What causes Earth’s climate and climate change?What causes Earth’s climate and climate change?

The Ocean

Recall that the ocean is a natural thermostat annual sea surface temperature variation

2 C in tropics, 8 C in middle latitudes, 4 C in polar regions

global average 17 C releases and absorbs heat over decades to centuries,

whereas the atmosphere does the same but in days to weeks

Water has a high specific heat requires high energy to raise its temperature: 1 calorie

(4.18 J) of energy to raise water temperature by 1 C

Why is seawater salty?Why is seawater salty?

Seawater is 96.5% water, the rest is sodium chloride (NaCl) (about 3%) and other dissolved salts

Why is seawater salty? land sediments carried by rivers into oceans

2.5 billion ton per year dissolved cations (Na+, Mg2+, Ca2+, etc.) leached

from rocks anions such as chloride (Cl-) and sulfate (SO4

2-) have accumulated over centuries from gases escaping from Earth’s interior through volcanic eruptions

less important are dust blown in from deserts and anthropogenic pollutants

Na+ resides longer in the sea than Ca2+ because marine animals remove Ca2+ to make carbonate skeletons

Na+ is removed by adsorption to clay minerals but slow process

The composition of seawater

E.A. Mathez, 2009, Climate Change: The Science of Global Warming and Our Energy Future, Columbia University Press.

Addition of salt into water decreases freezing point (-1.9 C instead of 0 C) and increases density (1.026 g cm-3 instead of 1.0 g cm-3)

In oceans, increased temperature decreases density Processes that alter salinity

evaporation removes water, so increases salinity precipitation or influx of fresh river water decreases

salinity freezing removes water, so increases salinity melting of ice adds water, so decreases salinity salinity of oceans varies from place to place

Salinity is low at equator and at poles because of high precipitation (equator) and low evaporation (poles) also at mouths of large rivers

High salinity at semiclosed seas in arid regions Persian Gulf, Red Sea, and Mediterranean Sea

Ocean depthsOcean depths

Density increases with higher salinity increases with lower temperature

So, deep water is typically denser, colder, and more saline than shallow water

Ocean is stratified by density into 3 major layers 0-20 m: thin warm surface layer

called a mixed layer because it affected by waves and temperature changes => rapid mixing/changes

20-500/900 m: thermocline zone where temperature and salinity change

rapidly with depth depth/thickness varies from location to location

and season to season

below the thermocline called the deep zone slight variation in temperature and salinity 65% of ocean water is in this layer but in winter at high latitudes, thermocline can

extend all the way to the ocean bottom (like in Norwegian-Greenland Sea)

Vertical profiles of density, temperature, and salinity through the upper several hundred meters of the ocean

E.A. Mathez, 2009, Climate Change: The Science of Global Warming and Our Energy Future, Columbia University Press.

The average annual salinity of ocean surface water, 2005

E.A. Mathez, 2009, Climate Change: The Science of Global Warming and Our Energy Future, Columbia University Press.

A conductivity, temperature, and depth measurement device

E.A. Mathez, 2009, Climate Change: The Science of Global Warming and Our Energy Future, Columbia University Press. Photograph by E.A. Mathez

The global ocean conveyor system

E.A. Mathez, 2009, Climate Change: The Science of Global Warming and Our Energy Future, Columbia University Press. Source: IPCC, 2001

Antarctic Bottom Water (AABW)

North Atlantic Deep Water (NADW)

The global ocean conveyor systemThe global ocean conveyor system

The global ocean conveyor system is also known as Thermohaline Circulation (THC)

This circulation is driven by differences in density of sea water, which is controlled by temperature and salinity generally less than 0.1 ms-1

overturns entire ocean depth every 100-1000 years Begins with the downwelling of water in the North

Atlantic and Southern Ocean water flows to and wells up in the Pacific Ocean and

flows as shallow water to replace the downwelling water

This system exerts/moderates a stabilising influence on global climate for hundreds to thousands of years but can change abruptly as well

Warm, near surface water forms in the Atlantic Ocean at about 35 N and flow northward at a depth of about 800 m

