Why the East Sea experienced extreme conditions in 2021

Extreme environmental events are becoming an increasingly pressing concern with continued pressures from climate change, both on land and in the marine realm. While heat waves on land tend to occur over a few days, those affecting the oceans often last weeks or even months.

A few anomalous weeks of extremely high temperatures occurred in the East Sea (Sea of ​​Japan, northwest Pacific) during the summer of 2021, when a particular current experienced record changes in its dominant patterns , initially intensifying considerably before weakening. This was the East Korean Warm Current (EKWC), which is a western boundary current off the east coast of South Korea that occurs due to intensifying marine conditions at the west of the ocean basin and which transports warm water from the tropics towards the poles.

New research published in Frontiers of Marine Science investigated the factors that likely led to this unusual event. Doctoral student Gyundo Pak, of the Korea Institute of Ocean Science and Technology, and colleagues determined a northward shift of the EKWC up to 40°N in late July 2021, alongside an increase in ocean current speed. 1.16 m/s, measured from moorings on the seabed. These moorings also measured an increase in the volume of water transported via the EKWC during this period (more than double normal levels) in the East Sea, as it is a key branch of entry into The Pelvis. However, by mid-August a rapid deceleration of the current saw it weaken to summer background levels of 0.5 m/s.

The cause of these abrupt oceanographic changes is attributed to temperature gradients between coastal and offshore areas and changes in wind patterns, linked to the descent of coastal waters and the flow of fresh water from rivers that began to develop in early July 2021. In these locations, warm surface waters are “pushed” to greater depth, while cold nutrient-rich waters cannot reach the surface along the coastal region, making them areas of low productivity for marine organisms. In mid-August, the wind pattern was further modified by a monsoon (Typhoon Lupit) which caused southerly wind-induced upwelling and thus altered surface temperatures.

Overall, the research team attributes an ocean temperature anomaly as the main driver of the 2021 EKWC marine heatwave. The subsurface layer located at a water depth of 200 to 300 ma the contributed more to this temperature change and was influenced by the warm temperature of nearby Ulleung. Eddy, a lens-shaped body of water that lies in the thermocline (transition between deeper cold water and warmer surface water). In August, this whirlwind was weakened by the passage of Typhoon Lupit.

Understanding the occurrence and cause of marine heatwaves is of vital importance not only to the health of our oceans, but also to the people who depend on them for survival, such as fishing communities around the world who consider the fish as an important source of protein for a large part. of the world population.

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