ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY

Lecture 19 Learning objectives: develop a physical

understanding of ocean thermodynamic processes

1. Ocean surface heat fluxes; 2. Mixed layer temperature equation.

Oceanic surface mixed layer: is subjected to wind & buoyancy (heat and salinity) flux forcing:

Properties (e.g., T, S,..) are pretty well mixed: Tm and Sm are often used to represent sea surface temperature (SST) and sea surface salinity (SSS).

1.  Ocean surface heat fluxes;

The mixed layer: in direct contact with the atmosphere; is subjected to forcing by winds, heat fluxes, salinity fluxes. Winds: momentum flux, drive oceanic circulation; Heat fluxes: directly affect SST. Importance for understanding SST change: Climate: Drives atmospheric circulation; air/sea interaction, Hadley and Walker circulation, atmospheric waves; global climate; SST & SSS (sea surface salinity) changes: affect deepwater – thermohaline circulation – climate. Marine biological activities, marine chemistry (CO2).

Ocean: important Component of Earth’s Climate System & Water cycle

Meridional distribution of net surface heat flux Processes that cause SST change?

TRMM (satellite observed) global SST Observations

Processes that determine the temperature change of a water parcel (Lagrangian) in the surface mixed layer are:

(1). Net surface radiative flux

Solar shortwave radiation Outgoing longwave

radiation

Units: J/s/m2=Watt/m2

(2). Surface turbulent sensible heat flux (3). Surface turbulent latent heat flux (4). Heat transfer directly by precipitation (usually small for long time scale) (5). Oceanic processes: entrainment cooling

2. Mixed layer temperature equation The first law of thermodynamics says: heat absorbed by a system is used to increase its internal energy and do external work. Example:

air Sliding door

heating For oceanic mixed layer, energy absorbed is used mainly to increase its internal energy (temperature). Apply the first law of thermodynamics to the mixed layer with depth for a unit area.

z

x

y

For a water column of with an area of , internal energy increase is:

where Is the specific heat of water ( )

density (J/s)

For a unit area, it is:

This energy increase is caused by net heat fluxes from both surface and bottom of the mixed layer.

here, we assume the shortwave radiation is completely absorbed within the surface mixed layer (without shortwave penetration into the deeper ocean).

(J/s/m2

= W/m2 )

Because

This is the mixed layer temperature equation. In a surface mixed layer model, however, the effects of entrainment and upwelling can not be separated.

Parameterize to

In a surface mixed layer model, however, the effects of entrainment and upwelling can not be separated.

is written as:

Where Qnet =Qnr +Qs +Ql +Qpr and w = (∂h∂t+!V •∇hm +w−hm

)

w−hmis the upwelling velocity (Ekman pumping velocity).

(2)

Equation (2) is used to calculate mixed layer heat budget in many studies.

∂Tm∂t

=Qnet

ρwcpwhm−!Vm •∇Tm −w

(Tm −Td )hm

In real ocean, properties within the surface mixed is not fully mixed and some vertical variations remain, even though small. A more accurate Tm equation that considers light penetration and turbulent transport is:

is shortwave radiation that is left at the bottom of the mixed layer and the last term is: turbulent heat transport convergence/divergence.

Q−hm

Qnet =Qnr +Qs +Ql +Qpr −Q−hm,

∂Tm∂t

=Qnet

ρwcpwhm−!Va •∇Ta −w

(Tm −Td )hm

−∇• (!V 'T ')

−h(x,y,t )

0∫ dz

atm radiative transfer

Oceanic surface

Top of atmosphere

a)

Q: Positive into the ocean

Below, we discuss each term in detail.

b)

Potential temperature

Top of PBL

SST Bulk formulae:

c)

d)

e) −wTm −Tdhm

It includes the effects of both upwelling & entrainment cooling.

Entrainment: turbulent mixing effect that entrains water from the thermocline into the surface mixed layer. Upwelling: vertical advection due to surface mass divergence; it involves isotherms tilting upward

TAO data in the eastern Pacific: Color: SST; black arrow: winds;White arrow: Ekman transport

Equatorial upwelling: meridional view

Zonal view