The role of the western North Pacific in modulation of climate over North America

The role of the upper-ocean heat-flux convergences in the Kuroshio region in modulation of climate anomalies over North America is investigated using atmospheric GCM coupled to a mixed layer ocean model. Heat flux convergence anomalies over tropical and western North Pacific are obtained from the Ocean GCM hindcast run in which the model was forced with the NCEP/NCAR Reanalysis wind stress anomalies, and are applied as forcing to the ocean of the coupled model. The underlying philosophy states that North Pacific/North American climate anomalies on interannual scales result from anomalous conditions in the Pacific equatorial upwelling regions, and the North Pacific Kuroshio-Oyashio extension (KOE), where anomalies in the ocean's thermocline are efficiently coupled to the surface. In these areas, changes of upwelling, thermocline depth, water mass properties and stratification, and advection impact a heat flux convergence of the ocean's surface mixed layer. In response to this forcing, the coupled atmosphere - ocean mixed layer system adjusts, and generates anomalies of sea surface temperature (SST) over the entire Pacific, including the central and eastern North Pacific where the oceanic mixed layer is effectively insulated from the thermocline. These SST anomalies are accompanied by the changes in the strength of the subtropical jet and the shift in the location of the storm tracks. The atmospheric response is global in nature and includes the response in the precipitation and surface temperature anomalies over North America. The probabilistic distribution functions of the precipitation and surface temperature averaged over particular regions exhibit the changes in median and standard deviation depending on the sign of the low-frequency variability of the forcing over the KOE region. The role of the forcing in the western North Pacific in modulating the tropical teleconnection patterns is investigated by comparing composites for different polarities of the NINO3.4 SST index and KOE SST index. Statistically significant change of the response to the tropical forcing is detected in both surface and free atmosphere variables. For instance, the positive SST anomalies in the KOE region lead to the enhancement of the 500-mb height pattern El-Nino response and to its longitudinal elongation. We also isolated the role of the tropical Pacific by comparing these integrations with the results obtained from the run with only tropical forcing. In addition the role of the seasonal cycle of the mid-latitude forcing in setting the atmospheric response over North America was studied by comparing the integrations mentioned above with the integrations of the coupled model forced with the low-pass filtered mixed layer heat flux convergence anomalies over tropical and western North Pacific. The inclusion of the seasonal cycle improves the skill of the hindcast of temperature and precipitation anomalies over Alaska, Western Canada and Northwestern United States.