ADRIATIC SEICHE DECAY AND ENERGY-LOSS TO THE MEDITERRANEAN

A salient feature of sea level records from the Adriatic Sea is the fr equent occurrence of energetic seiches of period about 21 h. Once exci ted by a sudden wind event, such seiches often persist for days. They lose energy either to friction within the Adriatic, or by radiation th rough Otranto Strait into the Mediterranean. The free decay time of th e dominant (lowest mode) seiche was determined from envelopes of bandp assed sea level residuals from three locations (Bakar, Split and Dubro vnik) along the Croatian coast during twelve seiche episodes between 1 963 and 1986 by taking into consideration only time intervals when the envelopes decreased exponentially in time, when the modelled effects of along-basin winds were smaller than the error of estimation of deca y time from the envelopes and when across-basin winds were small. The free decay time thus obtained was 3.2+/-0.5 d. This value is consonant with the observed width of the spectral peak. The decay caused by bot h bottom friction and radiation was included in a one dimensional vari able cross section shallow water model of the Adriatic. Bottom frictio n is parameterized by the coefficient k appearing in the linearized bo ttom stress term k rho(0)u (where u is the along-basin velocity and eo the fluid density). The coefficient k is constrained by values obtain ed from linearization of the quadratic bottom stress law using estimat es of near bottom currents associated with the seiche, with wind drive n currents, with tides and with wind waves. Radiation is parameterized by the coefficient a appearing in the open strait boundary condition zeta = auh/c (where zeta is sea level, h is depth and c is phase speed ). This parameterization of radiation provides results comparable to a llowing the Adriatic to radiate into an unbounded half plane ocean. Re peated runs of the model delineate the dependence of model free seiche decay time on k and a, and these plus the estimates of k allow estima tion of a. The principle conclusions of this work are as follows. (1) Exponential decay of seiche amplitude with time does not necessarily g uarantee that the observed decay is free of wind influence. (2) Winds blowing across the Adriatic may be of comparable importance to winds b lowing along the Adriatic in influencing apparent decay of seiches; ac ross-basin winds are probably coupled to the longitudinal seiche on ac count of the strong along-basin variability of across-basin winds forc ed by Croatian coastal orography. (3) The free decay time of the 21.2 h Adriatic seiche is 3.2+/-0.5 d. (4) A one dimensional shallow water model of the seiche damped by bottom stress represented by Godin's (19 88) approximation to the quadratic bottom friction law rho(0)C(D)u\u\ using the commonly accepted drag coefficient C-D = 0.0015 and quantita tive estimates of bottom currents associated with wind driven currents , tides and wind waves, as well as with the seiche itself with no radi ation gives a damping time of 9.46 d; radiation sufficient to give the observed damping time must then account for 66% of the energy loss pe r period. But independent estimates of bottom friction for Adriatic wi nd driven currents and inertial oscillations, as well as comparisons b etween quadratic law bottom stress and directly measured bottom stress , all suggest that the quadratic law with C-D = 0.0015 substantially u nderestimates the bottom stress. Based on these studies, a more approp riate value of the drag coefficient is at least C-D = 0.003. In this c ase, bottom friction with no radiation leads to a damping time of 4.73 d; radiation sufficient to give the observed damping time then accoun ts for 32% of the energy loss per period. (C) 1998 Elsevier Science Lt d. All rights reserved.