SEASONal Sea level OUTLOOKs
for the U.S-affiliated Pacific Islands

(i) Seasonal Sea Level Forecast for JFM, FMA, and MAM 2007

Forecasts of the sea-level deviations in the USAPI are presented using CCA statistical model. Based on the independent SST values in JAS 2006, the resulting CCA model has been used to forecast the sea-level of three consecutive month periods: JFM, FMA, and MAM (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
(JFM: Jan-Feb-Mar; FMA: Feb-Mar-Apr, and MAM: Mar-Apr-May)

With a mean skill greater than 0.65 (at 0 to 2-months lead time) in all the three consecutive seasons (JFM, FMA, and MAM), all the tide gauge stations are well predicted. However, the forecasts skill for JFM has been found to be slightly lower than the previous season (OND) (Fig. 3).

The forecast values of sea-level for JFM, FMA, and MAM (Table 1), which also determines fall in most of the north Pacific islands, is consistent with the present on-going weak El Niño conditions in the tropical Pacific. Note that, according to CPC, El Niño conditions are likely to continue through March-May 2007.

(ii) Tide Predictions (January 01 to March 31, 2007)

Below: Predicted water level plots from January 01-2007 to March 31-2007 for three major stations (a) Marinas, Guam (b) Kwajalein, RMI and (c) Pago Pago, American Samoa. Dotted line in each graph represents monthly mean water level.

Note: MLLW: Mean lower low water level (default datum)

(a) Marinas, Guam. Mean tide is likely to be 2 to 3 inches lower than the average, Maxima: 5 to 8 inches lower than the average, and Minima: 2 to 4 inches lower than the average.

(b) Kwajalein, RMI. Mean tide is likely to be close to average, Maxima: 4 to 6 inches lower than the average, and Minima: 2 to 3 inches lower than the average.

(c) Pago Pago, American Samoa. Mean tide is likely to be 5 to 8 inches lower than the average, Maxima: 5 to 8 inches lower than the average, and Minima: 0 to 2 inches lower than the average.

NOAA’s web site for tide and currents (http://tidesandcurrents.noaa.gov/station_retrieve.shtml?type=Tide+Predictions) has been used to generate these water levels plot for the next three months. When compared to other major El Nino years (like 1982-83 and 1997-98), the overall tide data in all these stations are found to record a moderate fall.

Other tide related monthly extreme data has been compiled from the above data records and reported in the following Table 1. Note that the monthly average values computed here are from the mean values of tides from 1975 to 2006.

Table 2: Monthly extremes of tide (Feet relative to MLLW)

As compared to the average water level in January, February, and March, the predicted water level seems to be slightly lower (2 to 3 inches) than the average values observed in Guam (see Table 2). The maximum water level is predicted to be considerably lower (5 to 8 inches) than the observed values, while the minimum water level is also likely to display a trend of several inches lower from the default datum, which indicates a value lower than average minimum for January-March (see Figure a, above).

The station Kwajalein displays a tide trends very close to the average values observed during January-March (Figure b, above). But, like Guam, the maximum water level is predicted to be considerably lower here (4 to 6 inches) than the observed values.

On the other hand, the predicted tide level at Pago-Pago is considerably lower (5 to 8 inches) than the average values observed in January to March (Figure c, above). In this case also, the maximum water level is predicted to be considerably lower (5 to 8 inches) than the observed values and the minima is also likely to touch the default datum indicating a lower than average minima for January-March.

(iii) Observed monthly sea level deviation in Oct-Nov-Dec (OND), 2006

The monthly time series (October - December) 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.

Table 3 provides the monthly observed sea level deviations (in inches). Unlike the 2nd quarter (AMJ) of 2006 where all the tide-gauge stations recorded considerable rise, the 3rd quarter (JAS) displayed a moderate to weak positive deviation among most of the tide gauge stations in the vicinity of USAPI (Table 3). Only Majuro at Marshalls and Pohnpei at FSM recorded some marginal negative deviations during July and August, respectively. While the sea level rise in July was very distinct in some of the stations (Malakal, Kwajalein and Kapingamarangi), the rise in August and September was marginal (Table 3). The sea-level variation in the western North Pacific islands has been identified to be sensitive to ENSO-cycle, with low sea-levels during El Niño and high sea-levels during La Niña events. Consistent with the on-going weak El Niño event, the sea-level has recorded some drop. Because the onset of the current El Niño occurred relatively late in the season (September 2006), considerable sea level drop is yet to be seen, but will most likely be visible in the months to come.

In the last issue, our forecasts provided positive deviations for all the tide gauge stations in JAS (Fig. 4 dotted line). Real-time observed sea-level data in JAS provided verification for the forecast values (Fig. 4 ). Other than very marginal variations in Majuro and Pohnpei, stations in Kwajalein, Kapingamarangi, and Pago-Pago maintained a close match with the forecast levels. Due to unavailability of data, the sea level variations of Guam and Yap could not be verified.

Table 3 : Monthly observed sea level deviations in inches
(year to year std deviation in parentheses)

As per CCA model forecasts, a slight fall of sea level was predicted in OND, NDJ, and DJF in the vicinity of the western North Pacific (Guam, Malakal, Yap, and Pohnpei) while the other tide gauge stations in the Central and Southern Pacific were predicted to be near normal. Real-time observed data provided slight variations here - most of the western North Pacific stations recorded several inches rise. This finding is not very consistent with the on-going El Niño event (August-December 2006), in which case the sea-level is likely to show a considerable fall.