Pacific ENSO Update

3rd Quarter, 2006 Vol. 12 No. 3

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

tide station location

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 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. Also, 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 AMJ, MJJ, and JJA 2006

Forecasts of the sea level anomalies in the USAPI are presented using CCA statistical model. Locations of all stations are shown in Fig 2. Based on the independent SST values in AMJ 2006, the resulting CCA model was used to forecast the sea level of three consecutive months:Jul-Aug-Sep (JAS), Aug-Sep-Oct (ASO), and Oct-Nov-Dec (OND) (Table 1). CCA cross-validation forecast skills for 0, 1, and 2-month leads are presented (Fig. 3 ).

Table 1 : Forecasts of sea level deviation in inches (JAS: Jul-Aug-Sep, ASO: Aug-Sep-Oct, and OND:Oct-Nov-Dec)

Tide Gauge Station JAS ASO SON Forecast quality1

1. Forecast quality is a measure of the expected CCA cross-validation correlation skill. In general terms, these forecasts are thought to be of useful skill (or at least fair skill) if the CCA cross-validation value is greater than 0.3 (Fig. 3 ). Higher skills correspond to greater expected accuracy of the forecasts. Skill levels greater than 0.4 and 0.5 are thought to be moderate and good, while skill levels greater than 0.6 and 0.8 are thought to be strong and very strong respectively.

2. The lead time is the time interval between the end of the initial period and the beginning of the forecast period. For example, lead-0, lead-1M, and lead-2M means ‘sea-level’ of target season 0 (JAS), 1 (ASO), and 2 (SON) month leads based on SSTs of Apr-May-Jun.
Lead Time2 0 1M 2M  
Guam +5 +3 ** Good
Malakal +2 +2 +2 V. Strong
Yap +2 +2 ** Strong
Pohnpei ** ** ** Strong
Kapingamari +2 +2 +2 Strong
Majuro ** +1 +2 Good
Kwajalein +1 +1 ** Good
Pago Pago +4 +3 +3 Strong

Note: (-) indicates negative deviations (fall of sea level from the mean), and (+) indicates positive deviations (rise of sea level from the mean), N/A: data not available. Deviations of +/-1 in. are considered negligible and denoted by **. Deviations +/- 2 in. are unlikely to cause any adverse climatic impact.

 

As in the previous season (AMJ), this season (JAS) provided very skillful forecasts (Fig. 3). Most of the tide gauge stations show strong skill level (Table 1). Only Majuro and Kwajalein displayed a relatively lower skill, which is still reasonably well predicted (skill level is 0.5 or more). With a mean skill greater than 0.67 (at 0 to 2-months lead time) in a ll three consecutive seasonal (JAS, ASO, and SON), the tide gauge stations are very well predicted.

In the last newsletter, all of the tide gauge stations were predicted to have higher than normal sea levels for AMJ. For the next three seasons (JJA, ASO and SON), the tide gauge stations will continue to have slightly higher than normal sea levels. The higher positive deviations are forecast for the westernmost stations (Guam, Malakal, Yap) and the near equatorial area near Kapingamaringi. Further east, Pohnpei, Kwajalein and Majuro are likely to display less positive deviations. In the South Pacific, the vicinity of Pago Pago is forecast to have a positive deviation in sea level.

(ii) Observed monthly sea level deviation in Apr-May-Jun (AMJ), 2006

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.

Table 2 provides the monthly observed sea level deviations (in inches). In the last quarter (AMJ: 2 nd quarter of 2006), all the tide gauge stations recorded a monthly sea level which was higher than normal. Majuro recorded only slight positive deviations. Amongst the others, Guam continued to record considerable rise in this quarter (12.8-, 9.4-, and 7.8-inches in April, May, and June respectively). Other stations that recorded considerable rises are: Malakal at Palau, Pohnpei at FSM, and Pago-Pago at American Samoa (Table 2).

In the last quarter’s issue, we forecasted positive deviations for all the tide gauge stations in AMJ (Fig. 4 dotted line). The observed Real-time sea-level data in AMJ was consistent with the forecast values (Fig. 4 ). In all cases, the observed values were found to be positive (higher than normal sea level). Yap, Kwajalein, Majuro, Kapingamarangi, and Pago-Pago maintained a close match with the forecast values. However, Guam and Malakal remained relatively under forecast.

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

Tide Gauge January February March
  Deviation SD Deviation SD Deviation SD
Guam +12.8 (4.4) +9.4 (4.1) +7.8 (3.6)
Malakal +5.4 (4.6) +7.8 (4.7) +7.5 (4.2)
Yap +1.6 (3.8) +5.5 (3.4) +6.4 (4.1)
Kwajalein +5.6 (2.1) +5.0 (2.5) +4.0 (2.2)
Majuro +2.7 (1.9) +1.1 (1.9) +0.1 (2.1)
Pohnpei +6.5 (2.0) +3.5 (2.3) +1.4 (2.7)
Kapingamarangi. +3.6 (3.0) +1.6 (2.9) +1.6 (2.6)
Pago Pago +6.8 (3.7) +5.2 (4.3) +4.8 (3.6)

Note: - indicate negative deviations (fall of sea-level from the mean), and + indicate positive deviations (rise of sea-level from the mean), n/a: data not available, Figures in parenthesis are year-to-year SD (standard deviations) for the month.