogger.com,1999:blog-1756017225505722248.post-79624009707461801282012-11-20T07:49:00.001-08:002012-11-20T07:49:33.083-08:00A shift in the European climate in the 1990 linked to the AMO<span style="font-family: Trebuchet MS;"><span style="font-family: Times New Roman;">
</span><span style="font-family: "Trebuchet MS", sans-serif;">During the 1990s there was a substantial shift in the
European climate. We experienced more wet summers in northern Europe and more
hot and dry summers in southern Europe relative to earlier summers. What caused
this anomalous shift in European climate? Was it related to anthropogenic
climate change or is it just a case of natural variability in the climate
system? </span></span><br />
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;"><br /></span>
<span class="Apple-style-span" style="font-family: 'Trebuchet MS', sans-serif;">A <a href="http://www.nature.com/ngeo/journal/v5/n11/pdf/ngeo1595.pdf" target="_blank">recently published paper</a> in Nature by Sutton & Dong explains
that it is the North Atlantic Ocean and a pattern of variability known as the
Atlantic Multidecadal Oscillation (AMO) that is responsible for this shift in European
climate. The AMO is a multidecadal variation in North Atlantic sea surface
temperature (SST) which fluctuates between anomalously warm and anomalously
cool phases, each lasting several decades at a time. Its instrumental record is
shown below, where you can see the shifts about the mean state over the years.</span><br />
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<span style="font-family: Trebuchet MS;"><br /></span>
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<span style="font-family: Trebuchet MS;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi127W8Vz18hVCLm0FD08NBjkpqaYBgfENZW9X7X0Yfhuv1CL8RNxbSoendwAcaO98dZiRlwPvKngGA4cHnhbK3p9QwC_4DDq9NcpDo2f16OebXuONiU2-lFtZLRSYjzncbRYN_58J0EfM/s1600/AMO+index.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="231" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi127W8Vz18hVCLm0FD08NBjkpqaYBgfENZW9X7X0Yfhuv1CL8RNxbSoendwAcaO98dZiRlwPvKngGA4cHnhbK3p9QwC_4DDq9NcpDo2f16OebXuONiU2-lFtZLRSYjzncbRYN_58J0EfM/s400/AMO+index.jpg" width="400" /></a></span></div>
<span style="font-family: Trebuchet MS;"><span style="font-family: "Trebuchet MS", sans-serif;"><br /></span></span>
<span style="font-family: Trebuchet MS;"><span style="font-family: "Trebuchet MS", sans-serif;">As you can see there was a shift towards a positive phase in
the mid 1990s, and Sutton & Dong finds evidence that this has caused the
climate shift in European summers from 1996-2010. AMO-like variations in SST
are closely related to the variations in the Atlantic Meridional Overturning Circulation
(AMOC) (an Atlantic circulation feature part of the global ocean circulation which
transports heat from lower- to higher latitudes, where the water gets so dense
that it sinks and deepwater is formed, thereby <i style="mso-bidi-font-style: normal;">overturning</i> circulation). Evidence suggests that the 1990s warming
of the Atlantic Ocean was largely caused by an acceleration of the AMOC in
response to the persistent positive phase of the winter NAO, which I’ve talked
about in <a href="http://www.climatechangeatlantic.blogspot.co.uk/2012/10/recent-trend-in-nao-index.html" target="_blank">previous posts</a>.</span></span></div>
<span style="font-family: Trebuchet MS;">
<div class="MsoNormal" style="margin: 0in 0in 10pt;">
<span style="font-family: "Trebuchet MS", sans-serif;">As you also can see from the above figure, there was another warm
period between about 1930 and 1960, with very similar pattern of North Atlantic
SST as for the recent warm period. Previous research has shown that this warm
state, relative to the cold period 1960-1990, forced a certain pattern of sea
level pressure (SLP), surface air temperatures (SATs) and precipitation over
Europe. So the similarities in North Atlantic SST anomalies between the two
warm periods suggest that similar climate impacts may have been excited in the
recent warm period. And that is true – very similar patterns in SLP, SATs and
precipitation does exist for the two warm periods for spring (MAM), summer (JJA) and autumn (SON).
