A climatology of tropical synoptic scale behavior from TOVS-estimated precipitable water

Abstract

Tropical synoptic scale behavior is examined using 3 to 8 day filtered precipitable water (PW) estimated from TOVS operational satellite observations for 24 three-month seasons. Zonally-oriented tropical convergence zones and regions of enhanced synoptic variance are quantified and found to be poorly correlated with each other. Time-longitude plots (Hovmoller format) identify spatially coherent PW anomalies that can often be tracked around the globe. The strongest and most consistent signal is of eastward propagation across northern hemisphere Africa. Other regions demonstrate both eastward or westward propagation according to season and location. A general shift from eastward to westward propagation occurs between 200 and 300 latitude in each hemisphere. Hovmoller composites suggest an additional eastward propagating mode between 5ON and 12.50N during SON across the entire Pacific Ocean. Seven tropical regions are chosen to perform climatological studies of synoptic scale behavior. Hovmoller composites reveal 10 m/s westward propagation across the North Atlantic (zonal wavelength is 6,000 km) and 7 m/s eastward propagation of over Sahel Africa (zonal wavelength is 2,500-3,000 km). Composites over the southern Indian Ocean suggest westward motion, while propagation in the ITCZ, SPCZ, west Pacific warm pool, and Amazon basin shows little preference of zonal direction. Interaction between the synoptic and the intraseasonal and interannual time scales is studied. The intraseasonal oscillation does not affect synoptic PW anomalies, even within the monsoon regions. Lag correlation plots reveal a directional reversal of zonal propagation of synoptic PW anomalies between warm and cold ENSO phases in the Pacific and Indian Ocean regions. Hovmoller plots of synoptically filtered OLR and PW data show large regions void of OLR anomalies, while PW anomalies remain coherent throughout the tropics. A seasonally averaged daily point-to-point correlation between PW and OLR data over the tropical domain shows weak negative correlation, with almost zero correlation along convergence zones and in the west Pacific warm pool region, implying the PW is both a more sensitive and a more reliable signal.