Spatial Distribution of Sound Scattering Layer and Density Estimation of Euphausia pacifica in the Center of the Yellow Sea Bottom Cold Water Determined by Hydroacoustic Surveying

The Yellow Sea Bottom Cold Water (YSBCW) refers to seawater with a water temperature of 10 °C or less found at the bottom of the center of the Yellow Sea. The spatiotemporal variability of the YSBCW directly affects the distribution of organisms in the marine ecosystem. In this study, hydroacoustic and net surveys were conducted in April (spring) to understand the spatial distribution of the sound scattering layer (SSL) and estimate the density of Euphausia pacifica (E. pacifica) in the YSBCW. Despite the shallow water in the YSBCW region, E. pacifica formed an SSL, which was distributed near the bottom during the daytime; it showed a diel vertical migration (DVM) pattern of movement toward the surface during the nighttime. The mean upward and downward swimming speeds around sunset and sunrise were approximately 0.6 and 0.3–0.4 m/min, respectively. The E. pacifica density was estimated in the central, western, and eastern regions; the results were approximately 15.8, 1.3, and 10.3 g/m2, respectively, indicating significant differences according to region. The results revealed high-density distributions in the central and eastern regions related to the water temperature structure, which differs regionally in the YSBCW area. Additional studies are needed regarding the spatial distribution of E. pacifica in the YSBCW and its relationship with various ocean environmental parameters according to season. The results of this study contribute to a greater understanding of the structure of the marine ecosystem in the YSBCW.

Air-sea heat flux during warming season determines the interannual variation of bottom cold water mass in a semi-enclosed bay

A bottom cold water mass (BCWM) is a widespread physical oceanographic phenomenon in coastal seas, and its temperature variability has an important effect on the marine ecological environment. In this study, the interannual variation of the BCWM in Iyo-Nada (INCWM), a semi-enclosed bay in the Seto Inland Sea, Japan, from 1994 to 2015 and its influencing factors were investigated using monthly observational data and a hydrodynamic model. The interannual variation in water temperature inside the INCWM showed a negative correlation with the area of the INCWM, and positive correlations with the local water temperature from April to July and with remote water temperature below 10 m in an adjacent strait in July. Differing from previously studied BCWMs, which had interannual variations depending closely on the water temperature before the warming season, the interannual variation of INCWM depends strongly on the air-sea heat flux during the warming season via local vertical heat transport and lateral heat advection. Further, by comparing several BCWMs, we found that the BCWM size is a key factor in understanding the mechanisms responsible for the interannual variation of BCWMs in coastal seas. These findings will help to predict bottom water temperatures and improve the current understanding of ecosystem changes in shelf seas under global climate change.

Satellites HY-1C and Landsat 8 Combined to Observe the Influence of Bridge on Sea Surface Temperature and Suspended Sediment Concentration in Hangzhou Bay, China

We analyzed the influence of a cross-sea bridge on the sea surface temperature (SST) and suspended sediment concentration (SSC) of Hangzhou Bay based on landsat8_TIRS data and HY-1C data using an improved single window algorithm to retrieve the SST and an empirical formula to retrieve the SSC. In total, 375 paired sampling points and 70 transects were taken to compare the SST upstream and downstream of the bridge, and nine transects were taken to compare the SSC. The results show the following. (i) In summer, when the current flows through the bridge pier, the downstream SST of the bridge decreases significantly, with a range of 3.5%; in winter, generally, the downstream SST decreases but does not change as obviously as in summer. The downstream SSC increases obviously. (ii) The range of influence of the bridge pier on the downstream SST is about 0.3–4.0 km in width from the bridge and that on the downstream SSC is approximately 0.3–6.0 km. (iii) When the current flows around the pier, a portion of the flow is dispersed in upward and downward directions; the downward flow generates local scour. When the scouring at the front end of the pier stops, the upward flow behind the pier brings the sediment and the bottom cold water downstream, causing the downstream SST to decrease and the SSC to increase. (iv) The other portion passes around the pier, which generates a wake vortex. Once a wake vortex is released, a low-pressure center appears, sucking the sediment and the bottom cold water to the downstream sea surface, reducing the downstream SST and raising the SSC. (v) The range of reduction of the SST downstream of the bridge is shorter than the range of increase in the SSC. This is because the wake vortices have an effect in the 0.3–4.0 km downstream but not in the 4.0–6.0 km. Therefore, the SST and SSC are affected within the range of 0.3–4.0 km by wake vortices, while in the 4.0–6.0 km region, the SSC is still high due to the transport of sediment by currents.

Unprecedented coastal upwelling in the southern coast of the Korean peninsula during summer 2013

<p>Unprecedented coastal upwelling in the southern coast of the Korean peninsula was reported in the summer of 2013. The offshore water temperature was 2℃ higher than that of climate (10-year mean) due to the hot summer in 2013. However, the water temperature at the coastal region was 2℃ lower. The upwelling continued for a month despite of weakening of upwelling-favorable wind. In this study, observational data and numerical model results were analyzed to investigate what caused the upwelling and sustained it for a long time. The upwelling was induced by upwelling-favorable wind in July. Coastal upwelling resulted in dynamic uplift of bottom cold water due to geostrophic adjustment. The dynamic uplift decreased sea level in the coastal region. The sea level difference between coastal and offshore regions resulted in an intensified cross-shore pressure gradient which induced geostrophic current accompanied by geostrophic adjustment along the coast. This positive feedback between dynamic uplift and geostrophic adjustment sustained the coastal upwelling for a long time regardless of upwelling-favorable wind.</p>