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4th Quarter, 2009 Vol. 15 No. 4


SEASONAL SEA LEVEL OUTLOOKS FOR THE U.S.AFFILIATED PACIFIC ISLANDS 
The following sections describe: (i) the Canonical Correlation Analysis (CCA) forecasts for seasonal (mean and maxima) sealevel deviations for the forthcoming seasons OND, NDJ, and DJF of 2009, (ii) .the observed monthly mean and maximum sealevel deviations for the season JAS 2009, (iii) forecast verifications for JAS 2009 (observed/forecast values). Note that the 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’. (See Figure 2 at right for location of these stations.) Also note that, the CCAforecasting technique adapted here does not account for sealevel deviations created by other atmospheric or geological conditions such as tropical cyclones, storm surges or tsunamis. 


(i) Seasonal sea level forecast (deviations with respect to climatology) for OND, NDJ, and DJF of 2009
Forecasts of the mean and maximum sealevel deviations in the USAPI are presented using CCA statistical model. Based on the independent SST values in JAS 2009, the resulting CCA model has been used to forecast the sealevel of three consecutive threemonth periods: OND, NDJ and DJF 2009 (see Table 1). CCA crossvalidation forecast skills for 0, 1, and 2month leads are presented in Fig. 3.
Table 1 : Forecasts of MEAN and MAXIMUM sea level deviation in inches for forthcoming seasons 

(1) Seasonal Mean Deviations 
(2) Seasonal Maximum Deviations 
Tide Gauge Station 
OND 
NDJ 
DJF 
(3) Forecast Quality 
OND 
NDJ 
DJF 
(3) Forecast Quality 
(4) RP for JAS Season 
(5) Lead Time 
0 
1M 
2M 

0 
1M 
2M 

20 yr 
100 yr 
Marianas, Guam 
0 
0 
0 
Good 
+17 
+16 
+16 
Good 
6.5 
9.1 
Malakal, Palau 
0 
2 
2 
Very Good 
+37 
+35 
+34 
Very Good 
6.1 
6.4 
Yap, FSM 
0 
2 
1 
Very Good 
+27 
+26 
+25 
Very Good 
8.2 
11.0 
Chuuk, FSM** 
0 
0 
0 
N/A 

+29 
+28 
N/A 
N/A 
N/A 
Pohnpei, FSM 
+1 
+1 
+1 
Very Good 
+32 
+32 
+32 
Very Good 
9.1 
11.8 
Kapingamarangi, FSM 
+3 
+3 
+2 
Good 
+32 
+31 
+31 
Fair 
5.7 
6.4 
Majuro, RMI 
+2 
+1 
+1 
Good 
+42 
+42 
+43 
Good 
6.6 
8.4 
Kwajalein, RMI 
+1 
0 
0 
Good 
+38 
+38 
+39 
Very Good 
4.9 
6.1 
Pago Pago, AS 
+3 
+3 
+1 
Good 
+26 
+27 
+27 
Good 
3.0 
3.7 
Honolulu, Hawaii (6) 
+3 
+2 
+2 
Fair 
+21.6 
+23 
+22 
Fair 
3.2 
5.2 
Hilo, Hawaii (6) 
+4 
+2 
+2 
Fair 
+24.4 
+28 
+29 
Fair 
5.5 
6.8 
Remarks: The forecasts values of sea level for the OND, NDJ, and DJF seasons (Table 1, above) indicate that sea levels will remain slightly elevated for some stations while most of the other stations will recorded fall—no further rise is expected. The Hawaiian stations, on the other hand, are likely to be elevated (2 to 3 inches) during the same time period. The forecasts values provide a clear indication of considerable fall in all stations. This trend is supportive to the condition of moderate El Nino condition. However, if the strength of El Nino changes to a stronger one then we might see more pronounced fall. At PEAC, we are constantly monitoring the ongoing El Nino event; we will report the outcome in bulletin format, if required. 
Note: () indicate negative deviations (fall of sealevel from the mean), and (+) indicate positive deviations (rise of sealevel from the mean), n/a: data not available; also note that any deviations from 1 to +1 inches are considered negligible and deviations from 2 to +2 inches are unlikely to cause any adverse climatic impact. Forecasts for Chuuk (**) are estimated subjectively based on information from WSO Chuuk and observations from neighboring stations of Pohnpei and Yap.
(1) Seasonal Mean Deviations is defined as the difference between the mean sea level for the given month and the 19751995 mean sea level value at each station. Likewise, (2) Seasonal Maximum Deviations is defined as the difference between the maximum sea level (calculated from hourly data) for the given month and the 19751995 mean sea level value at each station.
(3) Forecast Quality is a measure of the expected CCA crossvalidation correlation skill. In general terms, these forecasts are thought to be of useful (but poor) skill if the CCA crossvalidation value lies between 0.3 ~ 0.4 (Fig. 3). Higher skills correspond to a greater expected accuracy of the forecasts. Skill levels greater than 0.4 and 0.6 are thought to be fair and good, respectively, while skill levels greater than 0.7 are thought to be very good.
(4) Return Period (RP) of extreme values is calculated from hourly sealevel data. For example, the predicted rise of 6.5 inches at 20year RP at Marianas, Guam indicates that this station may experience an extreme tide event once every 20 years that could result in sealevel rise of up to 6.5 inches above the median of seasonal maxima during the OND season. Likewise, about once every 100 years we can expect the highest OND tide at Marianas, Guam to be as high as 9.1 inches above the median of seasonal maxima. During some seasons some stations display alarmingly high values at the 20 and 100 year RP. These high values are due to large and significant increases in the tidal range caused by the passage of past storm events during that season. Click here to view probability of exceedence graphs for the OND season.
(5) Lead time is the time interval between the end of the initial period and the beginning of the forecast period. For example, lead0, lead1M, and lead2M means ‘sealevel’ of target season 0 (OND), 1 (NDJ), and 2 (DJF) month leads based on SSTs of JAS 2009. 


