Intraseasonal air-sea interaction in the western Pacific and the onset of warm events
Harry Hendon
CDC Boulder, CO USA
The role of the Madden-Julian Oscillation (MJO) for the onset of El Nino is examined. Tropical Pacific variability is compared during three boreal winters which initially had similar distributions of sea-surface temperature (SST) but: one had strong MJO activity and later developed a strong El Nino (1996-97), one had strong MJO activity and produced no El Nino (1989-90), and the last had moderate MJO activity and produced a strong El Nino (1981-82). SSTs are observed to cool in the west Pacific after passage of the convectively active phase of the MJO. Wind stress anomalies also force equatorial Kelvin waves that propagate eastward across the Pacific. Those waves are associated with SST warming in the central and eastern Pacific. However, exceptions to that behavior are also found and the MJO/SST relationship is often obscured.
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 an oceanic Kelvin wave that was forced during the February MJO. That warming was primarily due to zonal temperature advection, promoted by strong eastward currents along with an east-west temperature gradient. After the passage of the Kelvin wave, zonal currents remained slightly eastward because trade winds did not resume their pre-Kelvin wave strength. This suggests that, if the MJO did contribute to this SST warming through nonlinear interactions, it did so through the coupling of atmospheric and oceanic dynamics.
