Pacific ENSO Update

4th Quarter, 2008 Vol. 14 No. 4

SEASONAL SEA LEVEL OUTLOOKS
for the U.S-Affiliated Pacific Islands

tide station location

The following sections describe: (i) the Canonical Correlation Analysis (CCA)-based forecasts for maximum and mean sea level deviations for the forthcoming seasons, (ii) the observed monthly sea level deviations from the previous season (JAS 2008), (iii) forecast verification from the previous season, and (iv) tide predictions for OND (October 1 - December 31, 2008). All units are in inches. 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’. (See Figure 2 at right for location of these stations.) 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) Mean and Maximum Seasonal Sea Level Forecasts for October-November-December (OND), November-December-January (NDJ), and December-January-February (DJF) 2008-09.

Forecasts of the mean and maximum sea-level deviations in the USAPI are presented using CCA statistical model. Based on the independent SST values in JAS 2008, the resulting CCA model has been used to forecast the sea-level of three consecutive month periods: OND, NDJ and DJF 2008-09 (see Table 1). CCA cross-validation forecast skills for 0, 1, and 2-month leads are presented in Fig. 3.

Table 1 : Forecasts of MEAN and MAXIMUM sea level deviation in inches for forthcoming seasons (OND, NDJ and DJF 2008-09)

 
(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 OND Season
(5) Lead Time
0
1M
2M
.
0
1M
2M
20 yr
100 yr
Marianas, Guam

+3
+3
+3
Very Good
+18
+18
+18
Good
6.5
9.1
Malakal, Palau
+4
+2
+2
Very Good
+40
+39
+37
Very Good
6.1
6.4
Yap, FSM
+3
+2
+2
Very Good
+30
+29
+28
Very Good
8.2
11.0
Chuuk, FSM**
+3
+2
+2
N/A
+30
+29
+28
N/A
N/A
N/A
Pohnpei, FSM
+4
+4
+4
Very Good
+34
+35
+33
Very Good
9.1
11.8
Kapingamarangi, FSM
+3
+3
+3
Good
+30
+31
+31
Fair
5.7
6.4
Majuro, RMI
+3
+2
+2
Very Good
+30
+31
+31
Very Good
6.6
8.4
Kwajalein, RMI
+2
+1
+1
Very Good
+40
+40
+39
Very Good
4.9
6.1
Pago Pago, AS
+1
+2
+1
Good
+24
+25
+25
Good
3.0
3.7
Honolulu, Hawaii (6)
+1
+1
0
Fair
+20
+20
+21
Fair
3.2
5.2
Hilo, Hawaii (6)
+1
0
0
Good
+23
+24
+24
Good
5.5
6.8

Remarks: The positive sea-level deviations forecast for the OND, NDJ, and DJF seasons (Table 1, above) indicate that sea-levels will remain slightly elevated at all USAPI and Hawaii stations for another 1 to 3 months. From OND 2008 to DJF 2009, mean sea-levels are expected to begin receding back toward normal levels, consistent with the transition from La Niña to ENSO neutral conditions that occurred in June. Mean and maximum sea-levels are expected to remain about 0 to 3 inches above median values at each station during the forthcoming seasons. (The median value is the 50th percentile value, or middle value, in the distribution of observed values.)

Click HERE to see Table 1 in graphical form.

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 from -1 to +1 inch are considered negligible (***) and 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 1975-1995 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 1975-1995 mean sea level value at each station.

(3) Forecast Quality is a measure of the expected CCA cross-validation correlation skill. In general terms, these forecasts are thought to be of useful (but poor) skill if the CCA cross-validation 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 sea-level data. For example, the predicted rise of 6.5 inches at 20-year RP at Marianas, Guam indicates that this station may experience an extreme tide event once every 20 years that could result in sea level 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, lead-0, lead-1M, and lead-2M means ‘sea-level’ of target season 0 (OND), 1 (NDJ), and 2 (DJF) month leads based on SSTs of JAS 2008.

