Strong
wintertime forcing by outbreaks of cold, dry Siberian air may drive intermediate
water formation and subduction at the subpolar front of the Japan/East Sea.
The subpolar front separates warm, saline southern waters from seasonally stratified,
colder, fresher waters that lie to the north. Previous observations taken south
of the front during spring and summer reveal subsurface pycnostads having elevated
levels of dissolved oxygen and watermass characteristics consistent with those
expected of waters formed in wintertime at the front. In January 2000, R/V Revelle
made a series of four intensive surveys of the subpolar front, spanning the
passage of three distinct cold air outbreaks. Upper ocean measurements were
conducted using a towed, undulating profiler (SeaSoar) equipped with extensive
physical and bio-optical sensors while ship-based observations included upper
ocean velocities, boundary layer meteorology and real-time, remotely sensed
sea surface temperature and ocean color. The surveys bracketed a particularly
strong cold-air event that occurred 24-26 January, between the first and second
surveys. Mixed layers deepened and cooled with the passage of each successive
storm system, consistent with a largely one-dimensional response in which intense
surface cooling and convective overturning play important roles. Between cold-air
outbreaks, advective effects associated with both the front and a nearby eddy
likely govern mixed layer evolution. Observations south of the front revealed
small (O(20 km) horizontal and O(20 m) vertical scales) regions of weakly stratified
water with T-S characteristics similar to those within the northern-side mixed
layer. These subsurface features were typically found between the 27.0 
and
26.7 isopycnals
(a layer that outcrops on the northern edge of the front) at distances as far
as 50 km south of the frontal interface. The features nearest the front also
exhibit elevated bio-optical signals similar to those found in the northern
mixed layer, suggesting that the waters have been recently subducted beneath
the southern mixed layer.
