Freshening and dense shelf water reduction in the Okhotsk Sea linked with sea ice decline

• Prominent freshening has occurred down to 500 m during the past four decades.
• Dense shelf water has reduced by about 30%, causing weakening of the overturning.
• Freshening may be caused by weakening of salt redistribution by sea ice decline.

A recently constructed hydrographic dataset from the Okhotsk Sea reveals a prominent freshening to depths of ∼500 m during the past four decades, with the maximum in the northwestern part of the sea. Averaged over the sea, this freshening corresponds to an input of 0.55 m of freshwater. This leads to a decrease in density of the intermediate water and deepening of the isopycnals, with the maximum deepening at 26.8 σθ of ∼60 m averaged over the sea. The intermediate water is significantly warmed along the pathway of dense shelf water (DSW). A simple box model shows that DSW production has decreased by ∼30% during the past four decades. We propose that the freshening and DSW reduction are caused by the weakening of salt/freshwater redistribution through sea ice decline as well as by the increase of excess precipitation over evaporation. Since the overturning in the North Pacific originates from the Okhotsk Sea through the DSW, these changes possibly weaken the shallow overturning of the North Pacific.