Monsoon

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

The Annual Cycle in Equatorial Convection and Sea Surface Temperature

Abstract: The coupled atmosphere-ocean system in the equatorial eastern Pacific and Atlantic exhibits a distinct annual cycle that is reflected in contrasting conditions at the times of the two equinoxes. The contrasts are so strong that they dominate the annual march of zonally averaged outgoing long wave radiation for the equatorial belt. The March equinox corresponds to the warm season when the equatorial cold tongues in the eastern Pacific and Atlantic area absent. With the onset of summer monsoon convection over Colombia, Central America, and West Africa in May-June, northward surface winds strengthen over the eastern Pacific and Atlantic, the equatorial cold tongues reappear, and the marine convection shifts from the equatorial belt to the intertropical convergence zones (ITCZs) along 8 deg N. On the basis of observational evidence concerning the timing and year-to-year regularity of the surface wind changes during the development of the cold tongues, it is argued that (1) the increase in the northward surface winds in response to the onset of the northern summer monsoon may be instrumental in reestablishing the cold tongues, and (2) positive feedbacks involving both the zonal and meridional wind components contribute to the remarkable robustness of the cold tongue-ITCZs complexes in both oceans.

Impact of ENSO on the Variability of the Asian–Australian Monsoons as Simulated in GCM Experiments

Abstract: The influences of El Nino–Southern Oscillation (ENSO) on the summer- and wintertime precipitation and circulation over the principal monsoon regions of Asia and Australia have been studied using a suite of 46-yr experiments with a 30-wavenumber, 14-level general circulation model. Observed monthly varying sea surface temperature (SST) anomalies for the 1950–95 period have been prescribed in the tropical Pacific in these experiments. The lower boundary conditions at maritime sites outside the tropical Pacific are either set to climatological values [in the Tropical Ocean Global Atmosphere (TOGA) runs], predicted using a simple 50-m oceanic mixed layer (TOGA-ML runs), or prescribed using observed monthly SST variations. Four independent integrations have been conducted for each of these three forcing scenarios. The essential characteristics of the model climatology for the Asian–Australian sector compare well with the observations. Composites of the simulated precipitation data over the outstanding...

Surface circulation in the South China Sea

Abstract: A three-dimensional, primitive-equation model with a free surface is used to simulate the monthly circulation in the South China Sea. The model has a resolution of 0.4° in the horizontal and 21 layers in the vertical in a region from 2°N to 24°N and from 99°E to 124°E. Inflow and outflow in the Kuroshio, through the Taiwan Strait, and between the Sunda Shelf and the Java Sea are prescribed bimonthly. At the sea surface, the model is forced by monthly-averaged climatological winds and temperature and seasonally-averaged salinity. Several important features are reproduced in the model simulation. First, a strong coastal jet is present at the western boundary. The current is southward along the continental margin from China to southern Vietnam in winter. In summer, the current is northward and separates from the coast between 11°N and 14°N. The transition in September begins as a southward undercurrent, which is remotely forced by the northeast monsoon in the northern reaches of the South China Sea. The undercurrent extends to the surface in about a month. Second, inflow through the Luzon Strait from October to February transports the Kuroshio water in the top of 300 m of the water column westward along the continental slope south of China. In summer, eastward flow in the Luzon Strait transport surface water west of Luzon to the region east of Taiwan. Finally, a subsurface current, which is opposite to the surface current, exists over the Sunda Shelf and is driven by a pressure gradient set up by monsoon winds. These simulated currents are in qualitative agreement with the circulation inferred from the available observations.

Indian Monsoon–ENSO Relationship on Interdecadal Timescale

Abstract: Empirical evidence is presented to support a hypothesis that the interdecadal variation of the Indian summer monsoon and that of the tropical SST are parts of a tropical coupled ocean-atmosphere mode. The interdecadal variation of the Indian monsoon rainfall (IMR) is strongly correlated with the interdecadal variations of various indices of El Nino-Southern Oscillation (ENSO). It is also shown that the interannual variances of both IMR and ENSO indices vary in phase and follow a common interdecadal variation. However, the correlation between IMR and eastern Pacific SST or between IMR and Southern Oscillation index (SOI) on the interannual timescale does not follow the interdecadal oscillation. The spatial patterns of SST and sea level pressure (SLP) associated with the interdecadal variation of IMR are nearly identical to those associated with the interdecadal variations of ENSO indices. As has been shown earlier in the case of ENSO, the global patterns associated with the interdecadal and interannual variability of the Indian monsoon are quite similar. The physical link through which ENSO is related to decreased monsoon rainfall on both interannual and interdecadal timescales has been investigated using National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis products. The decrease in the Indian monsoon rainfall associated with the warm phases of ENSO is due to an anomalous regional Hadley circulation with descending motion over the Indian continent and ascending motion near the equator sustained by the ascending phase of the anomalous Walker circulation in the equatorial Indian Ocean. It is shown that, to a large extent, both the regional Hadley circulation anomalies and Walker circulation anomalies over the monsoon region associated with the strong (weak) phases of the interdecadal oscillation are similar to those associated with the strong (weak) phases of the interannual variability. However, within a particular phase of the interdecadal oscillation, there are several strong and weak phases of the interannual variation. During a warm eastern Pacific phase of the interdecadal variation, the regional Hadley circulation associated with El Nino reinforces the prevailing anomalous interdecadal Hadley circulation while that associated with La Nina opposes the prevailing interdecadal Hadley circulation. During the warm phase of the interdecadal oscillation, El Nino events are expected to be strongly related to monsoon droughts while La Nina events may not have significant relation. On the other hand, during the cold eastern Pacific phase of the interdecadal SST oscillation, La Nina events are more likely to be strongly related to monsoon floods while El Nino events are unlikely to have a significant relation with the Indian monsoon. This picture explains the observation that the correlations between IMR and ENSO indices on the interannual timescale do not follow the interdecadal oscillation as neither phase of the interdecadal oscillation favors a stronger (or weaker) correlation between monsoon and ENSO indices.

Analysis of long-term rainfall trends in India

Abstract: The study of precipitation trends is critically important for a country like India whose food security and economy are dependent on the timely availability of water. In this work, monthly, seasonal and annual trends of rainfall have been studied using monthly data series of 135 years (1871–2005) for 30 sub-divisions (sub-regions) in India. Half of the sub-divisions showed an increasing trend in annual rainfall, but for only three (Haryana, Punjab and Coastal Karnataka), this trend was statistically significant. Similarly, only one sub-division (Chattisgarh) indicated a significant decreasing trend out of the 15 sub-divisions showing decreasing trend in annual rainfall. In India, the monsoon months of June to September account for more than 80% of the annual rainfall. During June and July, the number of sub-divisions showing increasing rainfall is almost equal to those showing decreasing rainfall. In August, the number of sub-divisions showing an increasing trend exceeds those showing a decreasing...