In the north, the water sinks because it loses heat to the atmosphere and being now cold and more dense, it sinks, and starts to flow southward, all the way to the Southern Ocean North Atlantic Deep Water (NADW)

Water in the Antarctica rises because the seas here are less dense, but sinks again as the NADW are cooled again

Water wells up at less salty, warmer, and shallower Indian Ocean and Pacific Ocean

Deep water forms in North Atlantic, rather than North Pacific because North Atlantic is saltier (by 5%) than North Pacific more precipitation in North Pacific than North Atlantic

(which has higher evaporation) the global conveyor system acts to redistribute the

salt to correct this salt imbalance between these two oceans

Northward water flow into the North Atlantic brings enormous amount of heat, equivalent to 30% of annual solar energy, to Europe even though Europe is at the high latitudes, its

weather is mild because of the conveyor system that brings heat here from the equator

Effect of global warmingEffect of global warming

More ice from the Arctic melting, adding freshwater into the North Atlantic

Retreating ice cover exposes more of the ocean surface, allowing more moisture to evaporate into the atmosphere and leading to more precipitation (rain and snow)

So, the increased freshwater into the North Atlantic increases the buoyancy of the ocean and makes it harder more the warm water from the equator to sink to the bottom hence, NADW might slow down or stop! ironically, causing global cooling (ice age, perhaps?) average Europe temperature might fall 5-10 C

(colder)

Mediterranean Sea has also high salinity and its water flows into Atlantic Ocean, making it saltier by 6% increased use of freshwater means less freshwater

flowing from rivers into the Mediterranean Sea, causing higher salinity in both the Mediterranean Sea and Atlantic Ocean

Region Evapotranspiration Precipitation Runoff

(in millimeters/year)

Europe/Asia 795 1353 558

Africa 582 696 114

North America 403 645 242

South America 946 1,564 618

All land 480 746 266

Atlantic Ocean 1,133 761 –372

Indian Ocean 1,294 1,043 –251

Pacific Ocean 1,202 1,292 90

All oceans 1,176 1,066 –110

The water balance of the continents and oceans

E.A. Mathez, 2009, Climate Change: The Science of Global Warming and Our Energy Future, Columbia University Press. Source: Hartmann, 1994

Ocean surface currentsOcean surface currents

Ocean currents driven by winds their interaction with the atmosphere have important

consequences for both climate and weather confined mostly to the upper kilometre or two of the

ocean typical speeds

horizontal flow or currents are 0.01-1.0 ms-1

vertical speeds within the stratified ocean are 0.001 ms-1

Ocean gyres correspond nearly to the wind gyres rotate clockwise in the Northern Hemisphere rotate counter clockwise in the Southern Hemisphere

Continents positions affect wind gyres, deflecting them into boundary currents flowing poleward, parallel to coastlines Gulf Stream in North Atlantic Kuroshio Current in North Pacific Brazil Current, along coats of South America others such as East Australian and Mozambique

Currents

http://www.crd.bc.ca/watersheds/protection/geology-processes/globaloceancurrents.htm

Boundary currents are important because they carry heat from the equator to the north, making

the weather in the north milder Gulf Stream can carry heat from the equator to the

mid latitudes in a month (mean flow: 100 mil. m3 of water per second)

they carry water vapour they help to remove CO2 from the atmosphere

warm waters has less CO2 than colder waters

Upwelling and downwelling: Ekman transportUpwelling and downwelling: Ekman transport

http://www.crd.bc.ca/watersheds/protection/geology-processes/globaloceancurrents.htm

Deeper waters are richer in nitrates and phosphates supports growth of plankton and, in turn, fish, such as occurring in Peru and Ecuador

In the Northern Hemisphere:

The food chain:Phytoplankton → Zooplankton → Predatory zooplankton → Filter feeders → Predatory fish

Upwelling zones

http://www.absoluteastronomy.com/topics/Upwelling

El Nino and La NinaEl Nino and La Nina

Every several years, coastal areas in Peru and Ecuador experience rapid warming, increasing from -2 to 4 C in a month usually occurs around Christmas and lasts until

