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Pacific ENSO Update

4th Quarter, 2011 Vol. 17 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 2011, (ii) the observed monthly mean and maximum sea-level deviations for the season JAS 2011, and (iii) forecast verifications for JAS 2011 (observed/forecast values) and discussion. Note that the deviations are defined 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’. Also note that the CCA-forecasting technique adapted here 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 2011-2012

Forecasts of the sea-level deviations in the USAPI are presented using CCA statistical model. Based on the independent SST values in JAS 2011, the resulting CCA model has been used to forecast the sea-level of three consecutive months: OND, DNJ, and DJF (see Table 1: left panel shows values for seasonal mean while the right panel shows the seasonal maxima). The forecast values of sea-level for OND, NDJ, and DJF displays a positive deviation in the vicinity of north and south Pacific Islands. All stations are likely to record higher than normal sea level during this time period. CCA cross-validated results show that forecasts are very skillful (at 0 to 2-months lead time) for the three consecutive seasons (Table 1: bottom panel). The just ended 2010-11 La Nina condition has returned and the atmospheric component of this event is active. Consistent with this La Nina impact (i.e., enhanced trade winds), the sea level in these islands is already higher than normal. It may rise further in the months to come. 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

 
Seasonal Mean Deviations1
Seasonal Maximum Deviations2
Tide Gauge Station
OND
NDJ
DJF

Forecast Quality3

OND
NDJ
DJF
Forecast Quality3
RP for OND Season4
Lead Time5
0
1M
2M
0
1M
2M
20 yr
100 yr
Marianas, Guam
+4
+4
+4
Very Good
+21
+20
+20
Good
6.5
9.1
Malakal, Palau
+4
+3
+3
Very Good
+40
+39
+39
Very Good
6.1
6.4
Yap, FSM
+3
+2
+2
Very Good
+31
+30
+29
Very Good
8.2
11.0
Chuuk, FSM**
+3
+2
+2
N/A
+30
+30
+30
N/A
N/A
N/A
Pohnpei, FSM
+4
+4
+4
Very Good
+36
+36
+36
Very Good
9.1
11.8
Kapingamarangi, FSM
+3
+3
+4
Good
+30
+31
+32
Poor
5.7
6.4
Majuro, RMI
+4
+3
+3
Very Good
+44
+44
+45
Very Good
6.6
8.4
Kwajalein, RMI
+3
+2
+2
Very Good
+41
+41
+41
Very Good
4.9
6.1
Pago Pago, AS
+4
+3
+3
Good
+27
+27
+28
Good
3.0
3.7
Honolulu, Hawaii
+1
+1
0
Fair
+21
+21
+20
Fair
3.2
5.2
Hilo, Hawaii
+2
+1
-1
Fair
+24
+25
+23
Fair
5.5
6.8

Remarks: As compared to the previous seasons, the forecasts values of sea level for OND, NDJ, and DJF seasons (2011-12) (Table 1: above) indicate a positive trend (3-5 inches rise) in the months to come. Currently all of these stations show about 2-9 inches higher-than-normal sea level.

Despite an elevated sea level of about 3-5 inches higher than normal in the forthcoming seasons, the forecasts clearly indicate that a further rise is also possible. This rising trend is supportive to on-going La Niña condition. According to CPC, La Niña conditions are expected to gradually strengthen and continue through the Northern Hemisphere winter 2011-12.

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 6.3 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 6.5 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 2010.

 

(ii) Observed monthly sea level deviation in July-August-September (JAS) 2011 Season

The monthly time series (July-September 2011) 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

Monthly Mean Deviations1
Monthly Maximum Deviations2
Jul
Aug
Sep
SD
Jul
Aug
Sep
SD
Marianas, Guam
*
+7.6
+6.2
3.3
*
+23
+24
3.4
Malakal, Palau
*
+9.0
+11.0
4.1
+44
+46
+48
4.3
Yap, FSM
+6.1
+7.2
+8.9
4.4
+31
+37
+37
4.0
Chuuk, FSM
*
*
*
*
*
*
*
*
Pohnpei, FSM
+6.9
+5.9
*
2.8
+36
+33
*
3.3
Kapingamarangi, FSM
*
*
*
2.4
*
*
*
2.6
Majuro, RMI
+4.5
+4.1
*
2.3
+43
+45
*
3.0
Kwajalein, RMI
+5.2
+4.2
+2.9
2.2
+43
+42
+41
2.8
Pago Pago, AS
+8.7
*
+9.9
2.8
+30
+33
+35
3.3
Honolulu, Hawaii
+2.1
+2.2
+1.3
1.9
+22
+20
+18
2.3
Hilo, Hawaii
+1.0
+2.8
*
1.8
+25
+26
*
2.4

* Data currently unavailable
1 Difference between the mean sea level for the given month and the 1975 through 1995 mean sea level value at each station
2 Same as 1 except for maxima
SD stands for standard deviations

Remarks: As compared to August 2011, the monthly mean sea level in September 2011 shows rise in some stations (Malakal, Pago Pago) and fall in other stations (Guam, Kwajalein, and Honolulu). It may be mentioned that during the month of June 2011, sea level in most of the stations recorded fall. In July, sea level of a few stations recorded rise, and in August sea level of all stations (except Kwajalein) recorded rise. Again in September, some stations recorded rise and others recorded fall. Currently, the sea level at all stations and in the North Pacific are 2-9 inches higher than normal. The monthly maxima also displayed similar trends. These trends are supportive to on-going La Niña condition.

 

 

(iii) Forecast Verification (Seasonal Mean) for JAS 2011

Observed and forecast seasonal sea level values for the JAS 2011 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, 2011)

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, 2011 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 2010 season at (a) Marianas, Guam (b) Kwajalein, RMI (c) Pago Pago, American Samoa, (d) Honolulu, Hawaii (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

f) Chuuk, FSM



Pacific ENSO Applications Climate (PEAC) Center
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Web Master's email: peac@noaa.gov
Page Last Modified: December 08 2011 02:28:18 GMT

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