Pacific ENSO Update

1st Quarter, 2009 Vol. 15 No. 1

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 (OND 2008), (iii) forecast verification from the previous season, and (iv) tide predictions for JFM (January 1 - March 31, 2009). 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 January-February-March (JFM), February-March-April (FMA), and March-April-May (MAM) 2009.

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 OND 2008, the resulting CCA model has been used to forecast the sea-level of three consecutive month periods: JFM, FMA and MAM 2009 (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 (JFM, FMA and MAM 2009)

 
(1) Seasonal Mean Deviations
(2) Seasonal Maximum Deviations
Tide Gauge Station
JFM
FMA
MAM

(3) Forecast Quality
JFM
FMA
MAM
(3) Forecast Quality
(4) RP for JFM Season
(5) Lead Time
0
1M
2M
.
0
1M
2M
20 yr
100 yr
Marianas, Guam

+5
+5
+7
Very Good
+19
+20
+21
Good
5.6
6.7
Malakal, Palau
+2
+3
+3
Very Good
+39
+39
+39
Very Good
9.6
14.3
Yap, FSM
+2
+3
+3
Very Good
+30
+32
+33
Very Good
16.7
33.0
Chuuk, FSM**
+3
+2
+2
N/A
+30
+32
+33
N/A
N/A
N/A
Pohnpei, FSM
+4
+5
+5
Very Good
+30
+34
+34
Very Good
5.8
7.1
Kapingamarangi, FSM
+4
+4
+3
Good
+30
+29
+28
Good
7.4
9.4
Majuro, RMI
+3
+4
+3
Good
+44
+44
+44
Good
4.1
5.1
Kwajalein, RMI
+3
+4
+5
Good
+42
+43
+43
Good
4.5
5.9
Pago Pago, AS
+3
+3
+3
V. Good
+27
+28
+27
Very Good
3.9
5.4
Honolulu, Hawaii (6)
-1
-1
-1
Fair
+18
+17
+17
Fair
4.1
5.9
Hilo, Hawaii (6)
-1
-1
0
Good
+23
+20
+20
Good
7.9
11.4

Remarks: The positive sea-level deviations forecast for the JFM, FMA, and MAM 2009 seasons (Table 1, above) indicate that sea-levels will remain slightly elevated at all USAPI stations for the next several months. However, the Hawaiian stations are expected to record a marginal fall during the same time period. Consistent with on-going La Niña conditions, which are likely to continue for at least another three months, both mean and maximum sea-levels are expected to be about 2 to 6 inches higher than the median values. (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 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 JFM season. Likewise, about once every 100 years we can expect the highest JFM tide at Marianas, Guam to be as high as 6.7 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 JFM 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 (JFM), 1 (FMA), and 2 (MAM) month leads based on SSTs of OND 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 October-November-December (OND), 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 October, November and December 2008, with year to year standard deviations (SD).

Tide Gauge Station

(1) Monthly Mean Deviations
(2) Monthly Maximum Deviations
Oct.
Nov.
Dec.
SD
OND Median
Oct.
Nov.
Dec.
SD
OND Median
Marianas, Guam
+8.1
+8.0
+7.8
(+4.2)
+7.9
+28.4
+24.8
+24.5
(+4.1)
+15.8
Malakal, Palau
+7.1
*
+7.0
(+4.1)
+7.1
+43.4
+42.4
+42.1
(+4.8)
+36.8
Yap, FSM
+5.3
+4.7
+4.6
(+4.7)
+4.8
+32.1
+33.2
+33.4
(+3.8)
+26.7
Chuuk, FSM **
*
*
*
(*)
*
*
*
*
(*)
*
Pohnpei, FSM
+3.0
+4.6
*
(+4.7)
+3.0
+31.8
+35.1
*
(+3.9)
+31.4
Kapingamarangi, FSM
+1.5
+1.0
+5.4
(+3.0)
+3.5
+29.7
+30.8
+34.2
(+3.9)
+30.5
Majuro, RMI
+5.1
+3.5
*
(+3.7)
+4.6
+47.0
+43.3
*
(+2.7)
+42.0
Kwajalein, RMI
+1.9
+3.0
+1.8
(+3.2)
+1.9
+40.2
+41.9
+40.5
(+2.8)
+38.0
Pago Pago, AS
+1.8
+2.4
*
(+2.2)
+2.4
+23.2
+26.7
+26.0
(+2.7)
+25.2
Honolulu, Hawaii
+1.8
+1.4
+1.0
(+1.8)
+1.2
+19.5
+24.5
+26.1
(+2.4)
+21.3
Hilo, Hawaii
+3.8
+1.9
+1.7
(+2.1)
+1.7
+23.5
+23.5
+28.4
(+2.8)
+24.5

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 display positive deviation at all the USAPI stations. As compared to November 2008, the mean/maxima of sea-level in December 2008 was slightly lower at most stations. However, some stations in FSM recorded a rise in sea-level (Pohnpei, Kapingamarangi). The rise is Pohnpei was quite considerable. The forecast values predict elevated sea-levels of 2 to 6 inches for the next three months. This trend is consistent with the recent resurgence of La Niña.

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

Observed and forecast seasonal sea level values for the OND 2008 season are presented in Figure 4. Forecasts were in general skillful; however, some locations (especially Guam amd Malakal) were under-forecast (forecast values were lower than observed values) in the OND season by several inches.

(iv) Tide Predictions (January 1 to March 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 JFM 2009 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.