Pacific ENSO Update

2nd Quarter, 2008 Vol. 14 No. 2

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 (JFM 2008), (iii) forecast verification from the previous season, and (iv) tide predictions for AMJ (April 1 - June 30, 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 April-May-June (AMJ), May-June-July (MJJ), and June-July-August (JJA) 2008

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 JFM 2008, the resulting CCA model has been used to forecast the sea-level of three consecutive month periods: AMJ, MJJ and JJA 2008 (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 (AMJ, MJJ and JJA 2008)

 
(1) Seasonal Mean Deviations
(2) Seasonal Maximum Deviations
Tide Gauge Station
AMJ
MJJ
JJA

(3) Forecast Quality
AMJ
MJJ
JJA
(3) Forecast Quality
(4) RP for AMJ Season
(5) Lead Time
0
1M
2M
.
0
1M
2M
20 yr
100 yr
Marianas, Guam
+ 7
+ 7
+ 7
Good
+22 (18)
+22
+21
Very Good
5.6
6.7
Malakal, Palau
+ 6
+ 6
+ 6
Good
+40 (37)
+40
+41
Good
9.6
14.3
Yap, FSM
+ 7
+ 7
+ 7
Good
+33 (29)
+33
+33
Good
16.8
33.1
Chuuk, FSM
+5
+5
+5
N/A
+33 (N/A)
+33
+33
N/A
N/A
N/A
Pohnpei, FSM
+ 5
+ 3
+ 2
Very Good
+33 (31)
+32
+31
Good
5.8
7.1
Kapingamarangi, FSM
+ 2
+ 1
+ 1
Very Good
+27 (27)
+27
+28
Very Good
7.4
9.4
Majuro, RMI
+ 2
+ 1
0
Good
+40 (39)
+38
+37
Fair
4.1
5.1
Kwajalein, RMI
+ 4
+ 4
+ 2
Good
+41 (39)
+40
+39
Good
4.5
5.9
Pago Pago, AS
+ 4
+ 3
+ 2
Very Good
+27 (25)
+26
+26
Very Good
3.9
5.4

Remarks: The positive sea-level deviations forecast for the AMJ, MJJ, and JJA seasons (above) indicate that sea levels will remain elevated at all USAPI stations for another 1 to 3 months. This trend of elevated sea level is consistent with the on-going La Niña conditions; however, as La Niña conditions weaken, the sea-level is expected to begin receding toward normal levels. Mean sea levels are expected to be 2 to 7 inches above average during the AMJ season. Maximum sea levels for the AMJ season are expected to be 2 to 4 inches above the median value (in parentheses) at each station.

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 are unlikely to cause any adverse climatic impact. Forecasts for Chuuk (**) are estimated subjectively based on information from WSO Chuuk and observations from neighboring tide gage 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.

(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 are calculated from hourly sea level data. For example, the predicted rise of 5.6 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 5.6 inches above the median of seasonal maxima during the AMJ season. Likewise, about once every 100 years we can expect the highest AMJ tide at Marianas, Guam to reach as high as 6.7 inches abot the median of seasonal maxima. Note that Yap displays some alarmingly high values at the 20-year and 100-year RP; these high values are due to large and significant increases in the tidal range during the passage of past storm events in the AMJ season. Click here to view probability of exceedence graphs for the AMJ 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 (AMJ), 1 (MJJ), and 2 (JJA) month leads based on SSTs of Jan-Feb-Mar 2008.

(6) Median values of maximum sea level (in parentheses) for the AMJ season. The median is defined as the 50th percentile (or middle value) in the distribution of observed values.

 

(ii) Observed monthly sea level deviation in January-February-March (JFM), 2008

The monthly time series (January - March) 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 January, February and March 2008, with year to year standard deviations.

Tide Gauge Station

(1) Monthly Mean Deviations
(2) Monthly Maximum Deviations
Jan.
Feb.
Mar.
Standard Deviations
Jan.
Feb.
Mar.
Standard Deviations
JFM Median
Marianas, Guam
+6.0
+8.2
+8.5
(+3.8)
+21.3
+23.1
N/A
(+3.7)
+15.3
Malakal, Palau
+4.0
+8.7
+4.2
(+4.5)
+37.0
+44.1
+39.8
(+4.8)
+32.3
Yap, FSM
+6.0
+6.2
+8.9
(+3.8)
+31.0
+30.6
+36.6
(+4.5)
+25.2
Chuuk, FSM **
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
Pohnpei, FSM
+6.6
+4.2
N/A
(+3.1)
+35.3
+33.4
N/A
(+3.3)
+31.4
Kapingamarangi, FSM
+6.0
+4.2
N/A
(+3.5)
+33.0
+30.2
+27.1
(+4.1)
+28.2
Majuro, RMI
+3.0
+3.0
+3.0
(+2.2)
+42.0
+44.2
+40.2
(+2.8)
+39.4
Kwajalein, RMI
+4.0
+4.0
+3.8
(+2.4)
+42.0
+41.7
+40.2
(+2.8)
+39.4
Pago Pago, AS
+3.0
+1.0
+2.6
(+2.6)
+29.0
+23.1
+22.7
(+3.1)
+24.8

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.

Remarks: The observed values for seasonal mean/maxima display positive deviation for all the stations in USAPI. The mean values in March are higher than February values at some stations and lower at the others. This trend is consistent with the weakening state of the present La Niña episode. As the La Niña continues to weaken in the coming months, we may expect sea levels to begin falling ( 2 to 6 inches) in the next two to three months. See Table 1 for other notes.

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

Observed and forecast seasonal sea level values for the JFM 2008 season are presented in Figure 4. Forecasts were in general skillful; however, Guam and the Micronesian islands were found to be relatively under-forecast in the previous season (forecast values were lower than observed values).

(iv) Tide Predictions (April 1 to June 30, 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 April 1 to June 30, 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 AMJ 2008 season at (a) Marianas, Guam (b) Kwajalein, RMI and (c) Pago Pago, American Samoa. 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 (MR: 1.6 ft, SR: 2.4 ft, and ML: 1.4 ft)

 

b) Kwajalein, RMI (MR: 3.6 ft, SR: 3.9 ft, and ML: 1.9 ft)

 

c) Pago-Pago, American Samoa (MR: 2.5 ft, SR: 2.7 ft, and ML: 1.3 ft)

 

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.