Publications

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2001
Oey, LY, Wang DP, Hayward T, Winant C, Hendershott M.  2001.  "Upwelling" and "cyclonic" regimes of the near-surface circulation in the Santa Barbara Channel. Journal of Geophysical Research-Oceans. 106:9213-9222.   10.1029/1999jc000129   AbstractWebsite

The observed near-surface circulation in the Santa Barbara Channel indicates in particular two patterns: a dominant cyclonic circulation mode and a less frequent upwelling flow mode. To explain the dynamics that may govern these two flow regimes, momentum balance from a hindcast model of currents in the channel, forced by observed hourly winds and hydrographic data, was calculated. The along-channel balance was found to be between wind, which was eastward (i.e., equatorward), sea level tilt, which was westward (i.e., poleward), and Coriolis, which was westward if the wind was (1) intense west and east of the channel and was eastward if the wind was (2) weaker in the east. Wind condition 1 produced southward cross-channel flow in the midchannel, connected by eastward currents upstream (downstream) along the northern (southern) coast of the channel, while wind condition 2 produced northward cross-channel flow connected by cyclonic recirculation in the west and westward inflow in the east. It is suggested that the former corresponds to the dynamical balance that may occur in the upwelling flow mode, while the latter corresponds to the cyclonic circulation mode.

1999
Winant, CD, Alden DJ, Dever EP, Edwards KA, Hendershott MC.  1999.  Near-surface trajectories off central and southern California. Journal of Geophysical Research-Oceans. 104:15713-15726.   10.1029/1999jc900083   AbstractWebsite

The near-surface circulation in the Santa Barbara Channel and off the coast of central and southern California is described based on 20 releases of drifters drogued 1 m beneath the surface from 12 sites within the channel at bimonthly intervals. This description includes small-scale features of the circulation which are not part of descriptions based on moored observations or of the statistics of the drifter releases. The eventual fate of drifters at long time intervals compared to the residence time in the channel (about 7 days) is also included. In the channel the trajectories document a persistent cyclonic circulation with a typical recirculation period between 3 and 5 days. In the spring, currents near the mainland are weaker than near the Channel Islands, and the overall flow is toward the southeast. Trajectories document the possibility for water parcels to leave the channel through the interisland passes. In the late fall and winter a poleward flow with velocities often exceeding 0.5 m s(-1) is confined within 20 km of the mainland. Between these two seasons the cyclonic tendency is enhanced, although most of the drifters eventually migrate westward. The trajectories of drifters released at the same time from sites only 20 km apart can be remarkably different. Once the drifters migrate out of the channel, their trajectories can be grouped into a few patterns. In spring and summer, drifters tend to remain in the Southern California Eight. Their trajectories often remain close over extended periods, as if they were caught in convergence zones. In fall the drifters often are caught in a poleward current.

Auad, G, Hendershott MC, Winant CD.  1999.  Mass and heat balances in the Santa Barbara Channel: estimation, description and forcing. Progress in Oceanography. 43:111-155.   10.1016/s0079-6611(99)00006-3   AbstractWebsite

Current meter, temperature and wind observations from the 1984 MMS experiment are used to estimate the mass and heat budgets in the Santa Barbara Channel. The mass transports estimated at the western, eastern and southern boundaries of the channel are characterized by fluctuations whose energy is concentrated around three different periods: 5, 14 and 2.8 days respectively. These three transports fluctuate along with the dominant EOF modes obtained at those 3 entrances respectively. The mean transport passing through the channel from east to west is about 0.28 Sv. There rue two frequency bands where winds and mass transports are coherent: 2.5-3.0 and 4.7-5.2 day bands. Winds on the northern shelf lead the transports in both bands by about 1.0 day. At the western half of the channel there is a recirculating (counterclockwise) mean transport of about 0.30 Sv. The time dependent part of the recirculating transport is coherent with the wind in the 4.7-5.2 day band where it also shows an absolute maximum of variance. nle recirculating transport lags the local downwelling-favorable winds by about 1.5 day and stems to be the channel response to wind relaxations with respect to its most persistent upwelling-favorable state. The main mean balance in the channel-integrated heat equation is between the heat transport passing through the western mouth, which cools off the channel, and the heat transport caused by the mass transport (the transport heat Bur), which warms: up the channel. This latter transport results from the advection of the temperature difference between the channel boundaries (mainly east and west) by the mass transport. There are no two terms that dominate the heat equation for the time dependent heat transports, but it can be simplified by balancing the along channel heat divergence (heat transport passing through the mouth plus transport heat flux), the vertical heat flux and the local change of heat. A clear thermal-wind balance at the eastern and western ends of the channel is found, in agreement with the work of Brink and Muench (1986) [Journal of Geophysical Research, 91, 877-895] and with the recent ADCP-CTD comparisons done by Richards (personal communication). All the terms in the heat equation show a variance peak in the 2-4 day band. It is found that when upwelling favorable winds blow on the northern shelf of the channel there is first a decrease or even a reversal, with respect to its mean, in the amplitude of thr transport heat flux. Next, a cooling of the Santa Barbara Channel is observed, followed by a loss of heat through the western mouth first and through the eastern mouth later, This whole process takes about 24 h to complete. (C) 1999 Elsevier Science Ltd, All rights reserved.

1997
Cerovecki, I, Orlic M, Hendershott MC.  1997.  Adriatic seiche decay and energy loss to the Mediterranean. Deep-Sea Research Part I-Oceanographic Research Papers. 44:2007-2029.   10.1016/s0967-0637(97)00056-3   AbstractWebsite

A salient feature of sea level records from the Adriatic Sea is the frequent occurrence of energetic seiches of period about 21 h. Once excited by a sudden wind event, such seiches often persist for days. They lose energy either to friction within the Adriatic, or by radiation through Otranto Strait into the Mediterranean. The free decay time of the dominant (lowest mode) seiche was determined from envelopes of bandpassed sea level residuals from three locations (Bakar, Split and Dubrovnik) along the Croatian coast during twelve seiche episodes between 1963 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 decay 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 both bottom friction and radiation was included in a one dimensional variable cross section shallow water model of the Adriatic. Bottom friction is parameterized by the coefficient k appearing in the linearized bottom 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 obtained from linearization of the quadratic bottom stress law using estimates of near bottom currents associated with the seiche, with wind driven 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 allowing the Adriatic to radiate into an unbounded half plane ocean. Repeated 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 estimation of a. The principle conclusions of this work are as follows. (1) Exponential decay of seiche amplitude with time does not necessarily guarantee that the observed decay is free of wind influence. (2) Winds blowing across the Adriatic may be of comparable importance to winds blowing along the Adriatic in influencing apparent decay of seiches; across-basin winds are probably coupled to the longitudinal seiche on account of the strong along-basin variability of across-basin winds forced 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 (1988) approximation to the quadratic bottom friction law rho(0)C(D)u\u\ using the commonly accepted drag coefficient C-D = 0.0015 and quantitative estimates of bottom currents associated with wind driven currents, tides and wind waves, as well as with the seiche itself with no radiation 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 per period. But independent estimates of bottom friction for Adriatic wind driven currents and inertial oscillations, as well as comparisons between quadratic law bottom stress and directly measured bottom stress, all suggest that the quadratic law with C-D = 0.0015 substantially underestimates the bottom stress. Based on these studies, a more appropriate value of the drag coefficient is at least C-D = 0.003. In this case, bottom friction with no radiation leads to a damping time of 4.73 d; radiation sufficient to give the observed damping time then accounts for 32% of the energy loss per period. (C) 1998 Elsevier Science Ltd. All rights reserved.