Gravitational circulation and its response to the variation in river discharge in the Seto Inland Sea, Japan

Abstract

[1] In this study gravitational circulation and its response to the variation in river discharge have been investigated in the Seto Inland Sea, Japan. Monthly hydrographic surveys have been conducted from 2002 to 2006 in Hiuchi-Nada, a basin located between two straits. Horizontal salinity gradients along the channel varied as the 9/10 power of the river discharge into one end of the basin. Significant volume fluxes which were proportional to the horizontal salinity gradients were found. The relationship between river discharge (R m 3 s À1 ) and the volume flux (Q m 3 s À1 ) was represented as Q = 13.9R 0.88 . When river discharge increased, vertical density gradients were intensified and then along-channel horizontal density gradients were gradually enhanced due to vertical mixing at the western strait. These gradients drove gravitational circulations along the channel. The gravitational circulations were not uniform across the channel and accompanied by horizontal circulations. In the northern part of the basin, low-salinity water at the eastern end of the basin was directed toward the west and high-salinity water was directed toward the east. The eastward flows arising from the intrusion of relatively heavy water at the western strait extended from the bottom to the surface. In the southern part, low-salinity water was directed toward the west. The westward flows arising from the intrusion of relatively light water at the eastern strait concentrated in the middle layer and formed a plume-like structure, accompanied by the eastward flow under the plume. These situations were responsible for the development of the bulk exchange flux.

Observation and Modeling of Seawater Exchange in a Strait-Basin System in the Seto Inland Sea, Japan

“…8g and h) is in general agreement with the results from a previous numerical simulation that incorporated realistic bottom topography and the observed density structure in Hiuchi Nada [12]. In the numerical simulation with realistic topography [12], the straits and associated density field were not located on opposite sides of the model domain at the same latitude (as in our simulation), which resulted in asymmetric residual flows that induced a significant flux of strait outflow into the upper layer of the basin. Although the large differences in topography between real sea and model domain, the facts that the residual currents from the strait to the basin were obtained in the numerical simulation, suggested that tides and the horizontal density gradients between the strait and the basin play essential role in driving those residual currents.…”

“…The horizontal circulation shown in the numerical simulation ( Fig. 8g and h) is in general agreement with the results from a previous numerical simulation that incorporated realistic bottom topography and the observed density structure in Hiuchi Nada [12]. In the numerical simulation with realistic topography [12], the straits and associated density field were not located on opposite sides of the model domain at the same latitude (as in our simulation), which resulted in asymmetric residual flows that induced a significant flux of strait outflow into the upper layer of the basin.…”

“…Moreover the volume fluxes associated with the surface outflow from Bisan Seto toward Hiuchi Nada (Fig. 1a) have been studied using the results from the observations and numerical model [12]. Those from Bisan seto toward the northern part of Harima Nada have also been studied using a ship-mounted Acoustic Doppler Current Profiler (ADCP) [13].…”

Impact of record flooding of a subtropical river on estuary/ocean exchange

“…Net seaward flows develop throughout the lower reaches of those estuaries with the largest annual river pulses. In contrast, the Vietnamese Ba Lat River (van Maren and Hoekstra 2004) and the Japanese Seto Inland Sea (Kobayashi et al 2010) show the opposite behavior, with largest gravitational exchanges occurring with increased river inputs. Development of two-layer gravitational exchange or unidirectional seaward flow in response to river pulses might be diagnosed with the Estuarine Richardson Number R E (Fischer et al 1979):…”

Drivers of Barotropic and Baroclinic Exchange through an Estuarine Navigation Channel in the Mississippi River Delta Plain

“…No substantial reversals in this pattern occurred during the period of data collection at Cocodrie and thus, mode 2 appears to be gravitationally-driven estuarine exchange flow, and this exchange flow responded strongly to variations in the longitudinal salinity gradient. The dependence of gravitational circulation on the longitudinal density gradient has been described analytically by [2], and numerous observational studies have documented this response, where enhanced density gradients drove stronger gravitational circulation [42] or even reversed it in the case of inverse estuaries where estuarine salinities exceeded those in shelf waters [43][44][45][46][47].…”