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Nat. Hazards Earth Syst. Sci., 12, 1607–1620, 2012
www.nat-hazards-earth-syst-sci.net/12/1607/2012/
doi:10.5194/nhess-12-1607-2012
© Author(s) 2012. CC Attribution 3.0 License.

Natural Hazards
and Earth
System Sciences

A study of meteorologically and seismically induced water level and
water temperature oscillations in an estuary located on the west
coast of India (Arabian Sea)
P.Mehra, R. G. Prabhudesai, A. Joseph, V. Kumar, Y. Agarvadekar, R. Luis, and B. Viegas
National Institute of Oceanography, Council of Scientific and Industrial Research (CSIR), Goa, India
Correspondence to: P. Mehra (pmehra@nio.org)
Received: 19 August 2010 – Revised: 12 May 2011 – Accepted: 11 January 2012 – Published: 21 May 2012

Abstract. The study examines the observed storm-generatedsea-level oscillations (June 2007 and November 2009) along
with the Sumatra geophysical tsunami (September 2007), indicating similarities in the sea-level response in the Mandovi estuary of Goa in the eastern Arabian Sea. Sea-level
and surface meteorological measurements collected during
storms exhibit strong synoptic disturbances leading to the
coherent oscillations in the estuary with significantenergy
bands centred at periods of 24, 45, and 80 min. In particular, during the sporadic atmospheric event of June 2007, the
atmospheric pressure dipped by ∼12 mb, the wind direction
stabilized to ∼249◦ with peak wind speed up to 16 m s−1 and
the positive sea-level surge swelled up by ∼40 cm. Also, the
water temperature cooled down by ∼4.5 ◦ C. Approximately
3 days prior to the 12September 2007 Sumatra earthquake,
the water temperature at Verem station started exhibiting a
distinctly stronger semidiurnal oscillation (with a relatively
larger variance of ∼17.9 ◦ C2 in contrast to a lesser variance
of ∼12 ◦ C2 during the preceding normal days) and these
well-defined oscillations continued to manifest for a week
after the earthquake. The pre-earthquake enhanced seawatertemperature oscillations observed at this tropical estuary provides an indication that routine monitoring of seawater temperature from tropical estuaries with fine temporal resolution may provide early information about impending coastal
earthquakes.

1

Introduction

The open ocean long waves generated in response to major forcing such as undersea seismic activity, atmospheric
disturbances and soforth, and approaching the coast occasionally, generate large amplitude seiches in certain regions.

Although these waves may have different sources and, therefore, different characteristics, the local topography may modify them in similar ways, thereby producing similar oscillations and sometimes even destructive consequences. The meteotsunamis are mainly associated with atmospheric gravitywaves, pressure jumps, frontal passages, squalls and other
types of atmospheric disturbances, which normally generate barotropic ocean waves in the open ocean and amplify
them near the coast through specific resonance mechanisms
(Proudman, Greenspan, shelf, harbour). Therefore, the observed sea-level oscillations near the coast are a combined
effect of one or more external forcing andtopographic influence. The periods of such sea-level oscillations may range
from minutes to hours. The synoptic atmospheric perturbations, such as atmospheric pressure or wind patterns, generally have characteristic spatial scales of a few hundred kilometres and may have time periods larger than a few hours.
The spatial and temporal scales normally associated with the
large-scale synoptic perturbations(500 km, >1 day) and the
observed high-frequency sea-level variability (50 km, minutes) are normally not well matched. Therefore, synoptic
atmospheric systems are not likely to excite directly highfrequency sea-level oscillations even if realistically they can
be considered to be quite broadband (both in frequency and
wave number), as they usually do not contain much energy
at the scales and...