Experimental Sea level Forecasts
(deviations w. r. t. climatology) for the U.S-affiliated Pacific Islands

The following sections describe: (i) the CCA-based forecasts for sea level deviations for the forthcoming season, and (ii) the observed monthly sea level deviations. All units are in inches. Note that the forecasting technique adapted here does not account for sea level deviations created by other atmospheric or geological conditions such as tropical cyclones, storm surges or tsunamis.

(i) Seasonal Sea Level Forecast for JFM, FMA, and MAM 2006

Forecasts of the sea level anomalies in the USAPI are presented using CCA statistical model. Locations of all stations are shown in Fig. 3. Based on the independent SST values in OND 2005 (for SST data, see http://iridl.ldeo.columbia.edu/expert/SOURCES/.NOAA/.NCDC/.ERSST/.version2/.SST/), the resulting CCA model was used to forecast the sea level of three consecutive months: Jan-Feb-Mar (JFM), Feb-Mar-Apr (FMA), and Mar-Apr-May (MAM) (Table 1). CCA cross-validation forecast skills for 0, 1, and 2-month leads are presented (Fig. 4).

Table 1 : Forecasts of sea level deviation in inches (JFM: Jan- Feb- Mar, FMA: Feb.-Mar-Apr, and MAM: Mar-Apr-May)

As in the previous season (OND), this season (JFM) also provided very skillful forecast (Fig. 4). Most of the tide gauge stations show strong skill level (Table 1). Only Saipan in the north Pacific displays a relatively lower skill level; however, it is still above 0.4. The majority of the tide gauge stations in all the three consecutive months (JFM, FMA, MAM) are very well predicted with a mean skill greater than 0.6 (at 0 to 2-month lead time). The effect of the spring predictability barrier is therefore less dominant for the prediction of sea level in this season.

Results of the CCA model forecasts revealed that all the tide gauge stations (except Malakal in MAM) are likely to rise in the forthcoming seasons (Table 1). The rising trend has also been observed consistently in all the USAPI islands located in the vicinity of north-west, north-east, and south Pacific Ocean. However, it is also worth noting here that —other than Guam—all the northeastern Pacific islands display very marginal positive deviations; the deviations gradually increased as we move towards north-central, north-eastern, and south Pacific.

The differences of atmospheric circulation in the equatorial western/central Pacific explains this phenomenon very well. Initially the tropical easterly winds were active in OND. As the season advanced, the westerly anomalies developed in the western Pacific became active in AMJ. They gradually strengthened and displaced eastward in JAS. As the year advanced, the westerlies continued to propagate eastward to the equatorial eastern Pacific and finally shifted towards the south Pacific. Due to the switching of the prevailing wind direction, piled-up water in the tropical western Pacific flowed back eastward toward South America. As a result, the northwestern Pacific Islands are experiencing a drop in sea level while the northeastern and south Pacific Islands are experiencing a rise in sea level from OND to JFM.

(ii) Observed monthly sea level deviation in Oct-Nov-Dec (OND), 2005

The monthly time series (October to 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. Locations of all these stations are shown in Fig. 3.

Table 2 provides the monthly observed sea level deviations (in inches). All tide gauge stations, with the exception of Guam, recorded positive deviations in this quarter (4th quarter). The sea level at Malakal and Kapingamarangi in the northwest Pacific Ocean, and Kwajalein and Majuro in the northeast Pacific Ocean recorded considerable positive sea level deviations (Table 2). Similiarly, Yap and Pohnpei recorded moderate positive deviations. In the south Pacific, Pago Pago recorded considerable positive deviations.

Unlike the previous three quarters (JFM, AMJ, and JAS) when Guam continuously displayed positive deviations, Guam tended to show negative deviations at the beginning of this quarter (-1.5 inches in Oct.). However, due to the unavailability of data for the later part of this quarter at print time, this trend could not be verified independently.

In the last quarter’s issue, our forecasts provided positive deviations for all the tide gauge stations –except Guam, Saipan, Yap, and Kwajalein— in OND (Fig. 5 dotted line). Negligible variations were predicted in Guam, Saipan, Yap, and Kwajalein. Real-time observed sea level data in OND provided consistency with the forecast values (Fig. 5). In all cases, the direction of the deviations—either positive or negative— has been found to match with the forecast values. However, quantitatively the forecasts values of Malakal, Kwajalein, and Pago-Pago were considerably lower than the real-time observed values.

Table 2 : Monthly observed sea level deviations in inches (std deviation in parentheses)