Diurnal cycling: Observations and models of the upper ocean response to diurnal heating, cooling, and wind mixing

Abstract

Measurements made from R/P Flip using rapid profiling conductivity, temperature, and depth probes and vector‐measuring current meters provide a new and detailed look at the diurnal cycle of the upper ocean. A diurnal cycle occurs when solar heating warms and stabilizes the upper ocean. This limits the downward penetration of turbulent wind mixing so that air‐sea fluxes of heat and momentum are surface trapped during midday. The central problem is to learn how the trapping depth D T (mean depth value of the diurnal temperature and velocity response) is set by the competing effects of wind mixing and surface heating. In this data set the diurnal range of surface temperature was observed to vary from , with most of the day‐to‐day variability attributable to variations of wind stress τ. Wind mixing causes a pronounced asymmetry of the T s response by limiting the warming phase to only about half of the period that the surface heat flux Q is positive. The associated wind‐driven current, the diurnal jet, has an amplitude of typically , with no obvious dependence upon τ. The diurnal jet accelerates downwind during the morning and midday. It is turned into the wind by the Coriolis force during early evening and is often erased by the following morning. Under the assumption that wind mixing occurs as an adjustment to shear flow stability, a scaling analysis and a numerical model study show that the daily minimum trapping depth goes like τ/Q1/2. It follows that goes like Q3/2/τ and that goes like Q1/2. These results, as well as the simulated time dependence of the diurnal cycle, are at least roughly consistent with the observations. The observed time‐averaged velocity profile has a spiral shape reminiscent of the classical Ekman spiral. However, its structure is a consequence of diurnal cycling, and its parameter dependence is in some ways just opposite that of the Ekman model; e.g., increased wind stress may cause decreased vertical shear between fixed levels in the upper ocean.