Ocean Newsletter

No.5 October 20, 2000

  • Global Warming and the Circulation in the Sea of Japan Jong-Hwan YOON
    Professor, Dynamics Simulation Research Center Research Institute for Applied Mechanics, Kyushu University
    Selected Papers No.1
  • Why The Whaling Issues So Important Now Joji MORISHITA
    Fisheries Agency, Far Seas Fisheries Division
    Selected Papers No.1
  • The Focus Desired in the Fishing Industry of the Future Keiji WASHIO
    Advisor, Hayashizaki Fisheries Cooperative Association

Global Warming and the Circulation in the Sea of Japan

Is the Sea of Japan, the mini-ocean, changing?

Although nowhere near the size of the Atlantic or Pacific Ocean, nearly all of the phenomena that occur in the world's great oceans pertain to the Sea of Japan as well. In this sense the Sea of Japan is the "mini-ocean." One distinguishing feature of the Sea of Japan is the structure of its masses of water. In the south, the upper layer is warm and salty, heated by the Tsushima Warm Current, while the northern half is a subarctic zone, whose surface waters are cold and relatively salt-poor. Beneath the surface layers, most of the volume (from 300m down to the seabed) is a unique water layer found only in the Sea of Japan, characterized by a temperature of 0°C and a salinity of 34.07psu. Dissolved oxygen levels are considerably higher than those in the Pacific, as great volumes of oxygen-rich surface waters replenish (or have replenished) the lower depths. It is believed that the cooling of the northern half of the sea during the winter forms this deep-water layer.
Today, however, this deep-sea layer in the Sea of Japan is changing. Beginning in the latter half of the 20th century, the temperature of the waters at a depth of 2000-3000m has been rising some 0.02-0.03°C every decade, while its store of dissolved oxygen is disappearing at a rate of approximately 1μmol/kg per year (see diagram). This means that, for half a century, the deep waters of the Sea of Japan have not been supplied with sufficient dissolved oxygen to replace the dissolved oxygen consumed in the decomposition of organisms and other organic processes. If this layer's dissolved oxygen continues to be depleted at this rate, as early as 2350 the deep waters of the Sea of Japan could be completely devoid of dissolved oxygen. Oxygen-rich water from the surface is transferred to the lower depths in the winter, as the surface water cools, becomes heavy, and sinks. This sinking water has begun to rise up again, driving a vertical circulation that returns the water to the surface. The atmospheric and ocean conditions that block the sinking of oxygen-rich waters have grown significantly pronounced since the middle of the 20th century.
The factors preventing the sea surface water from growing dense and sinking is the increasing temperature of the air and declining salinity at the surface. According to a recent report by the Intergovernmental Panel on Climate Change (IPCC), the change in air temperature reflects a rise in average air temperature of perhaps as much as 0.5°C over the past 20 years in the eastern Eurasian landmass at latitudes between 40°N and 70°N. Warming is particularly dramatic in the cities of the Russian coast on the Sea of Japan, where minimum temperatures in winter have been rising at a rate of 0.03-0.06°C per year. During the past 50 years, the temperature has risen 1.5-3.0°C, raising the strong possibility that global warming is the factor obstructing the sinking of surface seawater in the northern Sea of Japan. As for the loss of salinity at the surface level, lack of sufficient data precludes discussion at this time.

The importance of vertical circulation in the oceans

To understand why the formation of deep-water and concomitant sinking and vertical circulation are so important, it is useful to think in terms of global warming. In the Labrador Sea off the west coast of Greenland and the Weddell Sea on the Antarctic coast, the surface water cools and sinks, raising the world's deep waters to the top to effect a slow vertical circulation that gradually returns deep waters to the surface.
Deep waters carry nutrients to the sea surface from the deep sea. Phytoplankton form organic material from carbon dioxide and water by means of photosynthesis feeding nutrients. Some plankton also synthesize calcium carbonate at the surface level. When these organisms die, some of the dead matter sinks, changing to mineral carbonates in the deep seas. In effect, marine organisms carry carbon to the deep levels and expel them as minerals, serving to make the deep-water levels denser. If the surface waters were deprived of phytoplankton, the surface waters would only be able to absorb as much carbon from the air as supplied by passive solution equilibrium. Some researchers predict that this process would double the carbon dioxide in the atmosphere, causing drastic warming.
If, on the other hand, this vertical circulation and vertical mixing were to stop, the oceans would become rigidly stratified. The phytoplankton would be starved of the nitrogen and phosphorus they need to propagate, since these would no longer be carried from the deep seas to the surface, killing off the phytoplankton. Condensation of the deep waters by mineral carbonates would cease as the level of carbon in the atmosphere rises. Normally the physical circulation of the seas interacts with the circulation of substances in which living creatures take part, regulating the volume of carbon dioxide in the atmosphere.
As stated in the opening paragraph, the bottom water formation may already have partially or completely ceased in the Sea of Japan. The nutrients phytoplanktons require to bloom may not be transported from the depths to the surface, severely impacting the fish population. Also, if the oxygen-rich surface waters are not transported to the deep layers, the creatures that live at those depths will suffer a murky death. Presently vertical mixing in the Sea of Japan in winter proceeds to a depth of about 1,000m. Although the numerical models and ocean observation data on which these conclusions are based are limited, it seems likely that further global warming would blunt this vertical mixing still further, placing the flora and fauna of these seas in calamitous straits. If global warming continues to progress on present trends, within 50 years the sea level the Sea of Japan could rise some 30-50cm. The Mamiya Strait, which separates Hokkaido and Sakhalin, would cease to freeze over in winter, causing fresh water from the Amur River to preventing vertical mixing flood into the sea and drastically increase the volume of fresh water in the surface layer. Moreover, as the economies of East Asia continue to develop, large volumes of man-made pollution (agricultural chemicals, etc.)would be washed into the Sea of Japan, greatly exacerbating the pollution of these waters.

Establishing an international research system for the Sea of Japan is an urgent issue

The cessation of vertical circulation in the "mini-ocean" of the Sea of Japan would be an ominous portent of the cessation of vertical circulation in oceans on a worldwide scale. An immediate start of investigations to find out what is happening in the deep layers of the Sea of Japan and what is causing these phenomena is an issue of the utmost urgency.
With the end of the Cold War, in 1993 the coastal nations of the Sea of Japan-Japan, South Korea and Russia-launched a Circulation Research of the East Asian Marginal Seas (CREAMS) to investigate vertical circulation in the Sea of Japan. In 1998 the United States joined CREAMS, which is currently in the midst of a five-year, 1 billion-plus program of observation activities. Other observation activities are in progress in the Sea of Japan under international collaboration, with the participation of research agencies under the aegis of Japan's Ministry of the Environment and Ministry of Agriculture, Forests and Fisheries as well as the Japan Atomic Energy Research Institute. Moreover, a number of other international cooperative research bodies, such as the North Pacific Marine Science Organization (PICES) and the North-East Asian Regional Global Ocean Observing System (NEAR-GOOS) are conducting their own observation programs. At this point, however, observation and research in the deep waters of the northern Sea of Japan, where this deep-water formation and sinking occurs, is insufficient to shed light on these urgent questions. As apprehension grows over the rapid advance of global warming, the Federation of Economic Organizations urges the early formation of a framework to investigate circulation in the Sea of Japan.

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