Impacts of ENSO diversity on the western Pacific and North Pacific subtropical highs during boreal summer

This study examines the interannual variability of the North Pacific high during boreal summer of 1979-2008 to understand how its leading modes are related to the two types of El Nino-Southern Oscillation (ENSO). In the observations, the first empirical orthogonal function mode (EOF1) is characterized by an in-phase variation between the western Pacific subtropical high (WPSH) and the northeastern Pacific subtropical high (NPSH), while the second mode (EOF2) is characterized by an out-of-phase WPSH-NPSH variation. The EOF1 mode dominates during the post early-1990s period and is a forced response to sea surface temperature (SST) variations over the maritime continent and tropical central Pacific (CP) regions related to developing CP ENSOs. Its in-phase WPSH-NPSH relationship is established through the ENSO-induced meridional atmospheric circulation, Pacific-North American pattern and eddy-zonal flow interaction over the North Pacific. In contrast, the EOF2 mode dominates prior to the early-1990s and is partially a forced response to tropical Indian Ocean (IO) and eastern Pacific (EP) SST variations related to decaying EP ENSOs and partially a coupled atmosphere-ocean response to western North Pacific SST variations. Of the 28 Atmospheric Model Intercomparison Project models, most (71%) realistically simulate the EOF1 mode but only a few (14%) simulate the EOF2 mode. The roughly 50% underestimation in the strength of the EOF2 mode is due to model deficiencies in properly representing the atmospheric circulation responses to the IO and EP SST variations. This deficiency may be related to underestimations of the strength of the mean Walker circulation in the models.