Synoptic disturbances found in precipitable water fields north of equatorial Africa

Abstract

The origin and structure of tropical synoptic scale aphics. precipitable water (PWI anomalies estimated from TOVS satellite observations are analyzed as they propagate eastward across northern Africa during MAM 1988. Previous studies, accomplished by Mackey (1996), determined the location and season for the strongest and most coherent 3 to 8 day filtered PW signal. NCEP/NCAR reanalysis (2.50 resolution) provides data for a use study analysis of seven prominent anomalous PW events during the period. Six case wind field composite analysis is accomplished with both actual winds and reduced shear winds (zonal mean removed at each latitude band). Case study analysis revealed a doppler shifted propagating Rossby Wave (k = 6, c =-5 m/sec) at 500hPa and 250hPa that moved onto the west African coast imbedded within a mean zonal flow of approximately 17 m/sec. The wave then moved east across the continent at approximately 12 m/sec near 20ON. The wave's associated cusp-like feature adverts deep tropical moisture northward, resulting in Mackey's PW anomaly. Analysis of potential vorticity, specific humidity, temperature, and wind field composites at 850hPa, 500hPa and 250hPa fit well within the range of Rossby wave structure. Descriptive statistical analysis established confidence intervals for the composites at two resolutions within the localized grid (20OW to 40oE 00 to 30ON). A sample structure of the anomalies is displayed for each variable found to be statistically significant. The strongest correlation between observed PW and analyzed specific humidity and temperature is observed at 500mb and 250mb. Composite analysis of specific humidity and the appropriate standard deviation field supports this finding. PW amounts much greater than the climate mean state over the Gulf of Guinea are found within the PW anomaly structure, indicating an additional moisture source, possibly convection enhanced by the wave passage. 850mb upward vertical motion is also found to be slightly enhanced due to the anomalous flow associated with the wave passage. An empirical model is provided showing a four stage development of the PW anomalies during the five primary days of their life cycle and days prior.