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JAMSTEC 2002 Annual Report Frontier Research System for Global Change activity) over the eastern part of the warm pool region and associated changes in Hadley and Walker circulations over the Pacific sector. Drastic changes in the heating and circulation fields were noticed particularly since the / ENSO events. Climatological mean water balance in the Lena river basin is evaluated by atmospheric water balance, making use of moisture convergence evaluated from NCEP Reanalysis data and precipitation from GPCP. Summertime evaporation evaluated this way is compared with the weighted average of field observations at the three sites of GAME-Siberia, Tiksi, Yakutsk and Tynda, where there is close agreement. GAME Reanalysis product, which covers April to October , is evaluated in the context of water balance of continental-scale river basins. With temporal resolution of one month and spatial scale larger than km, precipitation from GAME Reanalysis agrees with observation-based data sets such as GPCP in most of the cold regions and in some (though not all) of the temperate and tropical zones. On the other hand, a case study for the Indochina peninsula reveals that both GAME Reanalysis and GPCP do not represent features of precipitation with spatial scales smaller than km. Following the success of the first phase of the global soil wetness project, the second phase was planned. The science plan and the implementation plan were prepared by P.A. Dirmeyer and T. Oki, and the inaugural meeting was held in Center for Ocean-Land- Atomosphere Studies (COLA), USA, in October, . More than land surface modeling groups are expected to join the second phase of the GSWP to perform the -year offline simulation of land surface models to develop the best estimates of the global water and energy balances, and to explore the landatmosphere interactions and their long term variations. T. Oki is co-chairing the project and contributing for the distributed data center and the hydrological validation part. a-. Land Surface (snow cover, soil moisture, vegetation)–atmosphere Interaction and Climate Variability From the analysis targeting the vegetation over north Asia using remote sensing data, a west-to-east phenological green wave was found. The results were summarized in a paper accepted by an international journal. The land surface and the atmospheric boundary layer (ABL) around Yakutsk, a city in eastern Siberia, was studied using airborne data acquired by the FORSGC field experiment. The distribution of diverse land surfaces was mapped and its relation to the water and energy flux in the ABL was discussed. The relationship between continental-scale snow cover variation and the dominant mode of the atmospheric variability on the seasonal to interannual timescale, i.e., the North Atlantic Oscillation (NAO) or Arctic Oscillation (AO) were examined. The result demonstrated that they have common preferred timescales (quasi-biennial and sub-decadal), and that snow cover leads atmosphere by several or more months on the sub-decadal period. a-. Climate Variability and Hydrological Processes Surface heat and water balance trends and climatic variation over Eastern Asia, including the Tibetan Plateau, were analyzed using meteorological data for points in the period to . Changes in heat and water balances were examined using potential evaporation, and a wetness index (Kondo and Xu, a, b). Climate zones in Eastern Asia identified by the wetness index matched well with the distribution of vegetation. Average monthly temperatures increased over the years, with the sharpest increase in February. The data showed that diurnal temperature ranges have decreased and diurnal surface temperature ranges have increased in recent years. From the Tibetan Plateau, through central China, to southern Northeast China, there has been an increase in potential evaporation and pan evaporation (Fig. ), which may be related to higher temperatures and a lack of surface water. 122
Japan Marine Science and Technology Center Frontier Research System for Global Change 80 90 100 110 120 130 140 80 90 100 110 120 130 140 50 50 50 50 40 40 40 40 30 30 30 30 20 80 90 100 110 120 130 140 20 20 80 90 100 110 120 130 140 20 Fig. 4 a) Trends of potential evaporation during 1971-2000. b) Trends of observed pan evaporation during 1971-2000. Changes in storm track activity were identified to examine how these changes affect the seasonal mean flow over northern Eurasia associated with an interannual seesaw in summer precipitation between east and west Siberia (ES and WS). In the two extreme phases of this oscillation, an east-west dipole pattern of precipitation anomalies prevails over much of Siberia. We focused on the years from – when such spatial–temporal characteristics of the oscillation are clear and consistent, as noted by Fukutomi et al. (). The two precipitation regimes that are manifest as out-of-phase dipole patterns are represented as the ES-wet--WS-dry regime and the WS-wet--ES-dry regime. We documented largescale atmospheric patterns and storm track activities associated with these contrasting regimes using linear regression and compositing, and highlighted the synoptic-scale eddy forcing on the mean circulations. The climatic distribution and its long-term variability of the low-level cloud observed by the surface observer over the Eurasian continent were investigated. The frequency of appearance of low-level clouds over China and Mongolia obviously decreased during the last two decades. While the suppressing of cumuliform clouds was evident over the whole of China, stratiform clouds increased in the southern China during the summer season (Fig. ). However these longterm trends of low-level cloud have small correlation to the precipitation trends over China. Decadal-scale SST variations in the mid-latitude North Pacific were examined to diagnose the relative importance between surface and subsurface forcing on FRQ FRQ 0.4 0.3 0.2 0.1 0.0 0.4 0.3 0.2 0.1 0.0 The Lower Reaches of Yangtze River Valley The Coastal Region of South China Sea Cu Clear 70 75 80 85 90 95 YEAR Fig. 5 Time series data showing the frequency of appearance of lower-level clouds in the lower reach of Yangtze River valley (top) and the coastal region of South China Sea (bottom) from 1971 to 1996. The linear trends which are significant at the 95% level are also shown (dashed lines). the variations. For quantitative discussion, a diagnostic equation based on the annual integration considering the large seasonal change of ocean mixed layer was proposed. It was found that ocean dynamics played an important role for the low-frequency SST variation in the Kuroshio-Oyashio extension where the ocean mixed layer is deep. b. The Hydrological Process on Land Group Sensitivity of frozen soil permeability and surface water storage was investigated using a one-dimensional land surface model that can be implemented in a climate model. The atmospheric conditions were assumed to be those in frozen ground regions of Russia. The results shows that evaporation is quite sensitive in these processes. The annual evaporation is Sc Cb 123
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