Introduction

GEOF110: Innføring i dynamikken til atmosfæren og havet

GEOF110, 2008

Who’s Who

Assoc. Prof. Ilker Ferilker.fer@gfi.uib.no

Room 126 at GFI

Assistant Sturla Svendsen

sturla.svendsen@gfi.uib.no

Room 103 at GFI

Administration: Kristin Kalvik (Studiekonsulent)

kristin.kalvik@gfi.uib.no

GEOF110, 2008

Course Description 10 Credits Language: English (Norwegian if needed) Contents:

introduction to the theory for motion in atm. and ocean. governing equations stability, diffusion, continuity, geostrophic winds/currents,

circulation, vorticity boundary layer of the atmosphere and ocean surface gravity waves and internal waves effect of the Earths rotation on the winds and currents

Assessment Midway exam, written, 2 hours. Counts 20% on the final grade. Final exam, written, 4 hours. Counts 80% on the final grade.  

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Lærebøker

Pond and Pickard: Introductory Dynamical Oceanography

Pensum: Hele boken unntatt Kap. 8.9-8.10, 9.5-9.6, 9.8-9.10, 9.12-9.13, 9.14.1, 11 og 12.10

Tor Gammelsrød: Tyngdebølger, kompendium.

Pensum: Hele (pdf on studentportal– but will be updated) Wallace and Hobbs: Atmospheric science

Pensum: Chapter 7 (pdf on studentportal) Anbefalt tilleggsbok:

Atmospheric science; Wallace and Hobbs Fluid Mechanics; P. Kundu Ocean Circulation. (The Open University).

GEOF110, 2008

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GEOF110, 2008

Introduction

Describe and interpret the structure and evolution of the large scale motions in the atmosphere and the ocean

Dynamical Oceanography is concerned with the forces acting on the ocean waters and with the motions that follow.

Ultimate objective is to predict Essence of dynamical approach is to deduce quantitative

information about the movements of the ocean and air from mathematical statements of the basic principles of physics.

Oceanography study of oceanPhysicists study of distribution of T,S, density, etc.,

water masses, the motions of the ocean

in response to forces acting on it.

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Typical questions:- Why are large scale mid-lat surface circulations are CW in

NH and CCW in the SH?- Why are these narrow and swift at the western sides (Gulf

Stream, Kuroshio etc.) but broad and slow elsewhere?- What is distribution with depth of ocean currents?- Why is the circulation around Antarctica is eastward?- What are the mechanisms of transfer of momentum and

energy between atmosphere and the ocean?- What are the characteristics and causes of surface and

internal waves?- What is the role of turbulent motions in the oceans?

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Overview of Geophysical Scales

Radius of the Earth = 6371 km Mean depth of the ocean = 3.795 km Area of ocean surface = 3.61 1014 m2

Area of ice sheets and glaciers = 1.62 1013 m2

Area of sea ice (March) = 1.75 1013 m2

Area of sea ice (September) = 2.84 1013 m2

Volume of the ocean = 1.37 1018 m3

Mass of the ocean = 1.4 1021 kg Mass of the atmosphere = 5.3 1018 kg Mass of water in lakes and rivers = 5.0 1017 kg

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Zonally Averaged Radiative Balance

Excess of radiation in the tropics. Deficit in middle and high latitudes. The atmosphere/ocean system is forced to move about by this imbalance, and bring heat by convection/advection from equator to the poles.

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Ocean is Deep

Left: Histogram of elevations of land and depth of the sea floor as percentage of area of Earth

Right: the hypsographic curve

[from R.H.Stewart (2005), Introduction to Physical Oceanography]

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T-S range

Major water masses of the ocean have a small T-S range

North Atlantic Deep Water

Indian Bottom Water

Pacific Bottom Water

South AtlanticBottom Water

Peak: 26x1015 m3

Worthington (1981)

COLD!!

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Oceanic Heat Transport

Ocean transports about 50% of the poleward flux in the A/O system.Heat transport in the ocean is done by both wind-driven and THC.Crude heat transport estimate for meridional volume flux of 20 Sv, deltaT=20deg is 1000x4000xdTxUV=1.6x10^15 W (1.6PW).

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Temperature, Salinity, Density

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Thermohaline Circulation

Thermo (heating/cooling) + haline (freshening/salinification) An overturning circulation – warm water flows poleward near

the surface and cold/dense water sinks and flows equatorward in the interior.

Time scale? Poleward transport of heat? Overturning of the Atlantic THC? Average heat flux to the atmosphere from warm water

flowing North? Why is the upper/northern part of the Atlantic THC water

salty? Role of thermal vs. haline forcing on THC?

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The Great Ocean Conveyor Belt

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(Schmitz, 1996)

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(From K. Heywood)

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Southern Ocean THC

(Speer et al., 2000)

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S-N Atlantic Ocean

Cold AABW fills the abyss

Fresh tongue of AAIW Saline tongue of NADW Saline signature of

MOW Dense waters formed

circum-Antarctica have no obstacle

Dense waters formed circum-Arctic, circulate and overflow GSR.

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External forcing…

Dissipation at coast [2.6 TW]

Dissipation to balancedeep ocean mixing [2.1 TW]

Earth tide [0.2TW] Surface

tide[3.5 TW]

Wind[1.2 TW]

Atm. tide[0.02 TW]

Moon[3.2 TW]

Sun[0.5 TW]

Internal tides

IW and deep turb. [0.2

TW]

Local turbulent patches [0.7

TW]

Munk & Wunsch (1998)

1 TW = 1012 W

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Mean winds for July

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Dramatic Example of Air-Sea Interaction: Tropical Cyclone

A low-pressure system causes convergence and lifting of the air mass at the centre of the depression.

As the air is lifted, it cools, and water condenses out to form clouds. The latent heat released by the condensing water warms the cloud and strengthens the convective motion.

Generation of tropical cyclones requires SST> ~26C. Saturation vapour pressure

increases dramatically with temperature, thus very high rates of evaporation occur over the warm tropical ocean.

Evaporation rate depends on the air-sea temperature difference and wind surface wind speed.

-2C 35CSea Surface Temperature

L

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The enhanced convective lifting deepens the depression, increasing in-flowing surface winds, and thus increasing the surface flux of water vapour.

Initially the convection takes the form of a collection of individual thunderstorms, but as the circulation strengthens they become organised into a single tropical storm

This positive feedback process causes the intensity of the circulation to increase exponentially until…

…Eventually the loss of kinetic energy by surface friction balances the generation by buoyant convection.

The hurricane can be maintained until it loses the strong surface forcing – moving over land, or colder water

GEOF110, 2008

Cold water trails in the wakes of hurricanes Fabian & Isabel2003/08/27-2003/09/23

NASA/Goddard Space Flight Center Scientific Visualization Studio Data from : Aqua/AMSR-E, GOES/IR4, Terra/MODIS http://svs.gsfc.nasa.gov/vis/a000000/a002800/a002824/index.html

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A mature tropical cyclone can produce surface sensible heat fluxes of ~500 W m-2 and latent heat fluxes of 1000 W m-2. All this energy is extracted from the ocean, resulting in a significant cooling of the ocean surface mixed layer.

The sea surface temperature (through translucent cloud imagery) is reduced by ~5-10C in the wake of hurricane Katarina (August 2005).