MiKlip first phase: ATMOS

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).

ATMOS - Fig. 1
Figure 1: Observed (HadISST) SST and simulated convective precipitation averaged over the Gulf Stream extension (65W:40W, 38N:40N) in boreal summer. Simulations were performed with ECHAM5. Both time series are 5 year low-pass filtered. The correlation between the two is high (0.7).

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.

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.
ATMOS - Fig. 2
Figure 2: ANOVA explained variance (in %) of convective precipitation due to the boundary forcing for summer (a) and winter (b). As before, the ANOVA was performed on 5-year low-pass filtered and seasonally averaged data from the 138-year time varying 5-member ensemble run. Underlying contours show the seasonal mean climatological SSTs (in °C).

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GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel
Prof. Dr. Richard Greatbatch
Ralf Hand

The use of a flow field correction technique for alleviating the North Atlantic cold bias with application to the Kiel

2015 - Climate Model. Ocean Dynamics, Vol. 65, pp. 1079-1093

Drews, A. | R. J. Greatbatch, H. Ding, M. Latif, and W. Park

Simulated response to inter-annual SST variations in the Gulf Stream region

2014 - Climate Dynamics, Vol. 42 (3), pp. 715-731

Hand, R., | Keenlyside, N.S., Omrani, N.-E. and Latif, M.