On the Annual Cycle of the Eastern Equatorial Pacific

Tianming Li Atmospheric and Oceanic Sciences Program Princeton University, Princeton New Jersey

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S. George H. Philander Atmospheric and Oceanic Sciences Program Princeton University, Princeton New Jersey

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Abstract

Although the sun “crosses” the equator twice a year, the eastern equatorial Pacific has a pronounced annual cycle, in sea surface temperature and in both components of the surface winds for example. (This is in contrast to the Indian Ocean and western Pacific where a semiannual oscillation of the zonal wind is the dominant signal on the equator.) Calculations with a relatively simple coupled ocean-atmosphere model indicate that the principal reason for this phenomenon is the marked asymmetry, relative to the equator, of the time-averaged climatic conditions in the eastern tropical Pacific. The important asymmetries are in surface winds, oceanic currents, and sea surface temperature: The time-averaged winds and currents have northward components at the equator and the warmest waters are north of the equator. Because of those asymmetries, seasonally varying solar radiation that is strictly antisymmetric relative to the equator can force a response that has a symmetric component. The amplitude of the resultant annual cycle at the equator depends on interactions between the ocean and atmosphere, and on positive feedbacks that involve low-level stratus clouds that form over cold surface waters.

Abstract

Although the sun “crosses” the equator twice a year, the eastern equatorial Pacific has a pronounced annual cycle, in sea surface temperature and in both components of the surface winds for example. (This is in contrast to the Indian Ocean and western Pacific where a semiannual oscillation of the zonal wind is the dominant signal on the equator.) Calculations with a relatively simple coupled ocean-atmosphere model indicate that the principal reason for this phenomenon is the marked asymmetry, relative to the equator, of the time-averaged climatic conditions in the eastern tropical Pacific. The important asymmetries are in surface winds, oceanic currents, and sea surface temperature: The time-averaged winds and currents have northward components at the equator and the warmest waters are north of the equator. Because of those asymmetries, seasonally varying solar radiation that is strictly antisymmetric relative to the equator can force a response that has a symmetric component. The amplitude of the resultant annual cycle at the equator depends on interactions between the ocean and atmosphere, and on positive feedbacks that involve low-level stratus clouds that form over cold surface waters.

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