International Pacific Research Center and Department of Meteorology, University of Hawaii Honolulu, Hawaii, USA

bwang@soest.hawaii.edu

　

Using extended singular value decomposition analyses we characterize the evolution of Asian-Australian monsoon (AAM) anomalies during El Nio as two low-level, off-equatorial anticyclones--one over the South Indian Ocean (SIO), the other over the western North Pacific (WNP). The SIO anticyclone, which is responsible for unusual climate conditions over Indian Ocean, India, and east Africa, originates in boreal summer while El Nio develops, reaches its height in fall, and decays before El Nio matures. On the other hand, the WNP anticyclone,S forms in fall, attains its maximum intensity when El Nio matures, and persists through the next spring and summer, providing a prolonged impacts on East Asian monsoon. Although the remote El Nio influences the monsoon by suppressing convection in maritime continent, the El Nio forcing alone cannot explain the extraordinary amplification of the SIO anticyclone in the developing stage of El Nio nor the maintenance of the WNP anticyclone in the decaying phase of El Nio.

　

The atmosphere-ocean conditions in the two regions of anticyclones are similar, namely, a SST dipole with cold water to the east and warm water to the west of the anticyclone center. These conditions result from a positive feedback between the anticyclone and SST dipole, which intensifies SIO anticyclone during El Nio development and maintains the WNP anticyclone during its decay. The outcome of this interaction depends crucially on the background flows. The coupled mode over the SIO is often triggered by El Nio, but also possibly by other local or remote forcing.

　

　

University of Hawaii, International Pacific Research Center (IPRC) Honolulu, Hawaii, USA

sen@soest.hawaii.edu

　

The objective of this study is to investigate the effect of surface cover change in the Indochina Peninsula (deforestation particularly) on the East-Asian summer monsoon through model sensitivity experiments. During summer months, the strong monsoon westerlies bring abundant moisture to the higher latitudes over China and may cause severe floods as in 1991 and 1998. These westerlies essentially flow over the Indochina Peninsula between the Tibetan plateau in the northwest and the subtropical high-pressure system over the western Pacific. It is, for this reason, interesting to investigate whether and how the changes in the surface characteristics of the Indochina Peninsula could modify this strong monsoonal flow and affect its downstream activities. Preliminary results from a regional climate model sensitivity experiment of the 1998 summer monsoon - in which the experimental run with replacement of the agricultural fields of Indochina Peninsula with tropical rainforest is compared to the control run with current vegetation cover - show that the reforestation causes significant changes in rainfall patterns over China. However, the regionally averaged monthly rainfall is not very different except a higher July and a lower September rainfall over Yangtze River basin (105-122E and 30-34N). It is worth reporting that an interesting phenomenon is observed over the South China Sea in September when the monsoon activity retreats to the south. The cyclonic circulation over southeast China and the South China Sea is intensified in September with the "forest" simulation. Consequently, the rainfall amount in September is significantly increased over the South China Sea and southeastern corner of China while it is decreased over the Indochina Peninsula and inland China.

　

　

¹Department of Earth and Planetary Science, Graduate School of Science University of Tokyo, Tokyo, JAPAN

hisashi@eps.s.u-tokyo.ac.jp

　

²Climate Variations Research Program, Institute for Global Change Research Yokohama, JAPAN

　

The wintertime atmospheric circulation over the Far East and Northwestern (NW) Pacific is characterized by persistent southward flow of cold, dry air between the Siberian High and Aleutian Low. Abundant latent and sensible heat is supplied to this airflow from the warm ocean surface over the Kuroshio and its extension. This heat supply and strong meridional temperature gradients caused by the cold monsoonal flow both act to sustain the extremely high near-surface baroclinicity that feeds baroclinic eddies to form a well-defined storm track downstream. The weakening of the winter monsoon, which occurred in the late 1980s, was found to be associated with substantial modifications in the heat and fresh water exchanges between the ocean and atmosphere over the entire North Pacific basin. These modifications were not only directly through the weakened monsoonal flow but also through modulated behavior of synoptic storms. The most profound signature of the latter was the disappearance of the well-known midwinter minimum in the seasonal march of the storm track activity over the NW Pacific owing to the marked midwinter intensification of synoptic eddies since the late 1980s. The intensification occurred despite the relaxed meridional temperature gradient associated with the weakened monsoonal flow and upper-level jet intensity. Structure of synoptic eddies seemed to be modified significantly in association with the relaxation of the excessive jet intensity since the late 1980s, so that they tended to convert energy from the mean flow more efficiently than before.

　

Vladimir Ponomarev¹, Dmitril Kaplunenko¹, Vladimir Krokhin² and Hajime Ish ida³

¹Laboratory for Modeling of Physical Processes in the Ocean, Pacific Oceanological Institute Far Eastern Branch of Russian Academy of Sciences Vladivostok, Primorsky Krai, RUSSIA

ponomarev@poi.dvo.ru

　

²Department of Long-term Weather Prediction

Far Eastern Regional Hydrometeorological Research Institute Vladivostok, Primorsky Krai, RUSSIA

　

³Laboratory of Coastal Engineering Kanazawa University Kanazawa, Ishikawa, Japan

　

The aim of this study is to estimate centennial/semi-centennial climatic tendencies of monthly mean surface air temperature and precipitation in the Northeast Asia as well as SST in the Northwest Pacific. Both mean-square-root and robust methods of trend estimation are applied. In the second case Kendall's rank approach is used. The air temperature and precipitation time series for the 20th century are examined. Tendency of both centennial and semi-centennial warming is revealed mainly in winter and spring for the most of regions in the mid-latitude. The most substantial centennial warming in the large-scale continental area of the mid-latitude band is found mainly for December - March. The semi-centennial/centennial cooling occurs in South Siberia and subarctic marginal zone in June-October.

　

Change of precipitation accompanies air temperature trends in some large-scale areas. Semi-centennial tendencies of SST in the Northwest Pacific have also opposite sign in different large-scale areas which being changed from winter to summer. Positive tendency of the SST predominates in both Kuroshio region and area of the northwest subarctic gyre (region of Aleutian and Kamchatka currents) in winter. Negative tendency of the SST dominates of southwest subarctic gyre and offshore area of subtropic gyre in summer. The SST trend distribution in the Northwest Pacific, tendency change from winter to summer are in agreement with climatic tendencies in the Northeast Asia and Pacific marginal seas. The processes of interaction between ocean and atmosphere associated with climate change in this area are discussed.