ATOC 5051 INTRODUCTION TO PHYSICAL OCEANOGRAPHY

class 4 1.  Indirect current measurement and application; 2. Integrated observing system; 3. Observed Ocean Circulation.

Previous class: e. Measurement of currents

•  Goals: 3-dimensional circulation => heat & salt transport => climate;

•  Typical horizontal currents vary from a fraction of 1 cm/s (deep ocean and much of near surface), 10-100 cm/s (equatorial current system), 200cm/s western boundary current.

•  Vertical speed:

Why? Prompt class.

Current Measurements •  Lagrangian methods: follows fluid parcels. •  Eulerian methods: velocity is stated at every

point in the fluid. (instrument is fixed in a space).

Direct measurements

•  Surface drifters •  Subsurface floats •  Current meters •  Acoustic Doppler Current Profiling (ADCP)

Surface drifters: Lagrangian method

•  Ship drift: earliest circulation map. •  Drift pole: near land mark •  Drift buoy: extend the pole idea to open ocean A

B current

ship

C

(begin)

(end)

(radio transmitor or tracked by satellite). Subsurface drogues: TOGA WOCE drifter: 15m

Subsurface floats •  SOFAR floats (sound source, moored

receiver) •  RAFOS floats (sound receiver, moored

sound source) •  Pop up floats: pop up regularly to

communicate with the satellite (bladder). WOCE subsurface floats: 800-1000m.

RAFOS float

Eulerian methods 1. Current meter

Rotor current Meter (RCM)

Accuracy~a few cm/s

A rotor & a vane & a compass

Acoustic Doppler Current Profiling (ADCP)

Using sound Wave’s Doppler Shifting effect

ADCP:

F

F reflect

source

Frequency shift:

WOCE: 75khz-150khz; up-look, down-look V (z)

1. Indirect current measurements a. Geostrophic method

•  T,S,P => density; => vertical shear of geostrophic current component perpendicular to the line connecting a station pair;

•  Make use of all observed data => 3-Dimensional oceanic circulation;

Geostrophic method

A B

Z

x

V1

V2

Shear: (V1-V2)

z

Current at a reference level is needed Northern Hemisphere(NH)

Indirect current measurements Geostrophic method

•  Large-scale motion satisfies “geostrophy”; •  Coriolis force; •  Pressure gradient force; •  Geostrophic balance: balance between the

Coriolis force and pressure gradient force.

NH, V moves with “high p” to its right, low to its left;

Geostrophic method

x

y

z

o PGF CF

V

NH

Geostrophic method

A B

z

x h1 p1

p2 x PGF CF

Geostrophic current: into the paper. 1000m

h2 p3 V3=0

Geostrophic method

b. Applying geostrophic method

P1

is latitude

P2

P3 P( r )

A B

Calculation using hydrographic observations

Step 1: Use observed T, S, P derive density and thus at stations A and B;

Step 2: Calculate from P(r ) to P (z ) at stations A and B by integrating equation:

€

Φ

Geostrophic current

Step 3: Calculate geostrophic current:

(i)

(ii) Level of no motion

Direct observations

Reference level of 500m, 1000m, and 1500m are often used.

Dynamic meter

Dynamic meter - dyn m:

Dynamic height:

1 dynamic m (D=1):

Because

is close to 1.

1 dyn meter is very close to the depth of 1m

Pressure level 1db is close to 1m.

z is very close to be 1 m.

Geostrophic current: example

P2

P3 P( r )

A B 0db

500db

1000db

T,S,P T,S,P

Northern Hemisphere

PGF CF

0

•  Advantage: use all useful hydrograhic data to infer general ocean circulation;

•  Disadvantage: (i) level of no motion – relative current; (ii) filter out barotropic mode.

•  Surface geostrophic current: sea level measurements: (i) satellite altimetry (TOPEX/POSEIDON/JASON); (ii) coastal sea level stations; [Both barotropic and baroclinic modes)

y

z V 0

V 500db

V 1000 db

Shear

Total motion: Barotropic Baroclinic

2. Integrated observing system http://www.pmel.noaa.gov Pacific.

TRITON array: east Indian & west Pacific (Japan)

TAO Buoy and ship

TRITON buoy

TAO/TRITON buoy data

•  moored buoy. Example: TAO/TRITON moored buoy; [Tropical Atmosphere Ocean (TAO); TRIangle Trans-Ocean buoy Network (TRITON)].

TAO ATLAS MOORING

•  ATLAS - (Autonomous Temperature Line Acquisition System). Low-cost deep ocean mooring; designed to measure surface meteorological and subsurface oceanic parameters; transmit all data to shore in real-time via satellite relay.

RAMA Array

Prediction and Research Moored Array in the Atlantic (PIRATA)

Argo floats T,S PROFILE & CURRENTS AT INTERMEDIATE DEPTH

Pop-up floats: ~10-days pop up, transmit data to ARGO satellite (~6hours up/down); on the way, collect T and S profile (0~2000m); (Surface: 6~12hours) currents at ~1000m (float drift; then dive down to 200m for T&S Profiling). Life cycles; ~150 repeated cycles. ARGO Floats Postions

http://www.argo.ucsd.edu/

3. Observed ocean circulation The Pacific

(Geostrophic Method)

The Pacific

EQ

S. Pacific current

ITF (Indonesian Throughflow)

Eastern Pacific (home of El Nino)

Geostrophic current: the Atlantic

The Atlantic

EQ

Indian Ocean

Equatorial Undercurrent (EUC)