Sack Lunch Seminar (SLS)

Abstract:
The termination of large El Nino events usually starts at the turn of the calendar year. This process limits the seasonal variance growth of eastern equatorial Pacific sea surface temperature anomalies and strongly contributes to seasonal variance modulation of ENSO. It is also a major source of predictability and shortens the duration of El Nino events to considerably less than half of the dominant interannual periodicity. I will present a new theoretical framework to explain the emergence of four key features that result from the tight interaction between ENSO and the annual cycle of zonal winds in the western tropical Pacific: ENSO's seasonal variance modulation, the seasonally locked termination of large El Nino events, spectral combination tones and 2:1 phase locking. The theoretical framework is supported by a hierarchy of atmosphere/ocean model experiments that demonstrate that a characteristic wind anomaly pattern, which arises as a nonlinear combination tone between ENSO and the annual cycle and involves the Philippine Anticyclone and a southward shift of ENSO-related wind anomalies, is the key terminator of strong El Nino events. The seasonal development of the South Pacific Convergence Zone plays an essential role in establishing the dynamics of this prominent wind pattern. The resulting ocean dynamical response includes the generation of an upwelling Kelvin wave and a meridionally asymmetric discharge process.