This is shown in the model simulation figures below (which show anomalies
relative to the intervening cool phases), which also show the typical climate pattern associated with a warm phase of the AMO:</span><br />
<br />
<div style="text-align: center;">
<b>SLP</b></div>
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<img border="0" height="312" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjRxkNIJMMTm1xKTjCAJz8HswVd1tbF7pn7xsSp1AqiCJqY9QWLqOiZ9wtSwlyQsOGnGzaIsYEeR-kghqlZ6jH_bwWhnISytM-ZMJSmURRJc6QyOZakCvUx7tsfkg5GQD38mRLpheLh9xQ/s640/SLP.jpg" width="640" /></div>
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<b>SAT</b></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjk6QBGbQMwKvoEGIXKl4Nl7ml9M3bvqMsiYa4XqyLmDwcTfUGE3jpmzhLIyVL9l2AmnZcddzVURvrNUo7Z4lalFu-FASvK67snodm8eetTDWn_jOZ5vg4_AEki88zBl2JnhJk_jYBMbUc/s1600/SAT.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="392" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjk6QBGbQMwKvoEGIXKl4Nl7ml9M3bvqMsiYa4XqyLmDwcTfUGE3jpmzhLIyVL9l2AmnZcddzVURvrNUo7Z4lalFu-FASvK67snodm8eetTDWn_jOZ5vg4_AEki88zBl2JnhJk_jYBMbUc/s640/SAT.jpg" width="640" /></a></div>
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<br /></div>
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<b>Precipitation</b></div>
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<br /></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi4XlnNQtGDo90dDVLLLhPQrCAL69wQnbsLGM-KbLpQ_UFyX3SVCx2GNwonby-tkMNiQTcMI9jLY3cQrShipvBvst7TjjxYKuXGuRU07BhwWDAMSk3xNam7-d_6pAHzc-kzzvNXAtjfn5Y/s1600/Precip.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="394" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi4XlnNQtGDo90dDVLLLhPQrCAL69wQnbsLGM-KbLpQ_UFyX3SVCx2GNwonby-tkMNiQTcMI9jLY3cQrShipvBvst7TjjxYKuXGuRU07BhwWDAMSk3xNam7-d_6pAHzc-kzzvNXAtjfn5Y/s640/Precip.jpg" width="640" /></a></div>
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</span><br />
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<span style="font-family: "Trebuchet MS", sans-serif;">This is in close agreement with the observed recent <a href="http://www.guardian.co.uk/uk/2012/aug/30/rain-chaos-summer-wettest-years" target="_blank">wet northern Europe summers</a> and <a href="http://www.farming.co.uk/news/article/7078" target="_blank">hot and dry southern Europe summers</a>. It
is also consistent with the observed conditions for recent springs and autumns.
As Sutton & Dong explains; </span></div>
<div style="text-align: center;">
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><span style="font-family: "Trebuchet MS", sans-serif;"><i style="mso-bidi-font-style: normal;">“The consistency between the two warm North Atlantic </i><i style="mso-bidi-font-style: normal;"><span style="mso-bidi-font-family: t1-mini-regular;">periods
in the patterns of anomalies in SAT, precipitation and SLP is strong
circumstantial evidence that the North Atlantic Ocean was an important driver
of these decadal changes in European climate.”</span></i><span style="mso-bidi-font-family: t1-mini-regular;"><o:p></o:p></span></span></div>
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><span style="mso-bidi-font-family: t1-mini-regular;"><span style="font-family: "Trebuchet MS", sans-serif;"></span></span><br />
<span style="mso-bidi-font-family: t1-mini-regular;"><span style="font-family: "Trebuchet MS", sans-serif;">So what can we expect into the future? How long will the AMO
stay in its warm phase, keeping the European climate locked in the same
pattern? And are there also external forcings contributing to the recent warming,
like anthropogenic greenhouse gasses? These are hard questions to answer given
that the AMO is a natural mode of variability internal to the climate system,
varying with no particular pattern, which makes it hard to predict in the
future. I will try to dig deeper into this for my next blog post and further
discuss these questions.<o:p></o:p></span></span>
</span><span style="font-family: Times New Roman;">
</span><br />
<br />Anonymoushttp://www.blogger.com/profile/16989515393582579555noreply@blogger.com2tag:blogger.com,1999:blog-1756017225505722248.post-55535851843621815292012-11-13T04:30:00.000-08:002012-11-13T04:46:52.093-08:00Atlantic hurricane activity and climate change<span style="font-family: "Trebuchet MS", sans-serif;">I think it is about time that I mention hurricane Sandy here
on my blog. The “Superstorm” is the largest Atlantic hurricane on record and
we’ve all heard about the devastating effects it had on parts of the Caribbean
and the US east coast. It is easy to conclude that this unusually intense storm
was caused by climate change and global warming, as a relatively </span><a href="http://www.guardian.co.uk/environment/blog/2012/oct/30/hurricane-sandy-supersized-climate-change" target="_blank"><span style="font-family: "Trebuchet MS", sans-serif;">newly released article</span></a><span style="font-family: "Trebuchet MS", sans-serif;"> in the Guardian Environment discusses. </span><br />
<span style="font-family: "Trebuchet MS", sans-serif;"></span><br />
<span style="font-family: "Trebuchet MS", sans-serif;">
