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C. S. Ramage

Circulation around the South Pacific anticyclone causes cold water to upwell along the South American coast and along the equator east of the date line. Normally the atmospheric circulation in the equatorial region is weakest in April, when the Northern Hemisphere near-equatorial convergence is closest to the equator.

Six times in the past 80 years, large positive sea surface temperature anomalies developed along the Peruvian coast and over a large part of the tropical Pacific and persisted for a year or more—the phenomenon now generally known as El Niño. In the past, data from isolated islands were used in attempts to explain the accompanying meteorological events; understandably a variety of hypotheses resulted.

In the intense 1972–73 El Niño (March 1972–March 1973) extensive weather satellite data have removed much of the observational uncertainty of earlier studies. The Northern Hemisphere near-equatorial convergence shifted about 3° closer to the equator near the Line Islands while the equatorial “doldrum” belt shifted from west of the date line to east of the date line. In the equatorial strip, the El Niño year was relatively wet in the central Pacific, but relatively dry in the western Pacific, while in the eastern Pacific no coherent pattern emerged. Certainly no positive correlation could be established between anomalies of sea surface temperature and of rainfall.

Our preliminary findings suggest that contrary to previous opinion, ocean-atmosphere feedback may not be important in maintaining El Niño, which could be a Pacific manifestation of a global phenomenon.

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C. S. Ramage

Abstract

Latest available compilations confirm that mean hurricane frequencies vary widely within the tropics. Referred to a standard unit area, hurricanes (typhoons) are three times as likely to develop in the western North Pacific as in any other generating area.

Numerous published analyses indicate that an intensifying upper trough in low latitudes may, through an energy dispersion mechanism, sharpen the next downstream trough. The resultant pressure fall in the downstream trough, should it overlie a low-level cyclonic disturbance, might be enough to trigger hurricane development in the disturbance.

Frequent energy dispersion from a vigorous persistent upper trough in the central North Pacific could account for the high frequency of west Pacific typhoons.

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C. S. Ramage

Abstract

In October 1970 the South China Sea experienced three typhoons. Meteorological and oceanographic data were examined in an attempt to explain why the typhoons underwent intensity changes while over the South China Sea. The clearest relationship was found with troughs in the upper tropospheric westerlies—intensification accompanied development of a middle and high cloud plume streaming northeastward from the storm area.

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C. S. Ramage

Abstract

Except for parts of the northwest Pacific, Indian Ocean and Pacific Ocean hurricanes always begin to develop in troughs located at high enough latitudes for low-level westerly winds to be observed on their equatorial sides. These near-equatorial troughs coincide with axes of maximum sea surface temperature, and in this respect they resemble continental heat troughs. Oceanic near-equatorial troughs follow the march of the sun. Generally, the farther they are from the equator the more often do hurricanes begin to form in them. The meridional rate of trough movement depends on the rate of ocean surface heating.

Seas east and equatorward of the continents. Through the spring, continental heating rapidly diminishes the equatorward winds of winter, reducing evaporational cooling of the sea surface and mixed-layer depth. The consequent sea surface temperature rise brings the near-equatorial troughs poleward and starts the hurricane season only. In the height of summer, monsoon winds by increasing evaporational cooling and mixed-layer depth cause sea surface temperatures to fall; the trough is largely over land (heat trough) and hurricanes become rare. In the autumn transition, surface winds are once again light and hurricane frequency attains a second maximum.

Seas west of the continents and the open oceans. Through the spring, continental heating helps prolong the equatorward winds of winter over neighboring seas. Elsewhere too, trade winds maintain a deep mixed layer and considerable evaporational cooling. Thus the near-equatorial trough remains too close to the equator for tropical cyclogenesis. However, by midsummer except over the central and southeast Pacific, the trough reaches high enough latitudes for hurricanes to develop, the season starts, and hurricane frequency soon attains a single maximum.

Oceanic near-equatorial troughs more than 5° from the equator satisfy most of the prerequisites for hurricane development. Because hurricanes cool the sea surface, time and space intervals between hurricane developments seem to be inversely related.

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C. S. Ramage

Abstract

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C. S. Ramage

Abstract

No abstract available.

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C. S. RAMAGE

Abstract

Thunderstorm frequency and amount of moisture above 500 mb. both indicate that the equatorial regions of South America and Africa and the “maritime continent” of Indonesia and the Carolines generate a much greater amount of heat for export than do equatorial oceanic regions.

Over the maritime continent in January 1963, heat generated from excessive rains was efficiently transported northward and through conversion of potential to kinetic energy proably helped maintain an intense subtropical jet stream. In January 1964 drought over the maritime continent was accompained by a relative accumulation of heat in the upper troposphere, associated with inefficient poleward transport, and a much weaker circulation. Most winters over the western Pacific and southeast Asia fluctuate between situations typical of January 1963 and January 1964.

Since the troposphere over the maritime continent in winter is probably the single greatest source of energy for the extratropical circulation, the proposed Marshall Island experiment should be modified to include Indonesia and be rescheduled to include winter.

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C. S. Ramage
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C. S. Ramage

Abstract

Parameterizations in numerical models of the general atmospheric circulation accord with the hypothesis advanced by lchiye and Petersen and Bjerknes—high sea surface temperatures “cause” high local rainfall. Observations at Canton Island and elsewhere in the tropics fail to support this attractively simple idea.

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C. S. Ramage

Abstract

No abstract available.

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