The Coupling between Harbor Seiches at Palawan Island and Sulu Sea Internal Solitons*

Abstract

Between 1989 and 1991 the authors made observations that confirm and elucidate the coupling between harbor seiches at Puerto Princesa on Palawan Island in the Philippines and tide-generated internal solitons in the Sulu Sea. Published tidal current predictions for Basilan Strait in the Sulu Archipeligo were used to produce an index to the daily ebb tidal flows near Pearl Bank where the solitons originate. The coherence between predicted maximum ebb tidal current speed and observed seiche activity was 0.60 and the phase lag between the two quantities closely matched published estimates of the time required for solitons to cross the Sulu Sea. Arrival of the Sulu Sea waves immediately offshore of Puerto Princesa in the form of internal bores corresponded in time to the initiation of harbor seiche activity. A more precise estimate of soliton travel time was determined from the time difference between predicted maximum ebb current and the initiation of seiche activity, and it was found to have a remarkably regular seasonal pattern, which can be largely explained by seasonal variations in Sulu Sea temperature and salinity stratification. Therefore, the timing of seiche events at Puerto Princesa can be forecast based on Sulu Archipeligo tide predictions and soliton travel time. In general, the fortnightly pattern of seiche magnitude follows that of tidal current predictions, but day-to-day deviations are common. Seiche magnitude varies seasonally with generally low activity during the winter (northeast) monsoon and usually maximum activity late in the summer (southwest) monsoon.

Continuous seiche in bays and harbors

“…Since tidally forced shelf modes are a plausible driver of seiches, we expect that tidal amplitude variance should be reflected in seiche amplitudes, a view consistent with the strong fortnightly modulation of seiche amplitude reported by Giese et al (1990) and Wijeratne et al (2010). To examine such a dependence, Fig.…”

“…An interesting feature is that while all stations exhibit near perfect correlation at times, they also episodically transition to near zero or statistically insignificant correlation. This suggests that the fortnightly modulation of tidally driven shelf resonances is also influenced by other factors, of which internal tide variability has previously been noted by Giese et al (1990) and Wijeratne et al (2010).…”

“…Internal waves are known to influence seiches as demonstrated by Giese et al (1990) who analyzed a 10-year time series of 6 min data at Magueyes Island, Puerto Rico, noting distinct seasonal and fortnightly distributions of shelf resonance and seiche amplitude, suggesting that stratification and its influence on internal waves generated by barotropic tides are important components of the observed seiche variability. Subsequent work led Giese and Chapman (1993) to conclude that, in locations where strong internal waves propagate to coastal regions, seiche sustenance is possible, and to question "Is this a general phenomenon to be expected wherever large coastal seiches are found, or is it specific to certain locations where large internal waves are known to occur?"…”

“…Golmen et al (1994) studied a coastal embayment near the headland Stad in northwest Norway and identified permanent seiching superimposed on the semidiurnal tide, concluding that tidal forcing was the only possible energy source of the observed oscillations. Continuing their exploration of internal waves and seiching, Giese et al (1998) examined harbor seiches at Puerto Princesa in the Philippines and found that periods of enhanced seiche activity are produced by internal bores generated by arrival of internal wave soliton packets from the Sulu Sea. However, as one would expect from soliton excitation, their analysis suggests that these seiches are not continually present.…”

Development of internal solitary waves in various thermocline regimes - a multi-modal approach

“…Internal solitary waves may show a remarkable longevity, like in the Sulu Sea , where they travel distances of over 450 km, until they reach the coast of Palawan Island, where they still are energetical enough to create considerable seiches in Puerto Princesa harbor (Giese et al, 1998). In this case a uni-modal approach seems appropriate: the solitary waves are well described by the first mode alone, which here has its maximum near the thermocline, giving the mode an interfacial character.…”