I decided to do my own little research in the field and read
a paper published in Nature by </span><a href="http://www.nature.com/ngeo/journal/v3/n3/pdf/ngeo779.pdf" target="_blank"><span style="font-family: "Trebuchet MS", sans-serif;">Knutson et al. (2010)</span></a><span style="font-family: "Trebuchet MS", sans-serif;"> on tropical cyclones and
climate change. In this study they try to answer whether past changes in
tropical cyclone activity have exceeded the variability expected from natural
causes. First they state that there is a relationship between tropical Atlantic
SSTs and the upward trend in the Atlantic hurricane activity. They then go on
by looking at cyclone- <b style="mso-bidi-font-weight: normal;">(1)</b>
frequency, <b style="mso-bidi-font-weight: normal;">(2)</b> intensity, <b style="mso-bidi-font-weight: normal;">(3)</b> rainfall, and <b style="mso-bidi-font-weight: normal;">(4)</b> genesis, tracks, duration and surge flooding separately. It is
hard to model tropical cyclone activity because it depends on so many
constantly changing factors (such as tropical SSTs), but with the improvements
in models and analyzing techniques, the authors could raise their confidence
level concerning cyclone-activity projections and conclude the following:</span><br />
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><br />
<div class="MsoListParagraphCxSpFirst" style="margin: 0in 0in 0pt 0.5in; mso-list: l0 level1 lfo1; text-indent: -0.25in;">
<span style="font-family: "Trebuchet MS", sans-serif;"><span style="mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;"><span style="mso-list: Ignore;">1.<span style="font-size-adjust: none; font-stretch: normal; font: 7pt/normal "Times New Roman";">
</span></span></span>It remains uncertain whether past changes in
tropical cyclone frequency have exceeded the variability expected through
natural causes. In the future it is likely that <u>global mean
tropical-cyclone-frequency will either decrease or remain essentially unchanged</u>
owing to greenhouse warming. Among the proposed mechanisms is the weakening of
the tropical circulation.</span></div>
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><br />
<div class="MsoListParagraphCxSpMiddle" style="margin: 0in 0in 0pt 0.5in; mso-list: l0 level1 lfo1; text-indent: -0.25in;">
<span style="font-family: "Trebuchet MS", sans-serif;"><span style="mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;"><span style="mso-list: Ignore;">2.<span style="font-size-adjust: none; font-stretch: normal; font: 7pt/normal "Times New Roman";">
</span></span></span>Future surface warming and changes in the mean
thermodynamic state of the atmosphere (as projected by climate models) will
lead to an <u>increase in tropical cyclone intensity</u> – both in the mean
intensities and in the frequency of cyclones at higher intensity levels. </span></div>
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><br />
<div class="MsoListParagraphCxSpMiddle" style="margin: 0in 0in 0pt 0.5in; mso-list: l0 level1 lfo1; text-indent: -0.25in;">
<span style="font-family: "Trebuchet MS", sans-serif;"><span style="mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;"><span style="mso-list: Ignore;">3.<span style="font-size-adjust: none; font-stretch: normal; font: 7pt/normal "Times New Roman";">
</span></span></span>Atmospheric moisture content has increased in
recent decades in many regions, and will continue to increase as the atmosphere
warms. This should increase rainfall rates in systems such as tropical
cyclones, but that has not been established by existing studies. <u>Tropical-cyclone-related
rainfall rates are likely to increase </u>with greenhouse warming<u> </u>though.
</span></div>
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><br />
<div class="MsoListParagraphCxSpLast" style="margin: 0in 0in 10pt 0.5in; mso-list: l0 level1 lfo1; text-indent: -0.25in;">
<span style="font-family: "Trebuchet MS", sans-serif;"><span style="mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;"><span style="mso-list: Ignore;">4.<span style="font-size-adjust: none; font-stretch: normal; font: 7pt/normal "Times New Roman";">
</span></span></span>There is no conclusive evidence that any
observed changes in tropical cyclone genesis, tracks, duration and surge
flooding exceeded the variability expected from natural causes. However, with
highly confident predictions of future sea-level rise, costal environments are
more vulnerable to storm-surge flooding.</span><br />
<span style="font-family: Trebuchet MS;"></span><br />
<div class="separator" style="clear: both; text-align: center;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhQxAyK9o4G82ZTZy3Zolh0Dm2yTUPiumqdqupPQ1xTSfy7-Hms5e92MtrMUs53DTS79w_H8_V_6a7jjAFzRT4aUgnF6vSAdaUlAalSczHTqGC72hdRBfno7j7wfhEcYv6b0gJWWF8dt1g/s1600/SST_PDI.png" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="229" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhQxAyK9o4G82ZTZy3Zolh0Dm2yTUPiumqdqupPQ1xTSfy7-Hms5e92MtrMUs53DTS79w_H8_V_6a7jjAFzRT4aUgnF6vSAdaUlAalSczHTqGC72hdRBfno7j7wfhEcYv6b0gJWWF8dt1g/s320/SST_PDI.png" width="320" /></a></div>
