Intraseasonal air-sea interactions at the onset of El Nino

The role of the Madden-Julian Oscillation (MJO) for the onset of El Nino is examined. A preliminary analysis compares tropical Pacific variability during three boreal winters that initially had similar distributions of sea surface temperature (SST). During the winter of 1996-97, strong MJO activity led to west Pacific cooling and central Pacific warming. Subsequently, convective activity migrated from the west Pacific into the central Pacific and the accompanying westerly surface wind anomalies promoted further central Pacific warming. Strong MJO activity was also evident during winter 1989-90 and the early stages of El Nino development were evident that winter with similar evolution to that during 1996-97. However, the development of El Nino was aborted in May 1990. It is speculated that a full El Nino did not develop during 1990 because the subsurface ocean structure would not support that development. The MJO was relatively quiescent during the winter of 1981-82. A strong El Nino developed during 1982, but not as rapidly as it did during 1997. Thus, MJO might be relevant to the timing and initial growth of El Nino rather than responsible for the event itself. A detailed analysis of dynamical interactions is performed for the winter of 1996-97, when two exceptionally strong MJOs accompanied substantial SST fluctuations in the Pacific. SST cooling in the west Pacific was, for the most part, forced by surface flux variations. Surface cooling was initiated by the reduction of short-wave surface fluxes due to enhanced cloud cover. Later, evaporative cooling during westerly wind anomalies reinforced that cooling. In February 1997, ocean dynamics were also important for the SST perturbation; off-equatorial upwelling, through an anomalously large vertical temperature gradient, contributed substantially to west Pacific cooling. During late March and early April 1997, central Pacific SSTs warmed in response to a downwelling Kelvin wave that was forced during the February MJO. That warming was primarily due to zonal temperature advection, promoted by strong eastward currents acting on an east-west temperature gradient. After the passage of the Kelvin wave, zonal currents, surface winds, and SST gradients did not revert to their pre-Kelvin wave values. As a result, temperature advection was negligible after the Kelvin wave, and SST continued to warm due to the positive surface heat flux that is typical for the region. So, if the MJO did contribute to this important SST warming through nonlinear interactions, then those interactions involve the coupling of atmospheric and oceanic dynamics.