Principal modes of Asian summer monsoon variability: Detection and changes
Abstract
Principal modes of Asian summer monsoon variability are identified. By using vertically integrated moisture flux, principal modes represent better separation than commonly used variables such as rainfall, winds and outgoing longwave radiation. An empirical orthogonal function of vertically integrated moisture flux within the South, Southeast and East Asia during summertime is analysed. Results of various analyses let us convince that the first and second EOFs of the moisture flux are the principal modes of the Asian monsoon variability. In summer, there are two modes dominant in the Asian monsoon region; one consists of low-level circulation over the subtropical western Pacific near Philippines and associated convective dipole centers located over the western Pacific and Indonesia. The other consists of El Nino-Southern Oscillation (ENSO) signal and the Pacific-Japan (PJ) pattern, called ENSO-PJ mixed mode. This pattern is detected as the first EOF mode of a simulation with an atmospheric general circulation model giving the climatological mean sea surface temperature. Furthermore, the pattern is dominant in both present climate simulation and global warming simulation using coupled GCM. A projected change shows increasing of precipitation over South China and Japan. The Pacific-Indo dipole pattern is found out to be excited without external forcing like a specific sea surface temperature anomaly. Moreover, the Pacific-Indo dipole pattern appears as the preferred structure of variability by giving small perturbations to a three-dimensionally varying basic state in summertime by using a linear baroclinic model. Factors of the basic state which help to excite and maintain the Pacific-Indo dipole pattern are examined. Free, stationary Rossby waves can be excited in the region of low-level westerly extending from the Indian Ocean to the South China Sea which blows as a part of the monsoonal flow in summer. Rossby waves at the eastern end of the low-level westerly where the basic state converges can extract the kinetic energy from the basic state. The circulation constituting the Pacific-Indo dipole pattern can be resonantly excited by a small perturbation given to the monsoon jet. The distribution of moisture of the basic state in summer is not uniform. Moisture is concentrated in the South China Sea and the Philippine Sea where the cyclonic circulation of the Pacific-Indo dipole pattern over the Philippines is centered. When the cyclonic circulation is formed, the moisture of the basic state converges and is advected to the northwestern Pacific, evaporation is activated at the southern part of the circulation anomaly, and then the field becomes unstable. Then a divergent circulation, an ascent over Philippines and a descent over Indonesia, are made, and the dipolar centers of convection are formed. The cyclonic circulation is enforced by the convection over Philippines. When an anticyclonic circulation over Philippines is formed, the reverse pattern of convection is formed. The circulation over the northwestern Pacific enforced by the convection composed of the Pacific-Indo dipole collects moisture and strengthens the dipolar convective anomalies. The Pacific-Indo dipole pattern is maintained by feedback between circulation and convection mentioned above.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.A13H0351Y
- Keywords:
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- 1620 GLOBAL CHANGE / Climate dynamics;
- 3305 ATMOSPHERIC PROCESSES / Climate change and variability;
- 3339 ATMOSPHERIC PROCESSES / Ocean/atmosphere interactions