The role of wind gusts in upper ocean diurnal variability

Abstract

Upper ocean processes play a key role in air‐sea coupling, with variability on both short and long time scales. The diurnal cycle associated with diurnal solar insolation and nighttime cooling, may act, along with stochastic wind variability, on upper ocean temperatures and stratification resulting in a diurnal warm layer and a nonlinear rectified effect on longer time scales. This study describes diurnal changes in upper ocean temperature for a location in the equatorial Indian Ocean, using observations from the Dynamics of the Madden‐Julian Oscillation field campaign, a high vertical resolution 1‐D process model, and a diurnal cycling scheme. Solar forcing is the main driver of diurnal variability in upper ocean temperature and stratification. Yet except during nighttime convection, winds with variability on the order of hours (here referred to as “wind gusts”) regulate how fast surface water is mixed to greater depths when daily mean winds are weak. Wind gusts are much stronger than diurnal winds. Even using stochastic wind gusts but no diurnal winds as input in a 1‐D process model yields an estimate of diurnal temperature that compares well with observations. A new version of the Large and Caron (2015) scheme (LC2015) provides an estimate of upper ocean diurnal temperature that is consistent with observations. LC2015 has the advantage of being suitable for implementation in a climate model, with the goal to improve SST estimates, hence the simulated heat flux at the air‐sea interface. Yet LC2015 is not very sensitive to the inclusion or omission of the high‐frequency component of the wind.

Integrated Observations of Global Surface Winds, Currents, and Waves: Requirements and Challenges for the Next Decade

“…Diurnal wind variability is most prominent along coastlines, where the landsea breeze circulation is driven by differential daytime warming of the land and ocean (Simpson, 1994), but the signatures of diurnal winds are detectable throughout the tropics (Dai and Deser, 1999). The diurnal cycle in the upper ocean is mainly forced by solar heating, yet diurnal and higher-frequency winds play an important role in regulating vertical mixing (e.g., Giglio et al, 2017), and air-sea fluxes of heat and gases. Highfrequency wind variability also impacts cross-shore exchanges (e.g., Hendrickson and MacMahan, 2009) and larvae transport (e.g., Fujimura et al, 2014).…”

Modeling the Near‐Surface Diurnal Cycle of Sea Surface Temperature in the Mediterranean Sea

“…In incorporating wave breaking into the TKE equation he improved the models' ability to replicate the diurnal cycle. However, other studies have not found this necessary (e.g., Giglio et al, ; Karagali et al, ). Our results show that we are likely underpredicting the magnitude of diurnal warming and to include wave breaking effects would further reduce the diurnal magnitude.…”

“…Similarly, a better characterization of wind bursts (cf. Giglio et al, ) would likely reduce diurnal magnitudes as the skin SST can be very sensitive to sharp wind bursts which generate vertical mixing, breaking down the buildup of near‐surface warming (e.g., Stuart‐Menteth et al, ). Another limitation of the current model setup is that we do not include freshwater fluxes.…”

“…GOTM has previously been tailored for modeling diurnal variability (Pimentel et al, ), and this is the version of GOTM which we improve further for this current study. More recently, GOTM has also been used by Karagali et al () and Giglio et al () to successfully reproduce diurnal variability of SST at isolated sites with in situ moorings.…”

Large-eddy simulation of gusty wind turbulence over a travelling wave

“…Additionally, the wind gustiness may affect the upper ocean temperature (Giglio et al. 2017) and the fluctuations in the electricity power output generated by wind turbines (Stevens & Meneveau 2017).…”