Atoll Rresearch Bulletin - 71. Microclimatic Observations at Enewetak

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Mh-odirnatic observatium at Eniwetok with a special section on Vegetation THE PACIFIC SCIENCE BOARD National Academy of Sciences—National Research Council

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Microclimatic observations at En.iwetok David I. Blumenstock and Daniel F. Rex, vith a s~?ecia.1section on Vegetation THE PACIFIC SCIENCE BOARD National Academy of Sciences--National Research Council

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It is a pleasure to commend the far-s i:hted policy of the Office of Naval Research, with its emnhasis on basic research, as a result of which a grant has made possible the continu:.tion of ‘the Coral Atoll Program of the Pacific Science Board. It is of interest to note, historically, that much of the fun~- mentil information on atolls of the Pacific vas gathered by the U. . Navy’s South Pacific E:,qloring Expedition, over one hundred years a~o, under the command of Captain Charles Uilkes. The continuing nature of such scientific interest by the Navy is show by the support for the Pacific Science Board’s research programs during the past thirteen years. The preparation and issuance of the Atoll Research Bulletin is assisted by funds from Contract N’i’onr-2300(12). The sole responsibility for all statements made by authors of necessarily represent the views of the Pacific Science Board or of the editors of the Bulletin. M.-H. Sachet, assistant editor Correspondence concerning the Atoll Research Bulletin should be addressed to the above c/o Pacific Science Board National Research Council

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for many years been obtained on various oceanic islands, many fundamentalquestionsCOncerntigthe 10Cal climates and microscaleweather on such islands remain unanswered. In particular,the problem of to what extent an oceanic atoll createsits own localweatherand microclbat= has not been satisfactorilyresolved. Is theresignifiAre theresignificantdifferencesin air +-ernperaturebetweenthe windwardand leeward sides? With a large deep lagoon air essentiallythe same and other questionshave undertakento provide at Initial tipetus for of Botany, Universityof like that at Eniwetok, is the heat exchange between lagoon water and as the exchangebetween water and air over the ocean nearby? These long given rise to considerablecontroversy. The present study was least somewhatbetter answers to such questions than have heretofore this study was provided by ProfessorMaxwell S. Doty of the Department Hawaii. Professor Doty had been conductingphytoplanktonproductivity studies at Eniwetok and wished to know whether there were significantdifferencesin mean rainfall from one to another part of the atoll. He suggestedto the authors that it might be worthwhile to establishraingages at several different sites and obtain comparativerainfall readings over a period of at least a year. After several discussionsamong Professor Doty and the authors, it was decided to carry this suggestionstill further and to obtain observations of several differentkinds on a micro-scale. Accordingly,a field plan was worked out and the Eniwetok MicroclimaticProject was formally establishedunder the joint auspices of the Task Force SEVEN. The period of investigationwas chosen so as to derive maximum possible support from Task Force operationsplanned for the spring and summer of 1958. The Operation HARDTACKwas conductedduring this period; and during as well as during the test period itself it was possible to draw support not usually availableat Eniwetok.

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Universityof Hawaii, the U. S. Weather Bureau, and Joint Task Force SEVEN each funds, equipment~md personnel in supp~t of the study” In addition those participatingin the observationalprogram at fiiwetok were: to the seriior 4. Prof. Irwin E. Lane ‘?. Prof. Jimmie Bob Smith Nithout the excellentwork of these other persons in the sponsoringagencies Pearl Harbor, Hawaii field personnel and the outstandingsupport of many and the U. S. Atomic Energy Commissionit would have been impossibleto conduct concernedwith the project we wish to acknowledgethe thank Professor Irwin Lane this study. de wish to express our sincere thank-s*o Ill ihose for their genuine interest and valuable assistance. Irlparticular active and continuing support provided by Mr. Ernest ‘Wynkoopana S. Atomic Energy Commissionand Professor Doty. k!ealso wish to for his special field investigationof the distributionof vegetation on two of the islets of Eniwetok and for his preparationof one of the principal sections of this study. Finally,we wish to thank the personnel of the USAF Air ‘deatherSerdetachment at Eniwetok, who made radarscope and other special observationsin direct supper+, of this study.

