Mean moist circulation for PEM‐Tropics missions

Abstract

Features of atmospheric water vapor and water vapor transport fields during the Pacific Exploratory Mission (PEM) in the Tropical Pacific were studied using the data from the National Centers for Environmental Prediction (NCEP) reanalyses, the Global Precipitation Climatology Project, the Climate Analysis Center, and the European Centre for Medium‐Range Weather Forecasts, with a focus on PEM‐Tropics B. Vertically integrated values of the moisture quantities such as the precipitable water and the zonal and meridional water vapor fluxes were examined. Moisture transports across boundaries of major sectors over the tropical Pacific and their budgets were estimated. The differences of the precipitable water between PEM‐Tropics B and PEM‐Tropics A were highlighted owing to its importance for atmospheric chemistry. During PEM‐Tropics B, the NCEP reanalyses showed that there existed clearly a moist Walker circulation over the northern tropical Pacific during the whole period and a moist Hadley circulation over the eastern Pacific during the first half of the mission. The southwest sector where the South Pacific Convergence Zone (SPCZ) was located had the largest areal mean precipitation and the northeast sector had the smallest, whereas the estimated evaporation rates for both regions were comparable. A more vigorous moist circulation during the second half period led to enhanced precipitation for all the regions except the southeast sector. An interesting and potentially very important meteorological phenomenon during PEM‐Tropics B was the double Intertropical Convergence Zone (ITCZ) structure over the eastern tropical Pacific. We suggest that this structure, along with the similar double convective zones over the western tropical Pacific (i.e., ITCZ and SPCZ), may act as double barriers for the interhemispheric exchanges for this particular season with implications for photochemistry in both hemispheres.