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Pacific ENSO Update

3rd Quarter, 2012 Vol. 18 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 of sea-level deviations for forthcoming seasons JAS, ASO, and SON of 2012, (ii) the observed monthly mean and maximum sea-level deviations for the season AMJ 2012, (iii) a synopsis of sea level rise and enhanced trade wind with suggestions for further reading on the subject, (iv) forecast verifications for AMJ 2012 (observed/forecast values), and (v) tide predictions. 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 JAS, ASO, and SON of 2012

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 2012, the resulting CCA model has been used to forecast the sea level of three consecutive three-month periods: JAS, ASO, and SON of 2012 (see Table 1). CCA cross-validation forecast skills for 0, 1, and 2-month leads are presented in Fig. 3.

All the tide gauge stations (at 0 to 2-months lead time) show skillful forecasts for these three consecutive seasons. Consistent with the on-going La Niña event, the sea levels in these islands are higher than normal.

Table 1 : Forecasts of MEAN and MAXIMUM sea level deviation in inches for forthcoming seasons

 
Seasonal Mean Deviations1
Seasonal Maximum Deviations2
Tide Gauge Station
JAS
ASO
SON

Forecast Quality3

JAS
ASO
SON
Forecast Quality3
RP for JAS Season4
Lead Time5
0
1M
2M
0
1M
2M
20 yr
100 yr
Marianas, Guam
+6
+5
+3
Good
+23
+21
+20
Good
6.3
10.9
Malakal, Palau
+6
+6
+5
Very Good
+42
+42
+41
Very Good
8.1
10.2
Yap, FSM
+6
+5
+4
Very Good
+32
+32
+32
Very Good
8.4
11.3
Chuuk, FSM**
+6
+5
+4
N/A
+32
+32
+32
N/A
N/A
N/A
Pohnpei, FSM
+2
+2
+3
Very Good
+30
+31
+32
Very Good
5.8
7.0
Kapingamarangi, FSM
+1
+1
+2
Good
+25
+26
+27
Fair
3.5
4.2
Majuro, RMI
+1
+2
+3
Good
+41
+42
+43
Fair
5.2
6.8
Kwajalein, RMI
+2
+2
+2
Good
+40
+40
+40
Fair
4.1
5.2
Pago Pago, AS
+2
+2
+2
Very Good
+26
+26
+26
Good
4.1
5.4
Honolulu, Hawaii
+2
+2
+2
Fair
+21
+20
+20
Fair
3.4
5.7
Hilo, Hawaii
+2
+2
+2
Good
+24
+24
+23
Fair
6.4
7.7

Remarks: The forecasts values of sea level for JAS, ASO, and SON seasons indicate that some of the stations (i.e., Guam, Malakal, and Yap) are likely to be about 4-6 inches higher than normal in the forthcoming seasons. Other stations Pohnpei, Majuro, Kwajalein, and Pago Pago are likely to be 1-3 inches higher than normal during the same time period. Here in Hawaii, both Honolulu and Hilo are likely to be 2 inches higher than normal.

Falling sea levels in some of the north Pacific Islands are supportive to the on-going ENSO conditions. According to CPC-IRI’s ENSO Alert System Status, there is an El Niño watch in place and the chance for an El Niño beginning in JAS 2012 is high. Currently the SST warming in the NINO 3.4 area appears to be consistent with a weakening of the low-level trade winds across the east-central equatorial Pacific. The observations are consistent with ENSO-neutral, but reflect a likely progression towards El Niño. This is the reason for which we see a trend of falling sea levels recently. This trend may continue in the coming months.

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 6.3 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 2012.

 

(ii) Observed monthly sea level deviation in AMJ 2012 Season

The monthly time series (AMJ 2012) 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, and June, with year to year standard deviations (SD).

Tide Gauge Station

Monthly Mean Deviations 1
Monthly Maximum Deviations 2
Apr
May
Jun
SD
Apr
May
Jun
SD
Marianas, Guam
+8.7
+9.0
+8.4
3.7
+24
+24
+24
3.9
Malakal, Palau
+5.3
+5.7
+9.5
4.0
+42
+42
+44
3.8
Yap, FSM
+5.2
+8.5
+7.7
3.4
+32
+37
+34
4.0
Chuuk, FSM
*
*
*
*
*
*
*
*
Pohnpei, FSM
+9.2
+8.1
*
2.2
+40
+37
*
2.7
Kapingamarangi, FSM
*
*
*
2.8
*
*
*
3.1
Majuro, RMI
+8.0
+7.0
*
1.8
+48
+48
*
2.9
Kwajalein, RMI
+8.5
+5.7
+6.9
2.3
+48
+44
+42
2.7
Pago Pago, AS
+10.0
+8.9
+6.4
3.7
+33
+35
+32
4.2
Honolulu, Hawaii
0.0
0.0
0.0
1.8
+16
+18
+20
1.9
Hilo, Hawaii
0.0
+2.0
-1.5
2.0
+18
+25
+24
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: A synopsis of the sea-level variability for the last six months (February-July 2012) is as follows: 1) In February, all stations recorded slight rise except Palau; 2) In March, all stations recorded moderate rise; 3) In April, most of the stations recorded fall except Pago Pago, which recorded slight rise; 4) In May, most of the stations recorded slight fall except Guam and Pohnpei; 5) In June, all stations recorded slight fall except Malakal and Kwajalein; and 6) Currently (in July), all north pacific stations are 5 to 10 inches higher than normal.

 

(iii) ENSO and Seasonal Sea Level Variability: A Synopsis

Table 3: Sea-Level Deviation in Current and Major ENSO Years

Seasonal Mean Deviations: Observed rise/fall (inches)

Seasons

AMJ 12(ENSO neutral-El Nino watch)

AMJ 11 (Moderate-to-weak La Nina)

JFM98 (Strong El Nino)

JFM99 (Strong La Nina)

OND 97 (Strong El Nino)

OND 98 (Strong La Nina)

Marianas, Guam

+9

+8

-6

+7
-7

+8

Malakal, Palau

+6

+3

-9

+8
-7

+9

Yap, FSM

+7
+2
-7
+6
-9
+7

Pohnpei, FSM

+9
+7
-5
+4
-10
+8

Majuro, RMI

+7
+4
-2
+2
-9
+6
Kwajalein, RMI
+7
+4
-4
+3
-7
+3

Pago Pago

+9

+9

-6

+4
+2

+7

Remarks: As the sea level in the USAPI is very sensitive to the phase of the ENSO climate cycle, a perspective of sea-level anomalies during the recent ENSO event (2011-12) and the historically strongest ENSO events of 1997-99 is presented in table 3, to the left. The objective is to provide context to the readers about the relative strength of the current phase of ENSO. Note that 1997 was a strong El Nino year and 1998 was a strong La Nina year.

 

 

(iv) Forecast Verification (Seasonal Mean) for AMJ 2012

Observed and forecast seasonal sea level values for the AMJ 2012 season are presented in Figure 4.

 

(v) Tide Predictions (July 2 to September 30, 2012)

NOAA's web site for tide and currents has been used to generate the water level plot for the next four months. Predicted water level plots from July 2 to September 30, 2012 for five 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 JAS 2012 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



Pacific ENSO Applications Climate (PEAC) Center
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Honolulu, HI 96822
(808) 956-2324

Web Master's email: peac@noaa.gov
Page Last Modified: December 12 2012 23:18:26 GMT

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