Abstract
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.
Highlights
This paper is a contribution to the special collection on El Niño–Southern Oscillation (ENSO) Diversity
Our analyses suggest that the Indian Ocean capacitor mechanism of Xie et al (2009) and the western North Pacific local air–sea coupling mechanism of Wang et al (2000) are probably more important in linking the ENSO influence to the EOF2 mode, but not the EOF1 mode, of Pacific high variability, because the both mechanisms emphasize the importance of the sea surface temperature (SST) anomalies in the IO and WNP regions
We find that the composite omega anomalies (Fig. 9c) show an enhanced meridional circulation that descends over the maritime continent (MC) region and ascends over the western Pacific subtropical high (WPSH) region, which is consistent with our suggestion that the developing central Pacific (CP) ENSO influences the WPSH intensity via an anomalous meridional circulation originating in the MC region
Summary
This paper is a contribution to the special collection on ENSO Diversity. The special collection aims at improving understanding of the origin, evolution, and impacts of ENSO events that differ in amplitude and spatial patterns, in both observational and modeling contexts, and in the current as well as future climate scenarios. Several physical mechanisms have been proposed to suggest how ENSO, which typically peaks in boreal winter (December–January–February; DJF) and decays afterward, can still exert an influence on the WPSH during the following summer These mechanisms emphasized the importance of sea surface temperature (SST) anomalies induced by the ENSO in different regions of the Indian and Pacific Oceans in producing this delayed influence. Due to the easterly shear-induced anticyclonic vorticity and surface friction, a subtropical divergence is produced on the northern flank of the Kelvin wave, which suppresses convection over the western North Pacific and intensifies the WPSH Through this Indian Ocean capacitor mechanism (Xie et al 2009), the ENSO-induced Indian Ocean SST anomalies can intensify the WPSH during the ENSO following summer.
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