ENSO and Sea-Level Variability: A Historical Perspective

(Guam, Marshalls, and American Samoa)

 

Among the many changes in oceanic and atmospheric conditions related to ENSO is a pattern of changes in mean (or average) sea level across the tropical Pacific.  PEAC Research Scientist, Dr. Rashed Chowdhury, is studying the connection between the ENSO Cycle and sea level variations.  By using a statistical analysis of the past events, Dr. Chowdhury has been able to look at the patterns between sea surface temperature (SST) and sea level variations (monthly or seasonal difference from mean sea level).  In locations where the SST and sea level variation correlate well, a seasonal outlook for sea level variation can be made based on predictions of SST.

 

The following is a summary of the effects of previous ENSO events on sea level variations in Guam, the Marshalls, and American Samoa.

 

Guam and Marshalls

   During strong El Niņo events, Guam and the Marshalls show a large negative anomaly, or lower than average sea levels.  Moderate El Niņo years also show lower than average sea level; however, the magnitude is less than during strong El Niņo events.  For El Niņo years, the amount of sea level variation is proportional to the strength of the El Niņo event.  In a strong El Niņo event, there is strong negative anomaly in sea level.  In a moderate El Niņo event, there is a moderate negative anomaly. 

 

   A similar relationship occurs during La Niņa years.  During both strong and moderate La Niņa events, Guam and the Marshalls show a positive anomaly, or higher than average sea levels.  Other than a few exceptional months, the rise was more significant during strong La Niņa years than moderate La Nina years.

 

American Samoa

   With a few exceptions, American Samoa did not show any significant variations in sea level during the strong and moderate El Niņo years.  In contrast, La Niņa years showed two distinct scenarios.  During strong La Niņa years, sea level was lower than average.  During moderate La Niņa years, sea level was higher than average.  The deviations were most pronounced from February to July during both strong and moderate La Niņa years.

 

 

For this study, Guam, the Marshalls, and American Samoa were used.  Other North Pacific Islands (Palau, CNMI, and FSM) have a response similar to Guam and the Marshalls; however, they were not included in this detailed analysis because they had a shorter record of sea level data.

 

Sea Level Outlook

   Most of the models predicted SST anomalies in the Niņo 3.4 region will likely to be positive (at or above +0.5 C) through the end of 2004; therefore a weak El Niņo situation is likely to develop. As a result, little or no upwelling is expected in the region of north Pacific Ocean during the next two to three months. Therefore, the north Pacific Islands (Guam and Marshalls) are likely to experience monthly sea levels slightly lower than the average  (within a few inches of normal) in these months.

   On the other hand, in American Samoa, no significant variation in sea-level is expected during the next two to three months; however, any definitive conclusion at this stage is difficult.

 

For the purposes of the study, the years for event ranking were:

 

Strong El Niņo Years: 1951, 1958, 1972, 1982, and 1997

Moderate El Niņo Years: 1963 1965, 1969, 1974, and 1987

Strong La Niņa Years: 1964, 1973, 1975, 1988, and 1998

Moderate La Niņa Years: 1956, 1970, 1971, 1984, and 1999

 

The process of ranking the strong/moderate events is subjective and based on Niņo 3.4 SST, average SOI for six months, and season-by-season cold and warm episodes. For more information on the ranking, see the following web sites.

http://iri.columbia.edu/climate/ENSO/background/pastevent

http://www.wrcc.dri.edu/enso/ensodef

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuf

 

Based upon this research, PEAC plans to develop a more detailed seasonal sea level outlook for Guam, the Marshall Islands and American Samoa.   When available, this outlook will be included in future issues of the Pacific ENSO Update.  For more information, please contact Dr. Chowdhury at rashed@hawaii.edu