Archives par mot-clé : NAO

Decadal predictability of North Atlantic blocking and the NAO

by Athanasiadis et al., June 3, 2020 in Nature (Open Access)


Abstract

Can multi-annual variations in the frequency of North Atlantic atmospheric blocking and mid-latitude circulation regimes be skilfully predicted? Recent advances in seasonal forecasting have shown that mid-latitude climate variability does exhibit significant predictability. However, atmospheric predictability has generally been found to be quite limited on multi-annual timescales. New decadal prediction experiments from NCAR are found to exhibit remarkable skill in reproducing the observed multi-annual variations of wintertime blocking frequency over the North Atlantic and of the North Atlantic Oscillation (NAO) itself. This is partly due to the large ensemble size that allows the predictable component of the atmospheric variability to emerge from the background chaotic component. The predictable atmospheric anomalies represent a forced response to oceanic low-frequency variability that strongly resembles the Atlantic Multi-decadal Variability (AMV), correctly reproduced in the decadal hindcasts thanks to realistic ocean initialization and ocean dynamics. The occurrence of blocking in certain areas of the Euro-Atlantic domain determines the concurrent circulation regime and the phase of known teleconnections, such as the NAO, consequently affecting the stormtrack and the frequency and intensity of extreme weather events. Therefore, skilfully predicting the decadal fluctuations of blocking frequency and the NAO may be used in statistical predictions of near-term climate anomalies, and it provides a strong indication that impactful climate anomalies may also be predictable with improved dynamical models.

Causes of the Rapid Warming of the North Atlantic Ocean in the Mid-1990s

by P. Homewood, April 9, 2020 in NotaLotofpeopleKnowThat


http://ocean.dmi.dk/arctic/icecover_30y.uk.php

Most of us are probably familiar with the pattern of Arctic sea ice decline between 1979 and 2007, followed by a period of relative stability. Most of the decline took place after the mid 1990s.

The decline is nearly always explained away as the result of global warming, but a couple of old studies show this not to be the case.

In 2011, Robson & Sutton found that the sub polar gyre underwent remarkable and rapid warming in the mid 1990s, and that this was linked to changes in the North Atlantic Oscillation:

North Atlantic Oscillation

by P. Homewood, March 19, in NotaLotofPeopleKnowThat


If you had not noticed (!), it has been a mild and wet start to the year here in the UK, and also across in NW Europe.

No doubt this will be linked to global warming in due course, but in fact it is simply weather, as the CET chart below proves:

Since the year started, temperatures have consistently been within the normal band. In other the sort of temperatures commonly seen at this time of year.

However, they have also been consistently in the top half of that band, rather than being spread between cold and warm, as would happen most years.

The reason for the weather we have had is, of course, the jet stream, or more precisely the North Atlantic Oscillation (NAO), which has been strongly and stubbornly positive all winter.

The Norwegian Centre for Climate Research CICERO spotted this mild weather coming back in December, and commented on 6th January:

The unusual warm temperatures this winter and forecasts indicating milder winter conditions for January, February and March in Europe are partly due to an atmospheric circulation pattern called the North Atlantic Oscillation, or NAO. This atmospheric circulation pattern explains well the weather we get in Europe, especially in winter.

https://cicero.oslo.no/no/posts/nyheter/unseasonal-temperatures-for-norway#

 

 

Our own Met Office explains the NAO phenomenon: