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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

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

Opinions differ on the nature of near-equatorial troughs and their associated weather. Two research aircraft traversed a South Indian Ocean near-equatorial trough on 2 February 1964. Their detailed observations support earlier, large-scale studies which postulated that trough lines above 5° latitude coincide with maximum sea-surface temperature and relatively fine weather.

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C. S. Ramage

Abstract

The Comprehensive Ocean-Atmosphere Data Set reveals that mean scalar winds decreased between 1854 and 1920 and increased since World War II. The latter increase is due to a change in estimating procedure and to the growing proportion of ships equipped with anemometers. The sign of the earlier decrease is attributed to the transition from sail to steam; although a true secular trend might also contribute, it cannot be isolated.

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C. S. Ramage

Abstract

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C. S. Ramage

Abstract

A new Comprehensive Ocean-Atmosphere Data Set for the period 1854–1979 will soon become available for studies of secular climate changes in ocean surface heat flux. Of the observed variables from which heat flux is calculated, wind speed and sea surface temperature have undergone indeterminate spurious changes due to modifications in estimating and measuring.

Analysis of a summertime ocean data set for the Philippine Sea revealed unacceptably large increases in air temperature and dew point readings resulting from daytime heating of the ship. Differences between ocean skin temperatures and subsurface temperatures lead to positive heat flux errors with light winds. Computing heat fluxes for individual ship reports and then averaging them improves matters.

These errors, as well as those arising from spatial and temporal inhomogeneities of individual monthly averages, require that studies of ocean climate change first be confined to the most heavily traveled ship routes. Criteria of consistency, pattern persistence and physical reasonableness would need to be satisfied before one could accept evidence of secular changes in surface heat flux.

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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

No abstract available.

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C. S. Ramage

Abstract

The summer heat low system extending from Somalia across southeast Arabia to northwest India is the most extensive and intense on earth. Although it develops in the normal way over desert regions in response to the sun's zenithal march, it is maintained and intensified through the summer by subsidence of air originally lifted and warmed by the release of latent heat in monsoon rain systems to the east and south. The subsidence not only dominates West Pakistan, Arabia, and Somalia, but severely restricts low cloud formation over the central and western Arabian Sea.

The heat low exports cyclonic vorticity in the middle and upper troposphere to the northern Arabian Sea. When a deep layer of moist air is present over the eastern part of this area, subtropical cyclogenesis occurs, producing a burst of west Indian monsoon rains. This in turn, by increasing subsidence above the heat low, intensifies the heat low and its associated low-level monsoon circulation. When the supply of moist air is cut off, the subtropical cyclone fills, the heat low weakens, and a break takes place in the monsoon rains. With renewal of the moisture supply, the sequence is repeated.

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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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