Journal
CLIMATE DYNAMICS
Volume 44, Issue 7-8, Pages 2215-2228Publisher
SPRINGER
DOI: 10.1007/s00382-014-2299-x
Keywords
SAM; ENSO; Tropical forcing; Partially coupled model; Relaxation technique
Categories
Funding
- BMBF MiKlip Project MODINI
- DFG under ISOLAA [1158]
- GEOMAR
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Tropical influence on the austral summer Southern Annular Mode (SAM) over the ERA-40 period 1960/1961-2001/2002 is investigated using (1) a partially coupled climate model (PCM) driven by observed wind stress and (2) a version of the ECMWF atmospheric model by means of a relaxation technique. We show that the tropical influence in the PCM is dominated by El Nino Southern Oscillation (ENSO) whereas the relaxation experiments suggest an additional influence independent of ENSO. In the observations, we find that the simultaneous influence of ENSO on the summer SAM was much stronger after 1979 than before, with the consequence that the ensemble mean of the PCM captures around 50 % of the interannual variance of the SAM after 1979 and less than 10 % before. Nevertheless, in the ensemble mean of the PCM, the relationship between ENSO and the summer SAM is stable throughout the whole period 1960/1961-2001/2002, and it is the individual ensemble members that exhibit a non-stationary relationship like that found in the observations. It follows that variability not related to the observed wind forcing used to drive the PCM is important for obscuring the ENSO/SAM relationship. The experiments using relaxation show that tropical forcing was important for both the interannual variability and the trend of the summer SAM, even before 1979. Adding the observed extratropical sea surface temperature and sea-ice (SSTSI) to the tropical relaxation runs improves the model performance, indicative of a positive feedback from extratropical SSTSI onto the SAM.
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