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Pacific ENSO Update

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) sea-level deviations for the forthcoming seasons OND, NDJ, and DJF of 2009, (ii) .the observed monthly mean and maximum sea-level 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 CCA-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) Seasonal sea level forecast (deviations with respect to climatology) for OND, NDJ, and DJF of 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 JAS 2009, the resulting CCA model has been used to forecast the sea-level of three consecutive three-month periods: OND, NDJ and DJF 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
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 on-going El Nino event; we will report the outcome in bulletin format, if required.

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; 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 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 2009.

 

(ii) Observed monthly sea level deviation in July-August-September (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
Aug
Sep
SD
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 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: In general, the sea-level in all the stations is still elevated and is slightly above normal (based on 1975-95 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/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

 

Predicted Water Level Plot for Chuuk

e) Chuuk, FSM



Pacific ENSO Applications Climate (PEAC) Center
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2525 Correa Road, suite 250
Honolulu, HI 96822
(808) 956-2324

Web Master's email: peac@noaa.gov
Page Last Modified: August 04 2011 22:30:58 GMT

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