Pacific ENSO Update

1st Quarter, 2008 Vol. 14 No. 1

SEASONAL SEA LEVEL OUTLOOKS
for the U.S-Affiliated Pacific Islands

tide station location

The following sections describe: (i) the Canonical Correlation Analysis (CCA)-based forecasts for sea level deviations for the forthcoming seasons, (ii) tide predictions for JFM (January 1 - March 31, 2008), (iii) the observed monthly sea level deviations from the previous season (OND 2007), and (iv) seasonal extremes for JFM at 20- and 100-year return periods. All units are in inches. 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’. (See Figure 2 at right for location of these stations.) Also, note that the forecasting technique adapted here does not account for sea level deviations created by other atmospheric or geological conditions such as tropical cyclones, storm surges or tsunamis.

(i) Seasonal Sea Level Forecasts for January-February-March, February-March-April, and March-April-May 2008

Forecasts of the sea-level deviations in the USAPI are presented using CCA statistical model. Based on the independent SST values in OND 2007, the resulting CCA model has been used to forecast the sea-level of three consecutive month periods: JFM, FMA, and MAM 2008 (see Table 1). CCA cross-validation forecast skills for 0, 1, and 2-month leads are presented in Fig. 3.

Table 1 : Forecasts of sea level deviation in inches for forthcoming seasons (JFM, FMA, and MAM 2008)

Tide Gauge Station
JFM
FMA
MAM

Forecast Quality1

Note: (-) indicates negative deviations (fall of sea level from the mean), and (+) indicates positive deviations (rise of sea level from the mean).

N/A: data not available. Deviations of +/-1 in. are considered negligible and denoted by **. Deviations +/- 2 in. are unlikely to cause any adverse climatic impact.

Lead Time2
0
1M
2M
.
Guam
+5
+6
+6
Good
Palau
+ 3
+ 3
+ 3
Good
Yap
+ 2
+ 3
+ 3
Very Good
Pohnpei
+ 5
+ 5
+ 4
Very Good
Kapingamarangi
+ 5
+ 5
+ 4
Good
Majuro
+ 4
+ 3
+ 2
Good
Kwajalein
+ 3
+ 4
+ 4
Good
Pago Pago
+ 4
+ 5
+ 5
Good

1. 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.

2. The 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 (JFM), 1 (FMA), and 2 (MAM) month leads based on SSTs of Oct-Nov-Dec 2007.

With a mean skill greater than 0.72 to 0.56 (at 0 to 2-months lead time) in all three consecutive seasons, all tide gauge stations are very well predicted. The sea level in the USAPI began to rise from January 2007 and levels currently remain above average. This rising trend is consistent with on-going La Niña conditions, which is expected to continue for another three months. Likewise, we expect the sea level in the vicinity of USAPI may remain elevated (i.e. 2 to 5 inches) for another two to three months. Based on current atmospheric and oceanic interactions, no further sea level rise is expected now (other than any marginal variations). The sea level in the vicinity of the USAPIs are sensitive to the ENSO cycle, with falling sea level observed during El Niño years and rising sea level during La Niña years.

(ii) Tide Predictions (January 1 to March 31, 2008)

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 January 1 to March 31, 2008 for three major stations (i) Marianas, Guam (ii) Kwajalein, RMI and (iii) Pago Pago, American Samoa are provided below. Observations reveal that the MR, SR, and ML for all these above stations are likely to remain within the range of average highest seasonal values during the next three months.

Figure 5 (below): Predicted water level for the OND season 2007 at (a) Marianas, Guam (b) Kwajalein, RMI and (c) Pago Pago, American Samoa. 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.

 

 

(iii) Observed monthly sea level deviation in October-November-December (OND), 2007

The monthly time series (October-December) 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 October, November and December 2007, with year to year standard deviations.

