Pacific ENSO Update

3rd Quarter, 2007 Vol. 13 No. 3


The pattern of sea surface temperatures in the tropical Pacific, along with the recent values of the Southern Oscillation Index (SOI) indicate ENSO-neutral conditions. Weather conditions in the tropical western North Pacific over the past three months can be best described as tranquil: fewer than normal tropical cyclones, a weak monsoon, and a general suppression of large scale convective activity. July continued the general trend of tranquil weather, with the exception early in the month of one large tropical depression that became a tropical storm when it passed to the southwest of Guam. This tropical depression (which later became Typhoon Man-Yi), brought minimal tropical storm conditions to the Mariana Islands and to Yap State. Yap and Palau received enhanced rainfall from the associated monsoon flow, but rainfall in the Marianas was not plentiful. The South Pacific Convergence Zone has been fairly active over the Samoa region, keeping conditions wetter than expected for the first half of the year.

Most of the island groups of Micronesia were drier than normal during the first half of 2007. Six-month rainfall totals were less than 75% of normal at some locations in the Republic of Palau, Yap State, Guam, the northern RMI (see figures, Fig. 1a, 1b). Driest of all were some locations on Guam and in the northern RMI, where six-month totals of less than 20 inches occurred. Only a few islands were wetter than normal in the first half of 2007. Six-month rainfall totals in excess of 125% of normal were seen at Aasufou, American Samoa, and (surprisingly) in Saipan where the relatively meager 25-inch island totals were about 5 inches greater than the normal of approximately 20 inches. The 127.38 inches of rain at Aasufou, American Samoa during the first half of 2007 was the highest recorded value in the USAPI, followed by the 94.10 inches of rainfall at the WSO Kolonia, Pohnpei Island and the 94.04 inches recorded at nearby Palikir. Six-month rainfall totals exceeded 90 inches at Nukuoro and at Kosrae SAWRS. Two typhoons (Kong-Rey and Yutu) occurred in the western North Pacific basin during the first half of 2007. Both of these typhoons evolved through their tropical depression stage in portions of Micronesia, contributing rainfall to some locations from Pohnpei westward to Yap. The Hawaiian islands have been quite dry for the first half of 2007, and dry conditions have continued to worsen through the summer, with most locations receiving below average rainfall in the 2nd quarter.

The underlying weather pattern in the Samoa region is expected to bring near normal rainfall to the islands of American Samoa as they enter the heart of their dry season. Easterly trade winds should continue to dominate the flow in eastern Micronesia (Pohnpei and eastward), and keep rainfall average to slightly below average. Monsoon and storm activity will have more influence in western Micronesia (Chuuk and westward), and these areas will likely see average to slightly above average rainfall. The Marianas will have near average rainfall, while Hawaii is expected to continue receiving below-average rainfall through the 3rd quarter. The trade winds and sub-surface ocean warmth in the western North Pacific will keep sea levels above normal.

Sea-level variation in the USAPI is sensitive to ENSO-cycle, with low sea-level observed during El Niño and high sea-level during La Niña years. All locations across the USAPI have continued to record a rise in sea level during the 2nd quarter of 2007, which is consistent with the possible transition from ENSO-neutral to La Niña conditions.

The following comments from the CPC EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION were posted on the U.S. Climate Prediction Center web site on July 12, 2007:

Synopsis: ENSO-neutral conditions are expected to continue for the next 2 months, with ENSO-neutral or La Niña conditions equally likely thereafter.

“ENSO-neutral conditions continued in the tropical Pacific during June 2007, with average to below-average sea surface temperatures (SSTs) extending from the date line to the west coast of
South America.”

“Nearly all of the model forecasts predict below-average SSTs in the Niño 3.4 region during the remainder of the year. A majority of the statistical models indicate a continuation of ENSO neutral
conditions through the summer months, with several statistical models forecasting weak La Niña conditions during the fall or winter. In contrast, most dynamical models, including the NCEP Climate Forecast System (CFS), continue to predict a transition to La Niña within the next three months. However, several of the dynamical models have recently been predicting a stronger and more rapid cooling than has actually occurred. Given the large spread in ENSO forecasts, along with the slower than expected decrease in observed SSTs over the past few months, it is reasonable to expect either a slower evolution toward La Niña conditions or the continuation of ENSO-neutral conditions.”