Contributions of Atmosphere-Ocean Interaction and Low-Frequency Variation to Intensity of Strong El Nino Events since 1979

摘要

Evolutions of oceanic and atmospheric anomalies in the equatorial Pacific during four strong El Ninos (1982/83, 1991/92, 1997/98, and 2015/16) since 1979 are compared. The contributions of the atmosphere-ocean coupling to El Nino-associated sea surface temperature anomalies (SSTA) are identified and their association with low-level winds as well as different time-scale variations is examined. Although overall SSTA in the central and eastern equatorial Pacific is strongest and comparable in the 1997/98 and 2015/16 El Ninos, the associated subsurface ocean temperature as well as deep convection and surface wind stress anomalies in the central and eastern equatorial Pacific are weaker during 2015/16 than that during 1997/98. That may be associated with a variation of the wind-SST and wind-thermocline interactions. Both the wind-SST and wind-thermocline interactions play a less important role during 2015/16 than during 1997/98. Such differences are associated with the differences of the low-level westerly wind as well as the contribution of different time-scale variations in different events. Similar to the interannual time-scale variation, the intraseasonal-interseasonal time-scale component always has positive contributions to the intensity of all four strong El Ninos. Interestingly, the role of the interdecadal-trend time-scale component varies with event. The contribution is negligible during the 1982/83 El Nino, negative during the 1997/98 El Nino, and positive during the 1991/92 and 2015/16 El Ninos. Thus, in addition to the atmosphere-ocean coupling at intraseasonal to interannual time scales, interdecadal and longer time-scale variations may play an important and sometimes crucial role in determining the intensity of El Nino.