[1] By relating the water vapor flow from both reanalysis data and satellite observations to the lower branches of the Hadley and Walker circulations, it was shown that the strength of both north-south and east-west tropical circulations has intensified since 1979. Consistent with the expected water vapor climatology under the strengthened north-south Hadley circulation, satellite observations of upper tropospheric humidity and total water vapor show that the subsidence region has become drier because of the increased sinking motion. In the east-west direction, also consistent with the strengthened Walker circulation drawn from water vapor flux, the difference of the sea level pressure between the east Pacific and west Pacific has become larger over the past three decades.Citation: Sohn, B. J., and S.-C. Park (2010), Strengthened tropical circulations in past three decades inferred from water vapor transport,
[1] In this study, we investigated whether long-term normalized difference vegetation index (NDVI) data show climate change signals after the mid-1990s which are inferred from other studies on changing trends in precipitation and dust frequencies. In doing so, mean NDVI data for the growing seasons (April-October) from 1982 to 2006 were used for examining the spatiotemporal variations in the vegetation over East Asia, in conjunction with precipitation and temperature data. Results indicate that there was a prominent change in the trend of NDVI around the mid-1990s: a pronounced positive trend over most of the East Asian domain before the mid-1990s (1982-1996) and a reverse (or weakened) trend after the mid-1990s (1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006). The reverse trend is evident over the higher-latitude regions north of 50°N and the eastern Mongolian border area. The EOF and SVD analysis suggest that the dominant warming trend until the mid-1990s led to the increased NDVI over the high-latitude regions. However, after the mid-1990s, the reverse NDVI trend found primarily in the east of Lake Baikal and the arid and semiarid regions south of 50°N seems to be closely linked to local precipitation changes occurred abruptly in the mid-1990s. However, precipitation influences on the reverse NDVI changes are not clear over the high-latitude regions north of 50°N.
In order to examine the changes in Walker circulation over the recent decades, we analyzed the sea surface temperature (SST), deep convective activities, upper tropospheric moistening, sea level pressure (SLP), and effective wind in the boundary layer over the 30-year period of . The analysis showed that the eastern tropical Pacific has undergone cooling while the western Pacific has undergone warming over the past three decades, causing an increase in the east-west SST gradient. It is indicated that the tropical atmosphere should have responded to these SST changes; increased deep convective activities and associated upper tropospheric moistening over the western Pacific ascending region, increased SLP over the eastern Pacific descending region in contrast to decreased SLP over the western Pacific ascending region, and enhanced easterly wind in the boundary layer in response to the SLP change. These variations, recognized from different data sets, occur in tandem with each other, strongly supporting the intensified Walker circulation over the tropical Pacific Ocean. Since the SST trend was attributed to more frequent occurrences of central Pacific-type El Niño in recent decades, it is suggested that the decadal variation of El Niño caused the intensified Walker circulation over the past 30 years. An analysis of current climate models shows that model results deviate greatly from the observed intensified Walker circulation. The uncertainties in the current climate models may be due to the natural variability dominating the forced signal over the tropical Pacific during the last three decades in the twentieth century climate scenario runs by CMIP3 CGCMs.
Abstract. Diurnal variations of upper tropospheric humidity (UTH) as well as middle tropospheric humidity (MTH) were examined in conjunction with the diurnal cycle of convection over tropical Africa and the adjacent tropical Atlantic Ocean using Meteosat-8 measurements. Cloud and humidity features were also tracked to document the diurnal variations of humidity and clouds in the Lagrangian framework.A distinct diurnal variation of UTH (and MTH) is noted over regions where tropical deep convective cloud systems are commonly observed. The amplitude of the UTH diurnal variation is larger over land, while its variations over convectively inactive subtropical regions are much smaller. The diurnal variation of UTH tends to reach a maximum during nighttime over land, lagging deep convection and high cloud whose maxima occurred in the late afternoon and evening, respectively. It was revealed that these diurnal variations over the African continent are likely associated with continental-scale daytime solar heating and topography, in which topographically-induced signals develop earlier around the mid-afternoon and merge into stronger and broader continental-scale convection clusters later, forming a precipitation maximum in the late afternoon. It was also revealed that advection effect on the diurnal variation appears to be insignificant.
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