Tropical Instability Wave

W. Timothy Liu, NASA Jet Propulsion Laboratory


Sea surface temperature (SST) observed by the Microwave Imager on the Tropical Rain Measuring Mission (TRMM), shown as color image, and surface wind speed (SWS), shown as isotachs, measured by the SeaWinds scatterometer on QuikSCAT, on 11 September 1999, clearly reveal the atmospheric manifestation of tropical instability waves (TIW) in the tropical Pacific. image The TIW were best observed by radiometers on geostationary satellites as meanders of the temperature front between the cold upwelling water of the Pacific equatorial cold tongue and the warm water to the north. The waves propagate westward, with period of approximately 30 days. The waves are stronger from June to November and during the anomalous cold events of La Nina. SST is often obscured from visible and infrared radiometers by cloud cover. Because the atmosphere and the clouds are transparent to microwave, the coincident measurements at microwave frequencies by TRMM and QuikSCAT during the La Nina episode in the second half of 1999 provided unprecedented opportunity to observe the influence of SST front on the atmosphere. A unique zonal-temporal band-pass filter enables the isolation of the propagating signals and the determination of their phase differences. The phase differences between the propagation of wind vector and SST observed from space and the vertical wind profiles measured from a research ship are consistent with the hypothesis the coupling between SWS and SST is caused by buoyancy instability and mixing which reduces the wind shear in the atmospheric boundary layer. Details of the result can be found in Liu et al. [Geophys. Res. Lett, 27, No 16, 2545-2548].