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

(i) Seasonal Sea Level Forecast for AMJ, MJJ, and JJA 2007

Forecasts of the sea-level deviations in the USAPI are presented using CCA statistical model. Based on the independent SST values in JFM 2007, the resulting CCA model has been used to forecast the sea-level of three consecutive month periods: AMJ, MJJ, and JJA 2007 (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
(AMJ, MJJ, and JJA 2007)

With a mean skill greater than 0.65 (at 0 to 2-months lead time) in all three consecutive seasons (AMJ, MJJ and JJA), all tide gauges stations are well predicted. The forecast values of sea-level for AMJ, MJJ, and JJA (Table 1) display rise in all the north Pacific islands. In the south Pacific, Pago Pago also displays rise. This rising trends is consistent with the on-going ENSO-neutral or transition from ENSO-neutral to La Niña conditions within the next 3 months. It may be note here that USAPIs are sensitive to ENSO, with rising sea level during La Nina years.

(ii) Tide Predictions (April 01 to June 30, 2007)

Tide related monthly extreme data has been compiled from the NOAA’s web site for tide and currents (http://tidesandcurrents.noaa.gov/station_retrieve.shtml?type=Tide+Predictions) to generate water level plot for the next three months. Predicted water level plot from April 01-2007 to June 30-2007 for the three major stations (i) Guam at Marinas, (ii) Marshalls at Kwajalein, and (iii) Pago-Pago at American Samoa are provided below. Observations reveals that the MR, SR, and ML for all these stations are likely to remain within the normal range during the next three months (April 01 to June 30, 2007). No abrupt deviations are observed from the simulation results.

Figure 5 (below, left): Predicted water level for the AMJ season 2007 at (a) Marinas , Guam (b) Kwajalein, RMI and (c) Pago Pago, American Samoa. Data from NOAA/NOA/CO-OPS. X-axis: date/time (GMT); Y-axis: height in feet relative to Mean lower low water level (MLLW); MR: Mean-difference between high and low; SR: Difference between high and low tide during full moon (spring tide); and ML: Arithmetic means of high and low tides. Figure 6 (below, right): Predicted monthly tide data for Marinas, Guam for (a) April, (b) May and (c) June 2007.

Figure 7 (below, left): Predicted monthly tide data for Kwajalein, RMI for (a) April, (b) May and (c) June 2007. Figure 8 (below, right): Predicted monthly tide data for Pago Pago, American Samoafor (a) April, (b) May and (c) June 2007.

(iii) Observed monthly sea level deviation in Jan-Feb-Mar (JFM), 2007

The monthly time series (January to March) 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 January, February and March 2007, with year to year standard deviations (SD).

Like the previous two quarters, a moderate to positive deviations has been observed in JFM also. Only Malakal at Palau recorded negative deviations during this time. It may be mentioned here that the sea-level variation in the USAPI is sensitive to ENSO-cycle, with low sea-level during El Niño and high sea-level during La Niña years. Consistent to a transition from ENSO-neutral to La Niña conditions, o bservations also revealed that the sea level in this year has already started to record a rise. Despite the CCA model forecasts for fall in Guam, Malakal, and Yap during JFM. FMA and MAM, the r eal-time observed data provided slight rise here.

(iv) Generalized Extreme Value (GEV) Analysis for Apr-May-Jun for 100, 20, and 1-year return periods.

With the objective to provide an improved outlook on seasonal sea-level variability, the GEV Analysis has been calculated from the 6-hourly sea-level data (available at the University of Hawaii sea level center) (Table 3, below). This product defines the thresholds beyond the seasonal tidal range that have low but finite probabilities of being exceeded on a seasonal scale. (Note: this product is based on historical data only and does not take into account any trends that may be present.)

How to correctly interpret GEV:

From Table 3 (above), we can see that the highest AMJ tidal deviation for Marinas, Guam in 100 years will most likely not exceed 9.54 inches above normal (100 year return period). Likewise, about once every 20 years we can expect the highest tide at Marinas to be 7.73 inches above normal for the AMJ season (20 year return period). From the last column we see that for any single given year, the highest tide at Marinas, Guam during AMJ will most likely not exceed 2.72 inches (1 year return period).

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.