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Ly of Hawaii ng support of many le of the princiPal JSAF Air weather Servi( zs in direct support TABLE OF CONTENTS GENERAL GECWAPHIC RELATIONSHIPS. .............2 DATA (Includ@ separate Table of Contents) ...... 25 APPENDIX II. INDICES FOR PHOTOGRAPHS ........... U9 FIGURESAND PL4TES (In numerical order followingp. 158)

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LIST OF FIGURES AND PLATES (Figuresand Plates appear in order as the last section of this publication.) BATHYRITHMSIN THE VICINITY OF ENIWE’IOK SURFACE WATER CURRENTS IN BIKINI LAGCON SURFACE WATER TEM2F&iTUREsIN JANUARY AND AUGUST MAP ND SITE DIAGRAN, FRED SKETCH MAPS OF JANET AND YVONNE MAP AND SITE DIAGRAN, KEITH TWER LAYOUT, SITE MACK

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MICROCLIMATIC OBSERVATIONS AT ENIWETOK For a one-Year period, from August, 1957, to August, 1958, the authors tog ether with their other field colleagues conducted a study of mi.croclimatic c!ondition3 at Enlwetok Atoll In the Marshall Islands. The primary purpose of the study was to determine to what extent a deep, large atoll lying far at sea in a trade wind zone creates its own weather and climate. Stated differently, ow and to what degree do the weather and climate of Eni.wetokdiffer from the eather and climate that would obtain if there were only open ocean where This rep~rt on our study does not attempt to answer the fundamental question aigetiabove. Instead, It merely presents our observational findings. It is a ata report, designed to make available to meteorologists and others data that e hope will be useful to them In many different kinds of’inquiries. We are including in this report not only the data themselves, together with nformation concerning the observational sites and procedures used, but also a odlcum of information concerning the nature of the atoll and of broad-scale “eather condltiong in the Eniwetok ares. This additional.Information is provided make our results most l~seful to as many different investigators as possible, eluding those unfamiliar with Eniwetok and with

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Observations varied from site to site, but among the usual kinds of wrface weather observations and also sites they included all tl cloud photographs, and radarscope photographs. During these two 2-week period. observations were also made on trans-lagoon runs aboard an M-boat (LCM) and on ocean runs outside the reef in an aircraft rescue boat (ARP). On lagoon and ocean runs surface water temperatures were measured through making bucket haul at frequent intervals. During the remainder of the year, outside these two intensive-study peric the observations were restricted to the usual comprehensive hourly observation at FRED and to daily, semi-monthly, and monthly rainfall observations at varit other sites2. Circumstances did not permit making regular rainfall observatl[ 2Except for She intensive-study periods, only the daily rainfall values presented for FRED. Sources of other data for FRED are given in Appendix III throughout the entire year at all of the sites listed above. It is hoped non theless that the observations obtained will be found to be useful in suppleme Ing the observations for the two intensive-study periods. Those who wish to use the primary data appearing in Appendix I or listec Appendix II may find Appendix III helpful to them. Appendix III llsta geverz major sources for additional meteorological data for Enlwetok. Eniwetok is situated in the Marshall Islands, a group of islands lying t of the Gilbert Islands and east of the Caroline Islands. It is located at 11.4°N., 162.3°E. Most of the atolls which make up the Marshall Islands are distributed along two chains which are nearly parallel and trend northwestwa 3A large part of the factual information contained in this section was obtained from “Geology of Bikini and Nearby Atolls” by Emery, Tracy, Ladd et USGS Prof. Paper No. 260-A, Part 1, 1954. The reader Is referred to this pr cation for a more detailed presentation. The easternmost Is the Ratak (Sunrise) Chain; the westernmost, the Ralik (SG

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Or listed in Chain. In addition to these two main chains there are several Izolated outlying atollS . Altogether the group contains twenty-nine atolls, five islands having no interior lagoon and two, known, submerged banks shallower than ten fathoms. The highest land elevation within the group is about twenty-eight feet. Eniwetok iS an isolated atoll lYing west of the Ralik Chain and is located 2,500 statute miles west-southwest of Honolulu, Hawaii and some 4,700 miles San Francisco. The atoll is some 190 statute miles due west of Bikini Atoll, which together with Ujelang, located some 130 miles southwestward from Eniwetok, are the closest exposed land areas. It appears that Eniwetok Atoll was originally a volcanic conej since basalt was found there in 1950 as a result of several deep drilling explorations. The cone probably initially emerged some feet above the water and later was eroded away and absorbed by wave and water action. When the critical depth of sea water required for coral existence and growth was reached by the emerging cone, coral growth probably began. surrounding an oval with a total lagoon exposed at low tide Atoll consists of a chain of about thirty small, low islets lagoon 25 miles long by about 20 miles wide (Figure 1). The of these islets is only 2.5 square statute miles compared area of 360 square statute miles. The total reef area is about 32 square statute miles. Most of the islets are less than 13 feet high but are, in some instances, covered by coconut palms reaching up to 80 to 100 feet above low tide level. Three entrances penetrate the reef. Deep Entrance at the southeast side is only about 3/4 but it has a depth of 31 fathoms between ELMER and Japtan Islets South Channel, on the other hand, is very wide, about six miles, of a mile wide and is usually known as Wide Passage. Charted depths in Wide Passage are only 6 to 12 fathoms. Southwest Passage on the west side is even shallower, having depths of only about 1 fathom. Maximum tidal currents of two knots in Deep Entrance and of 1 knot in Wide Passage have been observed. The Eniwetok lagoon is nearly elliptical with its long sxis trending northwestward. The deepest area tS in the north central part of the lagoon, which IS the area farthest from the ~in passes through the reef (Figures 1 and 2). If the numerous superimposed coral mounds were ignored, the bottom contours would show a smooth slope from depths of about 24 fathoms near FRED northwestward to the deepest point of the lagoon, about 35 fathoms. There appears to be no