<em>Time series of late summer tropical Atlantic sea surface temperature (blue) and the Power Dissipation Index (green), a measure of hurricane activity which depends on the frequency, duration, and intensity of hurricanes over a season. From <a href="ftp://texmex.mit.edu/pub/emanuel/PAPERS/Factors.pdf" target="_blank">Emanuel (2007).</a> As you can see there exists a very close relationship.</em></div>
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><br />
<span style="font-family: "Trebuchet MS", sans-serif;"></span><br />
<span style="font-family: "Trebuchet MS", sans-serif;"></span><br />
<span style="font-family: "Trebuchet MS", sans-serif;"></span><br />
<span style="font-family: "Trebuchet MS", sans-serif;"></span><br />
<span style="font-family: "Trebuchet MS", sans-serif;">So it seems like climate change and global warming is not
directly affecting hurricane activity in the Atlantic Ocean, but more
indirectly through ocean warming, sea-level rise and circulation changes. Concerning
hurricane Sandy, anthropogenic activity has definitely played at least a
supporting role in its intensity and destructiveness. </span><br />
<span style="font-family: "Trebuchet MS", sans-serif;"></span><br />
<span style="font-family: "Trebuchet MS", sans-serif;">
It was like Sandy was a sign from above to the American
people, right before the election, that they and their political leaders should
open their eyes and realize that climate change is happening and needs to be integrated
in their politics. If hurricane Sandy had a saying in who won the election, we
can literally thank God Obama won. That way </span><a href="http://www.guardian.co.uk/environment/damian-carrington-blog/2012/nov/07/obama-climate-change-us-election-president?CMP=twt_fd" target="_blank"><span style="font-family: "Trebuchet MS", sans-serif;">there will at least be some effort</span></a><span style="font-family: "Trebuchet MS", sans-serif;">
to slow down the rate of anthropogenic climate change, easing future damage by
superstorms like Sandy.</span>Anonymoushttp://www.blogger.com/profile/16989515393582579555noreply@blogger.com0tag:blogger.com,1999:blog-1756017225505722248.post-16698000759076672022012-11-07T05:51:00.001-08:002012-11-07T05:53:49.108-08:00Tropical SST relation to the strong NAO phase<span style="font-family: "Trebuchet MS", sans-serif;">In my last post I discussed and reviewed <span lang="EN-GB" style="mso-ansi-language: EN-GB;">a paper about the reason for the strong
positive phase in the NAO index and the associated warming over Europe, and
concluded that it is not just internal variability of NAO – forcing factors such
as anthropogenic greenhouse gasses do play a central role. <o:p></o:p></span></span><br />
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><br />
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="font-family: "Trebuchet MS", sans-serif;">I would
like to point out a couple of other studies done by <a href="http://www.sciencemag.org/content/292/5514/90.full.pdf" target="_blank">Hoerling et al. (2001)</a> and
<a href="http://acacia.ucar.edu/cas/asphilli/Docs/2004_climdyn_jhurrell_part1.pdf" target="_blank">Hurrell et al. (2004)</a> that investigates another forcing factor on winter North
Atlantic climate; low-latitude sea surface temperatures (SSTs). They
investigate tropical SST forcing on the NAO winter index by using atmospheric
general circulation models (AGCMs) forced with the observed evolution of global
SSTs since 1950. They find that variance in tropical SSTs in their models
simulates a positive trend in the NAO index, and the special pattern of the
simulated trend agrees with that observed. They present evidence that tropic-wide
changes in the atmospheric circulation associated with warming surface waters
over the tropical Indian and western Pacific Oceans and associated rainfall
over the tropical Indian Ocean produce a North Atlantic anomaly pattern very
much like the positive index phase of the NAO.<o:p></o:p></span></span><br />
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="font-family: "Trebuchet MS", sans-serif;"></span></span><br />
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="font-family: "Trebuchet MS", sans-serif;">Hurrell et
al. published a part two paper (<a href="http://www.cgd.ucar.edu/cas/asphilli/Docs/2004_climdyn_hoerling_part2.pdf" target="_blank">Hoerling et al. 2004</a>) following up their first
study, where they explore their previous findings. They support their earlier
arguments and state that SST warming over the Indian Ocean sector is the key
forcing mechanism for the observed trend in the NAO, pointing out the striking
similarity in the time series of Indian Ocean and North Atlantic climate
variations (see figure below). </span></span><br />
