Abstract

The influence of global warming on mid-depth water mass ventilation in the Japan Sea was investigated using both Argo-based and ship-based hydrographic datasets. The Argo-based dataset of the entire Japan Sea area revealed a warming trend during the past two decades in the upper portion of the Japan Sea Proper Water (UJSPW), which lies at intermediate depths from just under the main thermocline to approximately 1000 m. The warming rates in the southern Japan Sea are generally greater than those in the northern sea by a factor of 2–3. Long-term hydrographic data obtained over the last five decades in the northeast and southeast of the sea revealed that higher warming rates in the southern sea began from 2008, although a significant warming in both northern and southern seas was initiated from the late 1980s. A stagnation in the UJSPW formation from the late 1980s was suggested by a positive shift in the winter sea surface temperature in its formation region and a decreasing trend in dissolved oxygen concentration during the 1990s. In addition, a vertical multi-box model demonstrated that an imbalance between the heating from the upper layer and the cold water supply from its source region induces a warming in the UJSPW. We conclude that a significant change in the mid-depth water mass ventilation occurred in the entire Japan Sea in the late 1980s due to a stagnation in the UJSPW formation. Subsequently, a modest event in the mid-depth water mass ventilation have occurred since 2008. The higher warming rates in the southern sea than those in the northern sea in the event suggest a reduction in cold UJSPW supply to the southern sea from its formation region.

Highlights

  • Oceanic thermohaline circulation, which is an important part of the global climate system, is believed to be significantly influenced by global warming

  • Since the warming trends in JW and Tsushima Basin (TB) in the western sea are similar to those in JE and Yamato Basin (YB) in the eastern sea, respectively (Figure 4), it is suggested that the significant acceleration of warming from the late 1980s occurred concurrently in the northwestern and southwestern areas, which is in the entire Japan Sea area

  • Before the mid-1980s, when the influence of global warming in this region was not very serious, the cold, newly formed UJSPW was supplied to the entire Japan Sea area from its formation region via deep circulation (Senjyu et al, 2005b)

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Summary

Introduction

Oceanic thermohaline circulation, which is an important part of the global climate system, is believed to be significantly influenced by global warming. One of the unique features of the Japan Sea is a self-contained thermohaline circulation system that includes deep water formation. The thermohaline circulation system in the Japan Sea is sensitive to global warming and climate change owing to its limited size (Bindoff et al, 2007). A gradual warming and depletion in the dissolved oxygen (DO) concentration have been reported from the abyssal Japan Sea, suggesting a stagnation in the deep water formation (Gamo et al, 1986; Minami et al, 1999; Gamo, 2011). The Japan Sea can be regarded as a “canary in a coal mine” of climate change impacts in the global oceans (Gamo, 2011)

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