Influences of Tidal Fronts on Coastal Winds Over an Inland Sea

Abstract

A regional numerical model of the atmosphere was applied to an inland sea, the Seto Inland Sea in Japan, to study the influence of sea-surface temperature (SST) variations, accompanied by a tidal front, on the coastal winds in summer when tidal fronts fully develop. After confirmation of the model performance, two sensitivity simulations, which used spatially uniform SST with the highest and lowest values over the study area, were performed. The control and sensitivity simulations show that the mean wind speeds were apparently reduced by the low SST and the SST gradient accompanying the tidal front. The comparison of the terms in the momentum equations in control and sensitivity simulations indicates that the change of the perturbation pressure gradient force with the SST gradient is the most important factor in the modification of near-surface winds with SST variations. When the air flows across a tidal front, the air cools over the low SST area and warms over the high SST area. Consequently, the surface perturbation pressure increases over the low SST area and decreases over the high SST area. This adjustment in surface perturbation pressure produces an additional pressure gradient force with direction from the low SST area to the high SST area that decelerates the surface wind in the area upwind of the tidal front and accelerates the surface wind downwind of the tidal front.

Oceanic fronts and jets around Japan: a review

“…Among thermal fronts with different spatial scales (Yanagi 1987), tidal fronts are one of the smallest types of oceanic fronts that can influence the overlying PBL structure (Shi et al 2011). The surface wind speed reduces when the air travels from the warm to cooler water and vice versa, which is mainly because of the perturbation pressure gradient force with the SST gradient and not the stratification control on the turbulent vertical mixing of wind momentum (Shi et al 2011).…”

“…The surface wind speed reduces when the air travels from the warm to cooler water and vice versa, which is mainly because of the perturbation pressure gradient force with the SST gradient and not the stratification control on the turbulent vertical mixing of wind momentum (Shi et al 2011). …”

“…The SSH gradient, flow speed, transport, depth, and EKE are based on the general knowledge of tidal fronts found in the Seto Inland Sea from observations and numerical models (e.g., Takeoka 2002;Chang et al 2009). Heat fluxes are based on numerical simulations (Chang et al 2009;Shi et al 2011) for a limited period; thus, their interannual variability is not provided.…”

Fortnightly atmospheric tides forced by spring and neap tides in coastal waters

“…Recent studies using satellite microwave remote sensing data have shown that surface winds are positively correlated with SST (i.e., stronger winds occur above warmer water) in open oceans, such as the tropics 1 2 , the Gulf Stream 3 4 5 , the Kuroshio 6 , the Kuroshio Extension 7 , and the northwestern Indian Ocean 8 , partly as a result of vertical momentum mixing with intense winds at high altitudes 9 . However, few studies have investigated the modification of surface winds by SST in coastal waters 10 11 . Studies of coastal waters may be limited by contamination of microwave remote sensing observations by surrounding land, coarse spatial resolution and a lack of in situ wind measurements above coastal waters.…”

Ship observations and numerical simulation of the marine atmospheric boundary layer over the spring oceanic front in the northwestern South China Sea

“…The dynamical response of the MABL to an SST anomaly usually leads to the increased surface winds over warm water and decreased surface winds over cold water, termed “positive SST‐wind coupling.” There are two main hypotheses to explain the adjustment of sea surface wind to the sharp SST gradients associated with oceanic fronts: the generation of hydrostatic pressure gradients through adjustments of the MABL mass fields [ Lindzen and Nigam , ; Wai and Stage , ; Small et al ., ; Song et al ., ] and the stability‐dependent modification of the vertical mixing of momentum from aloft to the surface [ Wallace et al ., ; Hayes et al ., ; Tokinaga et al ., ; Minobe et al ., ]. In some cases, both these mechanisms are active in the positive SST‐wind coupling over oceanic fronts and eddies [ Tanimoto et al ., ; Takatama et al ., ; Shi et al ., ; Chelton , ; Frenger et al ., ].…”