Oceanic Rossby Waves over Eastern Tropical Pacific of Both Hemispheres Forced by Anomalous Surface Winds after Mature Phase of ENSO

摘要

The present study examined ENSO-related wind forcing contribution to off-equatorial Rossby wave formations in the eastern tropical regions of the North and South Pacific using satellite altimeter data and atmospheric reanalysis data during the period of 1993-2013. After mature phases of ENSO events, the sea surface height anomaly fields showed that off-equatorial Rossby waves propagated westward along 118N and 88S from the eastern Pacific. Starting longitudes of the westward propagation were distant from the eastern coast, especially for weak El Nino events in the 2000s, in contrast to the strong 1997/98 El Nino event in which the propagations started from the coast. Based on observational data, it was hypothesized that the Rossby waves could be formed by off-equatorial zonal belts of wind stress curl anomalies (WSCAs) in 1358-908W rather than by wave emissions from the eastern coast. A numerical model forced only by WSCAs, that is, without wave emissions from the coast, successfully reproduced observed features of the Rossby waves in 1808-1208W, supporting the study's hypothesis. During mature phases of El Nino events, equatorially symmetric negative sea level pressure anomalies (SLPAs) resulting from hydrostatic adjustment to the underlying warm sea surface temperature anomalies dominated over the eastern tropical Pacific. Anomalous surface easterlies blowing around the negative SLPA area as geostrophic winds were a major contributor in forming the anticyclonic WSCAs. The polarity of the anomalies is reversed during La Nina events. Therefore, spatial patterns of the SLPAs associated with the ENSO events are necessary to understand the Rossby wave formations.