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.