Physics of Diurnal Warm Layers

Upper ocean heat content changes daily due to radiative forcing from the sun that creates a thin diurnal warm layer extending several meters beneath the sea surface. The temperature of this layer can vary by several degrees C in the tropics. While sea surface temperature controls the instantaneous air–sea turbulent heat flux, upper ocean heat content provides a diurnally varying heat reservoir that regulates the phase between solar heat flux and the diurnal SST cycle. The thickness and average temperature of this reservoir, which constitutes the DWL, depend on net surface heating, wind speed, and subsurface ocean mixing.

Recent measurements in the equatorial Indian Ocean (DYNAMO) and the tropical western Pacific (PISTON) using unique home-brewed instrumentation have revealed new aspects of diurnal warm layers. Aspects shown in the simulation below have been confirmed from detailed measurements on our surface-following platform, SurfOtter. Here, near-surface shear is induced by a diurnal warm layer. An otherwise well mixed layer is forced by a weak wind of constant direction and magnitude. The bulk velocity oscillates at the Coriolis frequency (2-day period, 14°N). The addition of periodic penetrating solar radiation and heat loss induces near-surface stratification and, consequently, shear because the now-warmed layer traps the momentum input from wind.

Evolution of turbulence in the diurnal warm layer, J. Phys. Oceanogr.48(2), 383-396, doi:, 2018 (A.J. Moulin, J.N. Moum and E.L. Shroyer).

Wind limits on rain layers and diurnal warm layers, J. Geophys. Res., 124, 897– 924, doi:, 2018 (E.J. Thompson, J.N. Moum, S.A. Rutledge and C.W. Fairall).

Evolution of the velocity structure in the diurnal warm layer, J. Phys. Oceanogr.50(3), 615-631, doi:, 2020 (K.G. Hughes, J.N. Moum and E.L Shroyer).

Heat transport through diurnal warm layers, J. Phys. Oceanogr.50(10), 2885-2905, doi:, 2020 (K.G. Hughes, J.N. Moum and E.L Shroyer).