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="font-family: "Trebuchet MS", sans-serif;"><o:p></o:p></span></span><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjb65-TlyOCmUjXB3pfIxLqtK3f5BeH94cFxPrKFfh1jEBpeZMg208yQ8kfkZTWEEOAoyudk67T660zhii6yNFT6Bc-lSi5nLJlPgRCcGS7gu5O7mDsbs2QGa_t4LKohpg6uWr5jm0pM5A/s1600/indianSST-NAOindex.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="400" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjb65-TlyOCmUjXB3pfIxLqtK3f5BeH94cFxPrKFfh1jEBpeZMg208yQ8kfkZTWEEOAoyudk67T660zhii6yNFT6Bc-lSi5nLJlPgRCcGS7gu5O7mDsbs2QGa_t4LKohpg6uWr5jm0pM5A/s400/indianSST-NAOindex.jpg" width="347" /></a></div>
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="font-family: "Trebuchet MS", sans-serif;"></span></span><br />
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="font-family: "Trebuchet MS", sans-serif;">They also
discuss whether this warming contains an anthropogenic component and conclude
that it most likely contains a signature of anomalous greenhouse gas forcing. They’re
then using this as an argument to why models are not correctly simulating the
magnitude of the NAO phase (as I described in the previous post); <i style="mso-bidi-font-style: normal;">“Our theory for an anthropogenic, dynamical
oceanic forcing of North Atlantic climate change by Indian Ocean SST anomalies
might clarify why neither unforced AGCMs nor unforced coupled models are able
to produce winter NAO index trends of the magnitude observed since 1950”.<o:p></o:p></i></span></span><br />
<span style="font-family: "Trebuchet MS", sans-serif;">
</span><span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="font-family: Calibri;"><span style="font-family: "Trebuchet MS", sans-serif;"></span></span></span><br />
<span lang="EN-GB" style="mso-ansi-language: EN-GB;"><span style="font-family: Calibri;"><span style="font-family: "Trebuchet MS", sans-serif;">So again it
does look like we’re to some extent causing climate change and warming over
the Euro-Atlantic part of the hemisphere with our increasing emissions of
greenhouse gasses, though through a different mechanism from these
papers point of view.</span> <o:p></o:p></span></span>Anonymoushttp://www.blogger.com/profile/16989515393582579555noreply@blogger.com0tag:blogger.com,1999:blog-1756017225505722248.post-35395580635022677012012-10-31T08:26:00.002-07:002012-10-31T08:34:06.039-07:00The NAO - natural variability vs forcing factors<div class="MsoNormal" style="margin: 0cm 0cm 10pt;">
<span style="font-family: "Trebuchet MS", sans-serif;">Since we know that the NAO exerts a big influence on the wintertime climate over the Euro-Atlantic part of the hemisphere, one might think that winter warming in recent times is due to this natural mode of variability and that we’ve overestimated the effects of anthropogenic greenhouse gasses (GHGs) in the atmosphere. In my last post I talked about the pronounced positive trend in the NAO index and questioned whether this is part of its natural variability or if the NAO might be influenced by external forcing.<o:p></o:p></span></div>
<div class="MsoNormal" style="border-bottom: medium none; border-left: medium none; border-right: medium none; border-top: medium none; margin: 0cm 0cm 10pt;">
<span style="font-family: "Trebuchet MS", sans-serif;">I’ve read a paper by <a href="http://www.springerlink.com/content/a5gu08786m46m202/" target="_blank">Stephenson et al. (2006)</a> that is trying to answer this question by looking at the response of wintertime NAO to increasing concentrations of atmospheric carbon dioxide. In the study they’ve used 18 global coupled general circulation models (GCMs) with a 1% per year increase in concentrations of CO<sub>2</sub>. 15 of the models were able to simulate the NAO pressure dipole, which is one of its main features, but none of the models were able to reproduce a decadal trend as strong as that observed in its later time series. 14 out of those 15 models simulated an increasing trend in the NAO index with increasing CO<sub>2</sub> concentrations, but the magnitude of the response was generally small and highly model-dependent. Despite their different NAO responses, all the models showed a similar increasing NAO-like pattern in temperature and precipitation trends with increasing CO<sub>2</sub> concentrations, like warming and increasing precipitation over Northern Europe (which is fitting with my memories of “bad” Norwegian winters). <o:p></o:p></span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhAXE8_kzLHU3GTVQ5VJlAzfEU5QbIEJWrJtyEmIBnCDGvM9LEZ7yWeeF-5rtNNxD8u0snMa15AfK9ov0Mvs3iDjQkoOSoc1DsuZ3ax2dYTmhmdw2L4FP6m6X7HJv84WjqeKAE96UgxIfA/s1600/vinternorge.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="184" qea="true" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhAXE8_kzLHU3GTVQ5VJlAzfEU5QbIEJWrJtyEmIBnCDGvM9LEZ7yWeeF-5rtNNxD8u0snMa15AfK9ov0Mvs3iDjQkoOSoc1DsuZ3ax2dYTmhmdw2L4FP6m6X7HJv84WjqeKAE96UgxIfA/s640/vinternorge.jpg" width="640" /></a></div>
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Norway; what I would call a "good" winter....................and a "bad" winter</div>
