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Pacific ENSO Update

2nd Quarter, 2012 Vol. 18 No. 2


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 AMJ, MJJ, and JJA of 2012, (ii) the observed monthly mean and maximum sea-level deviations for the season JFM 2012, and (iii) a Synopsis of ENSO and seasonal sea-level variability . (The forecast verifications (observed/forecast values) for JFM 2012 can be found online at http://www.prh.noaa.gov/peac/sea-level.php) 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.

 

(i) Seasonal sea-level forecast (deviations with respect to climatology) for AMJ, MJJ, and JJA of 2012

Forecasts of the sea-level deviations in the USAPI (tide gauge station locations) are presented using a CCA statistical model. Based on the independent SST values in JFM 2012, the resulting CCA model has been used to forecast the sea-level of three consecutive months: AMJ, MJJ, and JJA (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 La Niña event, the sea level in these islands are higher than normal. 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
AMJ
MJJ
JJA

(3) Forecast Quality3

AMJ
MJJ
JJA
Forecast Quality3
RP for AMJ Season4
Lead Time5
0
1M
2M
0
1M
2M
20 yr
100 yr
Marianas, Guam
+7
+7
+7
Very Good
+21
+23
+23
Very Good
5.6
6.7
Malakal, Palau
+8
+8
+8
Very Good
+41
+43
+43
Good
9.6
14.3
Yap, FSM
+8
+8
+8
Very Good
+32
+34
+34
Good
16.7
33.0
Chuuk, FSM**
+8
+8
+8
N/A
+32
+33
+33
N/A
N/A
N/A
Pohnpei, FSM
+6
+5
+5
Very Good
+35
+35
+34
Very Good
5.8
7.1
Kapingamarangi, FSM
+1
0
-1
Good
+26
+27
+27
Fair
7.4
9.4
Majuro, RMI
+3
+2
+2
Good
+42
+41
+41
Fair
4.1
5.1
Kwajalein, RMI
+5
+5
+4
Good
+41
+41
+41
Good
4.5
5.9
Pago Pago, AS
+2
+1
+1
Very Good
+25
+25
+25
Very Good
3.9
5.4
Honolulu, Hawaii
0
+1
-1
Poor
+17
+16
+16
Poor
4.1
5.9
Hilo, Hawaii
0
+1
+1
Good
+22
+20
+20
Fair
7.9
11.4

Remarks: The forecast values of sea level for AMJ, MJJ, and JJA seasons indicate that sea levels for some of the stations (i.e., Guam, Malakal, Yap, and Pohnpei) are likely to be about 5-8 inches higher than normal in the forthcoming seasons. Kwajalein, at RMI, is also expected to be 5 inches higher than normal. However, stations like Majuro and American Samoa (Pago Pago) are expected to be marginally higher than normal in these seasons. Here in Hawaii, both Honolulu and Hilo are likely to remain normal.

Higher than average sea levels in some of the north Pacific Islands are slightly contradictory to the on-going La Niña condition, as according to CPC-IRI’s ENSO Alert System Status, La Niña is likely to turn to ENSO-neutral condition soon. However, significant anomalous low-level westerly winds developed in the western tropical Pacific in late March, associated with the MJO. Presently, the larger scale atmospheric circulation anomalies and the Southern Oscillation Index still retain the characteristics of La Niña. This is the reason for which we still see a trend of higher sea levels in the forthcoming seasons.

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 5.6 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 5.6 inches above the median of seasonal maxima during the AMJ season. Likewise, about once every 100 years we can expect the highest AMJ tide at Marianas, Guam to be as high as 6.7 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 AMJ 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 (AMJ), 1 (MJJ), and 2 (JJA) month leads based on SSTs of JFM 2012.

 

(ii) Observed monthly sea level deviation in the JFM 2012 Season

The monthly time series 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 can be found here.

 

Table 2: Monthly observed MEAN and MAX sea level deviations in inches for January, February, and March, with year to year standard deviations (SD).