May/June anchovies and sardines disappear birds, fur seals and other animals that depend on fish

die unusual heavy rains in coastal areas, and droughts in

Andes in south Peru and northeast Brazil caused by El Nino (boy child or Christ child in

Spanish)

El Nino is not a local phenomenon global: affects weather around the world severe droughts in Australia, Indonesia, southern

Africa, and Egypt milder weather (but stormier winter) in North America

El Nino cycle/occurrence is pseudo-periodic; cycle not consistent, but usually every 2 to 7 years

El Nino is caused by a change in atmospheric and ocean circulation across the entire equatorial Pacific Ocean

NOAA/PMEL/TAO

In normal years, easterly winds (from east to west) in the Northern and Southern Hemisphere converge along the equator, blowing westwards (toward west) this causes the usual upwelling in coastal areas such

as Peru and Ecuador the westward flowing surface water piles up in the

western Pacific, causing sea level to rise about 60-70 cm higher than in the eastern Pacific

the upwelling brings the thermocline nearer in the east, while in the west, the thermocline is much deeper

In the mean time, the warm, moist air in the western Pacific rise, causing rain, and warm air’s place is replaced by easterly winds a convection circulation that starts with rising air in the

west, flowing toward the east Pacific, then sinking in the east Pacific, then flows back to the west Pacific

In an El Nino year, this coupled air-ocean circulation cycle is reversed easterly winds weaken warm Pacific water flows toward east, sea level

flattens out (less difference between sea level at the west and east Pacific)

upwelling in coastal areas at South America stops thermocline along the equator deepens to tens of

meters in the east equatorial undercurrent stops

Warm air in the east Pacific rises, causing heavy rains in coastal areas in east Pacific but no rain in west Pacific, causing drought

Changes in circulation patterns are caused by changes in atmospheric air pressure Normal years: high pressure at east Pacific, low

pressure at west Pacific El Nino: low pressure at east Pacific, high pressure at

west Pacific this swing in air pressure is known as the Southern

Oscillation El Nino is an ocean phenomenom, but Southern

Oscillation an atmospheric phenomenom but the two (ocean and atmosphere) are coupled

to cause El Nino, they are known together as ENSO (El Nino-Southern Oscillation)

Atmospheric pressure:Normal years: low at Darwin, high at Tahiti,causing easterly (east-to-west) trade winds

El Nino year: high at Darwin, low at Tahiti,weakening easterly winds

The Southern Oscillation Index (SOI) measures the change in air pressure measured between the eastern (Tahiti) and western (Darwin) Pacific +ve SOI means

Darwin < Tahiti air pressure is lower in the west, higher in the east normal years (high index) La Nina (girl child in Spanish) (a very high index)

-ve SOI means Darwin > Tahiti air pressure is higher in the west, lower in the east El Nino year (low index)

http://www.pmel.noaa.gov/tao/elnino/faq.html

Climate variability and the global harvest : impacts of El Niño and other oscillations on agroecosystems by Cynthia Rosenzweig and Daniel Hillel, Oxford University Press, 2008

strongest period

2nd strongest period

Climate variability and the global harvest : impacts of El Niño and other oscillations on agroecosystems by Cynthia Rosenzweig and Daniel Hillel, Oxford University Press, 2008

El Nino has shown to give low grain yield in South Asia and Australia, but high grain yield in the North American prairies

Climate variability and the global harvest : impacts of El Niño and other oscillations on agroecosystems by Cynthia Rosenzweig and Daniel Hillel, Oxford University Press, 2008

El Nino of 1982-83 on Brazil

Climate variability and the global harvest : impacts of El Niño and other oscillations on agroecosystems by Cynthia Rosenzweig and Daniel Hillel, Oxford University Press, 2008

Long term effectsLong term effects

El Nino and La Nina, however, has no effect on global warming in the long run effects only on the short term

droughts or heavy rains will give poor yields in a short period

or may give good yields in some cases such as more rains in usually dry areas

El Nino and La Nina cancel each other out to give zero net change over the long run