Monsoon

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

The onset and cessation of seasonal rainfall over Africa

Abstract: Variation in the seasonal cycle of African rainfall is of key importance for agriculture. Here, an objective method of determining the timing of onset and cessation is, for the first time, extended to the whole of Africa. The method is applied to five observational datasets and the ERA-Interim reanalysis. Compatibility with known physical drivers of African rainfall, consistency with indigenous methods, and generally strong agreement between satellite-based rainfall datasets confirm the method is capturing the correct seasonal progression of African rainfall. The biannual rainfall regime is correctly identified over the coastal region of Ghana and the Ivory Coast. However, the ERA-Interim reanalysis exhibits timing biases over areas with two rainy seasons, and both ERA-Interim and the ARCv2 observational dataset exhibit some inconsistent deviations over West Africa. The method can be used to analyze both seasonal - interannual variability and long-term change. Over East Africa, we find that failure of the rains and subsequent humanitarian disaster is associated with shorter as well as weaker rainy seasons, e.g. on average the long rains were 11 days shorter in 2011. Cessation of the short rains over this region is 7 days later in El Nino and 5 days earlier in La Nina years with only a small change in onset date. The methodology described in this paper is applicable to multiple datasets and to large regions, including those that experience multiple rainy seasons. As such, it provides a means for investigating variability and change in the seasonal cycle over the whole of Africa.

Progress on upwelling studies in the China seas

Abstract: East Asian marginal seas surrounding China exhibit rich ocean upwelling, mostly in response to the southwesterly summer monsoon. Upwelling in the China seas, namely, the South China Sea, the Taiwan Strait, the East China Sea, the Yellow Sea, and the Bohai Sea, has become increasingly important because the potential changes in the upwelling may have dramatic ecosystem, socioeconomic, and climate impacts. This paper reviews the progress of upwelling studies in the China seas since the year 2000, by presenting the principal characteristics and new understanding of 12 major upwelling regions in the China seas. Upwelling exhibits long-term variability at intraseasonal to multi-decadal scales as well as short-term variability frequently caused by tropical cyclones. It is also associated with the El Nino Southern Oscillation, local environmental variation, and biogeochemical factors. The coastal upwelling around Hainan Island and the upwelling or cold dome northeast of Taiwan Island are specifically highlighted because they have attracted great interest for decades. This paper summarizes upwelling mechanisms in terms of wind, topography, tide, stratification, and background flow, with applications mostly to the China seas. Finally, we propose some topics for future upwelling research, i.e., potential intensification of coastal upwelling under global climate change, downwelling, intrusion of upwelling into coastal embayments, and the influence of upwelling on fishery and biogeochemical processes.

Abrupt Seasonal Change of Large-Scale Convective Activity over the Western Pacific in the Northern Summer

Abstract: Seasonal variations of large-scale convective activity and wind over the western Pacific are examined using Geostationary Meteorological Satellite infrared equivalent blackbody temperature (T BB ) and European Center for Medium range Weather Forecast (ECMWF) global analyses over a 10-year period from 1980 to 1989. In particular, this study describes an abrupt northward shift of large-scale convective activity over the western Pacific around 20°N, 150°E in late July. The enhanced convective activity is coincident with strong cyclonic circulation there which induces westerlies to the south of the cyclone and easterlies to the north of it. It is emphasized that this strong cyclonic circulation appears suddenly over the subtropical western Pacific region. Monsoon westerlies to the west of 110°E are not similary accelerated at the same time, indicating that this abrupt change is independent of the Asian monsoon system. To the north, an anticyclonic circulation is generated, which corresponds to the withdrawal of the Baiu season over Japan. Furthermore, this abrupt northward shift of large-scale convective activity is shown to be associated with tropical cyclone activity. In the mid latitudes, geopotential height pattern between pre- and post-northward shifts of the large-scale convective activity in late July exhibit equivalent barotropic vertical structure, suggesting the Rossbywave propagation emanating northeastward from the enhanced convective region around 20°N, 140°E (western Pacific) to as far north as 60°N, 180° (Bering Sea). Another feature is that the seasonal increase of sea surface temperature (SST) over the key area (20°N, 150°E) precedes abrupt convective enhancement by about 20 days, exceeding 29°C in early July. It is inferred that the northeastward extension of the warm SST tongue is intimately associated with the enhanced convection in late July. This result suggests that SST warming is not a sufficient condition but certainly one important ingredient for the abrupt northward shift of convections.

Sensitivity of Asian and African climate to variations in seasonal insolation, glacial ice cover, sea surface temperature, and Asian orography

Abstract: A general circulation model was used to investigate the sensitivity of Asian and African climate to prescribed changes in boundary conditions with the objective of identifying the relative importance of individual high-latitude glacial boundary conditions on seasonal climate and providing a physical basis for interpreting the paleoclimate record. The circulation model is described and results are presented. Insolation forcing increased summer Asian monsoon winds, while increased high-latitude ice cover strengthened winter Asian trade winds causing decreased precipitation. These factors had little effect on African climate. Cooler North Atlantic sea surface temperatures enhanced winter trade winds over North Africa, southern Asian climate was relatively unaffected. Reducing Asian orography enhanced Asian winter circulation while decreasing the summer monsoon. These model results suggest that African and southern Asian climate respond differently to separate elements of high-latitude climate variability.