SEASONAL SEA LEVEL OUTLOOKS
for the U.S-Affiliated Pacific Islands

(i) Seasonal Sea Level Forecasts for OND, NDJ, and DJF 2007-2008

Forecasts of the sea-level deviations in the USAPI are presented using CCA statistical model. Based on the independent SST values in JAS 2007, the resulting CCA model has been used to forecast the sea-level of three consecutive month periods: OND, NDJ, and DJF 2007-2008 (see Table 1). CCA cross-validation forecast skills for 0, 1, and 2-month leads are presented in Fig. 3.

Table 1 : Forecasts of sea level deviation in inches for forthcoming seasons
(OND, NDJ, and DJF 2007 - 2008)

With a mean skill greater than 0.73 (at 0 to 2-months lead time) in all three consecutive seasons (OND, NDJ and DJF), all tide gauges stations are very well predicted. All of the stations in the USAPI are forecast to observe a rise in sea level during the upcoming OND, NDJ and DJF seasons (Table 1, above). This rising trend is consistent with the current development of La Niña conditions, which, according to the CLimate Prediction Center (CPC), may continue for several months. Since the Pacific Islands are sensitive to ENSO events -- with rising sea level during La Niña years -- we may expect to observe several inches of sea level rise in all of these stations during the upcoming seasons.

(ii) Tide Predictions (October 1 to December 31, 2007)

NOAA's web site for tide and currents (http://tidesandcurrents.noaa.gov/station_retrieve.shtml?type=Tide+Predictions) has been used to generate the water level plot for the next three months. Predicted water level plots from October 1 to December 31, 2007 for three major stations (i) Marianas, Guam (ii) Kwajalein, RMI and (iii) Pago Pago, American Samoa are provided below. Observations reveal that the MR, SR, and ML for all these stations are likely to record several inches rise during the OND season. Guam, the Federated States of Micronesia, the Republic of the Marshall Islands, and American Samoa are likely to experience a higher tide level (i.e. rise by 2 to 4 inches), during the next 3 months (October 1 to December 31, 2007).

(iii) Observed monthly sea level deviation in July-August-September (JAS), 2007

The monthly time series (April to June) for sea level deviations have been taken from the UH Sea Level Center. The full time series (in mm) is available at: ftp://ilikai.soest.hawaii.edu/islp/slpp.deviations. Deviations are defined here as the difference between the mean sea level for the given month and the 1975 through 1995 mean sea level value computed at each station. Locations of all these stations are shown in Figure 2 (top of page).

Table 2: Monthly observed sea level deviations in inches for July, August and September 2007, with year to year standard deviations (SD).

Table 2 (above) provides monthly observed sea level deviations in inches. (Also, see Fig. 4 for JAS seasonal averages.) A considerable positive deviation has been observed in most of the tide-gauge stations in the JAS season. While the previous three quarters recorded rise, the rise in JAS is particularly great. As mentioned, the sea-level variation in the USAPI is sensitive to ENSO-events, with low sea-level typically recorded during El Niño and high sea-level during La Niña events. Consistent with the on-going La Niña conditions, the sea level has been rising in the vicinity of USAPI since January. The present amount of rise is not alarming, but it is quite considerable for some stations (such as Guam and Malakal). Due to technical problems, sea level data for Yap and Saipan was not available for the JAS season.

(iv) Seasonal Extremes for OND at 20- and 100-year return periods.

The Generalized Extreme Value (GEV) products define the thresholds beyond the seasonal tidal range that have low but finite probabilities of being exceeded on a seasonal scale. Results of the GEV analysis for the season JAS is presented in Table 3 (below). The extreme values are calculated from the 6-hourly sea-level data. The upper limit (at 90% confidence interval) of rise has been shown in the right side of the column, the left side is the estimated rise based on observations. For example, the predicted rise of 6.2 ~ 8.9 inches at Marianas (*) indicates that this station may experience sea level rise of 6.2 to 8.9 inches during JAS within any 20 year period (20 year RP). Likewise, about once every 100 years we can expect the highest JAS tide at Marianas to be as much as 17.2 inches above normal (100 year RP).

Note that Saipan displays a much higher rise than the other stations for both 20-year and 100-year return periods. These high values are due to large and significant increases in the tidal range duing the passage of past storm events in the OND season. Saipan was hit by Super Typhoon Kim on December 3, 1986 and Super Typhoon Wilda on October 25, 1994. The closest point of approach (CPA) intensity for Super Typhoon Kim was 135 kts (nautical miles per hour), and for Super Typhoon Wilda was 115 kts.

Click here to view probability of exceedence graphs for the OND season.

Chowdhury M. R , P-S Chu, and Schroeder T (2007): ENSO and Seasonal Sea-level Variability – A Diagnostic Discussion for the U.S-Affiliated Pacific Islands, Theoretical and Applied Climatology, 88: 213-224 (March 2007), Springer-Verlag Wien.