TropicalCyclonesAffectingDiegoGarcia
TROPICAL CYCLONES AFFECTING DIEGO GARCIA
TROPICAL CYCLONE CLIMATOLOGY FOR DIEGO GARCIA
The majority of tropical cyclones that pose a threat to Diego Garcia (any tropical cyclone approaching within 180 n mi is considered a "threat") occur during the months of October through February, with a peak in January (Figure XII-3).Because Diego Garcia is located in the development area for southwest Indian Ocean tropical cyclones, many threat tropical cyclones form very close to the island. This can be seen in Figure XII-4, which shows formation points and track segments of tropical cyclones (1944-76) that passed within 180 n mi of Diego Garcia. The tropical cyclones generally head west-southwest and approximately 80% develop or pass to the south of the island.
Figure XII-5 shows the percentages of tropical cyclones that have passed within 180 n mi of Diego Garcia during the months of September through April. These percentages can be interpreted as probabilities of threat; for example, a tropical cyclone located at 9S, 81E has a 60% probability of passing within 180 n mi of Diego Garcia. A timetable for a storm's arrival could be estimated based on the typical speeds of movement for tropical cyclones in the Diego Garcia area of approximately 6-8 kt.
In the slightly more than 31 years of data examined, 33 tropical cyclones were found to have threatened Diego Garcia, or about one threat per year. In two of the years, however, four tropical cyclones threatened the island. It should also be noted that approximately 11 tropical storms and hurricanes occur annually in the southwest Indian Ocean (Refer to Appendix I-C of Chapter I for the mean monthly and part monthly tropical storm and hurricane tracks for the southwest Indian Ocean.)
TROPICAL CYCLONE ASSOCIATED WINDS AT DIEGO GARCIA
Because tropical cyclones tend to develop near Diego Garcia, close passages of intense and fully developed tropical cyclones are a rarity. This situation is enhanced by the fact that low latitude tropical cyclones are, in general, weaker than higher latitude tropical cyclones. In addition, low latitude tropical cyclones have been observed to have smaller wind distributions and to affect a smaller area, in comparison with higher latitude tropical cyclones of equal maximum intensity.
The maximum sustained wind at Diego Garcia attributed to a tropical cyclone during the last 30 years has been approximately 40 kt. This does not imply that winds of this strength or stronger will not affect the island in the future. Diego Garcia is in the development area for southwest Indian Ocean tropical cyclones and the potential for strong winds always exists, but the probability of their occurrence is extremely low.
Although tropical cyclones passing south of the island would probably affect the harbor more often because these storms are more intense the further they are away from the equator, the most dangerous situation would be a direct pass over the island or a close passage to the north that would contribute strong northerly winds during passage. Since 80% of the tropical cyclones in the last 31 years developed or passed to the south, northerly winds associated with a tropical cyclone would be extremely rare. If strong winds (60 kt) from the north did occur, seas in the anchorage area could reach up to 6-7 ft. Strong southerly winds (60 kt) could contribute 7-8 ft seas in the anchorage area. Because of the nature of the tropical cyclones in this region, the strong winds and rough seas would be of short duration.
Source: http://www.nrlmry.navy.mil/port_studies/thh-nc/diegogar/text/sect2.htm
