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Abstract:
In this study the variability in the tropical Pacific and Atlantic Ocean connected to the wind driven shallow tropical and subtropical overturning cells is investigated.
The leading sea surface temperature (SST) mode in the western equatorial Pacific(the Nino4 region) is a decadal mode. The analysis of observed SST and simulations with the MPI-OM model forced with the NCEP reanalysis suggests that
the decadal SST variability is closely correlated with the variability of the tropical (TCs) and subtropical (STCs) cells. Changes in the strength of the cells lead the changes in SST. The lag becomes larger with increasing latitude, indicating that the STCs adjust slower than the narrow TCs. The correlation pattern between indices for the strength of the (S)TCs and the SST in the tropical Pacific shows an equatorial horseshoe-like pattern, which indicates that the influence of the cells is larger in the Nino4 than in the Nino3 region. For the TCs, a correlation between the cell strength and the SST is also found on interannual timescales.
Different processes play important roles in the connection between the cells and the SST. Apart from the anomalous upwelling of colder subsurface water, the zonal and meridional advection of water from the cold tongue by
anomalous currents contributes to the changes in SST. In the warm pool area, where the vertical temperature gradient in the upper layer is relatively weak, the horizontal temperature advection is of the same order as the vertical one. The surface heat flux acts in most regions as a damping. Thus, it is the ocean dynamics that drive the decadal SST variability in large regions of the tropical Pacific.
The analysis of a multi-century integration with the coupled atmosphere-ocean model ECHAM4/OPYC supports the results obtained from the simulations forced with the NCEP reanalysis. The coupled integration suggests the existence of a coupled feedback loop between the SST and the STCs, which can also be seen in the coupled model ECHAM5/MPI-OM.
Considering the influence of the Atlantic (S)TC onto the tropical SST, no uniform results are found for the different models and timescales. Since the variability in the tropical Atlantic is relatively small compared to the tropical Pacific, one reason might be that mainly noise is considered here.
Some influence from the tropical Pacific onto the Atlantic (S)TC via changes in the trade winds over the Atlantic Ocean can be found. During years with anomalously warm (cold) Nino3 SST the strength of the cells increases (decreases).
The results of a scenario integration of the ECHAM4/OPYC model show a relativ complex response of the (S)TCs to greenhouse warming. The analysis of the 1% integrations from the coupled model intercomparison project CMIP2 shows
different responses of the cells within the different models, making a prediction of (S)TC sensitivity to an increase in the CO2 concentration difficult.
