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Pacific ENSO Update

4th Quarter, 2012 Vol. 18 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 2012-2013, (ii) the observed monthly mean and maximum sea-level deviations for the season JAS 2012, and (iii) a Synopsis of ENSO and seasonal sea level variability. 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 2012-2013

Forecasts of the sea-level deviations in the USAPI are presented using CCA statistical model. Based on the independent SST values in JAS 2012, the resulting CCA model has been used to forecast the sea-level of three consecutive seasons: OND, NDJ, and DJF (see table 1). 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 borderline ENSO-neutral/ weak El Niño conditions, the sea level in these islands are projected to be closer to normal with some positive deviations. 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
+1
+1
+1
Very Good
+18
+18
+18
Good
6.5
9.1
Malakal, Palau
+2
+1
+1
Very Good
+38
+36
+36
Very Good
6.1
6.4
Yap, FSM
+1
+1
+1
Very Good
+29
+28
+28
Very Good
8.2
11.0
Chuuk, FSM**
+1
+1
+1
N/A
+29
+28
+28
N/A
N/A
N/A
Pohnpei, FSM
+2
+2
+2
Very Good
+33
+33
+33
Very Good
9.1
11.8
Kapingamarangi, FSM
+1
+1
+2
Good
+27
+28
+28
Fair
5.7
6.4
Majuro, RMI
+1
+1
+1
Very Good
+42
+41
+42
Very Good
6.6
8.4
Kwajalein, RMI
+1
+1
+1
Very Good
+38
+38
+38
Very Good
4.9
6.1
Pago Pago, AS
+2
+2
+2
Good
+26
+26
+26
Good
3.0
3.7
Honolulu, Hawaii
+1
0
-1
Fair
+20
+21
+20
Fair
3.2
5.2
Hilo, Hawaii
+2
+1
0
Fair
+24
+25
+24
Fair
5.5
6.8

Remarks: The forecasts values of sea level for OND, NDJ, and DJF seasons (Table 1, above) indicate that most of the stations in the north Pacific region are likely to be marginally (e.g., 1-2 inches) higher than normal in the forthcoming seasons. Other south Pacific station (e.g., Pago Pago) is likely to be 2-3 inches higher than normal during the same time period. Here in Hawaii, both Honolulu and Hilo are likely to be closer to normal during the same time period.

Falling sea levels in the USAPI region during the recent months are supportive to the on-going borderline ENSO-neutral/ weak El Niño conditions, as according to CPC-IRI’s ENSO Alert System Status, it is El Niño Watch now and chances for borderline ENSO-neutral/ weak El Niño conditions are expected to continue into Northern Hemisphere winter 2012-13, possibly strengthening during the next few 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 6.5 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) 2012 Season

The monthly time series (July-September 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) or here.

 

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
+9.2
+8.2
+8.6
3.3
+25
+25
+22
3.4
Malakal, Palau
+10.3
+11.2
+9.3
4.1
+43
+46
+46
4.3
Yap, FSM
+7.8
+7.4
+6.7
4.4
+34
+34
+31
4.0
Chuuk, FSM
*
*
*
*
*
*
*
*
Pohnpei, FSM
+8.4
+6.1
*
2.8
+38
+33
*
3.3
Kapingamarangi, FSM
*
*
*
2.4
*
*
*
2.6
Majuro, RMI
+6.0
+5.2
*
2.3
+45
+45
*
3.0
Kwajalein, RMI
+7.4
+6.4
+5.9
2.2
+44
+42
+42
2.8
Pago Pago, AS
+7.9
+6.8
+7.8
2.8
+32
+29
+29
3.3
Honolulu, Hawaii
-0.5
-0.5
+1.8
1.9
+19
+18
+16
2.3
Hilo, Hawaii
-1.0
-2.5
+1.5
1.8
+23
+21
+16
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 2012, the monthly mean sea level in September 2012 shows fall in all stations, except Guam and Pago Pago—where slight rise was recorded. Guam recorded +0.4 inches and Pago Pago registered +1.0 inches further rise. A synopsis of last 6-months sea level variability is as follows: In May, most of the stations recorded slight fall except Guam and Pohnpei; In June, all stations recorded slight fall except Malakal and Kwajalein; In July, all stations again recorded rise; In August, all stations recorded fall except Malakal at Palau; Currently, all USAPI stations are 6 to 9 inches higher than normal; The monthly maxima remained static; no considerable rise or fall was observed.

 

 

(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

JAS 12 (Borderline ENSO neutral)

JAS 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

+7

-6

+7
-7

+8

Malakal, Palau

+10

+10

-9

+8
-7

+9

Yap, FSM

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

Pohnpei, FSM

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

Majuro, RMI

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

Pago Pago

+8

+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. The objective is to provide an insight to the readers’ about the strength of on-going ENSO and the trend of rising sea level. Observations revealed that despite ENSO-neutral (JAS12) and weak La Niña (JAS11) conditions the sea level rise in these two years is even higher than the historically strong La Niña (OND98) year. This is an indication of trend of rising sea level which could be, among others, due to enhanced trend wind in the region of western Pacific.

 

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

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

 

(v) Tide Predictions (November 1, 2012 to January 31, 2013)

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 November 1, 2012 to January 31, 2013 for four stations provided below.

 

Figure 5 (below): Predicted water level for the NDJ 2012-2013 season at (a) Marianas, Guam, (b) Pago Pago, American Samoa, (c) Honolulu, Hawaii, and (d) 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 American Samoa

b) Pago-Pago, American Samoa

 

Predicted Water Level Plot for Honolulu

c) Honolulu, Hawaii

 

Predicted Water Level Plot for Hilo

d) Hilo, Hawaii



Pacific ENSO Applications Climate (PEAC) Center
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Honolulu, HI 96822
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Web Master's email: peac@noaa.gov
Page Last Modified: January 15 2013 03:50:03 GMT

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