Aerology and Amphibious Warfare
The Occupation of the Gilbert Islands
CHIEF OF NAVAL OPERATIONS
This is one of a series of pamphlets dealing with the weather aspects of Naval and Amphibious Warfare. The data on which these studies are based are taken from official documents and reports submitted to the Navy Department. The material has been collated and presented in a semi-technical form with particular attention given to the operational aspects of weather.
During the preparation of this study, it was found that weather data submitted by the various commands were occasionally at variance. An effort has been made to reconcile these differences in order to provide an accurate account of the sequence of weather conditions as forecast for the Force Commander, to describe the actual weather conditions observed during the operation, and to give a practical explanation of these weather conditions.
It is hoped that these studies will afford a clear view of the use of weather information during the planning, strategical, and tactical phases of the operation, and that they will form a basis for a better understanding of the application of weather information in future operations.
The primary objective of these analyses is to assist those officers who are charged with the responsibility for the planning and execution of similar operations.
[Signature: Aubrey Wray Fitch]
Vice Admiral, U.S.N.,
Deputy Chief of Naval Operations, (Air).
The Occupation of the Gilbert Islands
The significance of weather in this operation was noted by Commander, Central Pacific force. In his report he stated that:
- Landing conditions were ideal.
- Maximum air support was favored by the weather.
- Bad weather prevailed in the Southern Marshalls and between major enemy bases and the Gilbert Islands.
- Some protection from enemy air strikes after the operation resulted from the southward movement of the Equatorial Front.
- A tremendous saving in fuel was effected because of winds favorable for carrier operation.
- Weather in the Marshalls improved rapidly after the occupation of the Gilberts, providing a favorable period for operations in that area.
A STATEMENT OF THE WEATHER PROBLEM
The occupation of the Gilberts was particularly concerned with three possible weather conditions which could have seriously interfered with the operation. As stated by Commander, Central Pacific Force, these conditions were:
- "The existence of strong westerly winds in the Gilberts could prevent the accomplishment of the mission. This possibility would prevent a successful landing and would require a postponement of D-Day.
- A large area of bad weather north of the Gilberts might conceal major units of the Japanese Fleet.
- The existence of such extremely bad frontal weather as to make air support operations very difficult and dangerous."
Weather in a maritime equatorial region is controlled by the position of the Equatorial Front, and is much more stable in character than in northern latitudes. Because of the absence of migratory storms, weather on either side of the Equatorial Front may be predicted well in advance with some degree of assurance. A map (page 3) was drawn thirty days in advance of D-Day showing the predicted location of the Equatorial Front, and weather expected in the
Marshall and Gilbert Islands. As stated by Commander, Central Pacific Force, this map "was a combination of (a) the normal location of the frontal zone, and (b) the trend of the factors controlling its position." Its construction was based on three assumptions:
- "That the Equatorial Front was lying somewhat to the north of its normal position for this time of the year.
- That the pressure centers controlling the movement of the front would remain to the north of the Gilbert Islands during the entire occupation phase.
- That as long as the front remained north of the Gilberts, fair weather might be expected for the operation."
TACTICAL WEATHER INFORMATION
Because of the vital importance of wind direction to the success of this operation, a submarine was stationed to the west of the Gilberts, where it was possible to make weather observations necessary to give advance information on the possible unfavorable development of westerly winds. Another submarine, stationed near Tarawa, was able to obtain and transmit observations of surf conditions at that heavily defended objective.
WEATHER AND THE OPERATION
By 19 November, information from various sources indicated that the preliminary weather analysis had been correct and that the prognostic chart prepared in October (page 3) was an excellent representation of current weather conditions. The following is a review of the forecast with the verification evidence.
- Equatorial Front over the Southern Marshalls.
A number of bombing planes left the Ellice Islands on 16 November to complete a mission at Maleolap in the Marshall Islands. Several of the planes had to turn back due to bad weather over the Marshalls while others found the target covered by an overcast. Weather over the Gilbert Islands was found to be favorable.
- Surf Conditions at Tarawa - 19 November.
The submarine on patrol near Tarawa made the following (paraphrased) report:
SURF WEST AND SOUTH BEACHES TWO FEET. ON NORTHWEST CORNER FOUR FEET.
Chart of expected weather prepared by Commander Central Pacific Force.
- Cloud Cover, Visibility, Winds.
On the 19th, the following (paraphrased) forecast was received from the Fleet Weather Central, Pearl Harbor:
WEATHER FOR 20 NOVEMBER GILBERT ISLANDS AREA. PARTLY CLOUDY WITH CLOUD BASE AT 2000 FEET. SURFACE WINDS EASTERLY 15 TO 18 KNOTS. VISIBILITY 12 TO 20 MILES.
The initial landings at Makin and Tarawa were accomplished on 20 November under expected weather conditions: partly cloudy skies, with cloud bases at 2,000 feet; surface wind east-southeast 12 knots; sea slight. The temperature was 81.5° F and the relative humidity 82 percent. All phases of the initial assault - surface and air bombardment, strafing of the beaches, and the actual landings - took place under favorable weather conditions.
The weather continued satisfactory for the landings at Apamama on 21 November, and for all reconnaissance and landing activities in the remainder of the group on succeeding days. Throughout November, there was little change in the generally good weather conditions. Showers were observed on the 25th, the 27th, and the 29th, but these were local in nature, and not related to any unfavorable weather development.
The Commander, Central Pacific Force, in his report of the operations in the Gilberts, stated, "Weather conditions throughout were unusually favorable."
The weather requirements for the operation - satisfactory landing and flying conditions - were more than fulfilled. The southeast 12 knot wind, although giving the requisite light surf at the major assault points, was sufficient to facilitate the operation of carrier aircraft. Fair weather throughout the operation made it possible to attack any of the islands from the air. Unlimited ceiling and visibility allowed continuous bombing, strafing, and photography.
Existing weather conditions presented further advantages to our forces.
1. In addition to being north of its normal position, the equatorial front was also more active than usual. Thus, with weather over other Japanese bases unfavorable for air operations, air resistance was less than expected.
2. The southward motion of the equatorial front interposed a region of bad weather between the enemy and our new bases during the early part of December. Thus, our forces enjoyed some protection from enemy air strikes.
From the standpoint of weather considerations, the timing of the occupation of the Gilberts in relation to the major operation against the Marshalls was excellent. With the movement of the
Equatorial Front into the Southern Hemisphere, weather conditions in the Marshalls improved rapidly during December. This permitted the full employment of our air power in the preliminary phase, and favored the employment of a large amphibious force in the final phase of the occupation of these islands.