Tropical Central-eastern Pacific Research Articles

A hybrid coupled ocean— atmosphere model and enso prediction study1

A hybrid coupled ocean-atmosphere model is designed, which consists of a global AGCM and a simple anomaly ocean model in the tropical Pacific. Retroactive experimental predictions initiated in each season from 1979 to 1994 are performed. Analyses indicate that: (1) The overall predictive capability of this model for SSTA over the central-eastern tropical Pacific can reach one year, and the error is not larger than 0.8 degrees C. (2) The prediction skill depends greatly on the season when forecasts start. However, the phenomenon of SPB (spring prediction barrier) is not found in the model. (3) The ensemble forecast method can effectively improve prediction results. A new initialization scheme is discussed.

全球大気-海洋システムにおける十年規模変動

Decadal and interannual variations in the global atmosphere and ocean are investigated using the global sea surface temperature (SST), sea-level pressure, surface winds, precipitation and Northern Hemisphere 500hPa geopotential height data for 30 to 50 years. The most dominant interannual variation in the global atmosphere and ocean is associated with El Niño and Southern Oscillation (ENSO) phenomena.Two dominant decadal modes are obtained. The first mode mainly dominates over the entire Pacific Ocean and changed abruptly in the late 1970s. The spatial pattern of the tropical SST after 1978 contains large positive anomalies over most of the tropical ocean, particularly over the eastern Pacific. Negative SST anomalies are found in the mid-latitudes over the North and the South Pacific. The sea-level pressure in the tropics shows negative anomalies in the eastern Pacific, but positive anomalies in the western Pacific and Indian Oceans. There exist westerly anomalies in the equatorial western-central Pacific, indicating the weakening of the tropical Walker circulation. The surface precipitation increased in the tropical centraleastern Pacific where large increases of SST exist. Composite and singular value decomposition (SVD) analyses demonstrate that the Pacific/North American (PNA) pattern was strengthened and associated with this first decadal mode. It is suggested that the enhanced tropical convection due to the increase of SST over the tropical central-eastern Pacific may strengthen the PNA pattern.The second decadal mode has dominant SST anomalies mainly in the Northern Hemisphere midlatitudes. The Northern Hemisphere 500hPa height field associated with this mode shows a dominance of a north-south seesaw between middle latitudes and higher latitudes. A north-south dipole structure in the western Atlantic is especially noticeable. Large positive anomalies extending from East Asia to the North Pacific are also significant. The second mode has a time scale of about 15 years and changed its sign from negative to positive around 1987.Linear trends for SST, sea-level pressure, surface winds and 500hPa height are also investigated. Significant positive trends of SST are found over the most tropical regions and negative SST trends exist in the North Pacific. There exist large trends of sea-level pressure and surface winds over the North Atlantic. Easterlies are strengthened over most of the tropics. The PNA pattern in the 500hPa height field has tended to intensify during the last 50 years.

熱帯太平洋・インド洋における降水量の十年スケール変動

Interdecadal variations of precipitation over the tropical Pacific and Indian Oceans and their relationships with SST are investigated by using surface and satellite observation data. The analyses of surface precipitation indicated that the precipitation in the tropical central-eastern Pacific was increased during the period from the mid 1970s to the 1980s in accordance with the increase of SST in these regions. On the other hand, the precipitation in the tropical western Pacific was rather decreased during this period probably due to changes of tropical east-west circulations affected by the enhanced convection in the central-eastern Pacific. The precipitation in the South Indian Ocean was increased since the mid 1970s corresponding to the increase of SST over the Indian Ocean, but it was nearly constant in spite of gradual increase of SST before that period. Monthly mean high-cloud amount data derived from the GMS-IR observations during 16 years from April 1978 to February 1994 are analyzed to obtain long-term trend of the convective activity in the tropical western Pacific. The computation of the linear trend of the high-cloud amount for the 16 years reveals that decreasing trends dominate over most of the tropical western Pacific, but increasing trends can be seen in the equatorial central Pacific around the dateline. These results suggest that El Nino-like environmental conditions such as enhanced convection in the central Pacific, but suppressed convection in the western Pacific prevail over the tropical Pacific during recent decades. These findings are consistent with previous studies demonstrating that there occurred decadal scale changes in atmosphere-ocean systems in the tropical Pacific during the 1970s-1980s period. The results in this study support the scenario suggested by Nitta and Yamada (1989) that large atmospheric anomaly circulations in the North Pacific Ocean appeared in recent decades might be generated by the enhanced convective activity in the tropical central-eastern Pacific due to the increase of the tropical SST