Characterization of Preconditioning for Ocean Deep Convection in the Sea of Japan

Changes in the amount of dissolved oxygen concentration in the Sea of Japan (SOJ) over a number of years have been hypothesized to be due to the amount and depth of deep convection in this region. The first observations of the changes in oxygen concentration at depth due to deep convection in near-real time occurred during a recent Office of Naval Research-sponsored program, which included both ship tracks and floating buoy measurements. In general the process of deep convection has been divided into three general phases: preconditioning, deep convection, and lateral exchange and spreading (restratification). In this work we evaluate buoy and ship data between August of 1999 and August 2001 in order to understand the role and types of preconditioning evident in the SOJ, and compare with results from other regions with deep convection such as the Labrador Sea, the Mediterranean Sea, and the Green land Sea. Several aspects of preconditioning relative to other locations that we evaluated include the existence of a cyclonic gyre, doming of the isopycnals, and changes in mixed layer depth. The focus of our analysis was a region located south of the Peter the Great Bay and mainly north of the subpolar front (from 40.5°N to 42.5°N and from 131.0°E to 134.0° E). This has been one of the historically-designated regions of open ocean deep convection, due to its proximity to cold air outbreaks channeled through the opening in the mounts near Vladivostok. This is also the location of deep convection during the 1999 – 2001 time period from both observations and model simulations. The major findings of this study were that the surface cyclonic circulation has a primary role in preconditioning by transporting water transformed along the Siberian coast into the eastern portion of the deep convection region. We found that doming of isopycnals is not an important contributor to either preconditioning or deep convection, with a general uplifting of isopycnals of at most 100 meters. The evolution of the mixed layer properties and depth was found to be an important part of the preconditioning in SOJ. Changes in both temperature and salinity are affected by surface forcings (particularly in autumn) and by advection.