Intraseasonal Fluctuations at the onset of the 1997–98 El Nino

Air-sea interactions associated with two strong convective events in the equatorial (10S-10N) west Pacific during northern Winter 1996-97 are analyzed. During the first event (Dec. 1996) the equatorial west Pacific cooled by 1 degree C as a result of strong evaporative cooling (due to strong westerly winds) and reduced shortwave heating (due to enhanced cloud cover) at the ocean surface. This cooling took place over a one month time span, reducing anomalously warm waters to climatological values. Sea surface temperatures (SST) in the west Pacific did not recover from this cooling and, so, the event contributed to the slow (i.e., superseasonal) evolution of SST. During the second convective event (Feb. 1997), two important fluctuations of SST occurred. In the west Pacific, SST cooled by one degree as a result of off-equatorial (3N-10N) Ekman upwelling through an anomalously large vertical temperature gradient in the upper ocean. The cooling was short lived, however, and SSTs returned to climatological values after the convection subsides. In addition, the central Pacific warmed by one degree as a result of zonal temperature advection coincident with the passage of an oceanic Kelvin wave that was forced by surface stress anomalies during the convective event. These SST anomalies persisted thereby contributing to the slow evolution of SST in the central Pacific. This could indicate an important link between intraseasonal variations of convection and SST evolution during ENSO.