The tropopause cold trap in the Australian monsoon during STEP/AMEX 1987

Abstract

The relationship between deep convection and tropopause cold trap conditions is examined for the tropical northern Australia region during the 1986–1987 summer monsoon season. Particular attention is paid to the Australian Monsoon Experiment (AMEX) period from January 10 through February 14 when the NASA Stratosphere‐Troposphere Exchange Project (STEP) was conducting aircraft investigations at Darwin. Time mean values of the saturation mixing ratio at the local temperature minimum or cold point at stations in the AMEX radiosonde network were everywhere below the global lower stratospheric average of ∼3.5 ppmv and in places substantially so, thus fulfilling the first of two conditions necessary for stratospheric dehydration. The cold point potential temperature Θcp in the region was anticorrelated both spatially and temporally with mesoscale convection. However, there was no direct connection between surface layer moist entropy and Θcp. Time mean Θcp ranged from 366 to 374 K at stations in the monsoon trough zone along the northern coast and in the northern Gulf of Carpentaria, while the time mean of the average equivalent potential temperature in the surface layer Θe was no greater than 356 K. This difference of at least 10 K between trough mean Θcp values and the maximum potential temperature achieved in undilute moist ascent from the surface layer indicates that rapid incorporation of tropospheric air into the stratosphere was occurring in monsoonal convective systems through a combination of cumulonimbus overshooting and anvil radiative heating. Over the continent, where mean Θe was 6–10 K lower, stratosphere‐troposphere exchange was less widespread but more vigorous. Here the average Θcp approached 380 K and was likely due to deeper tropopause penetrations produced by continental convective systems. This elevation of Θcp in continental systems relative to monsoon trough systems is evident in the Θcp of tropopause layers which had never passed over the continental interior: these were 8–13 K lower than their counterparts of continental origin. This elevating tendency accounts for the upward trends in cold point temperature, saturation mixing ratio and Θcp in the trough zone following monsoon onset in mid‐January. Before onset, tropopause air in the north coastal zone beyond the immediate vicinity of convective systems appeared to have originated in the tropics upstream of the continent. With the establishment of the monsoon continental‐scale upper‐level anticyclone, tropopause air entering the monsoon trough zone had more often been processed by systems of the continental interior. Nonetheless, the necessary conditions for stratospheric dehydration, i.e., cold point saturation mixing ratios below the global average and significant stratosphere‐troposphere exchange, remained in force throughout the monsoon period.