Monsoon

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

1997: The El Niño of the Century and the Response of the Indian Summer Monsoon

Abstract: The year 1997 was characterized by the rapid development of an El Nino whose strength exceeded any previously observed this century. The basic understanding of the influence of El Nino on the Asian summer monsoon suggested that the monsoon should be substantially deficient, yet the all-India rainfall (AIR) was 2% above normal. The reasons for this have been investigated in terms of both the seasonal-mean, large-scale circulation anomalies and the subseasonal, regional weather events. By comparing the results with a similar analysis of two previous major El Nino events in 1982 and 1987, the common and disparate features of the response have been identified. On the large scale, the basic hypothesis that, in El Nino years, the strength of the monsoon is influenced by a modulation of the Walker circulation, in which there is implied additional subsidence over the west Pacific and southeast Asia, is generally supported by the results. However, the results have shown that the modulation of the local Ha...

Mid-Holocene climates of the Americas: a dynamical response to changed seasonality

Abstract: Simulations of the climatic response to mid-Holocene (6 ka BP) orbital forcing with two coupled ocean–atmosphere models (FOAM and CSM) show enhancement of monsoonal precipitation in parts of the American Southwest, Central America and northernmost South America during Northern Hemisphere summer. The enhanced onshore flow that brings precipitation into Central America is caused by a northward displacement of the inter-tropical convergence zone, driven by cooling of the equatorial and warming of the northern subtropical and mid-latitude ocean. Ocean feedbacks also enhance precipitation over the American Southwest, although the increase in monsoon precipitation there is largely driven by increases in land-surface temperature. The northward shift in the equatorial precipitation band that causes enhanced precipitation in Central America and the American Southwest has a negative feedback effect on monsoonal precipitation in northern South America. The simulations demonstrate that mid-Holocene aridity in the mid-continent of North America is dynamically linked to the orbitally induced enhancement of the summer monsoon in the American Southwest, with a spatial structure (wet in the Southwest and dry in the mid-continent) similar to that found in strong monsoon years today. Changes in winter precipitation along the west coast of North America, in Central America and along the Gulf Coast, caused by southward-displacement of the westerly storm tracks, indicate that changes in the Northern Hemisphere winter monsoon also play a role in regional climate changes during the mid-Holocene. Although the simulations with FOAM and CSM differ in detail, the general mechanisms and patterns are common to both. The model results thus provide a coherent dynamical explanation for regional patterns of increased or decreased aridity shown by vegetation, lake status and aeolian data from the Americas.

Equatorial currents and transports in the upper central Indian Ocean: Annual cycle and interannual variability

Abstract: The zonal circulation south of Sri Lanka is an important link for the exchange of water between the Bay of Bengal and the Arabian Sea. Results from a first array of three moorings along 80 degrees 30'E north of 4 degrees 10'N from January .1991 to March 1992 were used to investigate the Monsoon Current regime [Schott et al., 1994]. Measurements from a second array of six current meter moorings are presented here. This array was deployed along 80 degrees 30'E between 45'S and 5 degrees N from July 1993 to September 1994 to investigate the annual cycle and interannual variability of the equatorial currents at this longitude. Both sets of moorings contribute to the Indian Ocean current meter array ICM8 of the World Ocean Circulation Experiment. The semiannual equatorial jet (EJ) was showing a large seasonal asymmetry, reaching a monthly mean eastward transport of 35 Sv (1 Sv = 1 x 10(6) m(3) s(-1)) in November 1993, but just 5 Sv in May 1994. The Equatorial Undercurrent (EUC) had a maximum transport of 17 Sv in March to April 1994. Unexpectedly, compared to previous observations and model studies, the EUC was reappearing again in August 1994 at more than 10 Sv transport and was still flowing when the moorings were recovered. In addition, monthly mean ship drifts near the equator are evaluated to support the interpretation of the moored observations. Interannual variability of the EJ in our measurements and ship drift data appears to be related to the variability of the zonal winds and Southern Oscillation Index. The output of a global numerical model (Parallel Ocean Climate Model) driven by the winds for 1993/1994 is used to connect our observations to the larger scale. The model reproduces the EJ asymmetry and shows the existence of the EUC and its reappearance during summer 1994.

Development and application of a mesoscale climate model for the tropics: Influence of sea surface temperature anomalies on the West African monsoon

Abstract: [1] A mesoscale climate model (MCM) is developed from the Pennsylvania State University/National Center for Atmospheric Research mesoscale model 5 to simulate the West African summer monsoon. Results from the MCM are compared to observations, a reanalysis, and global climate model (GCM) output to show that the MCM reasonably simulates the West African monsoon climate and its variability and improves on many shortcomings of the GCM simulations. The MCM's ability to capture correctly processes that cause interannual variability, and thereby allow us to improve our physical understanding of that variability, is investigated by examining the influence of Gulf of Guinea sea surface temperature anomalies (SSTAs) on the West African monsoon. Similar to observations, precipitation decreases over the Sahel and increases along the Guinean coast in response to warm SSTAs in the gulf. The increase in rainfall along the Guinean coast is associated with an increase in lower tropospheric water vapor content due to enhanced evaporation over the warm SSTAs and northward moisture advection in the monsoon flow. This stronger precipitation on the coast occurs despite a decrease in the land/sea temperature gradient, which is the fundamental driver of the monsoon. The decrease in rainfall over the southern Sahel is associated with lower tropospheric subsidence replacing rising vertical motions as the monsoon circulation is shifted equatorward. Dynamically, the subsidence over the southern Sahel is associated with shrinking of both planetary and relative vorticity. The former is related to the equatorward shift of the monsoon, which results in an expansion of the equatorward flow from the Sahel to the northern Guinean coast. The latter occurs because the equatorward flow over the southern Sahel occurs lower in the troposphere (i.e., 850 mbar) than in the control (i.e., 700 mbar), where the meridional relative vorticity gradient is opposite to the gradient at 700 mbar.