Attribution of atmospheric variations in the 1997-2003 period to SST anomalies in the Pacific and Indian Ocean basins

The individual impacts of sea surface temperature (SST) anomalies in the deep tropical eastern/central Pacific (DTEP) and Indo-western/central Pacific (IWP) on the evolution of the observed global atmospheric circulation during the 1997-2003 period have been investigated using a new general circulation model. Ensemble integrations were conducted with monthly varying SST conditions being prescribed separately in the DTEP sector only, in the IWP sector only, and throughout the World Ocean.

During the 1998-2002 subperiod, when prolonged La Niņa conditions occurred in DTEP and the SST in IWP was above normal, the simulated midlatitude atmospheric responses to SST forcing in the DTEP and IWP sectors tend to reinforce each other. The anomalous geopotential height ridges at 200 mb in the extratropics of both hemispheres exhibit a distinct zonal symmetry. This circulation change is accompanied by extensive dry and warm anomalies in many regions, including North America.

During the 1997-98 and 2002-03 El Niņo events, the SST conditions in both DTEP and IWP were above normal, and considerable cancellations are simulated between the midlatitude responses to the oceanic forcing from these two sectors.

The above findings are contrasted with those for the 1953-58 and 1972-77 periods, which are characterized by analogous SST developments in DTEP, but by cold conditions in IWP. It is concluded that a warm anomaly in IWP and a cold anomaly in DTEP constitute the optimal SST configuration for generating zonally elongated ridges in midlatitudes.

Local diagnoses indicate that the imposed SST anomaly alters the strength of the zonal flow in certain longitudinal sectors, which influences the behavior of synoptic-scale transient eddies farther downstream. The modified eddy momentum transports in the regions of eddy activity in turn feed back on the local mean flow, thus contributing to its zonal elongation. These results are consistent with the inferences drawn from zonal mean analyses, which accentuate the role of the eddy-induced circulation on the meridional plane.