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.
1. Ocean surface heat fluxes; Oceanic surface mixed layer: is subjected to wind & buoyancy (heat and salinity) flux forcing: Properties (e.g., T, S,..) are pretty well mixed: T m and S m are often used to represent sea surface temperature (SST) and sea surface salinity (SSS).
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 (CO 2 ).
Ocean: important Component of Earth s Climate System & Water cycle
Processes that cause SST change? Meridional distribution of net surface heat flux
Observations TRMM (satellite observed) global SST
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/m 2 =Watt/m 2
(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 y x For a water column of with an area of, internal energy increase is: density (J/s) where Is the specific heat of water ( )
For a unit area, it is: (J/s/m 2 = W/m 2 ) 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).
Because Parameterize to This is the mixed layer temperature equation. In a surface mixed layer model, however, the effects of entrainment and upwelling can not be separated.
In a surface mixed layer model, however, the effects of entrainment and upwelling can not be separated. is written as: T m t = Q net ρ w c pw h m! V m T m w (T m T d ) h m Where Q net = Q nr +Q s +Q l +Q pr (2) and w = ( h t +! V h m + w hm ) w hm is the upwelling velocity (Ekman pumping velocity). Equation (2) is used to calculate mixed layer heat budget in many studies.
In real ocean, properties within the surface mixed is not fully mixed and some vertical variations remain, even though small. A more accurate T m equation that considers light penetration and turbulent transport is: T m t = Q net V! a T a w (T m T d ) 0 ( V! 'T ') dz h(x,y,t) ρ w c pw h m h m Q net = Q nr +Q s +Q l +Q pr Q hm, Q hm is shortwave radiation that is left at the bottom of the mixed layer and the last term is: turbulent heat transport convergence/ divergence.
Below, we discuss each term in detail. a) atm radiative transfer Top of atmosphere Oceanic surface Q: Positive into the ocean
b) Potential temperature Top of PBL Bulk formulae: SST
c) d) e) w T m T d h m 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