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Pacific ENSO Update

2nd Quarter, 2011 Vol. 17 No. 2


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 AMJ, MJJ, and JJA of 2011, (ii) the observed monthly mean and maximum sea-level deviations for the season JFM 2011, (iii) forecast verifications for JFM 2011 (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 AMJ, MJJ, and JJA of 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 JFM 2011, the resulting CCA model has been used to forecast the sea level of three consecutive three-month periods: AMJ, MJJ, and JJA of 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 Deviations1
(2) Seasonal Maximum Deviations2
Tide Gauge Station
AMJ
MJJ
JJA

(3) Forecast Quality3

AMJ
MJJ
JJA
(3) Forecast Quality3
(4) RP for AMJ Season4
(5) Lead Time5
0
1M
2M
0
1M
2M
20 yr
100 yr
Marianas, Guam
+7
+7
+7
Good
+22
+23
+23
Good
5.6
6.7
Malakal, Palau
+3
+3
+4
Good
+37
+37
+42
Good
9.6
14.3
Yap, FSM
+5
+5
+5
Good
+32
+32
+35
Good
16.7
33.0
Chuuk, FSM**
+5
+5
+5
N/A
+31
+32
+35
N/A
N/A
N/A
Pohnpei, FSM
+4
+2
+1
Very Good
+33
+32
+35
Good
5.8
7.1
Kapingamarangi, FSM
+3
+2
+2
Good
+29
+29
+30
Good
7.4
9.4
Majuro, RMI
+1
0
-1
Fair
+40
+38
+43
Fair
4.1
5.1
Kwajalein, RMI
+4
+3
+2
Good
+40
+39
+43
Fair
4.5
5.9
Pago Pago, AS
+5
+5
+4
Very Good
+28
+28
+30
Good
3.9
5.4
Honolulu, Hawaii
-1
0
+1
Fair
+20
+21
+16
Fair
4.1
5.9
Hilo, Hawaii
-1
0
+1
Fair
+21
+24
+19
Fair
7.9
11.4

Remarks: The forecasts values of sea level for AMJ, MJJ, and JJA seasons (Table 1, above) indicate that sea levels for most of the stations are likely to fall in the months to come. However, currently most of these stations show about 3-6 inches higher than normal sea level in the forthcoming seasons. As compared to other stations, Guam shows a higher rise of 6-7 inches while Majuro shows a lower rise 0-1 inches, or even fall in JJA season.

Despite an elevated sea level for about 3-6 inches higher than normal in the forthcoming seasons, the forecasts clearly indicate a fall in the same time horizon. This falling trend is supportive to on-going La Niña condition. According to CPC, a transition to ENSO-neutral conditions is expected by June 2011. La Nina weakened for the third consecutive month, as reflected by increasing surface and subsurface ocean temperatures across the equatorial Pacific Ocean. However, as observed, the atmospheric component is weakening slowly and, as a result, enhanced low-level easterly trade winds and anomalous upper-level westerly winds are still active over the equatorial Pacific. Collectively, these oceanic and atmospheric anomalies reflect a weakening La Niña.

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 2011.

 

(ii) Observed monthly sea level deviation in the JFM 2011 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 are shown in Figure 2 (top of page).

 

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
+5.6
*
*
4.2
+21
*
*
3.8
Malakal, Palau
+7.0
+8.1
+10.6
4.8
+42
+46
+49
4.8
Yap, FSM
+6.1
+5.5
+4.6
4.0
+34
+34
+33
4.4
Chuuk, FSM **
*
*
*
*
*
*
*
*
Pohnpei, FSM
+8.1
+6.5
*
2.6
+41
+35
*
3.2
Kapingamarangi, FSM
*
*
*
2.4
*
*
*
4.1
Majuro, RMI
+6.8
+9.5
*
3.3
+47
+54
*
2.5
Kwajalein, RMI
+3.0
+3.8
+5.3
3.7
+44
+45
+42
2.8
Pago Pago, AS
+10.0
+10.3
*
2.8
+37
+35
*
3.0
Honolulu, Hawaii
+1.0
-1.0
-2.0
1.7
+21
+17
+13
2.6
Hilo, Hawaii
+2.1
0.0
0.0
2.1
+27
+22
+18
3.0

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 footnotes.

Remarks: As compared to February 2011, the monthly mean sea-level in March 2011 recorded rise in Palau and Kwajalein, and recorded fall in Yap and Honolulu. There are several missing data; data for Guam is again unavailable now. Based on February data, it may be mentioned that Pohnpei recorded fall, Majuro recorded considerable rise, and Pago Pago remained stationary. The monthly maxima also displayed similar trends. Recent report by Hans (WSO, Pago Pago) stated that after the Tsunami of March 11, readings from the tide gauge indicates resonance for more than a day long. Resonance were also seen in CNMI, FSM.

 

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

Observed and forecast seasonal sea level values for the JFM 2010 season are presented in Figure 4. Because of missing data, we are not reporting for Guam in this issue of the newsletter. Forecasts were generally skillful. Pago Pago was slightly under forecasted.

(iv) Tide Predictions (January 1 to March 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 April 1 to June 30, 2011 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 JFM 2011 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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Web Master's email: peac@noaa.gov
Page Last Modified: August 04 2011 21:54:04 GMT

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