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Coastal Oscillations after the Passage of Hurricane Irene

By

Edwin Alfonso-Sosa, Ph. D.

24-FEB-2014

Summary

About 30 hours after the passage of Hurricane Irene over Puerto Rico, three tidal gauges located along

the Southwest Coast of Puerto Rico detected a sudden increase in coastal seiche amplitude of around

0.25 feet (7.6 cm). Rapid and small atmospheric pressure oscillations were observed simultaneously. The

distance and time lag between the maximum peak amplitudes suggest an edge wave trapped on the

shelf that was moving north at 36.7 MPH (16.4 m/s). These observations demonstrate that sudden

changes in water levels at the coast can occur one day after the hurricane event.

Observations

On the 22nd of August 2011, Hurricane Irene passed over the Island of Puerto Rico as a category 1 event.

At 1700 UTC its center was located north of the Mona Passage (Figure 1). Most of the island experienced

tropical storm force winds as shown in its wind history (Figure 2). In the southwest coast of Puerto Rico,

the Isla Magueye’s barometer measured the lowest atmospheric pressure (1003.8 mb) on August 22,

0842 UTC. Irene kept moving to the Northwest at a speed of 10.5 MPH. On August 23, 1200 UTC, was at

a distance of 278 miles NW from Bahia Salinas located in the southwest corner of Puerto Rico (Figure 3).

As Irene was moving NW, a smooth and steady rise in pressure was interrupted by sharp rapid

oscillations starting at 1424 UTC, almost 30 hours after Hurricane Irene passed Puerto Rico (Figure 4). In

18 minutes the atmospheric pressure increased 0.7 mb and 0.6 mb, at Magueyes and Mayaguez,

respectively. The small and rapid pressure fluctuations were observed between 1424 UTC and 1642 UTC.

Before the fluctuations, at Mona Passage, the strong flow of south winds across the passage decreased

from 40 knots (1051 UTC) down to 25 knots (1341 UTC) (Figure 5). Meanwhile at the coast, Mayaguez

and Magueyes were measuring moderate winds from the SE and SSE, 7-15 knots and 10-18 knots,

respectively. The rapid pressure oscillations started at 1424 UTC in Mayaguez Harbor, but 54 minutes

later (1518 UTC) the winds turn from southeast to southwest, to return back to southeast after 1.5

hours (1648 UTC).

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During the rapid pressure fluctuations period (between 1424 UTC and 1642 UTC), NOAA tide gauges

along the Southwest Coast of Puerto Rico measured 23-31 minutes sea level oscillations. These coastal

oscillations started at Bahia Salinas (1506 UTC) then followed at Puerto Real (1524 UTC) and ended on

Mayaguez Harbor (1536 UTC) (Figure 9). Bahia Salinas showed the largest oscillations and Mayaguez

Harbor the smallest. No oscillation was detected on Crash Boat (NW corner of Puerto Rico) and in Mona

Island. Magueyes Island showed a 48 minute oscillation. The Cabo Rojo Southwestern Insular Platform is

the widest and most important platform of Puerto Rico. The widest point is 16 miles from shore. Its

North-South extension from Punta Guanajibo down to Cabo Rojo’s Lighthouse is approximately 16 miles

(Figure 8). The distance separating Bahia Salinas and Puerto Real is about 10 miles, and from Puerto Real

to Mayaguez harbor there are another 10 miles. At Mayaguez Harbor the platform is much narrower.

Due to the nonlinear nature of the coastal oscillations we applied Empiral Mode Decomposition (EMD)

analysis to the water level record (Huang et al., 1998; Huang et. al., 1999, Alfonso-Sosa, 2010). This

method allowed us to extract the first Intrinsic Mode Function (IMF), corresponding to the nonlinear

coastal oscillation. The Normalized Hilbert-Huang Transform (NHHT) method allowed us to determine

changes in the Instantaneous Frequency (IF) (Huang et. al., 2009) of the coastal oscillations. The first IMF

and IF for each tide gauge station are shown in Figure 10. The three IMF plots show a sudden increase in

amplitude after 1424 UTC. The plot of IF for each station shows clearly changes in frequency, revealing

the true nonlinear nature of the signal. Figure 11 shows a zoom view of the previous Figure from 23-

Aug-2011 14:48 to 16:48 UTC, during the time of the large coastal oscillations. The largest amplitudes

were detected at Bahia Salinas (0.25 ft), Puerto Real (0.21 ft) and Mayaguez harbor (0.18 ft) and

occurred at: 1506, 1524 and 1536 UTC, respectively. The time lag suggests that the energy is moving

from South to North along the coast. An edge wave moving north with the coast at the right could be

responsible, but there is not additional information to support it. It is clear that the lack of coastal

oscillations in Mona Island and Aguadilla discard the possibility of a long barotropic ocean wave moving

north capable of generating a meteotsunami. This oscillation seems to be trapped in the shelf waters.

