Generation of Harmonics by Sea Breeze in Nontidal Water Bodies

Abstract

The sea breeze introduces harmonics of its diurnal cycle into water bodies, which can be notable for strong sea breeze and microtidal and nontidal conditions. The harmonics can be of the same magnitude as those produced by bottom friction and dominate those produced by nonlinear terms in the equations of motion. A closed-form analytic solution of the linearized depth-averaged equations of motion including friction is discussed for the situation of a sea breeze blowing on an idealized one-dimensional basin of constant depth. The solution reveals the generation of odd harmonics introduced by the quadratic wind stress and role of (linearized) friction. Sea-breeze forcing on the idealized basin is numerically modeled, and agreement with the analytic solution is found. The numerical model is then run with quadratic bottom friction and nonlinear terms to compare relative contributions to the generation of harmonics. Harmonics of the water motion are distinguished as forced, or arising from the wind forcing, and as response, or arising from the interactions within the water. The hydrodynamics of Baffin Bay, Texas, are modeled and spatial variation and relative strength of the harmonics investigated. Baffin Bay is a large shallow embayment with a weak connection to the Gulf of Mexico that experiences a strong southeast wind and sea breeze during the summer. The wind induces even and odd forced harmonics through the combined quasi-steady southeast wind and sea breeze. At Baffin Bay, ratios of the semidiurnal to diurnal amplitudes of water level and current speed are found to be comparable to M4/M2 ratios for U.S. Atlantic coast embayments.