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Stefan L. Hastenrath

Abstract

The flux of atmospheric water vapor has been examined as part of a more extensive study on general circulation and energetics in the area of the Central American seas. The analysis is based on twice daily radiosonde data for the entire year 1960. During the winter half of the year, a westward directed moisture transport is concentrated in a relatively narrow band over the southern Caribbean Sea. The flux of water vapor over the Gulf of Mexico is directed roughly eastward. An increase of the transport downstream indicates the excess of evaporation over precipitation. In summer, a strong moisture flux extends from the Caribbean Sea over the Gulf of Mexico into the interior of the North American continent. The water vapor transport decreases downstream, which means an excess of precipitation over evaporation. The eddy moisture flux and flux divergence over the Gulf of Mexico reaches considerable proportions in winter, while it is comparatively small over the Caribbean Sea throughout the year. The water vapor flux divergence is discussed with regard to estimates of lane-scale precipitation and independent computations of the sensible and latent heat flux at the sea-air interface.

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Stefan L. Hastenrath

Abstract

Various models of the mean meridional circulations are examined in terms of the vorticity equation. These include: 1) the traditional model of two Hadley cells joining in a region of ascending motion near the equator; 2) a model with two “equatorial cells” enclosed between the traditional Hadley cells, implying subsidence over the equator; and 3) a model with a single “equatorial cell” enclosed between the two Hadley cells, with ascending motion on one side, and subsidence on the other side of the equator. All three models are considered compatible with the vorticity equation, under specific conditions of latitudinal and vertical distributions of absolute vorticity, frictional force, and vertical and zonal wind components. The existence of twin equatorial cells requires the eastward directed frictional force in the lower layers to decrease from the kinematical equator poleward, within the subsiding portion of either of the two equatorial cells. It is suggested that this may be related to the existence of (eastward directed) equatorial undercurrents in the Pacific and Atlantic Oceans. The requirements for twin equatorial cells do not seem to be met over the interior of the tropical continents.

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Stefan L. Hastenrath

Abstract

The field of large-scale vertical motion and the atmospheric-oceanic energy budget in the areas of the Caribbean Sea and the Gulf of Mexico are studied with emphasis on seasonal and regional variations, using the available radiosonde data of the entire year 1960. The atmosphere over the Caribbean Sea exports latent heat during the winter half of the year, changing to import during summer, while divergence of the latent heat flux prevails over the Gulf of Mexico during most of the year with the exception of midsummer. The troposphere as a whole imports geopotential energy and sensible heat during winter in the Caribbean, and during most of the year in the Gulf area, this being effected by the upper-tropospheric westerly current originating over the equatorial regions of the eastern Pacific. During the summer half of the year, an export of geopotential energy and sensible heat takes place over the Caribbean Sea, being concentrated in the upper-tropospheric easterlies, this pattern also including the area of the Gulf of Mexico in midsummer. Regarding the total energy budget, the troposphere over the Caribbean Sea acts as an exporter of energy to other parts of the globe throughout the year, while import is indicated for the Gulf of Mexico during some winter months. Ocean currents export heat from the Caribbean Sea during the summer half of the year, while conspicuous import is indicated for the Gulf of Mexico throughout the year, with the exception of midsummer. The tropospheric energetics are discussed with respect to their role in the general circulation.

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Daryl L. Covey
and
Stefan Hastenrath

Abstract

Atmospheric–oceanic departure patterns in the tropical Atlantic and eastern Pacific associated with the Ecuador/Peru El Niñno and its antithesis are studied on the basis of long-term ship observations during 1911–71. Departure maps of sea level pressure (SLP), wind speed and sea surface temperature (SST) in March/April are presented for composite of ten extreme years of either regime. The evolution of SLP anomalies from the preceding to the event year is traced for large ocean areas. Gaussian and binomial probabilities are calculated as a measure of the importance of SLP departures.

El Niño years are characterized by abnormally low SLP over the eastern South Pacific and positive SLP anomalies over the Atlantic; winds are weaker than normal over the Pacific, and stronger over the Atlantic; SST anomalies are positive in the Pacific, but negative in a band of the Atlantic extending from West Africa toward the South American coast. During counter-El Niño years, departure patterns am approximately inverse. Departure patterns during droughts and floods in northeast Brazil are similar to, but not identical with, the ones obtained by stratification with regard to the Ecuador/Peru El Niño and counter-El Niño, respectively.

El Niño has a tendency to occur in a series of successive years, as does its antithesis. The seasonal development of departure patterns in initial El Niñ years (a single event or the first in a sequence) and sequential years (the second or later years in a sequence) differs, in that an initial El Niño tends to be heralded by anomalously high SLP and cold waters in the eastern South Pacific. In sequential years, the pre-season exhibits anomalies of the same sign as the peak season. Statistical probabilities indicate the possibility of foreshadowing initial El Niño from SLP anomalies in the Atlantic.

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STEPHEN K. COX
and
STEFAN L. HASTENRATH

Abstract

During the Line Islands Experiment in spring 1967, surface shortwave and net radiation was continuously recorded at Palmyra, and Snomi-Kuhn infrared radiationsondes were released daily at the islands of Palmyra, (5°53′ N., 162°05′ W.) and Christmas (1°59′ N., 157°22′ W.) as part of an extensive surface and upper air observation program. Data are evaluated in terms of the diurnal march of the surface radiation balance and the radiation budget characteristics of the troposphere-ocean system. These direct measurements indicate a substantially larger surface net radiation than is expected from available climatic mean charts based on empirical formulas. Implications for the tropical heat budget are pointed out.

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