Using the distance between the stations and the time lag, it was possible to estimate the wave speed as

36.7 MPH (16.4 m/s) (See Table 1). This value is equivalent to a long wave speed in waters with a 26 m

depth. The coastal oscillation’s instantaneous frequencies (IF) at the time of the largest peak in

amplitude ranged between 46 and 62 CPD, averaging 55 CPD. Based on the dimensions of the platform

and water depth, the first harmonic of oscillation for Puerto Real is 55.9 CPD. Coastal seiches with the

same frequency have been observed previously in Bahia Salinas, Puerto Real and Mayaguez Harbor

(Alfonso-Sosa, 2012).

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References

Alfonso-Sosa, E., 2012. Primer reporte de un meteotsunami en la costa caribeña de Puerto Rico.

https://www.academia.edu/4951196/Primer_Reporte_de_un_Meteotsunami_en_la_Costa_Caribena_d

e_Puerto_Rico

Alfonso-Sosa, E., 2010. Análisis de la marea barotrópica en Isla Magueyes usando el método de

descomposición en modos empíricos. Contribución a la Colección de Ciencias Marinas, UPRM,

Mayagüez, Puerto Rico. 11 pp.

Huang, N. E., Wu, Z., Long, S. R., Arnold, K. C., Chen, X., Blank, K., 2009. On Instantaneous Frequency.

Advances in Adaptive Data Analysis, Vol.1, No. 2, 177-229.

Huang, N. E., Shen Z. and S. R. Long. 1999. A new view of nonlinear water waves: the Hilbert Spectrum,

Annual Review of Fluid Dynamics 31:417-457.

Huang, N. E., Shen, Z., and S. R. Long, M. C. Wu, H. H. Shih, Q. Zheng, N-C. Yen, C. C. Tung, and H. H. Liu.

1998. The empirical mode decomposition and the Hilbert spectrum for nonlinear and non-stationary

time series analysis, Proc. R. Soc. London Series A 454: 903-995.

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Captions

Figure 1. Hurricane Irene over Puerto Rico. The center is located north of the Mona Passage. The

southern quadrant of the hurricane shows only high clouds and lack of thunderstorms. Most of the

thunderstorm activity is located at the northeastern quadrant. MODIS image taken on August 22, 2011

at 1700 UTC.

Figure 2. Hurricane Irene’s Wind History (Advisory 1-13) up to 23-Aug-2011 15:00 UTC by the NWS NHC.

All the Southwest Coast of Puerto Rico was under tropical storm force winds.

Figure 3. Best Track of Hurricane Irene by NWS National Hurricane Center.

Figure 4. Atmospheric pressure measured at Mayaguez Harbor and Magueyes Island. Lowest value at

Magueyes Island (1003.8 mb) was recorded on 22-Aug-2011 08:42 UTC. The dashed rectangle indicates

the rapid fluctuations in atmospheric pressure. (Below) Zooming into the rectangle the record from 23-

Aug-2011 14:24 UTC to 16:48 UTC.

Figure 5. Wind field measurements by WINDSAT NRT satellite sensor on 23-Aug-2011 10:51 UTC.

(Below) Wind field measurements by ASCAT satellite sensor on 23-Aug-2011 13:41 UTC.

Figure 6. Wind speed (blue) and gust (red) measured at Mayaguez Harbor between 14:48 and 16:48

UTC on 23-Aug-2011.

Figure 7. Wind velocity measured at Mayaguez Harbor between 14:48 and 16:48 UTC on 23-Aug-2011.

The black arrow shows the time of wind shift from southeast to southwest.

Figure 8. The Puerto Rico Southwestern Platform and the location of tide gauges.

Figure 9. Water level measured (in feets) by the tide gauges between 14:48 and 16:48 UTC on 23-Aug-

2011 for the following locations: Bahia Salinas, Puerto Real and Mayaguez Harbor. The time stamp

indicates when the largest oscillation occurs.

Figure 10. The amplitude (in feet) of the first IMF of water level and below its instantaneous frequency

in cycles per day (CPD) for the following locations: Bahia Salinas, Puerto Real and Mayaguez Harbor. The

dashed rectangles indicate the larger water level oscillations.

Figure 11. Zooming the previous figure from 23-Aug-2011 14:48 to 16:48 UTC.

Table 1. IMF’s maximum amplitudes and their respective instantaneous frequencies. The time interval

between maxima and their respective distances allowed us to estimate the wave speed. Average speed

is 36.7 MPH (16.4 m/s).

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Figure 1

Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

Figure 7

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Figure 8

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Figure 9

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Figure 10

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Figure 11

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Table 1

Table 2- Water level data available at NOAA/NOS/CO-OPS: http://tidesandcurrents.noaa.gov/

Primary Tidal Stations Secondary Tidal Station

9759110 Magueyes Island, PR 9759189 Puerto Real, PR

9759394 Mayagüez, PR 9759197 Bahía Salinas, PR

9759938 Mona Island, PR