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1.: Ideas of ocean circulation and biogeochemical responses

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Physical Oceanography

• Physical properties and processes that control the ocean motion;

• It is the major forcing that transport momentum, heat, material, and biogeochemical substances in the ocean;

• It largely controls the marine environment and climate change.

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Biological Oceanography• Concerns the biology and ecology of oceanic, marine, coastal and estuarine organisms.

•The organism range from viruses and bacteria to microbes and phytoplankton, from zooplankton and benthic invertebrates to shellfish, fish and marine mammals.

dynamic ocean motion

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Elevation of the Earth: Elevation. Positive: Land, Negative: Ocean

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Global Ocean Circulation

anti-clockwise

clockwise

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Subtropical gyre

Strong and narrow western boundary current

Subtropical gyre

Subpolar gyre

ACC

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Global Wind Pattern

Uneven heat distribution from sun is the primary energy source driving the atmosphere as well as oceans.

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Winds

Unevenly heating by the sun

Spinning sphere

Winds and wind-driven basin circulation

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Annual-mean isolation QSW (Kallberg et al 2005).

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Annual-mean net heat flux (Kallberg et al 2005).

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From Kallberg et al 2005.

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• Global sea surface temperature distribution

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Gulf Stream, western boundary current in north Atlantic Ocean

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Meanders, rings, eddies and gyres

gyre

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Large Scale Horizontal Circulation

• The surface ocean circulation is mainly formed by surface wind stress;

• It is clockwise and anti-clockwise in northern hemisphere and southern hemisphere, respectively

• It contains a strong western boundary currents, like Kuroshio in the Pacific Ocean, Gulf Stream in the north Atlantic Ocean, Brasil Current in the south Atlantic Ocean.

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The Thermohaline Circulation (north-south vertical circulation

Sinking of dense water due to cooling in mid to high latitude.

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Northward heat transport for 1988 in each ocean and the total transport summed over all oceans (Houghton et al., 1996).

Meridional Heat Transport

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•Meridional ocean circulation is mainly fulfilled by the thermohaline circulation

•Thermohaline circulation is a very slow and extremely deep movement of water in the oceans around the world.

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•A complete cycle can take thousands of years to complete from start to finish, with the cycle contributing a great deal to the mixing of the world's oceans.

•Thermohaline circulation appears to be linked to global climate and ocean health, as it mixes dissolved gases and minerals in the water and it may impact global temperatures as well.

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Global distribution of chlorophyll averaged over the period from 1 January 2002 to 28 February 2005 using data collected from MODIS on the Aqua satellite. Chlorophyll values range from 0.01 mg/m3 (purple) to 60 mg/m3 (red). From NASA Goddard Space Flight Center.

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Regional Characteristics

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Bathymetry in China Sea

meter

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Ocean Circulation in Pacific Ocean

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Sea surface temperature (SST) showing a western Pacific Warm pool in the Pacific Ocean

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Kuroshio, western boundary current, warm pool, heat transport and spatial variability of flow field in Pacific Ocean

Obtained from web)

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Surface Ocean Circulation (left) and wind stress (right) in the summer

Clockwise basin circulation embeded with eddies and local flow pattern in response to southwest monsoon.

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Surface Ocean Circulation (left) and wind stress (right) in the winter

Anti-clockwise basin circulation embeded with eddies and local flow pattern in response to southwest monsoon.

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Intrusion of Pacific surface waters into SCS is stronger in the winter (Gan et al., 2006).

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Circulation at depth 500 m (Gan et al., 2006).

winter summer

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winter summer

Circulation at depth 2000 m (Gan et al., 2006).

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winter

summer

Sea surface temperature reflects not only uneven atmosphere heating, but also the processes of oceanic circulation

(Gan et al., 2006).

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Elevation (color contour) and phase of semi-diurnal tide.

Tidal waves propagate from Pacific and amplitude in nearshore waters

(Zu et al., 2006).

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Internal solitary waves

Onshore propagation

Shoreward propagation of waves is one of the important ocean processes that affect coastal waters.

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Tides change flow direction in shallow waters periodically

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• Regional ocean circulation has its intrinsic characteristics, and is forced both by the local winds, remote forcing and local topography and by the interaction of forces with different frequency.

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Coastal wind driven currents

ENVR608.Lecture1 44Vectors of QuickScat wind stress (Pa) and MODIS SST (C) average over the cruise periods in 2000 and 2002

2000 2002

B B

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汕頭

汕尾

Summer

The surface and bottom currents behave differently in the coastal stratified (density changes with depth) waters.

