PHYSICAL OCEANOGRAPHY Lecture 21whan/ATOC5051/Lecture_Notes/... · 2019-11-14 · ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY Lecture 21 Learning objectives:should be able to

ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY

Lecture 21Learning 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.

Previous class: ocean is an important Component of Earth’s ClimateSystem & Water cycle

© 2010 Pearson Education, Inc.

Earth’s albedo is ~0.3, meaning it reflects/scatters 30% of solar to space

Atmospheric window. No GHGs to absorb at some frequencies

33% direct17% scattered

Ocean: in the context of Earth’s energy balance: global average

TOA

Atm.

Surface

x

Previous class:z

Unit:

Previous class: mixed layer temperature equation:

Positive: downward into the ocean.

H(w) – step function; 1 for w>0; 0 for w≤0.

Horizontal advection

Upwelling cooling: Ekman divergence𝑤 = 𝑤# = 𝑤$%&

Entrainment cooling(vertical mixing)

In a surface mixed layer model, however, the effects of entrainment and upwelling can not be cleanly separated; their effects are often represented by one term: the last term in the following equation.

Where Qnet =Qnr +Qs +Ql +Qpr

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

This equation is used to calculate mixed layer heat budget in many studies.

∂Tm∂t

=Qnet

ρwcpwhm−!Vm •∇Tm −w

(Tm −Td )hm

𝑤 = (𝜕ℎ*𝜕𝑡 + 𝑉* . ∇ℎ* +𝑤$%&)

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

+ horizontal mixing

In the above equations: cpd = 1004J/kg/°C is specific heat of air; CDE is close to CDH 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 = 2.44 × 106J/kg is latent heat of evaporation. qva is surface air humidity, and qvo is saturation specific humidity when Ta = To ; specific heat of water: Cpw = 4186 J/(kg⋅°C) (at 15°C, 101.325 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.

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?

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

Cold tongue: has an annual cycle

(1) Upwelling & entrainment:

(2) Advection: Zonal and meridional

(air/sea interaction: enhance cold tongue)(3) Qsw: shortwave flux/stratus cloud, cools SST;

(Qlw & wind-evaporative effect plays minor role)

(4) Eddy heat transport(Swenson & Hansen, 1999: JPO)

−w (Tm −Td )hm

∂Tm∂t

=Qnet

ρwcpwhm−!Va •∇Ta −w

(Tm −Td )hm

−∇• (!V 'T ')

−h(x,y,t )

0∫ dz

(a) Mean SST+wind

Fig. 1. (a)Observed long-term mean surface wind (arrow) and sea surface temperature (SST; color contour) for the Pacific Ocean. (b) Linear trend of satellite observed surface wind (arrow) and SST (color contour) for the 1993-2010 period. White areas show wind & SST trends below 90% significance. (c) Linear trend of satellite observed sea surface height (SSH) for the 1993-2010 period, with global mean sea level rise removed. White areas show SSH trend below 90% significance.

USA

USA WA OR CA

(b) 1993-2010 trend: SST&wind (c) 1993-2010 trend: SSH

!

Mean SST and surface wind

Question:(1) For an intensified surface wind pattern, will US west

coast become colder or warmer? Through what processes?

(2) How about sea level?

Importance: Sea Surface Salinity (SSS) ocean’s rain gauge: an indicator of global hydrological cycle

2. Ocean Surface salinity budget:

[4] Marine species

of mixed layer & surface trapping of heat & momentum

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

Satellite: sea salinity missionImportance: In the coupled climate system, SSS is also an indicator of global hydrological cycle

June 2011

(http://www.whoi.edu/sbl/liteSite.do?litesiteid=18912&articleId=28407)

AquariusSalinity

E-P

(for a water parcel)

Ocean surface salinity budget

z

x

y Sea iceP E R

entrainment

ρwhmΔxΔy× Sm,Salt: Salt change:ρwhmΔxΔy×

dSmdt

∂Sm∂t

=−ρr P

.r Sm − ρs P

.s Sm + ρw E

.0 Sm − ρrv R

.Sm + ρi

dhidt(Sm − Si )

ρwhm−!V •∇Sm −w

Sm − Sdhm

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.

E-P (solid) and salinity (dashed)

( )

3. Ocean surface buoyancy flux