4th Quarter, 2007 Vol. 13, No. 4 ISSUED: October 21, 2007 A Quarterly Bulletin of the Pacific El Nino/Southern Oscillation Applications Climate (PEAC) Center Providing Information on Climate Variability for the U.S.-Affiliated Pacific Islands www.soest.hawaii.edu/MET/Enso CURRENT CONDITIONS According to the U.S. Climate Prediction Center (CPC), Considerable sea level was recorded across all USAPI loca- the present oceanic and atmospheric anomalies are consis- tions in the July-August-September season, with some stations tent with the development of La Niña conditions in the tropi- (such as Malakal, Palau and Marianas, Guam) recording more cal Pacific. Many of the islands of Micronesia are typically than 8 inches of rise in a month. While many stations recorded slightly wetter than normal during a La Niña year, unless it is the rise in the previous three quarters, the rise during the 3rd Quarter year directly following El Niño, in which case most islands are of 2007 is particularly great. As mentioned, the sea-level vari- drier than normal (particularly during the first half of the year). ability in the Pacific Islands is sensitive to ENSO-events, with The month-to-month variability of rainfall is not as great during low sea-level typically recorded during El Niño and high sea- La Niña as it is during El Niño, primarily a result of the west- level during La Niña events. Consistent with the shift toward La ward shift of tropical cyclone development during La Niña. The Niña conditions, the sea level has been rising in the vicinity of typhoon threat is reduced at most islands during La Niña, espe- the USAPI since January. With the continued development (and cially for islands east of the longitude of Guam (145° E). possible strengthening) of La Nina conditions over the next sev- During the 3rd Quarter of 2007, most of the islands of Micro- eral months, we may expect to observe several inches of sea- nesia had rainfall totals that were below normal. Many locations level rise in all of these stations during the upcoming seasons. (especially those further to the east) had 3-month rainfall totals The following comments from the EL NIÑO/SOUTHERN OS- that were less than 80% of normal. Saipan (Capitol Hill), Palikir CILLATION (ENSO) DIAGNOSTIC DISCUSSION were posted (Pohnpei Island), and Wotje (Northern RMI) received less than on the U.S. Climate Prediction Center web site on October 11, 70% of the normal rainfall expected during these three months, 2007: which are in the heart of the rainy season. Only two locations Synopsis: La Niña will likely continue into early 2008. (American Samoa and Kapingamarangi, FSM) had 3rd Quarter “La Niña conditions strengthened during September 2007, as 2007 rainfall totals in excess of 120% of normal (Figs. 1a and negative SST anomalies along the equator expanded westward and 1b). The highest 3rd Quarter rainfall total recorded in Microne- now extend from 170ºE to the South American coast… The mag- sia was the 52.16 inches at the Palau International Airport, fol- nitude of the negative SST anomalies increased in all of the Niño lowed closely by the 51.78 inches at Luweech on Yap Island. rd regions... And the upper-ocean heat content in the central and east- The lowest recorded 3 Quarter 2007 rainfall total was the 17.73 central equatorial Pacific remained below average during Septem- inches of rain at Utirik in the northern RMI, followed closely by ber. Consistent with these conditions, the low-level easterly winds the 18.47 inches recorded at Wotje (also in the northern RMI). rd and upper-level westerly winds remained stronger than average During the 3 Quarter of 2007, individual monthly rainfall totals across the central equatorial Pacific, convection remained sup- exceeding 20 inches occurred only at some locations in the Re- pressed throughout the central and eastern equatorial Pacific, and public of Palau, some locations on Yap Island, at Kapingama- enhanced convection again covered parts of Indonesia and the far rangi, and in American Samoa. Individual monthly rainfall to- western Pacific. Collectively, these oceanic and atmospheric con- tals of less than 10 inches occurred at Guam, the CNMI, Chuuk ditions reflect a strengthening La Niña.” State, Pohnpei State, the central and northern RMI, and even at Kosrae where the 9.28 inches of rain during August was about “The recent SST forecasts (dynamical and statistical models) for 60% of the normal for that month. the Niño 3.4 region indicate a weak-to-moderate La Niña continu- ing into early 2008. Current atmospheric and oceanic conditions Near normal rainfall is anticipated throughout much of Micro- and recent trends indicate that La Niña will continue and may nesia during the next 3 to 6 months. Tropical cyclone activity, strengthen during the next 3 months.” which has been very quiet throughout the entire western North Pacific during 2007, should return to near normal in Micronesia “Expected La Niña impacts during October – December include a for the remainder of 2007. (See each island summary for the continuation of above-average precipitation over Indonesia and meaning of a “normal” tropical cyclone threat.) below-average precipitation over the central equatorial Pacific…” Page 2 Pacific ENSO Update Sea Surface Temperature and Southern Oscillation Index SST (Sea Surface Temperatures) Sea surface temperatures (SST) in the equatorial eastern Pacific have been cooler than average since January 2007. Over the past sev- eral months, below average SSTs have expanded westward, and negative anomalies now extend from the Date Line to the west coast of South America. By mid-October, equatorial Pacific SSTs were more than 0.5ºC below average east of 170°E, and more than 1ºC below average between 160ºW and the South American coast. SSTs were more than 0.5ºC above average in much of the western Pacific. Dur- ing August – September 2007 equato- 1a) rial sub-surface temperatures were 6 0 generally below average in the region July-August-September 2007 Rainfall east of the Date Line, while positive 50 ) Totals by Station temperature departures were confined ) S to the western Pacific. The most re- E 4 0 H cent period shows negative anomalies C N between the surface and 150 m depth I ( 3 0 across the central and eastern half of L L the equatorial Pacific Ocean, with the A 2 0 F largest departures between 120°W and N I 90°W. Current equatorial Pacific SST A 10 R trends indicate La Niña conditions will likely strengthen during the next sev- 0 eral months. m i i . r i i o O rt u O h rt H n ta O B O t O i p o e n O je il g S li S it a . a S F a S k a r g a i t a o e l le o C i o S w li l o n r le S o M W p l W U o p in R W A o W a e u a s a W P ir e ir n T A W l I g r o j W o u P p W a k o N P n k a a o a A a A p m m u P I i u K w r SOI (Southern Oscillation Index) g l u Y n i a a u P N P N m u a a a a a u u P a K j P P l p S G h g a a i G C in M P a p The 3-month average of the South- S a K ern Oscillation Index was -0.1 for the 3rd Quarter of 2007, with monthly values of -0.5, +0.1 and +0.2 for the 1b) 180 months of July – September 2007, re- spectively. Monthly SOI values will July-August-September 2007 Rainfall 160 likely become increasingly positive Percent of Normal over the next several months, as La L 140 A Niña conditions continue to develop M 120 into early 2008. R O N Normally, positive values of the SOI in 100 F excess of +1.0 are associated with La O T 80 Niña conditions, and negative values of N E the SOI below –1.0 are associated with C 60 El Niño conditions. The SOI is an R E index representing the normalized sea P 40 level pressure difference between Dar- 20 win, Australia and Tahiti, respectively. 0 Figure 1, left. 3rd Quarter 2007 Rain- i . o O t u O i t n a B O t O ir p o i e n e li g S r li S th a r .H a t O F a S k a r g i O tj i a o e li le o i o S S w li l o n ra e S M fall (a) totals in inches and (b) anoma- W p l W o p C in R A W a e a s l o P ir e U ir W A W lo I g u r ja W W o u P p W n T k o P n k a o a lies (expressed as percent of normal) at a A a A a m m u N I i u K ro g l u Y n ip a a u P P N P N m w u a a a a a u u P a K j P P l p S G h g a indicated stations.