Generation and Initial Evolution of a Mode Water Theta-S Anomaly

Gregory C. Johnson
NOAA/PMEL
7600 Sand Point Way NE Bldg. 3
Seattle, WA 98115 USA

Generation and evolution of an isopycnal potential
temperature-salinity (Theta-S), or spiciness, anomaly is studied around
20-23S, 110W in the austral winter of 2004. Two profiling CTD floats
deployed in the region in January 2004 provide the observations. The
anomaly (defined relative to water properties of the preceeding summer)
is quite large (initially about 0.25 in S and 0.9C in Theta). It is
associated with winter ventilation of a thick, low potential vorticity
layer known as South Pacific Eastern Subtropical Mode Water. Regional
lateral Theta and S distributions at the surface predispose the ocean to
formation of this water mass, and allow significant anomalies to be
generated there with relative ease. The water mass is potentially
important for climate in that, after northwestward advection in the
South Equatorial Current, it contributes to the Equatorial Undercurrent
and eventually resurfaces in the cold tongue of the eastern equatorial
Pacific. The anomaly studies is strong enough to predispose a portion
of the water column to salt fingering, increasing vertical mixing.
While lateral processes are doubtless important in the evolution of the
anomaly, the vertical mixing appear sufficiently vigorous to reduce it
significantly within six months after its formation by spreading it to
denser horizons through diapycnal fluxes. By that time the anomaly is
most likely sufficiently diffuse so that subsequent evolution from
diapyncal fluxes is significantly reduced as it makes its way toward the
equator.