Interannual variability in the onset of the summer monsoon over the Eastern Bay of Bengal |
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| Authors: | J Mao G Wu |
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| Institution: | (1) State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China |
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| Abstract: | Summary Climatological characteristics associated with summer monsoon onset over the eastern Bay of Bengal (BOB) are examined in terms
of the westerly-easterly boundary surface (WEB). The vertical tilt of the WEB depends on the horizontal meridional temperature
gradient (MTG) near the WEB, under the constraint of the thermal wind balance. The switch in the WEB tilt firstly occurs between
90 and 100°E during the first pentad of May. At this time the 850 hPa ridgeline splits over the BOB and heavy rainfall commences
over the eastern BOB, indicating the onset of the BOB summer monsoon (BOBSM). The area-averaged MTG (200–500 hPa) is proposed
as an index to define the BOBSM onset. A comparison of the onset determined by the MTG, 850 hPa zonal wind, and outgoing longwave
radiation (OLR) shows that the MTG index is the most effective in characterizing the interannual variability of the BOBSM
onset.
Strong precursor signals are found prior to an anomalous BOBSM onset. Composite results show that early (late) BOBSM onset
follows excessive (deficient) rainfall over the western Pacific and anomalous lower tropospheric cyclonic circulation which
extends zonally from the northern Indian Ocean into the western Pacific, and strong (weak) equatorial westerly anomalies in
the preceding winter and spring. Prior to an early (late) BOBSM onset, significant positive (negative) thickness anomalies
exist around the Tibetan Plateau, accompanied by anomalous upper tropospheric anticyclonic (cyclonic) circulation. The interannual
variations of the BOBSM onset are significantly correlated with anomalous sea surface temperature related to ENSO. These occurs
through changes in the Walker circulation and local Hadley circulation, leading to middle and upper tropospheric temperature
anomalies over the Asian sector. The strong precursor signals around the Tibetan Plateau may be partly caused by local snow
cover anomalies, and an early (late) BOBSM onset is preceded by less (more) snow accumulation over the Tibetan Plateau during
the preceding winter. |
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