A new method for estimating the turbulent heat flux at the bottom of the daily mixed layer

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Abstract
A new method is presented for estimating the vertical turbulent heat flux at the bottom of the daily mixed layer from the temperature data in the mixed layer and net solar irradiance data at the sea surface. We assume that fluctuations in the divergence of advective heat flux have longer than daily time scales. The method is applied to data from the eastern tropical Pacific, where the daily cycle in the temperature field is confined to the upper 10–25 m. The night‐to‐day difference of the turbulent heat flux calculated from the data obtained during nine daily cycles in November 1984 agrees well on average with the same quantity estimated from microstructure observations. The night‐to‐day difference of the turbulent heat flux, estimated at several mooring stations near the equator (an average over 100 to 300 daily cycles), varies from 120 to 220 W/m2 with larger values on the equator. Equatorial turbulence measurements show that the turbulent heat flux is much larger during nighttime than daytime. Therefore the present estimates give approximately the nighttime average, which is the major part of the turbulent heat flux. From the daytime heat budget we obtain divergence of the low‐frequency horizontal heat advection at 1°30′S, 140°W; it is governed by equatorial mesoscale fluctuations having a predominant period of 15–20 days.