80 



MERRILL AND DUCE 



the average. The highest frequency of brisk winds is in the 

 dry season, with over 45% of the hours having winds 

 >8.5 ms~' (19 mi h~'). During the wet season the wind 

 weakens substantially, particularly during August through 

 October when >50% of the hours have wind <5.4 ms~' 

 (12 mi h~^). Only during July through October are calms 

 at all common, i.e., greater than 1% occurrence. 



The dry season months exhibit the greatest constancy 

 of pattern: >50% from the east and >40% from the 

 northeast, with >75% frequency of speeds between 5.8 

 ms"' and 10.7 ms"' (13 and 24 mi h^'). April does not 

 differ much, except that the strength of the wind decreases 

 slightly. In May and June the winds are strong out of the 

 east, while in July through October the speed decreases 

 and the direction varies more. In November the wind 

 begins to shift back to the dry season pattern. 



As these are average winds, the pattern of variation 

 with time is lost. There is a consistent shift in the wind 

 associated with easterly waves, the most common distur- 

 bance type in the wet season. The correlation of wind 

 shifts with cloudiness and rainfall, obvious to anyone 

 present during such events, is lost. 



The annual average wind rose shown in Fig. 4 is easily 

 understood given the monthly distributions discussed previ- 

 ously. Note that % of the time the wind is from 5.8 to 

 10.4 m/s (13 to 24 mi h '), and over 60% of the time 

 the wind is from the east. Nevertheless, the annual aver- 

 age shows at least 0.1% winds from every direction. 



Tropical Storms and Disturbances 



While tropical storms strike the Marshall Islands infre- 

 quently, disturbances in the weather are a common and, 

 on occasion, regular occurrence. Tropical storms of the 

 greatest strength are called typhoons in the western 

 Pacific, and they are, of course, extremely dangerous and 

 destructive, particularly to exposed areas at low elevation 

 such as Enewetak Atoll. Such storms grow from and are in 

 fact the most fully developed form of tropical disturbance. 

 We discuss the disturbances first because they are more 

 numerous. 



Several types of tropical disturbances are recognized in 

 the literature; nevertheless, it is often impossible to classify 

 a given weather system as one of the several types, even 

 given estimates of the thermal structure and the movement 

 and growth of the system. We are concerned primarily 

 with the surface manifestation, so we shall only summarize 

 what is known about the most common disturbance types. 



During the wet season, particularly July through Sep- 

 tember, westward propagating wave-like systems are com- 

 mon in the tropics and have been observed and analyzed 

 in the western Pacific and in the Caribbean and North 

 Atlantic Ocean areas. In the western Pacific these easterly 

 waves, on average, have a horizontal scale of 3500 to 

 4000 km and travel toward the west an average of 7° 

 longitude per day (i.e., a mean velocity of 9 ms or 20 

 mi h^'); thus the disturbance affects a station for 4.5 to 

 5 days. During the passage of such a wave, there is a 



more or less systematic variation in the wind, cloud cover, 

 and rainfall. The north-south component of the wind 

 shifts, with maximum winds from the south of 1 to 2 

 ms"^ (2 to 5 mi h~') leading and maximum winds from 

 the north following the center of the disturbance. The max- 

 imum cloudiness and rainfall occur just after the pjassage of 

 the center of the disturbance. There is a temperature fluc- 

 tuation, but it is hardly discernable at the surface. These 

 waves can be observed with satellite images and are now 

 understood to be an inherent prop>erty of deep easterly 

 flow. The structure and detailed dynamic characteristics of 

 such waves in the Marshall Islands area were studied by 

 Reed and Recker (1971) using radiosonde and satellite 

 data. The waves are most common in the wet season 

 because the upper level winds are most favorable for their 

 growth then. About Vs of such waves increase in intensity 

 sufficiently to become classified as depressions or storms, 

 but this occurs most commonly well west of the Marshall 

 Islands. 



Other types of disturbances are more uniformly dis- 

 tributed through the year but are even less easily classi- 

 fied. One type, the upp)er level cold-core low, is similar to 

 the subtropical cyclone that is often observed in the 

 Hawaiian area. In the Marshall Islands area, it may have 

 no surface manifestation or may be accompanied by a 

 weak but long-lived period of disturbed weather. In addi- 

 tion, there arc squall lines and other short duration events 

 which may produce strong winds and intense rainfall over 

 limited areas as they pass. 



Although both the frequency and the destructive power 

 of tropical storms are greater in the far western Pacific 

 than in the Marshall Islands area, such storms can threaten 

 any tropical location. A sense of the seasonal distribution 

 and the range of impact possible can be obtained from 

 Table 2, which summarizes the depressions and storms 

 that affected Enewetak between 1959 and 1979. Of 

 course, the highest overall probability of tropical storm for- 

 mation in the area is during the wet season, particularly 

 July to October. However, there have been strong storms 

 well within the dry season (e.g., Alice in 1979). The high 

 winds and waves that extend to the periphery of such 

 storms can have devastating consequences. There is a sub- 

 stantial body of literature on the effects of such storms on 

 atolls, but the closest atoll so studied is Jaluit 

 (Blumenstock, 1961). Specific data about individual storms 

 are often sketchy, and prior to the operational use of satel- 

 lite images, the tracking of past storms when far from land 

 or shipping lanes may have been substantially in error. 

 Nevertheless, there are useful data on several storms over 

 the years, as indicated in Table 2. 



Winds Aloft 



The structure of the wind field above Enewetak Atoll is 

 complex and variable. At time scales longer than 2 years, 

 there are nearly periodic fluctuations at some levels, while 

 at other levels there are short-period variations as impor- 

 tant as those in mid-latitudes. In the following discussion. 



