EQUATORIAL WAVE SEQUENCE ASSOCIATED WITH WARM POOL DISPLACEMENTS DURING THE 1986-1989 EL-NINO LA-NINA

Citation
J. Picaut et T. Delcroix, EQUATORIAL WAVE SEQUENCE ASSOCIATED WITH WARM POOL DISPLACEMENTS DURING THE 1986-1989 EL-NINO LA-NINA, J GEO RES-O, 100(C9), 1995, pp. 18393-18408
Citations number
80
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Oceanografhy
Journal title
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS
ISSN journal
2169-9275 → ACNP
Volume
100
Issue
C9
Year of publication
1995
Pages
18393 - 18408
Database
ISI
SICI code
2169-9275(1995)100:C9<18393:EWSAWW>2.0.ZU;2-7
Abstract
In the Pacific equatorial band the zonal displacement of the eastern e dge of the warm pool, subject to insignificant seasonal variations, is dominated by strong interannual variations almost in phase with the S outhern Oscillation Index. Over the 1971-1973 El Nino-La Nina period, Gill (1983) suggested that such displacement was caused solely by hori zontal advection by zonal current anomaly. Basin-wide Geosat-derived z onal surface current anomalies are available during the 1986-1989 peri od, and they agree quite well with observed equatorial currents in the western and central Pacific. Within the 4 degrees N-4 degrees S equat orial band the cumulative contribution of first-baroclinic Kelvin and first-symmetric (m = 1) Rossby modes to surface zonal equatorial curre nt anomalies account for most of the Geosat-derived current variabilit y. From these Geosat-derived currents it is demonstrated that the east ward (westward) displacement of the eastern edge of the warm pool was primarily due to horizontal advection by zonal current anomalies durin g the 1986-1987 El Nino (1988-1989 La Nina). As a corollary, the El Ni no (La Nina) warm (cold) sea surface temperature (SST) anomaly in the central western Pacific was the result of anomalous zonal advection. T he 1986-1989 displacement of the warm pool appeared as a low-frequency movement resulting from high-frequency forcing, i.e., a succession of local wind forcing and its remote Kelvin and m = 1 Rossby wave respon ses in the equatorial waveguide. In the central western Pacific a m = 1 downwelling Rossby wave, issued from the eastern Pacific, shifted th e displacement from eastward to westward and therefore seems to have b een the cause for El Nino to turn to La Nina in boreal fall 1987. In t he eastern Pacific the simultaneous occurrence of free-propagating dow nwelling (upwelling) Kelvin waves with the seasonal SST warming (cooli ng) appeared necessary for the development of local El Nino (La Nina) SST anomaly. Over most of the equatorial Pacific, slow migration of SS T anomalies clearly showed up, distinct during El Nino and La Nina. Th ese zonal migrations appeared to result from the delayed association o f two phenomena: the zonal advection in the central western Pacific an d the arrival of downwelling (upwelling) Kelvin waves into the eastern Pacific, synchronous with the seasonal SST warming (cooling). Albeit different in terms of equatorial wave action, as compared to the delay ed action oscillator theory, our results suggest that zonal advection and wave dynamics were both important for the generation and migration of Fl Nino-Southern Oscillation SST anomalies in 1986-1989. Finally, our observational results about warm pool displacements, equatorial wa ve sequence, the shift from El Nino to La Nina, and the slow SST migra tions are presented in a schematic form involving sequential ocean-atm osphere coupling which appears continuously over the November 1986 to February 1989 period.