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The Annual Cycle and Interannual Variability in the Tropical Pacific and Indian Ocean Regions

Gerald A. MeehlNational Center for Atmospheric Research, Boulder, CO 80307

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Abstract

The annual cycle of outgoing longwave radiation (OLR), clouds, precipitation and sea level pressure is studied from satellite and station data in the tropical Indian and Pacific sectors. A region of heavy convection, termed the tropical convective maximum, moves from north to south and west to cast in the Indian and Pacific sectors as the mean annual cycle proceeds from northern summer to northern winter. During its return excursion northwestward from northern winter to northern summer in the Indian sector, it is not as strong as in the preceding half of the annual cycle. To study interannual fluctuations of the annual cycle in these regions, Indian monsoon rainfall is chosen as an indicator of precipitation and convection in the summer monsoon region. Relatively strong and weak years of monsoon rainfall are selected and used as a starting point to follow the evolution of the annual cycle in the two sets of years. More than two-thirds of the monsoon seasons since 1900 are classified as either relatively strong or weak indicative of the biennial tendency of monsoon rainfall. Examination of sea level pressure, precipitation, and sea surface temperatures shows the dynamically coupled ocean-atmosphere system in the Indian-Pacific region to be involved with producing Southern Oscillation-type signals in atmosphere and ocean in these sets of years, with extremes in the system being manifested as Warm and Cold Events. This is associated with the alternate strengthening and weakening of the mean west-to-east exchange of mass from the Indian to Pacific sectors in the atmosphere, and the interactive response of the ocean in helping reinforce and maintain those anomalies. For example, a year with a relatively strong Indian monsoon is characterized by lower pressure, warmer SSTs, and greater precipitation to the west of the South Pacific Convergence Zone (SPCZ) ahead of the convective maximum as it moves southeastward with the seasonal cycle from northern summer to northern winter. At the same time, higher pressure, decreased precipitation and low SSTs are in evidence to the east of the SPCZ in the tropical Pacific. In the extratropics, a weakened circumpolar trough in the Southern Hemisphere midlatitudes during May-July south of New Zealand and southwest of Australia is associated with strong subtropical highs in the Indian and Pacific oceans. A relatively weak yew is characterized by opposite conditions through the course of the annual cycle. A transition from strong to weak is made in northern spring, as low SLP and warm SST in the SPCZ area are associated with westerly wind anomalies in the equatorial western Pacific and a weakened South Pacific High. Warm SSTs then become established in the tropical eastern Pacific as a result of the dynamic response of the ocean, and a weak year begins. Since the system is not purely biennial, such transitions do not happen every year. Extremes in this oscillation, the Warm and Cold Events associated with the Southern Oscillation, happen less frequently and the anomaly patterns are of much greater amplitude than strong and weak years without Warm and Cold Events. Similar, albeit weaker, patterns are still in evidence throughout the composite annual cycles of these other years. These results point out that processes in the Indian-Pacific region are continually evolving from one annual cycle to the next and that Warm and Cold Events are not discrete occurrences but are extremes of patterns which appear in many other years as part of the dynamically coupled air-sea system in this region.

Abstract

The annual cycle of outgoing longwave radiation (OLR), clouds, precipitation and sea level pressure is studied from satellite and station data in the tropical Indian and Pacific sectors. A region of heavy convection, termed the tropical convective maximum, moves from north to south and west to cast in the Indian and Pacific sectors as the mean annual cycle proceeds from northern summer to northern winter. During its return excursion northwestward from northern winter to northern summer in the Indian sector, it is not as strong as in the preceding half of the annual cycle. To study interannual fluctuations of the annual cycle in these regions, Indian monsoon rainfall is chosen as an indicator of precipitation and convection in the summer monsoon region. Relatively strong and weak years of monsoon rainfall are selected and used as a starting point to follow the evolution of the annual cycle in the two sets of years. More than two-thirds of the monsoon seasons since 1900 are classified as either relatively strong or weak indicative of the biennial tendency of monsoon rainfall. Examination of sea level pressure, precipitation, and sea surface temperatures shows the dynamically coupled ocean-atmosphere system in the Indian-Pacific region to be involved with producing Southern Oscillation-type signals in atmosphere and ocean in these sets of years, with extremes in the system being manifested as Warm and Cold Events. This is associated with the alternate strengthening and weakening of the mean west-to-east exchange of mass from the Indian to Pacific sectors in the atmosphere, and the interactive response of the ocean in helping reinforce and maintain those anomalies. For example, a year with a relatively strong Indian monsoon is characterized by lower pressure, warmer SSTs, and greater precipitation to the west of the South Pacific Convergence Zone (SPCZ) ahead of the convective maximum as it moves southeastward with the seasonal cycle from northern summer to northern winter. At the same time, higher pressure, decreased precipitation and low SSTs are in evidence to the east of the SPCZ in the tropical Pacific. In the extratropics, a weakened circumpolar trough in the Southern Hemisphere midlatitudes during May-July south of New Zealand and southwest of Australia is associated with strong subtropical highs in the Indian and Pacific oceans. A relatively weak yew is characterized by opposite conditions through the course of the annual cycle. A transition from strong to weak is made in northern spring, as low SLP and warm SST in the SPCZ area are associated with westerly wind anomalies in the equatorial western Pacific and a weakened South Pacific High. Warm SSTs then become established in the tropical eastern Pacific as a result of the dynamic response of the ocean, and a weak year begins. Since the system is not purely biennial, such transitions do not happen every year. Extremes in this oscillation, the Warm and Cold Events associated with the Southern Oscillation, happen less frequently and the anomaly patterns are of much greater amplitude than strong and weak years without Warm and Cold Events. Similar, albeit weaker, patterns are still in evidence throughout the composite annual cycles of these other years. These results point out that processes in the Indian-Pacific region are continually evolving from one annual cycle to the next and that Warm and Cold Events are not discrete occurrences but are extremes of patterns which appear in many other years as part of the dynamically coupled air-sea system in this region.

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