Skip Navigation Links 
NOAA logo - Click to go to the NOAA home page National Weather Service   NWS logo - Click to go to the NWS home page
Climate Prediction Center

HOME > Expert Assessments > Climate Diagnostics Bulletin

Tropical Highlights - September 1999

Cold episode conditions continued across the tropical Pacific during September, as sea surface temperatures (SSTs) averaged 0.5-1.0C below normal across the central and eastern tropical Pacific (Fig. T18). However, the region of largest negative SST anomalies (more than 1.0 C below normal) shifted eastward to the eastern Pacific during the month. Over the past few months, the magnitude of the monthly SST anomalies across the central and eastern tropical Pacific has varied between -0.5C and -1.5C. These fluctuations, as well as variations in tropical convection and the equatorial low-level winds, are likely associated with intraseasonal (Madden-Julian Oscillation) activity, and do not indicate changes in the strength of the current cold episode. Elsewhere, SSTs across the equatorial Atlantic, which had been more than 1.0C above normal since May 1999, returned toward normal (Fig. T18).

The equatorial oceanic thermocline remained shallower than normal across the east-central and eastern Pacific during the month, and deeper than normal in the western and west-Pacific (Fig. T15). Consistent with this pattern, temperatures at thermocline depth remained more than 4 C below normal in the eastern Pacific and 1-2C above normal in the western Pacific (Fig. T17). This thermocline structure in consistent with the ongoing La Nia conditions.

Tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] was suppressed over Indonesia, as well as over both the western and central equatorial Pacific during September (Fig. T25). Convection has been suppressed across the central Pacific since mid-1998 (Fig. T8). In contrast, September marked the first time since April 1998 that monthly mean convection and rainfall was below normal over Indonesia (Fig. E4). This decrease in convection in the Indonesian sector was associated with strong intraseasonal activity (Fig. T11). Elsewhere, convection was again enhanced across the African Sahel, with well above-normal rainfall recorded in the region for the third consecutive month (Fig. E4).

Enhanced low level (850 hPa) easterly winds also persisted across the western Pacific during September (Fig. T20). Anomalous easterlies have prevailed in this region since May 1998 (Fig. T7), in association with ongoing La Nia conditions. Elsewhere, low-level westerly wind anomalies again covered the subtropical North Atlantic during the month, consistent with above-normal rainfall across the Africa Sahel. The upper-level atmospheric circulation (200 hPa) in the Tropics and subtropics remained consistent with continuing cold episode conditions, with upper level troughs observed over the low-latitudes of the western Pacific in both hemispheres (stronger in the Southern Hemisphere) and amplified low-latitude ridges observed across most of the remainder of both hemispheres (Figs. T21, T22).

The Southern Oscillation Index (SOI) was -0.1 (Table T1, Fig. T1), and the equatorial SOI was 1.3 (Fig. T2) during September. During the past five months, the SOI has averaged 0.1 (Fig. T1). However, during the same time period, the equatorial SOI has averaged 1.2, and has better depicted the ongoing cold episode.

Editor’s Note: Computer problems at NCEP have resulted in the temporary loss of certain forecast and monitoring products. The CCA SST forecasts (Figs. F1-F2) were not run, and the teleconnection indices (Table E1, Figs. E6, E7) were not computed. In addition, some of the weekly/daily monitoring files that appear on the CPC web site have also been temporarily lost. These interruptions are expected to last until mid-November.

NOAA/ National Weather Service
NOAA Center for Weather and Climate Prediction
Climate Prediction Center
5830 University Research Court
College Park, Maryland 20740
Page Author: Climate Prediction Center Internet Team
Page last modified: August 24, 2007
Information Quality
Privacy Policy
Freedom of Information Act (FOIA)
About Us
Career Opportunities