Observations of variability on synoptic timescales in the East Pacific ITCZ

Abstract

Data obtained in the eastern Pacific intertropical convergence zone (ITCZ) during the Tropical Eastern Pacific Process Study (TEPPS) show a 3-6-day variability. The NOAA ship Ronald H. Brown collected surface meteorological observations, C-band Doppler radar volumes, atmospheric soundings, and rainfall data while on station at 7.8°N, 125°W from 8-23 August 1997. The 3-6-day variability was a prominent timescale in the meridional wind and humidity data. The maxima of the surface-to-700-mb meridional wind anomalies were 5-12 m s-1. Maxima of the moisture anomalies at the same levels were ∼ 1-3 g kg-1, in phase with the low-level southerlies, while upper-level moisture lagged the southerlies by less than a day. The radar indicated a close connection between precipitation in the vicinity of the ship and the meridional wind, with the precipitation occurring when the surface southerlies were strongest. The meridional and zonal wind components had a maximum of 850-450-mb wind shear during the southerly events. The vertical profiles of wind, humidity, and temperature in each of the events at 3-6-day intervals were generally consistent with theoretical studies of eaterly waves. Satellite infrared (IR) data indicated significant variance at 3-6-day periods within the region 0°-20°N, 75°-150°W for July-August 1997. The variability seen by satellite was most pronounced off the coast of South America and over the warm waters north of the equator, which mark the latitude of the ITCZ. Satellite-observed cloudiness exhibited wavelike organization with disturbed areas moving westward at ∼ 8 m s-1 (6°-7° day-1), with wavelengths of 2000-3000 km. Consistent with previous descriptions of easterly waves, a chain of low clouds and moisture formed inverted V patterns in visible and water-vapor channels of satellite imagery. Mesoscale convective systems formed along this chain. Some of these systems appeared to have been associated with low-pressure regions that eventually led to hurricanes. However, others did not spin off tropical storms, thus implying that their existence did not necessarily depend on these events. One set of mesoscale convective systems moved eastward through the 3-6-day westward-moving waves in conjunction with a Kelvin wave. Surface meteorological data from the Woods Hole Oceanographic Institution's Improved Meteorological Instrumentation (IMET) buoy moored at 10°N, 125°W and the NOAA Tropical Atmosphere-Ocean (TOA) moored buoys in the eastern Pacific also indicated significant 3-6-day variability in the meridional wind component during August 1997. Spectral analysis of the buoy data were consistent with spectrally analyzed satellite IR data in showing a seasonal spatial pattern of 3-6-day variability with easterly waves most active during the July-September season. Together the ship, satellite, and buoy data show that the TEPPS ship data were collected in a large-scale environment that was characterized by an interference pattern of two wave types. A regular progression of easterly waves produced the dominant 3-6-day variability, which was modified during one time period by the passage of a Kelvin wave approaching the region from the west.