El Nino

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El Nino


     The Asian summer monsoon is the most vigorous, extensive and influential one of all monsoon systems with the vast Afro-Eurasian land mass, the Tibetan Plateau, and the East African Highlands. There are many researches to understand of the Asian monsoon system, but it has not fully understood yet.

1. Monsoon


    a) Definition of Monsoon

    - Etymologically, the Arabic "mausim" which means the season is the origin of the "monsoon". It originally meant southwesterly in summer and northeasterly in winter in the Arabian sea, but nowadays monsoon have the meaning of the seasonal wind.

    - In South and East Asia, "monsoon" means rainy season connected with the rice crop for a living.

    - Climatologically, "monsoon" means a constant winds group satisfying the next condition; It has to have high frequency in the season, occupy geographical space suitable for atmospheric general circulation wind system, and turn to the reverse direction or almost reverse direction from winter to summer.

       The features of monsoon climate are the seasonal change of wind, the humid summer and the dry winter. In a world-wide sense, there are many regions which have monsoon climate. It is known that Indian monsoon region is most dominant than others. In Asian region, summer monsoon starts to develop from mid-May and continues till September or October, and has much rainfall with flowed lower winds. In the case of summer monsoon, the features of continental space structure are the formation of low pressure gage in the lower layer, the convergence in the lower layer, existence of lower atmospheric inflow from ocean and increase in humidity, and increase in precipitation. In winter, there are reverse features.

       In this manner, although many regions lying in maritime continents are observed as monsoon regions, we are concerned in Asian monsoon having largest scale and great influences; That is Indian monsoon and East-Asian monsoon.

    b) Mechanisms

     - Heat Difference between Continents and Oceans
     - Coriolis Force by Rotation of the Earth
     - Role of Water : Energy Release/Absorption by Phase Change

    c) Monsoon Regions and Characteristics

    ① East Asian monsoon

     - East Asian monsoon is divided into to parts; summer and winter monsoon. It is very different feature from Indian monsoon having no winter monsoon. In the case of summer monsoon, there are heavy rain and humid weather under the influence of lower southwesterly in Central Asia and East Asia. Under winter monsoon, northeasterly originated in the vast anticyclonic circulation in Siberian region is superior wind in these regions in Siberian region.  This circulation shifts continental parcel characterized by cold temperature and low humidity to the east and south cost of China.

     - East Asian monsoon system

    ② Indian monsoon

     - Beginning from early-June and continuing to September

     - More than 90% of annual precipitation in central and western India, and more than 50-75% of southern and northwestern India

     - At peak of monsoon, July and August, spacial and monthly mean precipitation is 200 to 300㎜ (Fig. 45. a)

    - Comparison with recent cases of monsoon season
    monsoon season in 1997
           ridge takes its position from north of Africa to southwestern Asia
          - monsoon influences on north India and southeastern Asia during June and  July of 1997
    monsoon season in 1998
          - 1200-1600㎜ rainfall in western India
          - 200-600㎜ rainfall for total time average
          - upper level monsoon ridge extends from northeastern Africa to southeastern Asia 
          - The intensified ridge is connected with large circulation including the north Atlantic and the most Eurasia
          - In the downstream region of this monsoon ridge, cyclonic flow induces heavy rainfall in Yangtze River during June and July

    d) Monsoon Indices

    - Asian summer monsoon

    These three monsoon indices provide a succinct description of the Asian summer monsoon circulation variability for the broad scale South Asian monsoon, ISM, and WNP-EASM, respectively.  In order to conveniently measure Asian summer monsoon circulation anomalies, Webster and Yang (1992) proposed a circulation index, the Webster-Yang index (WYI), which is defined by the vertical zonal wind shear between 200 and 850 hPa (u 200-u 850) averaged over the region (40 ºE-110 ºE, 0-20 ºN). The WYI describes the broad-scale South Asian monsoon variability that is primarily driven by two convective heat sources, one lies in the Bay of Bengal, and the other in the vicinity of Philippines.

    Wang and Fan (1999) found that the variations of the two convective heat sources reflect, respectively, the variations of the Indian summer monsoon (ISM) and the western North Pacific-East Asian summer monsoon (WNP-EASM); but they are not significantly correlated on the interannaul and decadal time scales. Thus, Wang and Fan (1999) proposed two indexes that quantify the variability of the ISM and WNP-EASM, respectively. The ISM index (IMI) may be defined by the 850 hPa zonal wind averaged over (5 ºN-15 ºN 40 ºE-80 ºE) minus that averaged over (20 ºN-30 ºN, 60 ºE-90 ºE) (Fig. 2). This meridional shear of zonal winds depicts the intensity of the Indian monsoon trough and associated southwesterly monsoon. Similarly, the WNP-EAM index (WPEMI) can be defined by the 850hPa wind speed averaged over (5 ºN-15 ºN, 100-130 ºE) minus that over (20 ºN-30 ºN, 110-140 ºE). The IMI and the WPEMI are highly representative of the dominant EOF modes of the low-level circulation anomalies over the ISM (30 ºE-100 ºE, 0-30 ºN) and the WNP-EASM (100-170 ºE, 0-40 ºN) regions with correlation coefficients of 0.72 and 0.88, respectively for the period of 1958-1997 (Wang et al. 2001).

    The regional monsoon index, RM2 is index for east Asia monsoon region defined by Lau et al.(2000) obtained from upper circulation connected with rainfall. To identify circulation features that are most strongly correlated with rainfall over 5–25oN, 100–130oE for Southeast Asia, it is defined as u200mb[40–50oN, 110–150oE] minus u200mb[25–35oN, 110–150oE] based on the correlation map between rainfall and 200mb zonal wind field.


    - Australian summer monsoon

    McBride et al (1995) proposed a circulation index measuring the Australian summer monsoon variability using 850 hPa zonal wind anomalies averaged over  (0-10 ºS, 120 ºE-150 ºE), which is referred to as Australian monsoon index (AUSMI).

    - Five monsoon indices and its interannual variation (1950-2003)






U850(0-20N 40-110E)-U200(0-20N 40-110E)

Webster and Yang 1992

Broad scale South Asian monsoon


U850(5-15N 40-80E)-U850(20-30N 60-90E)

Wang and Fan 1999

Indian summer monsoon


U850(5-15N 100-130E)-U850(20-30N 110-140E)

Wang and Fan 1999

Western Pacific-East Asian monsoon


U200(42-52N 110-150E)-U200(27-37N 110-150E)

Lau et al. 2000

Regional scale East Asian monsoon


U850(0-10S 120-150E )

Mcbride et al. 1995

Australian summer monsoon



2. East Asian Monsoon


3. Our Research



4. Monsoon Links