Monsoon

About: Monsoon is a research topic. Over the lifetime, 16087 publications have been published within this topic receiving 599888 citations.

Orbitally driven east-west antiphasing of South American precipitation

Abstract: The variations of tropical precipitation are antiphased between the hemispheres on orbital timescales. A comparison between a speleothem record of precipitation in northeast Brazil and rainfall reconstructions from the rest of tropical South America shows that a similar antiphasing operated in the same hemisphere during the Holocene. The variations of tropical precipitation are antiphased between the hemispheres on orbital timescales. This antiphasing arises through the alternating strength of incoming solar radiation in the two hemispheres, which affects monsoon intensity and hence the position of the meridional atmospheric circulation of the Hadley cells1,2,3,4. Here we compare an oxygen isotopic record recovered from a speleothem from northeast Brazil for the past 26,000 years with existing reconstructions of precipitation in tropical South America5,6,7,8. During the Holocene, we identify a similar, but zonally oriented, antiphasing of precipitation within the same hemisphere: northeast Brazil experiences humid conditions during low summer insolation and aridity when summer insolation is high, whereas the rest of southern tropical South America shows opposite characteristics. Simulations with a general circulation model that incorporates isotopic variations support this pattern as well as the link to insolation-driven monsoon activity. Our results suggest that convective heating over tropical South America and associated adjustments in large-scale subsidence over northeast Brazil lead to a remote forcing of the South American monsoon, which determines most of the precipitation changes in the region on orbital timescales.

A revised picture of the structure of the “monsoon” and land ITCZ over West Africa

Abstract: This article presents an overview of the land ITCZ (Intertropical Convergence Zone) over West Africa, based on analysis of NCAR–NCEP Reanalysis data. The picture that emerges is much different than the classic one. The most important feature is that the ITCZ is effectively independent of the system that produces most of the rainfall. Rainfall linked directly to this zone of surface convergence generally affects only the southern Sahara and the northern-most Sahel, and only in abnormally wet years in the region. A second feature is that the rainbelt normally assumed to represent the ITCZ is instead produced by a large core of ascent lying between the African Easterly Jet and the Tropical Easterly Jet. This region corresponds to the southern track of African Easterly Waves, which distribute the rainfall. This finding underscores the need to distinguish between the ITCZ and the feature better termed the “tropical rainbelt”. The latter is conventionally but improperly used in remote sensing studies to denote the surface ITCZ over West Africa. The new picture also suggests that the moisture available for convection is strongly coupled to the strength of the uplift, which in turn is controlled by the characteristics of the African Easterly Jet and Tropical Easterly Jet, rather than by moisture convergence. This new picture also includes a circulation feature not generally considered in most analyses of the region. This feature, a low-level westerly jet termed the African Westerly Jet, plays a significant role in interannual and multidecadal variability in the Sahel region of West Africa. Included are discussions of the how this new view relates to other aspects of West Africa meteorology, such as moisture sources, rainfall production and forecasting, desertification, climate monitoring, hurricanes and interannual variability. The West African monsoon is also related to a new paradigm for examining the interannual variability of rainfall over West Africa, one that relates changes in annual rainfall to changes in either the intensity of the rainbelt or north–south displacements of this feature. The new view presented here is consistent with a plethora of research on the synoptic and dynamic aspects of the African Easterly Waves, the disturbances that are linked to rainfall over West Africa and spawn hurricanes over the Atlantic, and with our knowledge of the prevailing synoptic and dynamic features. This article demonstrate a new aspect of the West Africa monsoon, a bimodal state, with one mode linked to dry conditions in the Sahel and the other linked to wet conditions. The switch between modes appears to be linked to an inertial instability mechanism, with the cross-equatorial pressure gradient being a critical factor. The biomodal state has been shown for the month of August only, but this month contributes most of the interannual variability. This new picture of the monsoon and interannual variability shown here appears to be relevant not only to interannual variability, but also to the multidecadal variability evidenced in the region between the 1950s and 1980s.

