Monsoon

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

A High-resolution record of Late Quaternary upwelling along the Oman Margin, Arabian Sea based on planktonic foraminifera

Abstract: Planktonic foraminiferal abundances, fluxes, test sizes, and coiling properties are influenced in various ways by the southwest monsoon winds and associated upwelling in the western Arabian Sea. To determine the short-term changes in the southwest monsoon, we have carried out a high-resolution time-series analysis of three upwelling indices (total flux of planktonic foraminiferal tests and flux and relative abundance of the planktonic foraminiferal species Globigerina bulloides) from Ocean Drilling Program (ODP) Site 723A (Oman Margin, western Arabian Sea) spanning the last 19 kyr. In addition, we have determined the relationships between upwelling intensity and the relative abundance, fluxes, and shell concentrations of various planktonic foraminiferal species. Upwelling indices suggest that from 19 to 16 ka (22 to 18.2 cal kyr B.P.) the SW monsoon was relatively strong compared to the period 15.8 to 12.5 ka (17.8 to 13.8 cal kyr B.P.). The intensification of the SW monsoon took place at 12 ka (13.1 cal kyr B.P.) and reached a peak between 10 and 5 ka (10.6 and 4.8 cal kyr B.P.). The high-resolution data further demonstrate that the SW monsoon has started weakening from 5 ka (4.8 cal kyr B.P.) and the weakest phase was in place at 3.5 ka (3 cal kyr B.P.), which coincides with evidence of an arid climate in western Tibet. Fluxes and shell concentrations of many of the planktonic foraminiferal species increased between 12 and 5 ka in response to the intensification of the SW monsoon winds after the last glacial period. Globigerina bulloides shows a fivefold to tenfold increase in flux during this period of intense upwelling. The other species whose fluxes are influenced by this upwelling change are (in order from strongest to weakest response) Globigerinita glutinata, Globigerinoides ruber, Neogloboquadrina dutertrei, Globigerinella aequilateralis, Globigerina falconensis, and Globigerinoides sacculifer. The relative abundances of G. bulloides and G. ruber increased during intense upwelling, whereas the relative abundances of G. glutinata, N. dutertrei, G. falconensis, and G. sacculifer did not increase during this period, which might be due to differences in the productivity of various species in relation to upwelling change. Therefore the fluxes and shell concentrations provide better and more reliable information about the changes in the monsoon system in the Arabian Sea than relative abundance data.

Observed seasonal variability of barrier layer in the Bay of Bengal

Abstract: [1] The observed formation of barrier layer (BL) and the seasonal variability of BL thickness (BLT) in the Bay of Bengal are examined utilizing the most comprehensive data set. Thick BL (∼40 m) first appears in the coastal region of the northeastern bay in June and spreads westward as the summer monsoon progresses. Along the east coast of India the BL formation and its variability are controlled by the East India Coastal Current (EICC). Thick BL (∼50 m) appears along the east coast of India in November when the EICC flows equatorward and gets spatially organized by December. By January it weakens when the EICC reverses. The mature phase of BLT, both in amplitude (∼60 m) and in spatial extent occurs during February, when the Subtropical Anticyclonic Gyre (SAG) is well established in the bay. During both the summer and winter monsoon seasons the surface circulation and the redistribution of low saline waters show a dominant influence on the observed BLT distribution. Other processes such as Ekman pumping and propagating Rossby waves forced by the propagating Kelvin waves along the eastern boundary also contribute significantly in modulating its variability. The annual mode of BLT shows maxima during November–December, whereas the semiannual mode peaks during February–March and August–September. The peak in February–March is attributed to the interior Ekman pumping and the associated convergence in the central bay, whereas the peak during August–September is due to the westward propagating downwelling Rossby waves from the eastern boundary.

Interannual Variability of the Australian Summer Monsoon at Darwin: 1952–82

Abstract: Fluctuations in the Australian summer monsoon over the period 1952–82 are described. The basis of the study is an objective definition of the major summer monsoon components based on the low-level zonal winds at Darwin; this is shown to be in good agreement with other large-scale indicators. Statistics are presented and discussed for the interannual variation in summer monsoon onset, extent, active and break conditions, circulation strength, and vertical structure. Some relationships with the Southern Oscillation are also described. These indicate that the Southern Oscillation Index (SOI) is highly correlated with the intensity and degree of convergence in the low-level monsoonal shear zone, and with the mean daily rainfall rate over northern Australia. There is also a significant correlation between the summer monsoon onset date and the SOI in the following spring, which has implications for El Nino teleconnections.