Tide Gauge

October
November

December

Standard
Deviations

 
Marianas, Guam
Max:
+23.0
+21.0
n/a
--
Mean:
+4.9
+7.0
+5.5
+4.0
Saipan, CNMI
Max:
n/a
n/a
n/a
--
Mean:
n/a
n/a
n/a
n/a
Malakal, Palau
Max:
+42.0
+42.0
+39.0
--
Mean:
+7.4
+6.6
+4.0
+4.0
Yap, FSM
Max:
+38.0
+36.0
+35.0
--
Mean:
+8.9
+7.5
+5.5
+4.1
Chuuk, FSM **
Max:
n/a
n/a
n/a
--
Mean:
n/a
+8.0
+8.0
n/a
Pohnpei, FSM
Max:
+42.0
+40.0
n/a
--
Mean:
+4.8
+9.8
n/a
+4.7
Kapingamarangi, FSM
Max:
+29.0
+39.0
+31.0
--
Mean:
+2.0
+6.1
+4.1
+3.1
Majuro, RMI
Max:
+45.0
+45.0
+45.0
--
Mean:
+4.3
+4.0
n/a
+3.7
Kwajalein, RMI
Max:
+44.0
+42.0
+38.0
--
Mean:
+5.9
+4.3
+3.5
+3.3
Pago Pago, AS
Max:
+29.0
+30.0
+26.0
--
Mean:
+4.5
+4.8
+2.0
+2.2

** Sea level data for Chuuk is based on estimates from neighboring tide stations (Yap and Pohnpei) and observations from WSO Chuuk.

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; SD: Year-to-year standard deviations for the month.

'Max' is defined here as 'the observed difference between the monthly maximum and the climatological monthly mean values (from the period 1975- 1995) computed at each station'. Likewise, 'mean' is defined as 'the observed difference between the monthly mean and the climatological monthly mean values (from the period 1975- 1995) computed at each station'.

Table 2 (above) provides the monthly observed sea level deviations (in inches). Consistent with the on-going La Niña conditions, a positive deviation has been observed in all the stations since January 2007. Compared to November 2007, a slight fall has been observed in December 2007. Observed maximum values are considerably high for some of the stations. Due to technical problems, sea level data for Saipan was not available for the OND season.

(iv) Seasonal Extremes for JFM at 20- and 100-year return periods.

The Generalized Extreme Value (GEV) products define the thresholds beyond the seasonal tidal range that have low but finite probabilities of being exceeded on a seasonal scale. Results of the GEV analysis for the season JFM is presented in Table 3 (below). The extreme values are calculated from the 1-hourly sea-level data. The upper limit (at 90% confidence interval) of rise has been shown in the right side of the column, the left side is the estimated rise based on observations. For example, the predicted rise of 4.6 ~ 6.3 inches at Marianas (*) indicates that this station may experience sea level rise of 4.6 ~ 6.3 inches during JFM within any 20 year period (20 year RP). Likewise, about once every 100 years we can expect the highest JFM tide at Marianas to be as much as 5.4 ~ 7.9 inches above normal (100 year RP).

Table 3: Seasonal Extremes (GEV) for JFM
Sea Level Rise (inches)
Station
20 year RP
100 year RP
Marianas, Guam
4.6 ~ 6.3*
5.4 ~ 7.9*
Saipan, CNMI
3.8 ~ 7.4
4.9 ~ 11.3
Malakal, Palau
6.7 ~ 11.8
9.6 ~ 18.9
Yap, FSM
8.3 ~ 24.6
11.8 ~ 54.9
Pohnpei, FSM
4.2 ~ 7.0
4.9 ~ 9.6
Kapingamarangi, FSM
4.4 ~ 9.6
5.3 ~ 13.8
Majuro, RMI
2.8 ~ 4.8
3.5 ~ 6.4
Kwajalein, RMI
3.8 ~ 5.1
4.8 ~ 6.9
Pago Pago, American Samoa
2.7 ~ 5.2
3.5 ~ 7.7
Note: Upper boundaries of rise are calculated at the 90% confidence interval. RP stands for Return Period. Bootstrap methods with 5000 iterations were used to estimate these upper limit values of sea level rise.

Note that Yap displays the highest GEV deviations in JFM; These high values are due to large and significant increases in the tidal range during the passage of past storm events in the JFM season.

Click here to view probability of exceedence graphs for the JFM season.

Chowdhury M. R , P-S Chu, and Schroeder T (2007): ENSO and Seasonal Sea-level Variability – A Diagnostic Discussion for the U.S-Affiliated Pacific Islands, Theoretical and Applied Climatology, 88: 213-224 (March 2007), Springer-Verlag Wien.