ENSO components of the Atlantic multidecadal oscillation and their relation to North Atlantic interannual coastal sea level anomalies

J Park,G Dusek

doi:10.5194/os-9-535-2013

Abstract

Abstract. The El Niño Southern Oscillation (ENSO) and the Atlantic Multidecadal Oscillation (AMO) are known to influence coastal water levels along the East Coast of the United States. By identifying empirical orthogonal functions (EOFs), which coherently contribute from the Multivariate ENSO Index (MEI) to the AMO index (AMOI), we characterize both the expression of ENSO in the unsmoothed AMOI, and coherent relationships between these indices and interannual sea level anomalies at six stations in the Gulf of Mexico and western North Atlantic. Within the ENSO band (2–7 yr periods) the total contribution of MEI to unsmoothed AMOI variability is 79%. Cross correlation suggests that the MEI leads expression of the ENSO signature in the AMOI by six months, consistent with the mechanism of an atmospheric bridge. Within the ENSO band, essentially all of the coupling between the unsmoothed AMOI and sea level anomalies is the result of ENSO expression in the AMOI. At longer periods we find decadal components of sea level anomalies linked to the AMOI at three southern stations (Key West, Pensacola, Charleston), but not at the northern stations (Baltimore, Boston, Portland), with values of coherence ranging from 20 to 50%. The coherence of MEI to coastal sea level anomalies has a different structure and is generally weaker than that of the ENSO expressed AMOI influence, suggesting distinct physical mechanisms are influencing sea level anomalies due to a direct ENSO teleconnection when compared to teleconnections based on ENSO expression in the AMOI. It is expected that applying this analysis to extremes of sea level anomalies will reveal additional influences.

Highlights

The El Nino Southern Oscillation (ENSO) has been identified as an important forcing on sea level anomalies of the western Pacific Ocean and the west coast of the United States (Sweet et al, 2009)
Since coherence between AMO index (AMOI)(ENSO) and the sea level anomaly response is near zero, the analysis suggests this component of sea level variability is coupled to ENSO forcings that are not expressed in the AMOI, for example, atmospheric pressure changes not reflected in North Atlantic SST
Concerning AMOI coupling without ENSO forcing, we find evidence that Atlantic SST variability as expressed in the unsmoothed AMOI is partially coherent with coastal sea level anomalies in a broadband centered on a period near 9 yr at the three southern stations of Key West, Pensacola and Charleston

Summary

Introduction

The El Nino Southern Oscillation (ENSO) has been identified as an important forcing on sea level anomalies of the western Pacific Ocean and the west coast of the United States (Sweet et al, 2009). Along the east coast of the United States storm surges have a positive correlation with the El Nino phase of ENSO where patterns of anomalously high sea levels are attributed to El Nino related changes in atmospheric pressure over the Gulf of Mexico and eastern Canada, and to the wind field over the continental shelf of the northeast United States (Sweet and Zervas, 2011). Guan and Nigam (2009) analyzed twentieth century Atlantic sea surface temperature

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Discussion

Conclusion