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Twice wind onsets of monsoon over the western North Pacific and their simulations in AMIP models
Author(s) -
Zhang Li,
Li Jianping
Publication year - 2009
Publication title -
international journal of climatology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.58
H-Index - 166
eISSN - 1097-0088
pISSN - 0899-8418
DOI - 10.1002/joc.1908
Subject(s) - climatology , subtropical ridge , southern hemisphere , outgoing longwave radiation , equator , atmospheric circulation , geology , monsoon , environmental science , atmospheric sciences , latitude , meteorology , geography , convection , precipitation , geodesy
The absolute angle between wind vectors is used to investigate and describe the seasonal evolutions of wind direction, which is able to indicate the abrupt seasonal shift and stable seasonal state of winds. The results show that the wind directions over the western North Pacific (WNP, 10° –22.5°N, 125° –145°E) undergo two abrupt changes and three stable states from winter to summer. Here we name the phenomenon as twice wind onsets of monsoon. The first wind onset occurs near mid‐May at 15°N, and the second one happens with the wind direction shifting southwesterly in mid‐July and ends in late September. The variations of Outgoing Longwave Radiation (OLR) and rainfall in the northward 5° latitudes of the neighbouring region show similar abrupt features as those of the twice wind onsets. The western Pacific subtropical high (WPSH) retreats eastward in the second stable state (pentad 29 to 37), which differs from the first stable state (pentad 1 to 24). As the strong cross‐equator flows from the Southern Hemisphere reach the WNP, the eastward retreat of WPSH consequently triggers the first wind onset. Then the WPSH ridge suddenly jumps to 30°N and results in the second wind onset. Meanwhile, the propagation of the second wind onset is consistent with the migration of the WNP monsoon trough. Seven atmospheric general circulation models (AGCMs) of Atmospheric Model Intercomparison Project (AMIP) from Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) are validated against the observations mentioned earlier. The results show that there are still notable shortcomings for the models to simulate the WNP summer monsoon, especially the twice onsets. There is no significant improvement for the simulation of multi‐models ensemble mean. Only MPI‐ECHAM5 captures partial characteristics of similar pattern. The poor simulation of the migration of the WPSH ridge and monsoon trough may be important factors for the shortcomings. Moreover, none of the models reproduces corresponding rainfall patterns over the WNP. Copyright © 2009 Royal Meteorological Society