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

(i) Seasonal Sea Level Forecast for OND, NDJ, and DJF 2006/2007

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: OND, NDJ, and DJF (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 (OND: Oct-Nov-Dec, NDJ: Nov-Dec-Jan, and DJF: Dec-Jan-Feb)

Although all the previous forecasts were skillful, this season (OND) has provided the most skillful forecasts (Fig. 3). All of the tide gauge stations show strong or good skill level (Table 1). With a mean skill greater than 0.73 (at 0 to 2-months lead time) in all the three consecutive seasons (OND, NDJ, and DJF), the tide gauge stations are very well predicted.

Consistent with the onset of El Niño in August-September 2006, the sea-level in all the North Pacific islands has shown some sign of receding trends; however, a full-fledged sea-level drop (as recorded in previous El Niño years) is yet to be observed. Results of the CCA model forecasts reveal that most of the tide gauge stations in the western North Pacific (Guam, Malakal, Yap, and Pohnpei) are likely to experience falling sea level (negative deviations) during the forthcoming OND, NDJ, and DJF seasons (Table 1). Conversely, the central part of the North Pacific (Majuro and Kwajalein, RMI) is likely to display a very marginal rise in sea level (positive deviation) in the forthcoming seasons, while Pago Pago in the South Pacific is expected to show a moderate rise during the same timeframe. According to CPC, the present oceanic and atmospheric anomalies are consistent with the early stages of El Niño in the tropical Pacific, and existing El Niño conditions are likely to continue into early 2007.

(ii) Observed monthly sea level deviation in Jul-Aug-Sep (JAS), 2006

The monthly time series (Jul to Sep) 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 2 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 2). 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 2). 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 2 : Monthly observed sea level deviations in inches
(year to year std deviation in parentheses)