Long-Term Trends and Forcing Mechanisms of Circulation and Climate in the Equatorial Pacific

Scott Curtis Department of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, Madison, Wisconsin

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Stefan Hastenrath Department of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, Madison, Wisconsin

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Abstract

Trends of circulation and climate in the equatorial Pacific during 1948–92 were analyzed from observations of sea surface temperature (T), cloudiness (C), sea level pressure (P), specific humidity (Q), and zonal wind component. From indices compiled for the cold tongue in the east, the central equatorial Pacific, the warm pool in the west, and the eastern equatorial Indian Ocean, interannual relationships were compared with long-term trends. On the interannual timescale the significant negative T–P correlations in the eastern Pacific indicate a hydrostatic forcing of T on P, and the negative P–C correlations in the western Pacific and eastern Indian Oceans are consistent with anomalously high P through subsidence favoring clear sky. The tropical Pacific is warming and most markedly in the east; there is a weakening of the westward pressure gradient; and increasing Q and C in the east. Warming in the eastern section of the basin may force falling pressure hydrostatically, the trend of P being largest near 155°W. In the domain 155°–90°W easterlies accelerate, favoring enhanced evaporation and humidity. The strengthening of the winds east of 155°W and slowdown to the west imply enhanced convergence and upward motion, consistent with increasing C near 155°W. Also warming may thermodynamically force increasing humidity and cloudiness in the realm of the intertropical convergence zone. The observed basin-wide warming and weakening of the westward pressure gradient, with the consequent slackening of easterlies over the central equatorial Pacific, reflect a tendency toward patterns characteristic of the low/warm phase of the Southern Oscillation.

* Current affiliation: Joint Center for Earth Systems Technology, University of Maryland, Baltimore County, NASA/Goddard Space Flight Center, Greenbelt, Maryland.

Corresponding author address: Dr. W. R. Curtis, JCET, University of Maryland Baltimore Co., NASA/Goddard Space Flight Center, Code 912, Greenbelt, MD 20771.

Email: curtis@agnes.gsfc.nasa.gov

Abstract

Trends of circulation and climate in the equatorial Pacific during 1948–92 were analyzed from observations of sea surface temperature (T), cloudiness (C), sea level pressure (P), specific humidity (Q), and zonal wind component. From indices compiled for the cold tongue in the east, the central equatorial Pacific, the warm pool in the west, and the eastern equatorial Indian Ocean, interannual relationships were compared with long-term trends. On the interannual timescale the significant negative T–P correlations in the eastern Pacific indicate a hydrostatic forcing of T on P, and the negative P–C correlations in the western Pacific and eastern Indian Oceans are consistent with anomalously high P through subsidence favoring clear sky. The tropical Pacific is warming and most markedly in the east; there is a weakening of the westward pressure gradient; and increasing Q and C in the east. Warming in the eastern section of the basin may force falling pressure hydrostatically, the trend of P being largest near 155°W. In the domain 155°–90°W easterlies accelerate, favoring enhanced evaporation and humidity. The strengthening of the winds east of 155°W and slowdown to the west imply enhanced convergence and upward motion, consistent with increasing C near 155°W. Also warming may thermodynamically force increasing humidity and cloudiness in the realm of the intertropical convergence zone. The observed basin-wide warming and weakening of the westward pressure gradient, with the consequent slackening of easterlies over the central equatorial Pacific, reflect a tendency toward patterns characteristic of the low/warm phase of the Southern Oscillation.

* Current affiliation: Joint Center for Earth Systems Technology, University of Maryland, Baltimore County, NASA/Goddard Space Flight Center, Greenbelt, Maryland.

Corresponding author address: Dr. W. R. Curtis, JCET, University of Maryland Baltimore Co., NASA/Goddard Space Flight Center, Code 912, Greenbelt, MD 20771.

Email: curtis@agnes.gsfc.nasa.gov

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