Rectification of the intraseasonal wind forcing on ENSO and the interannual long equatorial waves

The rectification of the intraseasonal wind forcing on the El Niño-Southern Oscillation (ENSO) and associated interannual oceanic equatorial Kelvin and Rossby waves during 1990-2011 are investigated using an oceanic general circulation model, which is forced with daily wind of the National Centers for Environment Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis including and excluding intraseasonal exponents. The results show that the rectifications of the intraseasonal wind stress forcing on sea surface temperature anomalies (SSTA) and sea level anomalies (SLA) in the equatorial Pacific are much smaller than the total interannual variability. The oceanic equatorial Kelvin and Rossby waves decomposed from the model simulation show that the rectification of the intraseasonal wind stress forcing on the interannual Kelvin and Rossby waves is generally small in the equatorial Pacific, which provides a dynamical explanation for the above weak rectification of the intraseasonal wind forcing on interannual SSTA and SLA. Furthermore, the rectified interannual Kelvin waves are found to be much smaller than reflected Kelvin waves at the Pacific western boundary, thus the western boundary wave reflection plays a more important role in ENSO cycling compared with the intraseasonal wind stress forcing in the western equatorial Pacific Ocean.

Although the nonlinear rectification of the intraseasonal wind on the low-frequency wind stress can induce a small increase of amplitude of SSTA, SLA and interannual oceanic equatorial Kelvin and Rossby waves during the strong El Niño events, they are also much smaller than the total interannual anomalies. Besides the dynamics (wind stress), the imposed intraseasonal wind forcing acted on the model through the thermodynamics (wind speed). Rectification due to heat flux change induced by the intraseasonal wind speed is more important to the SSTA over the western equatorial Pacific, which is larger than the rectifications of the intraseasonal wind stress forcing. The influence the intraseasonal wind speed, however, can be negligible for the SLA, the interannual oceanic equatorial Kelvin and Rossby waves, and the SSTA over the eastern equatorial Pacific.