Variable winter moisture in the southwestern United States linked to rapid glacial climate shifts

Abstract: The last glacial period was characterized by large, rapid climate fluctuations. An analysis of a speleothem from New Mexico shows that the coldest conditions over Greenland coincide with increased winter precipitation in the southwestern United States, which can be attributed to a southward displacement of the polar jet stream and the North American storm track. During the last glacial period, the climate of the Northern Hemisphere was characterized by rapid, large-amplitude temperature fluctuations through cycles lasting a few thousand years1,2,3. These fluctuations are apparent in Greenland temperature reconstructions2,3, and corresponding temperature and hydrological variations have been documented throughout the Northern Hemisphere4,5. Here we present a record of precipitation in the southwestern United States from 56,000 to 11,000 yr ago, on the basis of δ18O measurements of speleothem calcite from New Mexico. Our record shows that increased winter precipitation in the southwestern United States is associated with Northern Hemisphere cooling, which we attribute to a southward shift in the polar jet stream, which modulated the position of the winter storm track over North America. On the western side of the Pacific Ocean basin, decreases in summer monsoon precipitation are associated with Northern Hemisphere cooling, due to southward displacement of the intertropical convergence zone4. We conclude that cooling and warming excursions in the Northern Hemisphere lead to concurrent latitudinal displacement of both the intertropical convergence zone and the polar jet stream over the Pacific Ocean. Our data are consistent with modern evidence for a northward shift of the polar jet stream in response to global warming6,7,8, which could lead to increasingly arid conditions in southwestern North America in the future.

Changes in global monsoon precipitation over the past 56 years

Abstract: [1] Changes in the global monsoon rainfall over land were examined using four sets of rain-gauge precipitation data sets compiled for the period of 1948–2003 by climate diagnostic groups around the world. Here, we define a global monsoon rain domain according to annual precipitation range, using simple objective criteria; then, we propose metrics for quantifying the intensity of the global monsoon precipitation. The results suggest an overall weakening of the global land monsoon precipitation in the last 56 years, primarily due to weakening of the summer monsoon rainfall in the Northern Hemisphere. However, since 1980, the global land monsoon rainfall has seen no significant trend, which contrasts with the rapid intensification of global warming during the same period. Meanwhile the oceanic monsoon precipitation shows an increasing trend after 1980. The results provide a rigorous test for climate models that will be used in future climate-change assessment.

The diurnal cycle of the West African monsoon circulation

Abstract: SUMMARY Using numerical model analyses, it is shown that there is a coherent diurnal cycle of the West African monsoon winds. As has been observed in previous studies of arid and semi-arid areas, the winds are at their weakest in the afternoon when the convective boundary layer (CBL) is deep, and intensify overnight when the boundary-layer turbulence is much weaker. This diurnal cycle is maximized in the northern part of the monsoon layer, where the meridional pressure gradient and the diurnal cycle of the CBL are both strong. The diurnal cycle can also be resolved in surface and upper-air data, which show how the nocturnal meridional circulation acts to stratify the lower part of the monsoon layer. In contrast, mixing in the daytime CBL acts to maintain the baroclinicity, as has been observed in laboratory flows. This pattern has implications for the efficiency of the monsoon circulation in the continental water budget, as well as in mixing of trace gases and aerosols between the surface layer and the free troposphere. Vertical mixing occurs by day, while meridional advection, with isentropic upgliding and downgliding, is most efficient at night. Finally, high-resolution observations from the JET2000 experiment are used to show that there is mesoscale structure in the diurnally varying monsoon circulation. In the nocturnal flows, local circulations have been observed and appear to represent a response to recent deep convective events. In contrast, the daytime CBL properties at these scales have been shown in a previous study to map closely onto patterns of soil moisture, with horizontal advection playing a weaker role.