Decadal Variations of the Global Atmosphere-Ocean System

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Abstract
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 500 hPa geopotential height data for 30 to 50 years. The most dominant interannual variation in the global atmosphere and ocean is associated with El Nino 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 central-eastern 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 500 hPa height held 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 500 hPa 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 500 hPa height field has tended to intensify during the last 50 years.