Modeling of the intermediate water and iron transport in the Sea of Okhotsk and the northern North Pacific

Humio Mitsudera
Institute of Low Temperature Science, Hokkaido University


The Amur River mouth is located at the northwestern shelf of the Sea of Okhotsk.  Dense Shelf Water (DSW) is produced over the area, when brine is rejected as sea ice forms.  DSW intrudes into intermediate depths (200-500m) along the Sakhalin coast.  This intermediate layer intrusion forms the lower limb of a meridional overturning circulation in the Sea of Okhotsk that is enhanced by strong tidal mixing near the Kuril Islands and closed by a surface wind-driven circulation that returns relatively saline water to the DSW formation regions. DSW mixes a lot of materials such as iron when it forms over the shelf. This water supplies a lot of materials including iron to the Sea of Okhotsk, and finally exports to the intermediate layer of North Pacific Ocean. 

A series of numerical experiments is conducted to investigate dynamics of this overturning that incorporats the intermediate-layer circulation.  The effects of wind, air temperature, Amur River discharge and tidal mixing along the Kuril Island were examined and were all found to influence the overturning.  In particular, it was found that stronger wind forcing enhances the DSW intrusion because 1) intensified circulation increases northward salinity flux from the Kuril Islands where saline water upwells from the intermediate layer, and consequently raises background salinity in the northern shelves where DSW forms, and 2) the DSW volume flux from the northern polynyas increases under increased winds.  Then, simulations with observed forcing were carried out to investigate which effects are responsible for recent warming in the intermediate layer.  It was found that not only air temperature rise but also fresh water flux increase onto the sea surface are important for the intermediate layer warming.

We are currently coupling this model with material circulation, aiming toward representing the iron circulation.  For the first step, CFC circulation was simulated and validity of model physics was evaluated.  It is confirmed that the model simulated the observed CFC well.  This model is now developed to couple with a P-Fe circulation model.