Temporal–Spatial Distribution of Thin Moist Layers in the Midtroposphere over the Tropical Eastern Pacific

Abstract

Abstract The temporal–spatial distribution of thin moist layers in the midtroposphere over the tropical eastern Pacific is studied by data analyses of radiosonde soundings and downscaling numerical experiments with a regional model. Radiosonde soundings at San Cristóbal, Galápagos, show frequent existence of thin moist layers between 2 and 10 km in altitude, with a local minimum at 7–8 km. The downscaling experiments with global objective analyses are completed for 2005–06, September and December of 1999–2004, and March of 2000–04. The vertical distribution of thin moist layers has three local maxima at 5, 10, and 16 km, where bimodality of the frequency distribution of water vapor is evident. Between 4 and 7 km, an annual variation is dominant in the occurrence ratio of thin moist layers, which tend to appear in nonconvective regions. In boreal winter, the layers appear to the north of the intertropical convergence zone (ITCZ), whereas in boreal summer the layers appear in the equator-side of the ITCZ. Interannual variations of the appearance of thin moist layers are also studied in 1999–2006, based on the experiments for particular months (March, September, and December). The occurrence ratio is generally high in December and March and low in September. In La Niña years, the annual variation is smaller than that in El Niño years; the occurrence ratio is higher in boreal summer to the south of the ITCZ.