Tide Gauge Station

Monthly Mean Deviations 1
Monthly Maximum Deviations 2
Jan
Feb
Mar
SD
Jan
Feb
Mar
SD
Marianas, Guam
+5.8
+8.4
+10.0
4.2
+21
+22
+25
3.8
Malakal, Palau
+5.9
+5.5
+6.7
4.8
+40
+40
+45
4.8
Yap, FSM
+3.3
+3.8
+5.2
4.0
+30
+30
+33
4.4
Chuuk, FSM
*
*
*
*
*
*
*
*
Pohnpei, FSM
+7.8
*
*
2.6
+37
*
*
3.2
Kapingamarangi, FSM
*
*
*
2.4
*
*
*
4.1
Majuro, RMI
+5.7
+5.8
*
3.3
+45
+45
*
2.5
Kwajalein, RMI
+5.9
+7.6
+9.2
3.7
+44
+46
+47
2.8
Pago Pago, AS
+9.3
+9.4
+8.5
2.8
+31
+31
+32
3.0
Honolulu, Hawaii
-2.0
-1.0
0.0
1.7
+16
+16
+16
2.6
Hilo, Hawaii
-1.0
-1.2
-3.0
2.1
+21
+22
+16
3.0

Notes: * data not available. Standard deviations describe how widely spread the values are in the dataset. See Table 1 for other footnotes.

Remarks: As compared to February 2012, the monthly mean sea level in March 2012 shows sight rise in all stations except Pago Pago. A synopsis of last 6-months sea level variability is as follows:
1) In October, most of the stations recorded a marginal fall while a few stations recorded marginal rise;
2) In November, most of the stations recorded fall;
3) In December all stations except Yap and Kwajalein recorded fall;
4) In January 2012, all stations recorded fall except Guam and Kwajalein;
5) In February, all stations recorded slight rise except Palau;
6) In March, all stations recorded moderate rise;
7) Currently, all stations are 3 to 10 inches higher than normal.

The monthly maxima also displayed similar trend.

 

(iii) Forecast Verification (Seasonal Mean) for JFM 2012

Observed and forecast seasonal sea level values for the JFM 2012 season are presented in Figure 4. Forecasts were in general skillful. However, few stations (e.g., Guam, Kwajalein and Pago Pago) are found to be under forecasted. The probable reason for this under forecasts is the recent trend of enhanced trade winds, which is not well correlated to the corresponding SSTs.

 

(iv) ENSO and Seasonal Sea level Variability: A Synopsis

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), as well as the ENSO event of 1998-99, is presented in Table 3. Data for the season OND is also presented here. Note that 1997 was a major El Nino (strong) year and 1998 was a major (strong) La Nina year.

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

 

Seasonal Mean Deviations: Observed rise/fall (inches)

Seasons
JFM12 (Weak La Nina transition)
JFM11 (Moderate-to-weak La Nina)
JFM98 (Strong El Nino)
JFM99 (Strong La Nina)
OND97 (Strong El Nino)
OND98 (Strong La Nina)
Marianas,
Guam
+8
+5
-6
+7
-7
+8
Malakal,
Palau
+6
+9
-9
+8
-7
+9
Yap,
FSM
+4
+5
-7
+6
-9
+7
Pohnpei,
FSM
+8
+7
-5
+4
-10
+8
Majuro,
RMI
+6
+7
-2
+2
-9
+6
Kwajalein,
RMI
+7
+4
-4
+3
-7
+3
Pago Pago,
AS
+9
+10
-6
+4
+2
+7
 

(v) Tide Predictions (April 1 to June 30, 2012)

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 April 1 to June 30, 2012 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: 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
c/o NOAA NWS - Weather Forecast Office Honolulu
2525 Correa Road, suite 250
Honolulu, HI 96822
(808) 956-2324

Web Master's email: peac@noaa.gov
Page Last Modified: June 09 2012 00:01:02 GMT

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