I. Cerovecki et al., ADRIATIC SEICHE DECAY AND ENERGY-LOSS TO THE MEDITERRANEAN, Deep-sea research. Part 1. Oceanographic research papers, 44(12), 1997, pp. 2007-2029
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.