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<span style="font-family: "Trebuchet MS", sans-serif;">Although the models in the study do suggest that the NAO show a weak positive response to increasing amounts of CO<sub>2</sub>, the authors note that with the large amounts of model uncertainty as they found in this study <i style="mso-bidi-font-style: normal;">“one has to be exceedingly careful about making inferences concerning future climate change”</i>. Since the models simulate differences in NAO response, but similar responses in temperature and precipitation over Europe, the authors suggest that NAO is not the key determining factor for such changes. <o:p></o:p></span></div>
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<span style="font-family: "Trebuchet MS", sans-serif;">So it seems like we cannot blame warming over Europe solely on the NAO's natural variability. There are so many factors interacting in the climate system and it is hard to know what is responsible for what. But it looks like there are external forcing mechanisms, including anthropogenic greenhouse gasses, contributing to the extreme winter climate over recent decades. <o:p></o:p></span></div>
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<span style="font-family: "Trebuchet MS", sans-serif;">Various studies have investigated NAO trends and ended up with similar results. I would like to quote one of them; <i style="mso-bidi-font-style: normal;">“[…]most authors agree that greenhouse gases are likely to be at least partly responsible for the long-term trend in the boreal winter NAO index”</i>, <a href="http://www.cccma.ec.gc.ca/papers/ngillett/PDFS/AGU_GM134_CH9_NEW.pdf" target="_blank">Gillett et al. 2003</a>. It is clear though, that a lot more research needs to be done on this subject to understand to what extent anthropogenic forcing from greenhouse gasses influence wintertime climate over Europe.<o:p></o:p></span></div>
Anonymoushttp://www.blogger.com/profile/16989515393582579555noreply@blogger.com0tag:blogger.com,1999:blog-1756017225505722248.post-25946405688614786702012-10-23T05:19:00.000-07:002012-10-23T08:43:27.661-07:00Recent trend in the NAO index<span style="mso-bidi-font-family: TimesNewRomanPS;"><span style="font-family: "Trebuchet MS", sans-serif;">Below you can
see the winter station-based index of the NAO, based on the difference
of normalized sea level pressure (SLP) between Lisbon, Portugal and Reykjavik,
Iceland. Red bars indicate positive index and blue bars negative index. (Figure found
</span><a href="http://climatedataguide.ucar.edu/guidance/hurrell-north-atlantic-oscillation-nao-index-station-based#" target="_blank"><span style="font-family: "Trebuchet MS", sans-serif;">here</span></a><span style="font-family: "Trebuchet MS", sans-serif;">).</span></span><br />
<span style="font-family: "Trebuchet MS", sans-serif; mso-bidi-font-family: TimesNewRomanPS;"><o:p></o:p></span><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhtdh8zCSNCxIdua0nAeox1_DKIfn897I4ePvPOd4z2kmhz_sZdh7bqYmebdyeDVZ7k575HHv7Jx1-HPStMSsDbf93nV1bwVWeuzuxRcQ16PngTglQUhG2632_wKIM3uiv1aEZj0gOmqXc/s1600/NAO+index.gif" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: "Trebuchet MS", sans-serif;"><img border="0" height="348" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhtdh8zCSNCxIdua0nAeox1_DKIfn897I4ePvPOd4z2kmhz_sZdh7bqYmebdyeDVZ7k575HHv7Jx1-HPStMSsDbf93nV1bwVWeuzuxRcQ16PngTglQUhG2632_wKIM3uiv1aEZj0gOmqXc/s640/NAO+index.gif" width="640" /></span></a></div>
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<span style="mso-bidi-font-family: TimesNewRomanPS;"><span style="font-family: "Trebuchet MS", sans-serif;">As you can see
from this figure, over recent decades the winter index has exhibited a
pronounced trend toward a strong positive index, especially prominent around year 1990. As described in my last blog post, this positive trend
corresponds to lowered surface pressure over the Arctic and increased surface
pressure over the subtropical North Atlantic, with anomalously strong
westerlies. I also talked about the related climatic conditions and weather
patterns.<o:p></o:p></span></span></div>
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<span style="font-family: "Trebuchet MS", sans-serif;"><span style="mso-bidi-font-family: TimesNewRomanPS;">I am originally
from Norway, a country thought of being a freezing cold, snow-covered place during
winter. But as I look back, I have a hard time remembering those really snowy, proper
winters where we could go skiing all winter long. I mostly remember grey skies,
drizzling rain, and "slush" snow in the streets, which obviously is a
consequence of the recent positive trend of the NAO, which typically give
northern Europe mild and wet winter conditions.<o:p></o:p></span></span></div>
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<span style="mso-bidi-font-family: TimesNewRomanPS;"><span style="font-family: "Trebuchet MS", sans-serif;">So it is pretty
clear that the NAO controls, in large parts, climate conditions during winter
around the North-Atlantic basin. But don’t you find this recent positive index
phase a bit striking? Yes, the NAO is a <u>natural</u> mode of climate
variability, but why does it now exhibit this upward trend, prominent in its
time series? To me this seems a bit “unnatural”. Are there external factors, such as anthropogenic forcing,
