Monsoon of South Asia

About: Monsoon of South Asia is a research topic. Over the lifetime, 1660 publications have been published within this topic receiving 81602 citations.

A Dipole Mode in the Tropical Indian Ocean

Abstract: For the tropical Pacific and Atlantic oceans, internal modes of variability that lead to climatic oscillations have been recognized1,2, but in the Indian Ocean region a similar ocean–atmosphere interaction causing interannual climate variability has not yet been found3. Here we report an analysis of observational data over the past 40 years, showing a dipole mode in the Indian Ocean: a pattern of internal variability with anomalously low sea surface temperatures off Sumatra and high sea surface temperatures in the western Indian Ocean, with accompanying wind and precipitation anomalies. The spatio-temporal links between sea surface temperatures and winds reveal a strong coupling through the precipitation field and ocean dynamics. This air–sea interaction process is unique and inherent in the Indian Ocean, and is shown to be independent of the El Nino/Southern Oscillation. The discovery of this dipole mode that accounts for about 12% of the sea surface temperature variability in the Indian Ocean—and, in its active years, also causes severe rainfall in eastern Africa and droughts in Indonesia—brightens the prospects for a long-term forecast of rainfall anomalies in the affected countries.

Monsoons: Processes, predictability, and the prospects for prediction

Abstract: The Tropical Ocean-Global Atmosphere (TOGA) program sought to determine the predictability of the coupled ocean-atmosphere system. The World Climate Research Programme's (WCRP) Global Ocean-Atmosphere-Land System (GOALS) program seeks to explore predictability of the global climate system through investigation of the major planetary heat sources and sinks, and interactions between them. The Asian-Australian monsoon system, which undergoes aperiodic and high amplitude variations on intraseasonal, annual, biennial and interannual timescales is a major focus of GOALS. Empirical seasonal forecasts of the monsoon have been made with moderate success for over 100 years. More recent modeling efforts have not been successful. Even simulation of the mean structure of the Asian monsoon has proven elusive and the observed ENSO-monsoon relationships has been difficult to replicate. Divergence in simulation skill occurs between integrations by different models or between members of ensembles of the same model. This degree of spread is surprising given the relative success of empirical forecast techniques. Two possible explanations are presented: difficulty in modeling the monsoon regions and nonlinear error growth due to regional hydrodynamical instabilities. It is argued that the reconciliation of these explanations is imperative for prediction of the monsoon to be improved. To this end, a thorough description of observed monsoon variability and the physical processes that are thought to be important is presented. Prospects of improving prediction and some strategies that may help achieve improvement are discussed.

Interdecadal changes in the ENSO-monsoon system

Abstract: The El Nino–Southern Oscillation (ENSO) and Indian monsoon are shown to have undergone significant interdecadal changes in variance and coherency over the last 125 years. Wavelet analysis is applied to indexes of equatorial Pacific sea surface temperature (Nino3 SST), the Southern Oscillation index, and all-India rainfall. Time series of 2–7-yr variance indicate intervals of high ENSO–monsoon variance (1875–1920 and 1960–90) and an interval of low variance (1920–60). The ENSO–monsoon variance also contains a modulation of ENSO–monsoon amplitudes on a 12–20-yr timescale. The annual-cycle (1 yr) variance time series of Nino3 SST and Indian rainfall is negatively correlated with the interannual ENSO signal. The 1-yr variance is larger during 1935–60, suggesting a negative correlation between annual-cycle variance and ENSO variance on interdecadal timescales. The method of wavelet coherency is applied to the ENSO and monsoon indexes. The Nino3 SST and Indian rainfall are found to be highly coherent, ...

Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon

Abstract: Convective removal of lower lithosphere beneath the Tibetan Plateau can account for a rapid increase in the mean elevation of the Tibetan Plateau of 1000 m or more in a few million years. Such uplift seems to be required by abrupt tectonic and environmental changes in Asia and the Indian Ocean in late Cenozoic time. The composition of basaltic volcanism in northern Tibet, which apparently began at about 13 Ma, implies melting of lithosphere, not asthenosphere. The most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere. The initiation of normal faulting in Tibet at about 8 (± 3) Ma suggests that the plateau underwent an appreciable increase in elevation at that time. An increase due solely to the isostatic response to crustal thickening caused by India's penetration into Eurasia should have been slow and could not have triggered normal faulting. Another process, such as removal of relatively cold, dense lower lithosphere, must have caused a supplemental uplift of the surface. Folding and faulting of the Indo-Australian plate south of India, the most prominent oceanic intraplate deformation on Earth, began between about 7.5 and 8 Ma and indicates an increased north-south compressional stress within the Indo-Australian plate. A Tibetan uplift of only 1000 m, if the result of removal of lower lithosphere, should have increased the compressional stress that the plateau applies to India and that resists India's northward movement, from an amount too small to fold oceanic lithosphere, to one sufficient to do so. The climate of the equatorial Indian Ocean and southern Asia changed at about 6–9 Ma: monsoonal winds apparently strengthened, northern Pakistan became more arid, but weathering of rock in the eastern Himalaya apparently increased. Because of its high altitude and lateral extent, the Tibetan Plateau provides a heat source at midlatitudes that should oppose classical (symmetric) Hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons. For the simple case of axisymmetric heating (no dependence on longitude) of an atmosphere without dissipation, theoretical analyses by Hou, Lindzen, and Plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal (monsoonlike) circulation than such a source centered on the equator, but only if heating exceeds a threshold whose level increases with the latitude of the heat source. Because heating of the atmosphere over Tibet should increase monotonically with elevation of the plateau, a modest uplift (1000–2500 m) of Tibet, already of substantial extent and height, might have been sufficient to exceed a threshold necessary for a strong monsoon. The virtual simultaneity of these phenomena suggests that uplift was rapid: approximately 1000 m to 2500 m in a few million years. Moreover, nearly simultaneously with the late Miocene strengthening of the monsoon, the calcite compensation depth in the oceans dropped, plants using the relatively efficient C4 pathway for photosynthesis evolved rapidly, and atmospheric CO2 seems to have decreased, suggesting causal relationships and positive feedbacks among these phenomena. Both a supplemental uplift of the Himalaya, the southern edge of Tibet, and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the Himalaya that, in turn, enhanced extraction of CO2 from the atmosphere. Thus these correlations offer some support for links between plateau uplift, a downdrawing of CO2 from the atmosphere, and global climate change, as proposed by Raymo, Ruddiman, and Froehlich. Mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts, but might also play an important role in altering regional and global climates.

The East Asian summer monsoon: an overview

Abstract: The present paper provides an overview of major problems of the East Asian summer monsoon. The summer monsoon system over East Asia (including the South China Sea (SCS)) cannot be just thought of as the eastward and northward extension of the Indian monsoon. Numerous studies have well documented that the huge Asian summer monsoon system can be divided into two subsystems: the Indian and the East Asian monsoon system which are to a greater extent independent of each other and, at the same time, interact with each other. In this context, the major findings made in recent two decades are summarized below: (1) The earliest onset of the Asian summer monsoon occurs in most of cases in the central and southern Indochina Peninsula. The onset is preceded by development of a BOB (Bay of Bengal) cyclone, the rapid acceleration of low-level westerlies and significant increase of convective activity in both areal extent and intensity in the tropical East Indian Ocean and the Bay of Bengal. (2) The seasonal march of the East Asian summer monsoon displays a distinct stepwise northward and northeastward advance, with two abrupt northward jumps and three stationary periods. The monsoon rain commences over the region from the Indochina Peninsula-the SCS-Philippines during the period from early May to mid-May, then it extends abruptly to the Yangtze River Basin, and western and southern Japan, and the southwestern Philippine Sea in early to mid-June and finally penetrates to North China, Korea and part of Japan, and the topical western West Pacific. (3) After the onset of the Asian summer monsoon, the moisture transport coming from Indochina Peninsula and the South China Sea plays a crucial “switch” role in moisture supply for precipitation in East Asia, thus leading to a dramatic change in climate regime in East Asia and even more remote areas through teleconnection. (4) The East Asian summer monsoon and related seasonal rain belts assumes significant variability at intraseasonal, interannual and interdecadal time scales. Their interaction, i.e., phase locking and in-phase or out-phase superimposing, can to a greater extent control the behaviors of the East Asian summer monsoon and produce unique rythem and singularities. (5) Two external forcing i.e., Pacific and Indian Ocean SSTs and the snow cover in the Eurasia and the Tibetan Plateau, are believed to be primary contributing factors to the activity of the East Asian summer monsoon. However, the internal variability of the atmospheric circulation is also very important. In particular, the blocking highs in mid-and high latitudes of Eurasian continents and the subtropical high over the western North Pacific play a more important role which is quite different from the condition for the South Asian monsoon. The later is of tropical monsoon nature while the former is of hybrid nature of tropical and subtropical monsoon with intense impact from mid-and high latitudes.