Chapter 6 



Meteorologi; and Atmospheric Chemistry; 

 of Enewetak Atoll 



JOHN T. MERRILL and ROBERT A. DUCE 



Center for Atmospheric Chemistr\/ Studies 



Graduate School of Oceanography^ 



University^ of Rhode Island, Kingston, Rhode Island 02881 



INTRODUCTION 



The Marshall Islands area has a marine climate that 

 varies from tropical to subtropical; near Enewetak Atoll 

 the weather is characterized by brisk steady winds, 

 moderate rainfall, and unvarying high temperatures with 

 typical partial cloudiness. The atoll lies near the northern 

 edge of the tropical zone dominated by the migrating equa- 

 torial trough of low pressure, with its heavy rains. It lies 

 well within the northeast trade wind area of the North 

 Pacific; that is, the surface winds are from the east and 

 northeast on average. There have been more than 20 

 years of careful meteorological observations at the airstrip 

 on Enewetak Island, and we make use of some of the 

 archived data to discuss, in turn, the various aspects of the 

 weather. In the section on climate and weather, we cover 

 briefly the mean and variation for each observed quantity 

 of interest and note our state of knowledge of these fac- 

 tors. Also in that section we set out an annotated bibliog- 

 raphy of sources of additional data and of specialized dis- 

 cussions. In the section on the atmospheric chemistry of 

 the atoll, we make use of the extensive data collected dur- 

 ing experiments there in 1979. 



We discuss both the mean value and exfjected range of 

 variation because neither alone covers all of the weather. 

 The variability of the weather is the combined effect of dis- 

 turbances of various scales which may have well-defined 

 structures in space and time and of phenomena that can 

 be taken as random. We begin by discussing some of the 

 more common structured disturbances. Over the years 

 diurnal variations at island sites have been discussed and 

 analyzed. While there is no doubt that there are diurnal 

 cycles in cloudiness and precipitation, no attempt is made 

 here to provide explanations for them in terms of first 



causes because the interaction can be both subtle and com- 

 plex. Also, at short periods there is the atmospheric tide, 

 primarily a thermally driven effect that produces global 

 pressure fluctuations and wind patterns that are rather 

 complex. The influence of the atmospheric tide at the sur- 

 face, though greatest in the tropics, is relatively small, and 

 we mention it only briefly. The semidiurnal fluctuation is 

 the dominant mode of the tide and has an amplitude of 

 about 1 mbar, or 85% of the diurnal variance about the 

 annual mean of 1010 mbar pressure at Enewetak. Chap- 

 man and Lindzen (1970) developed the presently accepted 

 theory of the tide. The discussion by Lavoie (1963) of cal- 

 culations available at that time is superseded, despite the 

 absence of seasonal effects in Chapman and Lindzen's 

 basic model. Nevertheless, the data presented by Lavoie 

 for the monthly variation of tide parameters are correct 

 and illustrative, despite the relatively short record. 



Disturbances lasting a day or more are common in the 

 tropics, and we discuss them in the sections entitled "Trop- 

 ical Storms and Disturbances" and "Winds Aloft." There 

 are two seasons at Enewetak, the dry season from 

 December through March and the wet season from April 

 through November. Annual variation is crucial to under- 

 standing the weather in the tropical marine environment, 

 and this influence is included in each section, particularly 

 in the section on precipitation. 



Although there has been much work recently on the 

 variability of climate over periods of a year to a decade, 

 we cannot say much yet about how such changes affect 

 the tropical islands. It is known that there are quasi- 

 periodic fluctuations in the strength of the Pacific trade 

 winds correlated with equatorial sea surface temperature 

 variations at very large scales and that there follows a 

 chain of consequences that includes changes in both tropi- 

 cal and mid-latitude circulations. A clear exposition on this 

 subject, the Southern Oscillation, is to be found in Tren- 

 berth (1976). While much of the present interest stems 

 from the possibility that disturbances in mid-latitude 

 weather and coastal upwelling could be forecast months 

 ahead, we will certainly learn much about the tropical cli- 

 mate itself from the numerous studies now under way. 



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