Persistence of coastal upwelling after a plunge in upwelling-favourable wind

Abstract

Unprecedented coastal upwelling off the southern coast of the Korean Peninsula was reported during the summer of 2013. The upwelling continued for more than a month after a plunge in upwellingfavourable winds and had serious impacts on fisheries. This is a rare phenomenon, as most coastal upwelling events relax a few days after the wind weakens. In this study, observational data and numerical modelling results were analysed to investigate the cause of the upwelling and the reason behind it being sustained for such an extended period. Coastal upwelling was induced by an upwellingfavourable wind in July, resulting in the dynamic uplift of deep, cold water. The dynamic uplift decreased the steric sea level in the coastal region. The sea level difference between the coastal and offshore regions produced an intensified cross-shore pressure gradient that enhanced the surface geostrophic current along the coast. The strong surface current maintained the dynamic uplift due to geostrophic equilibrium. This positive feedback between the dynamic uplift and geostrophic adjustment sustained the coastal upwelling for a month following a plunge in the upwellingfavourable wind. Coastal upwelling is a process that brings deep, cold water to the ocean surface. It can play an important role both in physical processes and in chemical and biological variability in coastal regions by transporting nutrients to the surface layer. Coastal upwelling can be induced by various mechanisms, but it generally results from Ekman transport due to the alongshore wind stress 1,2. Wind stress curl can also induce coastal upwelling 3. The current along a coastal region may enhance the onshore Ekman pumping through the bottom boundary layer 4,5. Upwelling occurs as a form of dynamic (isotherm) uplift that results from geostrophic equilibrium, which is a balance between the pressure gradient force (PGF) and the Coriolis force 6,7. The southern sea region off the Korean Peninsula connects the East China Sea and the East/Japan Sea. The mean depth of this offshore region is approximately 100 m. There is an eastward alongshore flow throughout the year 8,9. A two-layer structure, comprising warm water in the upper layer and cold water in the lower layer, forms during the summer. The surface waters that originate from the Kuroshio and East China Sea are heated by the atmosphere. The deep, cold water originates from the west 10,11. Unprecedented coastal upwelling was reported in various observations during August 2013. The sea surface temperature (SST) in the coastal region was 2 °C lower than the climatic SST (10 years mean) in the coastal region, whereas the offshore SST was 2 °C higher due to a hot summer in 2013 (Fig. 1c). The cold SST in the coastal region persisted for more than a month after the upwelling-favourable wind weakened (Fig. 1e). The upwelling had serious impacts on the fish farms in this area. In this study, observational data analyses and numerical modelling were performed to investigate the reason for the unprecedented coastal upwelling, ...

Quantification of the extremely intensified East Korea Warm Current in the summer of 2021: offshore and coastal variabilities

“…During summer, wind-driven upwelling due to monsoonal southerly winds occurs at the east coast of Korea (Park and Kim, 2010;Jung and Cho, 2020;Kim et al, 2023). We thus expect climatological northward wind stress in that area (Figure 11A).…”

Effects of Surface Heating on Coastal Upwelling Intensity

“…The air pressure and relative humidity for the bulk-flux formula were set to constants of 1007.5 hPa and 86.7%, respectively. The ranges of wind speed, air temperature, and shortwave radiation were based on the upwelling conditions on the southern coast of the Korean Peninsula during the summer (Jung & Cho, 2020). Shortwave radiation was imposed using a diurnal cycle.…”

“…However, a recent observational result demonstrated that the SST difference between the cold coastal and warm offshore regions increased during strong surface heating (Jung & Cho, 2020). A strong coastal upwelling along the southern coast of the Korean Peninsula was reported during the hot summer of 2013 (Figure 1).…”

“…The upwelling intensity was measured via offshore transport, isopycnal slope, and the SST difference between coastal and offshore regions. The change in upwelling intensity, at various wind speeds and surface heating levels, was investigated based on the upwelling condition in the southern coast of the Korean peninsula (Jung & Cho, 2020).…”

Numerical study on advective fog formation and its characteristic associated with cold water upwelling

“…Under these conditions, the surface sea water is swept away to the open ocean, and a CWM is formed by upwelling of the intermediate and deep-sea waters. These events can cause mass die offs in fisheries, or the formation of sea fog [ 21 ]. Kim et al have reported that an average of approximately 24 days of CWM has appeared every year for the past 7 years (2012 to 2018) along the southeastern coast and that the frequency tends to increase in June compared to that in July and August in accordance with the climatic changes such as the change in atmospheric pressure patterns, and the weakening of the monsoon [ 22 ].…”