Pacific climate and climate change as revealed by the modern historical records of the U.S.-affiliated Pacific Islands

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Thursday, 6 February 2014: 11:45 AM
Room C101 (The Georgia World Congress Center )
Mark A. Lander, University of Guam, Mangilao, Guam; and C. Guard

Continuous measurements of rainfall, sea level, and temperature exist for most of the U.S.-affiliated tropical Pacific islands (US-API) since at least the end of World War II. A meticulous record of weather variables (e.g., temperature, rainfall and wind) was kept by the Japanese across the western Pacific during the period of the Japanese League of Nations South Sea Mandate. Most of the island groups (e.g., Guam, the Federated States of Micronesia and the Republic of the Marshall Islands) have at least one station where a nearly complete record of climate variables is available for the period 1953 through the present. The record can be extended back to 1915 at some locations where the Japanese made weather observations. Climate observations over the past six decades show a relatively steady increase of temperature in both the daily maximum and nighttime minimum. Some locations show abrupt temperature changes that are likely a manifestation of physical changes to the station infrastructure or location. Throughout much of the western Pacific, the temperature peaked in 1998, and has been steady or cooling since then. Changes of sea level across the Micronesian islands of the western Pacific are highly coherent, and show major inter-annual variations that are mostly related to ENSO. After the very strong 1997 El Nino event, the sea level across Micronesia rose substantially, and has been well above normal for most of the years since that event. From the decade of the 1990s to the decade of the 2000s, the 12 cm rise of sea level across Micronesia was far above the global rise noted for the same time period. This abrupt rise of sea level has been accompanied by a major shift in ENSO behavior that was dominated by El Nino in the 1990s and by La Nina in the 2000s a signal of a possible shift of phase of the Pacific Decadal Oscillation (PDO). Annual rainfall amounts vary substantially from year-to-year at all US-API, and 60-year trends are mostly small and not statistically significant. Only the eastern part of Micronesia (e.g., the Republic of the Marshall Islands) and Hawaii have seen statistically significant drying trends in annual rainfall. At both Kwajalein and Majuro, there has been a substantial drying over the past six decades so that the latter years of the record are about 10-15% drier than the early years. Similar drying is also noted in Hawaii, where annual rainfall and stream flow have been declining for 100 years. A notable climate shift has recently occurred in the typhoon distribution of the western North Pacific. The past decade has seen an abrupt decrease in the number of tropical cyclones across the basin and in the Micronesia region. The year 2010 had only 14 named tropical cyclones in the whole basin, which was half of normal and a record low. Simulations of the Pacific climate in a warmer world suggest that rainfall amounts increase at most locations, particularly in the western portion, with a lesser increase, or even slight drying in the east. Simulations of tropical cyclone behavior in a warmer world indicate a 10 to 15% reduction in the annual number of cyclones on a 100-year time scale. Because some of the recent climate shifts have been so large and so abrupt, it is still too early to link them directly to the observed rise of air and sea surface temperature.