On Connection of Northern Hemispheric Circulation to ENSO.pdf

On Connection of Northern Hemispheric Circulation to ENSObyKwang-Y. KimFlorida State University, Florida submitted to Monthly Weather ReviewFebruary 16, 2000 AbstractThe linkage of Northern Hemisphere (NH) circulation, best known in terms of the Pa-cic/North American (PNA), Tropical/Northern Hemisphere (TNH), and North AtlanticOscillation (NAO) patterns, to El Ni~no-Southern Oscillation (ENSO) remains elusive andis still in scientic controversy. In this study, 200-mb height anomaly and tropical andNorth Pacic sea surface temperature anomaly (SSTA) have been analyzed using cyclosta-tionary EOFs (CSEOFs), which accounts for the seasonal dependence of spatial patterns.Then, 200-mb height anomaly patterns have been related to the tropical and the NorthPacic SSTA modes via regression analysis of the principal component (PC) time series.Results indicate a well-established connection of the PNA pattern to ENSO and also asecondary link to North Pacic SST variability. Henceforth, neither of the two sources ofPNA variability can be rebued. Results also show seasonally-dependent and complex re-sponse of NH circulation to tropical Pacic SST variability involving all three fundamentalmodes of NH circulation. 1 1. IntroductionENSO connection to the NH circulation has been reported in earlier studies (Trenberthand Hurrell 1994; Graham 1994; Jacobs 1994). Recent studies, however, report only a weaklink of the PNA pattern to ENSO (Deser and Blackmon 1995; Zhang et al. 1996). Instead,PNA variability is explained as the response to a SST mode in the North Pacic, whichis independent of ENSO (Deser and Blackmon 1995). The present study attempts toinvestigate and resolve these controversial views using the CSEOF technique which is bestsuited for time-dependent patterns of variability. One of important design considerationsof the experiment was to resolve seasonal dependence of spatial patterns. Despite thestrong seasonal dependency of NH circulational changes and also ENSO modes,