The zonal gradient structures of wintertime SST anomalies in the equatorial Pacific and their connection to the Walker circulation

Abstract

Tropical Pacific (TP) air–sea coupling is generally accompanied by significant zonal sea surface temperature anomalies (SSTAs) gradient. Here, the wintertime zonal SSTA gradient structures were highlighted in the zonal mean departure SSTA (ZMD-SSTA) field, and separated into two orthogonal dominant components by empirical orthogonal function (EOF) analysis: the west–east dipole (EOF1, 70.7%) and the zonal tripole (EOF2, 23.0%) structure. EOF result comparisons, singular value decomposition (SVD), correlation and composite analyses shows that both the two gradient structures are highly coupled with Walker circulation (WC) but in obviously different spatiotemporal features, and together constitute the vast majority of the coupling variability. Further, the dipole structure can be largely reflected by the indices for eastern-type El Niño-Southern Oscillation (ENSO), but the central-type ENSO indices appears as a superposition of both dipole and tripole structures signals. In rare events of “uncoupled El Niño warming”, the two gradient structures can further describe the weaker zonal SSTA gradient under the significant local SSTA in eastern Pacific. These implies that the dipole and tripole structures here may be beneficial to independently reflect the two distinct sea-air coupling structures and supplement the gradient information during ENSO. In addition, the dipole zonal SSTA gradient structure and air-sea feedback associated with it decay rapidly and disappear before the following summer, while the tripole one shows a longer persistence lasting until autumn. The lead–lag relationship with Niño3.4 index indicated that the wintertime tripole SSTA gradient structures precedes the development of ENSO by 1-year.