Forcing of the equatorial ocean by atmospheric Kelvin waves

Convectively coupled Kelvin waves are equatorially-trapped disturbances that propagate eastward with a mean phase speed of around 17 m/s. They are detectable over the tropical continents and most of the tropical oceans where sea surface temperature (SST) is high enough to support deep convection. Over the tropical central and eastern Pacific, their dynamical structures are maximized on the equator, consistent with linear theory, although the maximum convective signal is seen off the equator in the ITCZ. The Madden-Julian Oscillation (MJO) is another convectively coupled, eastward moving disturbance. The MJO propagates at at a much slower phase speed of around 5-10 m/s, although its convective envelope is much larger spatially. While the envelopes of convection associated with both the MJO and Kelvin waves propagate eastward, individual cloud clusters are observed to propagate both eastward and westward within the disturbances themselves. Although the MJO accounts for substantially more variance in convection than Kelvin waves over the warm pool, Kelvin waves are more important than the MJO for modulating convection within the central and eastern Pacific ITCZ. We first examine the modulation of the higher frequency Kelvin waves by the MJO. We show that during northern summer, Kelvin wave activity typically precedes the development of MJO convection over the Indian Ocean. On average, as the MJO develops over the Indian Ocean, the Kelvin wave activity propagates through the MJO envelope, and is subsequently enhanced over the tropical Pacific, well to the east of the residual MJO convection over Asia. A case study of such an interaction during May-June 1998 is examined in detail. In the months prior to this period, heat content over the tropical Pacific slowly decreased as a result of oceanic wave dynamics, although the SST remained quite high. Then in May equatorial Pacific SST plummeted by up to 8C, bringing a dramatic end to the highly anomalous 1997-98 El Nino conditions. We show that this SST tendency was primarily related to the rapid onset of oceanic upwelling induced by the sudden increase in trade wind flow associated with the MJO/Kelvin wave complex. In late April 1998, a convectively coupled Kelvin wave signal propagated from the Atlantic, through Africa, and into the Indian Ocean where it appears to have triggered an MJO. As the MJO developed, surface winds over the tropical Pacific went from westerly to easterly. At the same time the Kelvin wave continued rapidly across the Pacific in the convective and dynamical fields. The combination of wind forcing from the MJO and Kelvin disturbances resulted in the upwelling of cold subsurface water in the central and eastern Pacific, decreasing the SST dramatically. Once the SST decreased to levels unable to support convection, the convective field was re-established farther west in its more usual position, and the 1997-98 warm event was terminated. The potential role of such waves in the demise of the 1982-83 and 1991-92 warm events will also be discussed.