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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? 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 2 anions such as chloride (Cl ) and sulfate (SO4 ) 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 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 North Atlantic Deep Water (NADW) Antarctic Bottom Water (AABW) 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 The 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 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 The water balance of the continents and oceans 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 E.A. Mathez, 2009, Climate Change: The Science of Global Warming and Our Energy Future, Columbia University Press. Source: Hartmann, 1994 Ocean 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 -1 horizontal flow or currents are 0.01-1.0 ms 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 transport In the Northern Hemisphere: Deeper waters are richer in nitrates and phosphates supports growth of plankton and, in turn, fish, such as occurring in Peru and Ecuador The food chain: Phytoplankton → Zooplankton → Predatory zooplankton → Filter feeders → Predatory fish http://www.crd.bc.ca/watersheds/protection/geology-processes/globaloceancurrents.htm Upwelling zones http://www.absoluteastronomy.com/topics/Upwelling El 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 2nd strongest period 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 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 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