Wednesday 7 November 2012

Tropical SST relation to the strong NAO phase

In my last post I discussed and reviewed 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.

I would like to point out a couple of other studies done by Hoerling et al. (2001) and Hurrell et al. (2004) 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.

Hurrell et al. published a part two paper (Hoerling et al. 2004) 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).


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); “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”.

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.

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