Modelling of the air-sea interaction over the Gulf Stream and North Atlanic Current fronts

The goal of ATMOS is to improve the understanding of processes affecting North Atlantic climate, concentrating on air-sea interaction over the Gulf Stream and North Atlantic Current fronts. These regions are associated with the release of a large amount of heat and moisture to the atmosphere at the entry region to the North Atlantic storm track and thus could be important players for determining low-frequency variability (decadal time scales) and predictability of the climate in the North Atlantic region and beyond.

The role of this region for making long-range (seasonal and decadal) predictions of the atmosphere has been topic of several studies in the past (e.g. Ratcliffe and Murray (1970), Palmer and Sun (1985), Rodwell et al. (1999), Rodwell and Folland (2002)). Interest has recently been revived by Minobe et al. (2008) who have shown the importance of resolving the Gulf Stream front for the successful simulation of the rainfall climatology in this region, contrasting the atmospheric response to a high versus a low resolution representation of the front in an atmospheric general circulation model (AGCM).
 

Hand (2010) showed, that the ECHAM AGCM (Roeckner et al., 2003) which will be used in the MiKlip prediction system, is able to reproduce the climatological atmospheric patterns in the Gulf Stream Region. Using a 5-member ensemble-run forced by 138 years of observed SSTs (HadISST), evidence could be found, that a consiberadle part of the models multiannual to decadal atmospheric variability can be linked to that of the SST.
Goals

The questions to be adressed within ATMOS are in particular:

• What role does the resolution of the SST forcing play for the response of the AGCM?
• What local atmospheric patterns go along with specific SST patterns and is there any evidence for teleconnections emerging from this region in the model?

To answer these questions, it is planned to perform two different kinds of experiments using the ECHAM5 AGCM:

• Experiments in different resolution using time varying observed satellite SST/reanalyis SST.
• Sensitivity experiments, using fixed SST anomalies. These SST patterns will be retrieved from the time varying runs/observations and represent different idealized states of the SST front.