Oceans and Climate

Dr Mark Cresswell

FOLLOWED BY: Sea-level Practical

Oceans and Climate

69EG5513 – Climate & Climate Change

Lecture Topics• Introduction to the World’s Oceans

• Interactions between Ocean/Atmosphere

• El Nino Southern Oscillation (ENSO)

• Measuring SSTs from Space

• Ocean-Climate Indices

• Oceans & Climate Change

Introduction1. Most of planet Earth is covered with water (71%) or

361 million square km

2. Over 60% of global ocean surface is in the southern hemisphere

3. ¾ of the ocean is between 3 – 6km in depth

4. The deepest part of the oceans is the Mariana Trench – east of the Philippines at over 11km

5. The oceans are 96.5%, by volume, water

6. Remaining 3.5% is dissolved salts, particles, organic material and gases

Introduction

Thermohaline

circulation

Ocean-Atmosphere InteractionsIncident angle (° from vertical) Reflectance (%)

0 2

10 2

20 2.1

30 2.1

40 2.4

50 3.4

60 5.9

70 13.3

80 34.9

90 100

Reflectance of radiation from calm water (Bigg, 1996)

Ocean-Atmosphere Interactions1. Heat exchange between the oceans and

atmosphere is mainly through radiation processes

2. Another dominant mechanism is latent heat transfer

3. Water evaporating from the ocean surface uses energy to allow them to free their bonds from liquid water below the surface

4. Condensation within clouds releases energy which increases cloud buoyancy

5. Latent heat flux (transfer) therefore contributes to energy recycling via ocean-atmosphere

Ocean-Atmosphere Interactions1. Direct physical contact between the atmosphere

and oceans leads to energy being exchanged by conduction

2. This energy exchange is called sensible heat flux

3. Sensible heat flux causes heat to be transferred from warmer to cooler fluid

4. This process occurs at sea-surface and near-surface air

5. Turbulence and high wind speeds enhance sensible heat flux processes

Ocean-Atmosphere Interactions

Five Basic forces within Ocean and Atmosphere:

• Gravity

• Pressure gradients (due to density/mass differences)

• Drag (loss or gain across air/sea interface)

• Coriolis force due to Earth rotation

• Tidal motion due to Moon and Sun gravity

1. ENSO is an SST anomaly

2. Normally easterly trade winds prevents eastward movement of warm water from western Pacific to eastern Pacific

3. When trade winds weaken, warm water propagates eastward, the low pressure over Indonesia breaks down and so too does the Walker circulation

4. It is brought about by a change in circulation

El Nino Southern Oscillation

Read about Kelvin and Rossby waves

TOPEX-POSEIDON

• For much of our oceans, temperature is not measured directly – but by proxy

• Warmer water expands – if surrounded by cooler water it rises. Its height is therefore an indication of its temperature

El Nino Southern Oscillation

1. TOPEX is an altimetric satellite

2. Return time of pulses of energy sent by TOPEX to the ocean surface are measured

3. Distance between satellite and water surface can be accurately measured

4. TOPEX used to measure El Niño by proxy

El Nino Southern Oscillation

1. Southern Oscillation Index (SOI). The SOI is defined as the normalized pressure difference between Tahiti and Darwin

Ocean-Climate Indices

2. North Atlantic Oscillation (NAO). Both phases of the NAO are associated with basin-wide changes in the intensity and location

of the North Atlantic jet stream and storm track

Ocean-Climate Indices

3. Pacific Decadal Oscillation (PDO). The Pacific Decadal Oscillation (PDO) is a long-lived El Niño-like pattern of Pacific climate

variability

Ocean-Climate Indices

Oceans & Climate Change

Source: IPCC, 2007

Oceans & Climate Change

Source: IPCC, 2007

Southern Ocean region

Oceans & Climate Change

Source: IPCC, 2007

Oceans & Climate Change

Source: IPCC, 2007

Bigg G (1996). The oceans & climate. By Cambridge University Press

Climate Research Unit (UEA): http://www.cru.uea.ac.uk/cru/data/soi.htm

Univ. of Washington (PDO information): http://jisao.washington.edu/pdo/

Climate prediction Center: http://www.cpc.noaa.gov/data/teledoc/telecontents.shtml

KNMI Data Explorer: http://climexp.knmi.nl/start.cgi?someone@somewhere

IPCC: http://www.ipcc.ch

Reading List

Today’s Practicalhttp://jcm.chooseclimate.org/