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  • Author or Editor: P. C. Joshi x
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V. Sathiyamoorthy
,
P. K. Pal
, and
P. C. Joshi

Abstract

Using the top-of-the-atmosphere radiative flux and cloud data from satellites, as well as atmospheric data from NCEP–NCAR reanalysis, this paper investigates the reason for the unusually large high-cloud amount in the Asian monsoon region during the summer monsoon season (June–September). Earlier studies attributed the large negative net cloud radiative forcing in the Asian monsoon region to the unusually large high-cloud amounts with high optical depth. Analysis during 1985–89 suggests that the unique upper-tropospheric easterly wind shear [tropical easterly jet (TEJ)], present over the Asian monsoon region during the summer monsoon season, may be responsible for the unusual increase in cloud amount. This strong wind shear sweeps the cloud tops and may be unfavorable for cloud growth beyond about 300 hPa. The spreading of cloud tops by wind may increase the high-cloud amount. A significant association is found between the high-cloud amount and the speed of the easterly jet. In addition, magnitudes of the shortwave, longwave, and net cloud radiative forcing also strongly depend upon the variations in the speed of TEJ.

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Amanda C. Maycock
,
Manoj M. Joshi
,
Keith P. Shine
, and
Adam A. Scaife

Abstract

Observations show that stratospheric water vapor (SWV) concentrations increased by ~30% between 1980 and 2000. SWV has also been projected to increase by up to a factor of 2 over the twenty-first century. Trends in SWV impact stratospheric temperatures, which may lead to changes in the stratospheric circulation. Perturbations in temperature and wind in the stratosphere have been shown to influence the extratropical tropospheric circulation. This study investigates the response to a uniform doubling in SWV from 3 to 6 ppmv in a comprehensive stratosphere-resolving atmospheric GCM. The increase in SWV causes stratospheric cooling with a maximum amplitude of 5–6 K in the polar lower stratosphere and 2–3 K in the tropical lower stratosphere. The zonal wind on the upper flanks of the subtropical jets is more westerly by up to ~5 m s−1. Changes in resolved wave drag in the stratosphere result in an increase in the strength of tropical upwelling associated with the Brewer–Dobson circulation of ~10% throughout the year. In the troposphere, the increase in SWV causes significant meridional dipole changes in the midlatitude zonal-mean zonal wind of up to 2.8 m s−1 at 850 hPa, which are largest in boreal winter in both hemispheres. This suggests a more poleward storm track under uniformly increased stratospheric water vapor. The circulation changes in both the stratosphere and troposphere are almost entirely due to the increase in SWV at pressures greater than 50 hPa. The results show that long-term trends in SWV may impact stratospheric temperatures and wind, the strength of the Brewer–Dobson circulation, and extratropical surface climate.

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