An observational study of atmosphere-ocean interactions in the northern oceans on interannual and interdecadal time-scale
This study focuses on the air-sea interactions over the northern oceans from an observational point of view. Atmosphere-ocean interactions exhibit a strong seasonality in both the North Pacific and the North Atlantic. In winter, the atmospheric forcing of the ocean through modulating the latent and sensible heat fluxes is the dominant process of air-sea interaction for both oceans, although the interaction is stronger over the North Pacific. In summer, cloud-SST feedback between marine stratus clouds (MSC) and SST is the dominant interactive process over the North Pacific, whereas simple thermodynamic equilibration is dominant over the North Atlantic as represented by the strong correlation between SST anomalies and hemispheric-mean lower tropospheric temperature.The dominant large-scale SST pattern in the North Pacific seen in summertime persists for on the order of 9 months, longer than in other seasons. It is argued that the strong positive feedback between SST and MSC is responsible for the enhanced persistence of summertime SST. The strong persistence of summertime SST anomalies is also manifested in the strong season-to-season persistence of the large-scale SST anomaly pattern in the North Pacific, especially from summer to the following winter.The linear relationship between ENSO and extratropical climate variability is reevaluated. It is shown that a significant fraction of the variance of wintertime 500-mb height and North Pacific SST cannot be accounted for by ENSO and other tropical variability. It follows that the strong connection between these two fields is independent of ENSO. The SST variability in different parts of the Pacific is characterized by different time scales, namely interdecadal in the extratropics and interannual in the tropics (ENSO). The interdecadal and interannual variations are separable in time but not in space. They share similar features in several atmospheric and oceanic variables including SST, SLP, windstress, cold-season 500-mb height and land surface air temperature.The 1976-77 "regime shift" related to the warming in tropical SST and cooling in central North Pacific SST is shown to be but one manifestation of interdecadal variability. By using observations spanning the whole century it is demonstrated that a similar shift with opposite polarity around 1942-43 is just as significant, especially in the North Pacific.
- Atmospheric sciences