The response of a stochastically forced ENSO model to observed off-equatorial wind stress forcing
McGregor, S and Holbrook, NJ and Power, SB, The response of a stochastically forced ENSO model to observed off-equatorial wind stress forcing, Journal of Climate, 22, (10) pp. 2512-2525. ISSN 0894-8755 (2009) [Refereed Article]
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 12.58
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 18C. Moreover, off-equatorial wind stress anomalies increased the modeled amplitude of
the two biggest El Nin˜ o events in the twentieth century (1982/83 and 1997/98) by more than 0.58C, 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.