(6) Hawaii stations of Honolulu and Hilo are newly added and should be considered experimental. Any feedback regarding the usefulness of these forecasts will be appreciated.

(ii) Observed monthly sea level deviation in July-August-September (JAS), 2008

The monthly time series (July - September) 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 2008, with year to year standard deviations.

Tide Gauge Station

(1) Monthly Mean Deviations
(2) Monthly Maximum Deviations
July
Aug.
Sept.
Standard Deviations
July
Aug.
Sept
Standard Deviations
JAS Median
Marianas, Guam
+9.3
+9.8
+9.1
(+3.6)
+26.4
+25.5
+23.6
(+3.6)
+17.9
Malakal, Palau
+7.5
+8.1
+8.3
(+4.3)
+40.5
+42.6
+42.5
(+4.3)
+36.5
Yap, FSM
+7.5
+7.2
+8.3
(+4.3)
+30.3
+33.2
+31.8
(+4.3)
+29.0
Chuuk, FSM **
*
*
*
(*)
*
*
*
(*)
*
Pohnpei, FSM
+4.4
+3.7
*
(+3.0)
+33.5
+32.9
*
(+3.7)
+31.4
Kapingamarangi, FSM
***
***
+1.1
(+2.5)
+27.8
+27.3
+22.8
(+2.7)
+27.2
Majuro, RMI
+1.7
+1.6
*
(+2.5)
+40.6
+41.9
*
(+3.1)
+38.9
Kwajalein, RMI
+3.3
+3.5
+2.9
(+2.4)
+39.2
+42.5
+37.8
(+2.8)
+39.8
Pago Pago, AS
+3.6
+3.5
+2.2
(+2.9)
+28.1
+28.1
+22.8
(+3.4)
+29.9
Honolulu, Hawaii
+3.0
+4.6
+3.4
(+1.8)
+26.1
+25.7
+18.7
(+2.4)
+18.0
Hilo, Hawaii
+3.2
+2.4
+3.9
(+1.8)
+27.0
+28.8
+21.8
(+2.4)
+21.5

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. ** 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: The observed values for seasonal mean/maxima displayed positive deviations in all the USAPI stations during the July - August - September 2008 season. No significant fall of sea-level was recorded during JAS 2008, although levels have been expected to start receding back toward normal. Sea-levels remain influenced by the lingering La Niña effects in the atmostpheric circulation pattern over the western and central tropical Pacific.

(iii) Forecast Verification (Seasonal Mean) for JAS 2008

Observed and forecast seasonal sea level values for the JAS 2008 season are presented in Figure 4. Forecasts were in general skillful; however, all locations were under-forecast (forecast values were lower than observed values) in the JAS season by 1 to 3 inches.

(iv) Tide Predictions (October 1 to December 31, 2008)

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, 2008 for three major stations (a) Marianas, Guam (b) Kwajalein, RMI and (c) Pago Pago, American Samoa are provided below. Observations reveal that the MR, SR, and ML for all these above stations are likely to be 2-4 inches higher than average during the next three months.

Figure 5 (below): Predicted water level for the OND 2008 season at (a) Marianas, Guam (b) Kwajalein, RMI (c) Pago Pago, American Samoa, (d) Honolulu, HI and (e) Hilo, Hawaii. Data from NOAA/NOA/CO-OPS. X-axis: date/time (GMT); Y-axis: height in feet relative to Mean lower low water level (MLLW); MR: Mean-difference 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) Pago-Pago, American Samoa

d) Honolulu, Hawaii

e) Hilo, Hawaii

 

Chowdhury M. R , P-S Chu, and Schroeder T (2007): ENSO and Seasonal Sea-level Variability – A Diagnostic Discussion for the U.S-Affiliated Pacific Islands, Theoretical and Applied Climatology, 88: 213-224 (March 2007), Springer-Verlag Wien.