Abstract The Sea Surface Temperature Anomaly (SSTA) in tropical Atlantic during boreal spring and summer shows two dominant modes: a basin-warming and a meridional dipole mode, respectively. Observational and coupled model simulations indicate that the former induces a Pacific La Niña in the succeeding winter whereas the latter cannot. The basin-warming forcing induces a La Niña through a Kelvin wave response and the associated wind-evaporation-SST-convection (WESC) feedback over the northern Indian Ocean (NIO) and Maritime Continent (MC). Anomalous Kelvin wave easterly interacts with the monsoonal westerly, leading to a warm SSTA and a northwest-southeast oriented heating anomaly in NIO/MC, which further induces easterly and cold SSTA over the equatorial Pacific. In contrast, the dipole forcing has little impact on the Indian and Pacific Oceans due to the offsetting of the Kelvin wave to the asymmetric Atlantic heating. Further observational and modeling studies towards the Tropical North Atlantic (TNA) and Equatorial Atlantic (EA) SSTA modes indicate that the TNA (EA) forcing induces a CP- (EP-) type ENSO. In both cases, the Kelvin wave response and the WESC feedback over the NIO/MC are important in conveying the Atlantic’s impact. The difference lies in distinctive Rossby wave responses – A marked westerly anomaly appears in the equatorial eastern Pacific (EEP) for the TNA forcing (due to its westward location) while no significant wind response is observed in EEP for the EA forcing. The westerly anomaly prevents a cooling tendency in EEP through anomalous zonal and vertical advection according to a mixed-layer heat budget analysis.
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Mechanisms Determining Diversity of ENSO-Driven Equatorial Precipitation Anomalies
The longitudinal location of precipitation anomalies over the equatorial Pacific shows a distinctive feature with the westernmost location for La Niña, the easternmost location for eastern Pacific (EP) El Niño, and somewhere between for central Pacific (CP) El Niño, even though the center of the sea surface temperature anomaly (SSTA) for La Niña is located slightly east of that of CP El Niño. The mechanisms for such a precipitation diversity were investigated through idealized model simulations and moisture and moist static energy budget analyses. It is revealed that the boundary layer convergence anomalies associated with the precipitation diversity are mainly induced by underlying SSTA through the Lindzen–Nigam mechanism, that is, their longitudinal locations are mainly controlled by the meridional and zonal distributions of the ENSO SSTA. The westward shift of the precipitation anomaly center during La Niña relative to that during CP El Niño is primarily caused by the combined effects of nonlinear zonal moist enthalpy advection anomalies and the Lindzen–Nigam mechanism mentioned above. Such a zonal diversity is further enhanced by the “convection–cloud–longwave radiation” feedback, the SST-induced latent heat flux anomalies, and the advection of mean moist enthalpy by anomalous winds. This diversity in the longitudinal location of precipitation anomalies has contributions to the diversities in the longitudinal locations of anomalous Walker circulation and western North Pacific anomalous anticyclone/cyclone among the three types of ENSO.
more » « less- NSF-PAR ID:
- 10361937
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 35
- Issue:
- 3
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- p. 923-939
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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