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indication whatsoever of submerged terraces or cllff’son the seep portion of the lagoon floor. The mean depth of the lagoon is 26.2 fathoms, ‘withdepths between 24 and 32 fathoms most common. Bottom samples and tinderwaterphotograph show that the lagoon floor Is chiefly covered with Foraminifera, shellz, Halimeda debris, coral and other miscellaneous fine debris. In the MarshalIs, the atolls rise out of water about 15,000 feet in depth. The slopes of the atolls are steepest in the upper portiong near the Burface. At Eniwetok the contour gradient reaches a rate of about 4,000 feet per mile. Figure 3 shows the ocean bottom contours in the vicinity of Enlwetok Atoll. The original native population of Eniwetok Atoll was Micronesia and in 1930 consisted of 121 inhabitants who raised chiefly pigs, chickens and coconutg, and caught the abundam fish available in the Eniwetok area. In 1947 Eniwetok Atoll was selected for an expansion of the permanent Pacific Proving Ground because of Its isolated position, stable weather and the geography of its land masses. At this time the Eniwetok people were moved to Ujelang-,where nearly 200 natives live today. Since that time Enlwetok has been populated exclusively with American personnel associated with atomic test operations. The number of persons present varies from tens of thousands during active operations to several hundreds during interim periods. The development of’the atoll for test purposes has consisted principally of the construction of permanent base camps on FRED and ELMER Islets and of the utilization of the northern islets, extending from Runit to Bogallua, for shot-site and technical instrumentation purposes. 3. GENERAL WEATHER SETTING Although detailed studies of the microclimate of the Marshall Islands area and of Eniwetok in particular are available in the literature (Appendix III), it was thought desirable to include in this report a general description of the weather setting of Eniwetok. It is the purpose of this section to present a general description that will be especially useful to those not familiar with Eniwetok is located on the south side of the Pacific high pressure belt, in what is commonly called the north-east trade wind zone, and to the north of the equatorial trough of low pressure. Wind Structure. Eniwetok is overlain with three nearly independent wind

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an and in 19, systems . The lowest of feet, is the well known these, extending from the surface up to about 20,000 trade wind current. The Trades are deepest and strongest during the winter months, December through February, with an average strength at the surface of about 18 knots from an east-northeasterly direction. Maximum speeds occur at about the fOUr to five thousand foot level, where speeds greater than 25 knots are not uncommon. The top of the current during this season may often extend to 30,000 feet or more. During the spring and summer the Trades become gradually weaker and more variable. At the same time their average or most typical direction veers from east-northeasterly to easterly. During August amd September the average surface wind is 11 knots from the east. During these two months, frequent periods of very light winds, especially coming from the southeast, are often observed. During March, April and May the trade wind current becomes shallowest, often not extending above the 8,OOO or 9,000-foot level. Figure 4, on which is plotted the zonal or east-west component of the wind as a function of height and of month, shows these different changes. Surface wind statistics by month are gtven in Table I. Above the trades and extending up to the tropopause, which is generally located between 55,000 and 60,000 feet, are westerly winds which are usually called the Upper Westerlies. This wind stream may be thought of as the southward extension of the strong clrcumpolar jet stream of mid-latitudes. At the latitude of Enlwetok this southward extension of the polar westerlies overlies the trade wind current. The Upper Westerlies are quite variable due to the presence of numerous cyclonic and antlcyclonic vortices which are typically carried along in the basic current. Such a vortex, in the proper position relative to Eniwetok, often produces east winds for periods of two to four days at these upper levels. The upper westerly current, whose core is normally located at about the 40,000foot level, is strongest in the spring, from the month of March through May, at which time average velocities reach 25 knots. At the same time this current is deepest and most well developed. As the season progresses through summer into autumn, the thickness and strength of the current diminishes to average Values …