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Pacific ENSO Update

3rd Quarter, 2009 Vol. 15 No. 3


SEASONAL SEA LEVEL OUTLOOKS FOR THE U.S.-AFFILIATED PACIFIC ISLANDS


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 (AMJ 2009), (iii) forecast verification from the previous season and (iv) tide predictions for the current (JAS 2009) season. 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.

tide station location
 

(i) Mean and Maximum Seasonal Sea Level Forecasts for July-August-September (JAS), August-September-October (ASO), and September-October-November (SON) 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 AMJ 2009, the resulting CCA model has been used to forecast the sea-level of three consecutive month periods: JAS, ASO and SON 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

 
(1) Seasonal Mean Deviations
(2) Seasonal Maximum Deviations
Tide Gauge Station
JAS
ASO
SON

(3) Forecast Quality

JAS
ASO
SON
(3) Forecast Quality
(4) RP for AMJ Season
(5) Lead Time
0
1M
2M
0
1M
2M
20 yr
100 yr
Marianas, Guam
+4
+2
0
Good
+20
+20
+18
Good
6.3
10.9
Malakal, Palau
+2
+2
+1
Very Good
+38
+39
+38
Good
8.1
10.2
Yap, FSM
+2
+2
0
Very Good
+28
+29
+28
Good
8.4
11.3
Chuuk, FSM**
+2
+2
0
N/A
+28
+29
+28
N/A
N/A
N/A
Pohnpei, FSM
0
0
0
Good
+29
+29
+30
Very Good
5.8
7.0
Kapingamarangi, FSM
+1
+1
+2
Good
+27
+27
+28
Very Good
3.5
4.2
Majuro, RMI
0
+1
+1
Good
+40
+41
+41
Good
5.2
6.8
Kwajalein, RMI
+1
+1
+1
Good
+39
+39
+38
Good
4.1
5.2
Pago Pago, AS
+4
+3
+3
Very Good
+28
+28
+27
Very Good
4.1
5.4
Honolulu, Hawaii (6)
+2
+3
+3
Fair
+22
+21
+21
Fair
3.4
5.7
Hilo, Hawaii (6)
+3
+3
+3
Fair
+25
+25
+25
Fair
6.4
7.7

Remarks: The forecast for the JAS, ASO, and SON 2009 seasons (Table 1, above) indicates that sea levels will continue to gradually fall toward normal levels over the next three months. Consistent with the recent transition from ENSO-neutral to El Niño, sea levels will remain slightly elevated at all USAPI and Hawaiian stations for the next three months. However, no further rise is expected.

As El Nino conditions develop further, sea levels across the USAPI and Hawaii are expected to fall to normal or below normal levels over the next 3-6 months, depending on the strength of the event.

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 6.3 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 6.3 inches above the median of seasonal maxima during the JAS season. Likewise, about once every 100 years we can expect the highest JAS tide at Marianas, Guam to be as high as 10.9 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 JAS 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 (JAS), 1 (ASO), and 2 (SON) month leads based on SSTs of AMJ 2009.

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

 

(ii) Observed monthly sea level deviation in April-May-June (AMJ), 2009

The monthly time series (April - June) 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 April, May, June 2009, with year to year standard deviations (SD).

Tide Gauge Station

(1) Monthly Mean Deviations
(2) Monthly Maximum Deviations
April
May
June
SD
April
May
June
SD
Marianas, Guam
+7.8
+7.2
+7.2
(+4.1)
+22.3
+20.8
+22.8
(+3.5)
Malakal, Palau
+5.7
+6.6
+6.8
(+4.5)
+39.7
+41.8
+39.9
(+4.3)
Yap, FSM
+8.2
+6.7
+7.5
(+3.8)
+33.9
+33.5
+34.4
(+4.2)
Chuuk, FSM **
*
*
*
(*)
*
*
*
(*)
Pohnpei, FSM
+6.4
+4.2
*
(+2.4)
+36.5
+33.3
*
(+3.2)
Kapingamarangi, FSM
*
*
*
(+2.8)
+30.8
+29.5
+21.8
(+2.6)
Majuro, RMI
+4.0
+2.2
*
(+2.0)
+39.8
+39.7
*
(+3.1)
Kwajalein, RMI
+2.8
+2.3
+2.2
(+2.3)
+39.7
+37.6
+39.5
(+2.8)
Pago Pago, AS
*
*
*
(+3.9)
*
*
*
(+3.3)
Honolulu, Hawaii
-2.8
-3.3
-1.9
(+1.7)
+15.7
+17.5
+19.5
(+2.4)
Hilo, Hawaii
-2.5
0.0
0.0
(+2.2)
+20.5
+27.3
+23.5
(+2.4)

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 sea level remain slightly elevated at all the USAPI stations. The Hawaiian stations, on the other hand, remain near or slightly below normal. These trends (falling sea levels) are consistent with the transition from ENSO-neutral to El Niño that is occurring.

 

(iii) Forecast Verification (Seasonal Mean) for AMJ 2009

Observed and forecast seasonal sea level values for the AMJ 2009 season are presented in Figure 4. Forecasts were generally skillful; however, most locations were under-forecast by one to four inches.

(iv) Tide Predictions (July 1 to September 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 July 1 to September 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 JAS 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/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.

 

Predicted Water Level Plot for Guam

a) Marianas, Guam

 

Predicted Water Level Plot for RMI

b) Kwajalein, RMI

 

Predicted Water Level Plot for American Samoa

c) Pago-Pago, American Samoa

Predicted Water Level Plot for Honolulu

d) Honolulu, Hawaii

Predicted Water Level Plot for Hilo

e) Hilo, Hawaii



Pacific ENSO Applications Climate (PEAC) Center
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Web Master's email: peac@noaa.gov
Page Last Modified: August 04 2011 22:33:32 GMT

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