Lecture 20 ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY

Transcription

1 ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY Lecture 20 Learning objectives: should be able to apply mixed layer temperature equation to explain observations; understand buoyancy forcing & salinity budget 1. Mixed layer temperature equation: cold tongue formation, etc.; 2. Ocean surface salinity budget; 3. Ocean surface buoyancy flux.

2 z Previous class: x Unit:

3 Previous class: mixed layer temperature equation: H(w) step function; 1 for w>0; 0 for w 0. Positive: into the ocean.

4 Horizontal advection 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

5 In the above equations: c pd = 1004J/kg/ C is specific heat of air; C DE is close to C DH under ordinary conditions. Va is 10m wind speed, Vo us oceanic surface current in the wind direction, Ta us surface air temperature, and To is SST and is Tm if the surface layer is well mixed. Llv = J/kg is latent heat of evaporation. q va is surface air humidity, and q vo is saturation specific humidity when Ta = To. C P = 4186 J/(kg C) (at 15 C, kpa) Note: potential temperatures should be used but at the oceanic surface, potential temperature is equivalent to in situ temperature so people often use T instead of θ.

6 In a surface mixed layer model, however, the effects of entrainment and upwelling can not be separated, and their effects present in one term: the last term in the following equation. 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 and w = ( h t +! V h m + w hm ) w hm is the upwelling velocity (Ekman pumping velocity). It is this equation that is used to calculate mixed layer heat budget in many research studies.

7 Questions (1)When wind speed reduces, the ocean increases or decreases its heat loss? Through what possible processes? (2) When SST is colder, does the ocean increase Or decrease heat loss? Why? (3) Both upwelling and entrainment can cool the SST; physically, what s the major difference between the two processes?

8 1: Cold tongue: TAO data in the eastern Pacific: Color: SST; black arrow: winds;white arrow: Ekman transport Cold tongue: has an annual cycle

9 (1) Upwelling & entrainment: w (T m T d ) h m (2) Advection: Zonal and meridional (air/sea interaction: enhance cold tongue) (3) Qsw: shortwave flux/stratus cloud, cools SST; (Ql, wind-evaporative effect plays minor role) (4) Eddy heat transport - diffusion (Swenson & Hansen, 1999: JPO)

10 Mean SST and (a) Mean SST+wind surface wind USA Fig. 1. (a)observed long-term mea surface temperature (SST; color c (b) Linear trend of satellite observ (color contour) for the SST trends below 90% significanc (c) Linear trend of satellite observ period, with global mea areas show SSH trend below 90% Question: (b) trend: SST&wind (c) trend: SSH (1)For an intensified surface wind pattern, will US west coast become colder or warmer? Through what USA processes? (2)How about sea level?

11 2. Ocean Surface salinity budget: [4] Marine species of upper ocean Importance: ocean s rain gauge: an indicator of global hydrological cycle

12 Importance: In the coupled climate system, salinity is also an indicator of global hydrological cycle Satellite: sea salinity mission June 2011 A Delta II rocket launches with the Aquarius/SAC-D spacecraft payload from Space Launch Complex 2 at Vandenberg Air Force Base, Calif. - Image credit: NASA/Bill Ingalls

13 E-P ( Aquarius Salinity

14 Ocean surface salinity budget (for a water parcel)

15 z y P E R Sea ice x entrainment Salt: ρ w h m ΔxΔy S m, Salt change: ρ w h m ΔxΔy ds m dt

16

17 S m t = ρ r Ṗr S m ρ s Ṗs S m + ρ w Ė 0 S m ρ rv Ṙ S m + ρ i dh i dt (S m S i ) ρ w h m! V S m w S m S d h m

18 Processes for local (Eularian) salinity change: (1) Precipitation (rain or snow); P (2) Evaporation; E (3) River runoff; R (4) Sea ice freezing/melting; (5) Horizontal advection; (6) Entrainment & Upwelling. P-E plays a deterministic role in open ocean; R can be important in coastal regions where river discharge is large; Sea ice: important in high latitude: say arctic ocean; Advection, entrainment, upwelling can be important in any regions.

19

20 E-P (solid) and salinity (dashed)

21 3. Ocean surface buoyancy flux ( )