Upper Ocean Response to Cold Air Outbreaks in the Japan/East Sea:
SeaSoar Surveys at the Subpolar Front

Craig M. Lee, Applied Physics Laboratory, University of Washington.
      (craig@sahale.apl.washington.edu)
 
Burton H. Jones, Department of Biological Sciences, University of
      Southern California. (bjones@usc.edu)

Kenneth H. Brink,  Department of Physical Oceanography, Woods
      Hole Oceanographic Institution. (kbrink@whoi.edu)

Robert Arnone, Naval Research Laboratory, Code 7343 Stennis Space
      Center. (arnone@nrlssc.navy.mil)

Richard Gould, Naval Research Laboratory, Code 7343 Stennis Space
      Center. (gould@csips5.nrlssc.navy.mil)
   
Clive Dorman, Center for Coastal Studies, Scripps Institution of
      Oceanography. (cdorman@ucsd.edu)

Robert Beardsley, Department of Physical Oceanography, Woods
      Hole Oceanographic Institution. (rbeardsley@whoi.edu)

ABSTRACT

   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
kg/m$^{3}$ and 26.7 kg/m$^{3}$ 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.