(ii) Observed monthly sea level deviation in JulyAugustSeptember (JAS), 2009
The monthly time series (Jul to Sep) for sea level deviations have been taken from the UH Sea Level Center. Note that ‘deviation’ is defined here as ‘the observed or forecast difference between the monthly mean [or maximum] and the climatological monthly mean values (from the period 1975 1995) computed at each station’.. Locations of all these stations are shown in Figure 2 (top of page).

Table 2: Monthly observed MEAN and MAX sea level deviations in inches for July, August, and September, with year to year standard deviations (SD). 
Tide Gauge Station 
(1) Monthly Mean Deviations 
(2) Monthly Maximum Deviations 

Jul 



Jul 
Aug 
Sep 
SD 
Marianas, Guam 
+8.2 
+5.9 
+4.7 
(+3.3) 
+23.8 
+21.3 
+18.9 
(+3.5) 
Malakal, Palau 
+5.7 
+4.3 
+5.1 
(+4.1) 
+39.4 
+41.6 
+40.4 
(+4.3) 
Yap, FSM 
+4.1 
+4.3 
+2.7 
(+4.4) 
+31.6 
+32.4 
+29.8 
(+4.2) 
Chuuk, FSM ** 
* 
* 
* 
(*) 
* 
* 
* 
(*) 
Pohnpei, FSM 
+3.9 
* 
* 
(+2.8) 
+33.6 
* 
* 
(+3.2) 
Kapingamarangi, FSM 
* 
* 
* 
(+2.4) 
* 
* 
* 
(+2.6) 
Majuro, RMI 
+2.0 
* 
* 
(+2.3) 
+43.5 
* 
* 
(+3.1) 
Kwajalein, RMI 
+2.7 
+2.0 
+1.1 
(+2.2) 
+40.9 
+38.9 
+37.2 
(+2.8) 
Pago Pago, AS 
* 
* 
+4.2 
(+2.8) 
* 
* 
+27.2 
(+3.3) 
Honolulu, Hawaii 
+0.7 
+2.8 
+2.4 
(+1.9) 
+22.6 
+23.2 
+17.7 
(+2.4) 
Hilo, Hawaii 
+0.6 
+5.1 
+1.6 
(+1.8) 
+26.8 
+27.7 
+18.8 
(+2.4) 
Note:  indicate negative deviations (fall of sealevel from the mean), and + indicate positive deviations (rise of sealevel from the mean); N/A: data not available. ** Sea level data for Chuuk is based on estimates from neighboring tide stations (Yap and Pohnpei) and observations from WSO Chuuk. Standard deviations describe how widely spread the values are in the dataset. See Table 1 for other notes.
Remarks: In general, the sealevel in all the stations is still elevated and is slightly above normal (based on 197595 mean). The Hawiian stations also displayed a picture of positive deviations. This trend is consistent with the strengthening effect of El Niño conditions in the forthcoming season.



(iii) Forecast Verification (Seasonal Mean) for JAS 2009
Observed and forecast seasonal sea level values for the JAS 2009 season are presented in Figure 4. Forecasts were in general skillful; however, Malakal, Pohnpei, and Majuro were slightly under forecasted.
(iv) Tide Predictions (October 1 to December 31, 2009)
NOAA's web site for tide and currents has been used to generate the water level plot for the next three months. Predicted water level plots from October 1 to December 31, 2009 for six stations [(a) Marianas, Guam (b) Kwajalein, RMI (c) Pago Pago, American Samoa (d) Honolulu, Hawaii (e) Hilo, Hawaii and (f) Chuuk, FSM] are provided below. 

Figure 5 (below): Predicted water level for the OND 2009 season at (a) Marianas, Guam (b) Kwajalein, RMI (c) Pago Pago, American Samoa, (d) Honolulu, HI (e) Hilo, Hawaii and (f) Chuuk, FSM. Data from NOAA/NOA/COOPS. Xaxis: date/time (GMT); Yaxis: height in feet relative to Mean lower low water level (MLLW); MR: Meandifference 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.

a) Marianas, Guam 
b) Kwajalein, RMI 
c) PagoPago, American Samoa 
d) Honolulu, Hawaii 
e) Hilo, Hawaii 
e) Chuuk, FSM 

