A meteorological overview of the Pacific Exploratory Mission (PEM) Tropics period

NASA's Pacific Exploratory Mission‐Tropics (PEM‐T) experiment investigated the atmospheric chemistry of a large portion of the tropical and subtropical Pacific Basin during August to October 1996. This paper summarizes meteorological conditions over the PEM‐T domain. Mean flow patterns during PEM‐T are described. Important circulation systems near the surface include subtropical anticyclones, the South Pacific Convergence Zone (SPCZ), the Intertropical Convergence Zone (ITCZ), and middle latitude transient cyclones. The SPCZ and ITCZ are areas of widespread ascent and deep convection; however, there is relatively little lightning in these oceanic regions. A large area of subsidence is associated with the subtropical anticyclone centered near Easter Island. PEM‐T occurred during a period of near normal sea surface temperatures. When compared to an 11 year climatology (1986–1996), relatively minor circulation anomalies are observed during PEM‐T. Some of these circulation anomalies are consistent with much stronger anomalies observed during previous La Nina events. In general, however, the 1996 PEM‐T period appears to be climatologically representative. Meteorological conditions for specific flights from each major operations area are summarized. The vertical distribution of ozone along selected DC‐8 flights is described using the DIAL remote sensing system. These ozone distributions are related to thermodynamic soundings obtained during aircraft maneuvers and to backward trajectories that arrived at locations along the flight tracks. Most locations in the deep tropics are found to have relatively small values of tropospheric ozone. Backward trajectories calculated from global gridded analyses show that much of this air originates from the east and has not passed over land within 10 days. The deep convection associated with the ITCZ and SPCZ also influences the atmospheric chemistry of these regions. Flights over portions of the subtropics and middle latitudes document layers of greatly enhanced tropospheric ozone, sometimes exceeding 80 ppbv. In situ carbon monoxide in these layers often exceeds 90 ppbv. These regions are located near, and especially south of Tahiti, Easter Island, and Fiji. The layers of enhanced ozone usually correspond to layers of dry air, associated with widespread subsiding air. The backward trajectories show that air parcels arriving in these regions originate from the west, passing over Australia and even extending back to southern Africa. These are regions of biomass burning. The in situ chemical measurements support the trajectory‐derived origins of these ozone plumes. Thus the enhanced tropospheric ozone over the central Pacific Basin may be due to biomass burning many thousands of kilometers away. Middle‐latitude portions of the PEM‐T area are influenced by transient cyclones, and the DC‐8 traversed tropopause folds during several flights. The flight area just west of Ecuador experiences outflow from South America. Thus the biomass burning that is prevalent over portions of Brazil influences this area.