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The Nearshore Heat Budget

  • Author(s): Sinnett, Gregory
  • Advisor(s): Feddersen, Falk
  • et al.
Abstract

The nearshore region (surfzone and adjacent inner-shelf) are biologically and economically important and are affected by temperature changes from a variety of unique nearshore processes. Three such processes are explored here, along with their affect on the nearshore heat budget. Surfzone incident wave energy flux is dissipated by wave breaking which through viscosity generates heat (termed wave heating). A primative surfzone heat budget (including wave heating) closes on diurnal and longer time scales. Solar radiation was the dominant term, and wave heating the second most variable with mean heating contribution roughly one fourth that of solar radiation.

Breaking waves also generate foam, creating an albedo-induced surfzone solar heating reduction. Surfzone albedo observations were elevated above open-ocean observations, with average albedo of 0.15 and one-minute averaged albedo as high as 0.45. New image- and wave- based surfzone albedo parameterizations were tested versus observations and have high skill (r2 = 0.90 and r2 = 0.68 respectively).

Nonlinear internal waves propagate across the inner-shelf, sometimes into the surfzone. The cross-shore evolution of nonlinear internal waves were tracked from 8-m depth to shore. Coherently propagating fronts had associated temperature drops up to 1.7◦C with frontal velocities between 1.4 and 7.4 cms−1. Front position was quadratic in time, decelerating in a manner consistent with gravity current scalings. Frontal temperature drop and equivalent two-layer height collapsed as a linearly decaying function of normalized cross-shore distance. During the rundown, near-surface cooling and near-bottom warming at 8-m depth coincide with a critical gradient Richardson number, indicating shear-driven mixing.

Together, waves act to both warm the surfzone (through wave heating) at a daily-averaged rate of ≈ 28 W m−2 and cool the surfzone (through albedo-induced reduction of solar heating) at a daily-averaged rate of ≈ 41 Wm−2. Both wave-heating and albedo-induced reduction in solar heating varied predictably with commonly observed nearshore parameters.

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