Relationship between subsurface diurnal warming and wind speed

Abstract

Observations and numerical simulations show an inverse relationship between sea surface diurnal warming and wind speed. However, the dependence of subsurface diurnal warming on wind speed remains largely unexplored. Here, we investigate the relationship between subsurface diurnal warming and wind speed using field observations, numerical simulations, and theoretical analysis. With increasing wind speed, subsurface diurnal warming decreases at the shallower depths where the daily maximum temperature occurs in the afternoon, while it first increases and then decreases at the deeper depths where the daily maximum temperature moves from afternoon to night, reaching a maximum when the daily maximum temperature occurs at about 18:00 local time (sunset). The relationship is mainly governed by the time-accumulated ocean heat gain from the air-sea heat flux (the difference between time-accumulated shortwave absorption and time-accumulated surface heat loss), which represents the downward heat transfer. Increasing wind speed increases surface heat loss and decreases the timing of the daily maximum temperature (t2h). For the shallower depths, both the decreased t2h and the increased surface heat loss reduce the accumulated ocean heat gain from the air-sea heat flux, i.e., the downward heat transfer, and thus reduce subsurface diurnal warming. For the deeper depths, the decreased t2h with increasing wind speed initially reduces the accumulated surface heat loss (the accumulated shortwave absorption almost the same) when t2h is during the night, causing the increase in subsurface diurnal warming, and subsequently reduces the accumulated net ocean heat gain along with the increased surface heat loss when t2h is during the afternoon, leading to the decrease in subsurface diurnal warming. This study extends our understanding of the dependence of diurnal warming on wind speed.