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Pacific ENSO Update

4th Quarter, 2010 Vol. 16 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 2010-2011, (ii) the observed monthly mean and maximum sea-level deviations for the season JAS 2010, (iii) forecast verifications for JAS 2010 (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 accounts for ENSO-related sea-level deviations and does not account for sea-level deviations created by other atmospheric or geological factors 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 2010-2011

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 2010, the resulting CCA model has been used to forecast the sea level of three consecutive three-month periods: OND, NDJ, and DJF of 2010-2011 (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 OND Season
(5) Lead Time
0
1M
2M
0
1M
2M
20 yr
100 yr
Marianas, Guam
+7
+8
+8
Very Good
+22
+22
+22
Good
6.5
9.1
Malakal, Palau
+8
+7
+6
Very Good
+45
+45
+45
Good
6.1
6.4
Yap, FSM
+7
+6
+5
Very Good
+35
+35
+35
Good
8.2
11.0
Chuuk, FSM**
+7
+5
+5
N/A
+35
+35
+33
N/A
N/A
N/A
Pohnpei, FSM
+9
+9
+8
Very Good
+40
+33
+33
Good
9.1
11.8
Kapingamarangi, FSM
+5
+6
+6
Good
+32
+24
+24
Fair
5.7
6.4
Majuro, RMI
+7
+6
+5
Very Good
+48
+45
+45
Good
6.6
8.4
Kwajalein, RMI
+6
+5
+5
Very Good
+44
+45
+45
Very Good
4.9
6.1
Pago Pago, AS
+3
+3
+4
Good
+28
+32
+32
Good
3.0
3.7
Honolulu, Hawaii (6)
+1
0
-3
Fair
+22
+22
+23
Fair
3.2
5.2
Hilo, Hawaii (6)
+1
0
-1
Good
+40
+40
+39
Good
5.5
6.8

Remarks: The forecast values for the OND, NDJ, and DJF seasons (Table 1 above) indicate that sea levels for most of the stations in the north Pacific are likely to record further rise in the forthcoming seasons. The maxima will also be higher than normal during the same time period. This will be due to prevailing stronger-than-average westerly wind anomalies in the vicinity of western and central tropical Pacific. The Hawaiian stations, on the other hand, are likely to be close to normal during the same time period. The forecasted values are supportive of moderate La Niña conditions, which have recently strengthened in August, 2010.

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 JAS 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 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 (OND), 1 (NDJ), and 2 (DJF) month leads based on SSTs of JAS 2010.

 

(ii) Observed monthly sea level deviation in July-August-September (JAS) 2010 Season

The monthly time series (July-September 2010) 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
+6.6
+6.6
+6.4
(+3.3)
+22.3
+19.2
+22.2
(+3.4)
Malakal, Palau
+10.6
+10.5
+7.8
(+4.1)
+47.5
+47.5
+44.7
(+4.3)
Yap, FSM
+6.4
+6.4
+6.4
(+4.4)
+32.2
+34.9
+34.9
(+4.0)
Chuuk, FSM **
*
*
*
(*)
*
*
*
(*)
Pohnpei, FSM
+4.1
+3.6
*
(+2.8)
+32.1
+31.4
*
(+3.3)
Kapingamarangi, FSM
*
*
*
(+2.4)
*
*
*
(+2.6)
Majuro, RMI
+2.4
+3.6
*
(+2.3)
+42.0
+45.4
*
(+3.0)
Kwajalein, RMI
+3.9
+3.8
+4.2
(+2.2)
+41.8
+42.9
+44.6
(+2.8)
Pago Pago, AS
+6.6
+5.5
+5.6
(+2.8)
+30.9
+29.7
+22.2
(+3.3)
Honolulu, Hawaii
+2.8
+2.7
+3.6
(+1.9)
+23.9
+22.9
+22.2
(+2.3)
Hilo, Hawaii
+1.7
+2.7
+3.9
(+1.8)
+23.8
+25.1
+26.4
(+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: As compared to August 2010, the monthly mean sea-level in September 2010 didn’t change much. It remained mostly steady in most of the stations. However, Malakal recorded a fall after three months of recorded high sea level. The monthly maxima also displayed similar trends (mostly steady) in most stations, except Guam where it recorded a rise. Again in Palau, the maximum sea level recorded a fall.

The current trend of sea level, which is 2-6 inches higher than normal in the vicinity of the western Pacific, is supportive of an ongoing La Niña condition. According to CPC, La Niña is expected to strengthen and last through Northern Hemisphere winter 2010-11. Enhanced low-level easterly trade winds and anomalous upper-level westerly winds continued over the western and central equatorial Pacific. Collectively, these oceanic and atmospheric anomalies reflect the development and strengthening of La Niña conditions.

 

(iii) Forecast Verification (Seasonal Mean) for JAS 2010

Observed and forecast seasonal sea level values for the JAS 2010 season are presented in Figure 4. Forecasts were in general skillful; however, Malakal and Pago Pago were under forecasted.

(iv) Tide Predictions (October 1 to December 31, 2010)

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, 2010 for six stations [(a) Marianas, Guam (b) Kwajalein, RMI (c) Pago Pago, American Samoa (d) Honolulu, Hawaii and (e) Hilo, Hawaii are provided below

 

Figure 5 (below): Predicted water level for the OND 2010 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.

 

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



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Web Master's email: peac@noaa.gov
Page Last Modified: August 04 2011 22:20:01 GMT

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