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Bottom

The response of tracers (e.g. density) in the surface and bottom in the coastal waters

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Surface and bottom flow pattern in the entire NSCS, indicating locations of energetic currents in response to local topography

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River plume (fresh and nutrient rich waters) and internal solitary waves from SAR

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(c) (d)

(b)(a) bulgeBuoyancy of river plume alternates the surface currents

U changev change

Gan et al. 2008

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Barotropic (depth-average) current, showing the circulation in the estuary and on the shelf modulated by estuary and shelf geometry and topography (summer)

River + wind

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River + Tide

(summer)

River + Wind

(summer)Responses of nearshore aters to different regimes

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from North South China Sea

from East China Sea

Pearl River

from western

South China

Sea

HK

Winter

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Summer

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Buoyancy- and wind-driven currents jointly controlled the flow field in the NSCS

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without river

without river

with river

with river

+ADV.+mixingriver

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without river with river

Shoreward currentat the bottom

Seaward currentat the bottom

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without river with river

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• Near-shore ocean circulation is greatly controlled by winds, bottom topography, river buoyancy, tides and intrusion from remote forcing.

• The circulation is three-dimensional time- and space-dependent;

• Circulation is largely regularly by coastline as well as bottom (friction);

• It is physically and biologically most dynamic regions in the ocean.

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Ocean driven by atmospheric fluxes as intrinsic forcing (dynamics)

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1mm=10-3m

High Frequency，Small Scale

气候尺度Climate Scale

全球暖化

中尺度过程

近岸涌升

湍流表面重力波

内波和惯性震荡

分子过程

Tim

e Sc

ale

空间尺度

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Transfers of nutrients and waste products by molecular diffusion

Transfers of nutrients and waste products by faster turbulent diffusion, mesoscale, upwelling, eddy, front, surface and internal waves

Internal Rossby deformation radius=force Coriolis

forcen gravitatiosin2

'12

=Ω

−

==φ

ρρρ h

fhg

fc o

h: water depth; ρ: water density; Ω: angular velocity of earth rotation; Φ: latitute

(m)

Transfers of nutrients and waste products by eddy turbulent diffusion,ba sin scale circulation, plantary waves, tides

Molecular

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About 100 yrs1-10 yrs

days-1wks

days

hours Time scale

Large marine mammal

Temporal scales determined from organisms

Increase fluctuations in number

Process of trophic transfer

Molecular

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HK

(Δx,Δy)<1km

(Δx,Δy)<3km

(Δx,Δy)<10km

Nested with Pacific Ocean model

Developed the first South China Sea (SCS) multi-scale coupled circulation-ecosystem modeling system that is capable to resolve the oceanic processes with different temporal and spatial scales that dynamically links the coastal embayment (Hong Kong waters), Pearl River Estuary, continental shelf of northern SCS, SCS basin and Pacific Ocean together. The modeling system represents an advanced approach in ocean flows and ecosystem studies.

South China Sea Basin

ContinentalShelf of SCS

Pearl RiverEstuary

HK

HK

Temporal and spatial dow

nscaling

HK

(Δx,Δy)<0.1km

HK Embayment

Coupled Coast-Estuary- Embayment (HK) circulation-ecosystem modeling system

Multi-Scale System in South China Sea

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Sun

Food Web

Photosynthesis by phytoplankton

Passed from organism to organism by feeding transfers

Currents, tides, turbulence and stratification by heating or associated wind*

* 1. Alters boundary layers, stratification and euphotic layer, causes retention of planktonic organisms and modifies light penetration.

2. Transports nutrients and waste products, assists migrations, affects the rate of encounter between planktonic predators and their prey.

3. Rate of biological process is affected by T (C) and organisms colonizing is determined by v

(m/s).

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Remarks• Physical and biological processes are inter-

related aspects that govern the productivity of marine ecosystems.

• Physical processes that control abundance of marine organisms in the ocean.

• Turbulence in surface and bottom boundary layers and its roles on the lives of plants and animals; buoyancy- and wind-induced circulation, mixing, coastal upwelling/ downwelling processes, fronts, river plume, tides, internal waves, circulation and eddy and their effects on biological transport, primary production; zooplankton, fish and the regeneration of nutrients;

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Summary

• Ocean circulation has different temporal and spatial scales which are inter-related but characterized by their distinct forcing mechanisms as well.

• The biogeochemical processes in the ocean in closely coupled with motion in the ocean which is driven through various forcing processes.

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Study Questions:• What is physical oceanography? Is it related to fisheries or marine biology?

Is it important in the global climate change?

• What are the main driving force in the ocean? Name some of them.

• How much can you describe the major characteristics of global ocean circulation?

• What does the ocean bottom look like? Does ocean current change with depth?

• Is the ocean motion in the Hong Kong waters same as that in the central part of the Pacific ocean? What would be the major difference?