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Estimating Diffusivity from the Mixed Layer Heat and Salt Balances
Meghan F Cronin1, et al.1NOAA Pacific Marine Environmental Laboratory
4 April 2013 - Station P Science Workshop - NOAA PMEL, Seattle WA
A companion paper to: Cronin et al., 2013: Formation and erosion of the seasonal thermocline in the Kuroshio Extension recirculation gyre. DSR II, 85, 62-74.
KEO
Papa
KEO
Papa
KEO
Large Sfc Heat Loss
Large CO2 Uptake
Fronts & Eddies
Typhoons
Deep wintertime mixed layer
Mode water
Papa
Weak Sfc Heat Gain
Weak CO2 Uptake
Subsfc Fronts & Eddies?
Shallow halocline
WindATRH
http://www.pmel.noaa.gov/ocs/
UV at 15m, & 35m
http://www.pmel.noaa.gov/ocs/disdel/disdel.html
TS Seacat strapped to release(Starting 2012)
T upper 300mS upper 200m
SWR-LWRRain
WindATRHBP
SST&SSS
LongrangerADCP at 835m(2008-2010)
NarrowbandADCP at ~150m(2007-2010)
Q0= Net Surface Heat Flux (+ = into Ocean) Q0 = Qsw – Qlw – Qsen - Qlat
Q0= Net Surface Heat Flux (+ = into Ocean) Q0 = Qsw – Qlw – Qsen - Qlat
Surface Heat Flux Warms Ocean
Surface Heat Flux Cools Ocean
WindATRH
http://www.pmel.noaa.gov/ocs/
UV at 15m, & 35m
http://www.pmel.noaa.gov/ocs/disdel/disdel.html
TS Seacat strapped to release(Starting 2012)
T upper 300mS upper 200m
SWR-LWRRain
WindATRHBP
SST&SSS
LongrangerADCP at 835m(2008-2010)
NarrowbandADCP at ~150m(2007-2010)
Mixed Layer DepthIsothermal Layer Depth
MethodologyFor h = mixed layer depth, i.e.
where
Sfc Mooring Sfc +ADCP Moorings+ satellite or glider
Sfc Mooring+ satellite
ResidualSfc Mooring
Rate of MLT change
Due to SFCheat flux
Due to horizadvection
Due to entrainment(diapycnal w)
Due todiffusion
Methodology
w-h is Ekman balance with turbulent stress vanishing at z=-h=MLD
small small
Assume steady state linear momentum equation…
assume negligible
MethodologyFor h = mixed layer depth, i.e.
Sfc Mooring Sfc +ADCP Moorings+ glider (or as residual)
Sfc Mooring+ satellite
Residual(or using κT)
Sfc Mooring
Rate of MLS change
Due to Evap & Precp
Due to horizadvection
Due to entrainment(diapycnal w)
Due todiffusion
If salinity gradients can be estimated directly from glider data, then the residual of the salt budget provides an independent estimate of diffusivity.
Heat Budget Salt Budget
The correspondence between the diffusivity estimate from the heat & salt budgets gives us confidence that it can be used for other mixed layer budgets.
Papa diffusivity is 1x10-4 m2/s during summer, 3x10-4 m2/s during fall, and higher during spring.
Diffusivity at Base of Mixed Layerinferred from mixed layer temperature budget
KEO Papa
THIS IS BIG !
At Papa, strong Near Inertial Oscillation (NIO) shear is observed during winter. Winter 2010 was stronger than 2009.
At KEO, strong NIO shear is observed from May-March due to typhoons and winter storms.
+ Diffusivity at base of ML (weak advective periods) Error
Inferred κ at Papa
Inferred κ at KEO is
101 higher during summer and
101-102 higher during winter
Relation between diffusivity and stratification
KEO Papa
Both sites show an approximate N-2 relationship for diffusivity.
Slope = -1.9Slope = -2.5
Summaryhttp://www.pmel.noaa.gov/OCS
Heat gained through the surface by solar warming at Papa is exported into the deeper ocean, where it is presumably transported away.
Diffusivity is not as large as at KEO. At KEO during summer κ ≈ 3x10-4 m2/s. During winter, κ is > 5x10-2 m2/s.Papa diffusivity is roughly 1x10-4 m2/s during summer and 3x10-4 m2/s during fall.
Salinity advection appears to be an order 1 process in the mixed layer.