The response of a stochastically forced ENSO model to observed off-equatorial wind stress forcing

This study investigates the response of a stochastically forced coupled atmosphere-ocean model of the equatorial Pacific to off-equatorial wind stress anomaly forcing. The intermediate-complexity coupled ENSO model comprises a linear, first baroclinic mode, ocean shallow water model with a steady-state, two-pressure level (250 and 750 mb) atmospheric component that has been linearized about a state of rest on the B plane. Estimates of observed equatorial region stochastic forcing are calculated from NCEP-NCAR reanalysis surface winds for the period 1950-2006 using singular value decomposition. The stochastic forcing is applied to the model both with and without off-equatorial region wind stress anomalies (i.e., poleward of 12.5 latitude). It is found that the multiyear changes in the equatorial Pacific thermocline depth background state induced by off-equatorial forcing can affect the amplitude of modeled sea surface temperature anomalies by up to 1C. Moreover, off-equatorial wind stress anomalies increased the modeled amplitude of the two biggest El Nino events in the twentieth century (1982/83 and 1997/98) by more than 0.5C, resulting in a more realistic modeled response. These equatorial changes driven by off-equatorial region wind stress anomalies are highly predictable to two years in advance and may be useful as a physical basis to enhance multiyear probabilistic predictions of ENSO indices. (c) 2009 American Meteorological Society.