Role of forcing and three major modes of variability on decadal climate variability and change

Abstract:

We investigate the role of three key modes of variability, the North Atlantic Oscillation, El-Niño Southern Oscillation and Southern Annular Mode as pacemakers of climate variability over the instrumental period. This is based on a particle filter data-assimilation technique that constrains the observed variability in a global climate model without the use of nudging, producing a near free running model simulation with modes of variability similar to those observed. As expected, the assimilated model is closer then free-running model simulations (without any assimilation), to annual temperature observations over many areas of the globe, in particular the tropics, the north Atlantic and Europe. Assimilating these three modes also reconciles simulated global volcanic cooling with that observed in addition to the warming observed in Northern European winters following volcanic eruptions. We focus on areas of the globe where the assimilation model shows a clear increase in explained decadal variance above what would be expected from just external forcing. In particular long-term trends in Eurasian temperature and the Asian winter monsoon as well as North Atlantic ocean sea surface temperature and the link with the overturning circulation. The results suggest where skilful predictions of these modes may lead to potential skill in decadal predictions.