influencing the NAO and causing this positive trend? These are
questions I will be discussing in my next blog post, so stay tuned!<o:p></o:p></span></span></div>
Anonymoushttp://www.blogger.com/profile/16989515393582579555noreply@blogger.com0tag:blogger.com,1999:blog-1756017225505722248.post-36863603954145564742012-10-18T09:59:00.000-07:002012-10-18T10:06:32.515-07:00The North Atlantic Oscillation<div class="MsoNormal" style="margin: 0cm 0cm 10pt;">
<span style="font-family: Calibri;">I thought I would dedicate this second blog post to the North Atlantic Oscillation (NAO)– one of the leading modes of natural variability in the Northern Hemisphere (NH). It is important to get to know this phenomenon a bit better when looking into what is causing the extreme weather and climate around the North Atlantic basin, and that because the NAO has shown to exert a strong influence on the climate over large parts of the Northern Hemisphere.</span></div>
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The NAO is characterized by a fluctuation in sea level pressure (SLP) between the Arctic basin and the mid-latitudes. The differences in SLP drive a westerly flow that moves through the high- and low pressure systems, creating a zonal planetary-scale wave pattern across the Northern Hemisphere. Therefore the NAO is also characterized by an out-of-phase relation in the strength of this zonal flow along <span style="mso-bidi-font-family: 'Times New Roman';">~55° and 35°. The westerly flow is strongest during winter when the largest anomalies in SLP occur, as you can see in the figure below (<a href="http://www.cgd.ucar.edu/cas/jhurrell/Docs/naobook.ch1.pdf" target="_blank">from Hurrell et. al</a>). Dark (light) shading indicate negative (positive) departures from the mean . So the characters of the NAO are most pronounced during the Northern Hemisphere winter months (boreal winter).</span></div>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjlFftoBJ-OobZMIhCMpV3BHiCevDpSwmNzaPZH2L3mj1qKWMEFXQX3a6Db0ly5KJGppRMPDsVZHfEnr1ez8LyU_5FN3Fj6xxLZGIjbXLfYCY8gZ9InC-gguP9_ImT6SuzCrKxuF6eZDoE/s1600/NAM1.bmp" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="355" nea="true" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjlFftoBJ-OobZMIhCMpV3BHiCevDpSwmNzaPZH2L3mj1qKWMEFXQX3a6Db0ly5KJGppRMPDsVZHfEnr1ez8LyU_5FN3Fj6xxLZGIjbXLfYCY8gZ9InC-gguP9_ImT6SuzCrKxuF6eZDoE/s640/NAM1.bmp" width="640" /></a></div>
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Where these high- and low-pressure systems are located, and how strong they are, determine the phase or the index of the NAO. All results are based on the winter months when the variability is largest. A high index polarity is defined as anomalous strong subpolar westerlies, with a deeper than normal low pressure over the polar region and a higher than normal subtropical high pressure . A low index polarity is defined as anomalous weak westerlies and pressure systems. <span lang="EN-US" style="mso-ansi-language: EN-US;">A consequence of these contrasting polarities is that anomalies in climate on seasonal time scales typically occur over large geographical regions.<span style="mso-spacerun: yes;"> </span>These anomalies in climate include surface air temperature, SST, changes in storminess and precipitation, ocean heat content, ocean currents and their related heat transport, and sea ice cover. <o:p></o:p></span></div>
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<span lang="EN-US" style="mso-ansi-language: EN-US;">High index conditions are characterized by a northeastward shift and an increased intensity in the stormtrack across the North Atlantic, from northeastern North America to northern Europe, which tend to give wet and mild conditions in these areas. Stronger northerly winds over Greenland and northeastern Canada carry cold air southward and decrease land temperatures and SST over the northwest Atlantic. Warming over North America associated with the stronger clockwise flow around the subtropical Atlantic high-pressure center is also notable. </span>Evaporation exceeds precipitation over much of Greenland and the Canadian Arctic during high NAO index winters. Drier conditions also occur over much of central and southern Europe, the Mediterranean and parts of the Middle East, whereas more precipitation than normal falls from Iceland through Scandinavia. <span style="font-family: 'Calibri','sans-serif'; font-size: 11pt; line-height: 115%; mso-ansi-language: EN-GB; mso-ascii-theme-font: minor-latin; mso-bidi-font-family: 'Times New Roman'; mso-bidi-language: AR-SA; mso-bidi-theme-font: minor-bidi; mso-fareast-font-family: 'MS Mincho'; mso-fareast-language: JA; mso-fareast-theme-font: minor-fareast; mso-hansi-theme-font: minor-latin;">During low index periods the weaker (and fewer) winter storms crossing the Atlantic on a more west-easterly path bring moist air into the areas surrounding the Mediterranean. Northern Europe and the eastern part of the US experiences cold air outbreaks and hence snowy weather conditions.</span><span lang="EN-US" style="mso-ansi-language: EN-US;"> Below you can see a (much generalized) picture of the climate patterns created by the NAO in its different phases (found on Google).</span><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEigySJA3fZOXnzOvHj5o0ZqW8OQDDouUGQHnOyKC31nQCloKBzVCHImFF0Xct7HDYif4Pr6V7WFPpjvrlUVkrTFCm1N5d9Ou8W1QnrwfwOheiGvr_ENrXJrL1pBz5UBJQXnkxqYcMC2bAs/s1600/NAM2.bmp" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="332" nea="true" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEigySJA3fZOXnzOvHj5o0ZqW8OQDDouUGQHnOyKC31nQCloKBzVCHImFF0Xct7HDYif4Pr6V7WFPpjvrlUVkrTFCm1N5d9Ou8W1QnrwfwOheiGvr_ENrXJrL1pBz5UBJQXnkxqYcMC2bAs/s640/NAM2.bmp" width="640" /></a></div>
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<span lang="EN-US" style="mso-ansi-language: EN-US;">Phu, I know that was a lot in one post, but it covers the most essential information about the NAO, which is important to know in order to be able to look at how anthropogenic activity is influencing our weather versus these natural modes of variability. In my next post I will look into if/ how anthropogenic activity is influencing the NAO and what consequences that may have/are having on our weather and climate. </span></div>
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<span lang="EN-US" style="mso-ansi-language: EN-US;"><em>If you're hungry for more or want a more detailed overview of the NAO, I suggest you read the paper I already referenced to; </em><a href="http://www.cgd.ucar.edu/cas/jhurrell/Docs/naobook.ch1.pdf" target="_blank"><em>An Owerview of the North Atlantic Oscillation</em></a><em>.</em></span></div>
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</o:p></span>Anonymoushttp://www.blogger.com/profile/16989515393582579555noreply@blogger.com2tag:blogger.com,1999:blog-1756017225505722248.post-59282656638133482912012-10-12T04:01:00.000-07:002012-10-19T02:21:24.153-07:00My very first post!<span lang="EN-GB"><span style="font-family: inherit;">Welcome to my blog! Here I will be posting
news stories, scientific research and personal thoughts about the topic of the
blog. I am going to discuss whether or not anthropogenic activity has got
anything to do with the extreme weather events that have occurred in Europe and
North America over the past years, like wet summers, flooding, heat waves,
droughts, extreme winters, snow chaos and storms, or if it is solely due to the
natural modes of variability in the climate system.</span></span><br />
<span style="font-family: inherit;">
</span><br />
<span lang="EN-GB"><span style="font-family: inherit;">For example, this summer has been a season
of record braking extreme weather. It has been the wettest summer in the UK for
100 years, and it has generally been an unusually wet summer across northern
and central Europe. In the Black Sea region of Russia at least 103 people were
killed by intense flooding caused by sudden heavy rainfall. In the US nearly
two thirds of the nation experienced some level of drought, with 39 percent of
the nation suffering from severe to extreme drought, destroying farmers’ crops
and livestock. Colorado experienced its worst wildfire season in a decade, with
half a dozen lives lost. In June Florida was hit by tropical storm Debby, which
caused extensive flooding, several tornadoes, and high winds. </span></span><br />
<br />
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<span lang="EN-GB"><span style="font-family: inherit;"></span></span><br />
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjl_vC9Hxs8uhLkCsuaEcWF1B30cd1rPhrKtYCWR37nBnMukS04jvX1reYqf1HpW4ShRGuamrLNiyXd8TMQjRsF4lrzcjWU52RSlub4ithVx-jakaIW797SDh6nDmxgFwleju8P1xGD7C4/s1600/blog1.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><span style="font-family: inherit;"></span></a><br /></div>
<span lang="EN-GB"><span style="font-family: inherit;"></span></span><br />
<span style="font-family: inherit;"><span lang="EN-GB">What is up with this weather? Can we blame
it on ourselves and our emissions? How are the predictions for the future? Is
it only going to get worse? These are questions I’ll be discussing further on
this blog, and I hope you will find it interesting. </span></span><br />
<span style="font-family: inherit;"><span lang="EN-GB"></span></span><br />
<span style="font-family: inherit;"><span lang="EN-GB">I will leave you with this video from the Global Climate News channel on YouTube that is trying to explain one mechanism behind the extreme climate of 2012. Although focusing on America, it will give a taste of what this blog will be about. </span></span><br />
<span style="font-family: inherit;"><span lang="EN-GB"></span></span><br />
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<iframe allowfullscreen='allowfullscreen' webkitallowfullscreen='webkitallowfullscreen' mozallowfullscreen='mozallowfullscreen' width='320' height='266' src='https://www.youtube.com/embed/Vd7sAQWQzso?feature=player_embedded' frameborder='0'></iframe></div>
<span style="font-family: inherit;"><span lang="EN-GB"></span></span><br />Anonymoushttp://www.blogger.com/profile/16989515393582579555noreply@blogger.com0