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Environmental Change 1 - Lecture 6 Ocean Circulation Ron Kahana 1 Ocean Circulation • What drives the Ocean circulation? • Two kinds of circulation (?) – Wind-driven (surface) circulation. – Thermohaline circulation (deep), driven by fluxes of heat and freshwater across the sea surface. • Role of Oceans in climate 2 Atmosphere and Ocean circulations are all about the redistribution of heat on a rotating planet 3 Transporting the heat to the poles Northward heat transport across each latitude • Atmospheric and oceanic circulation (1PW=1015W) is about transporting heat from the equator to the poles. • The maximum energy transport is similar. Oceans peak at 200N • A direct effect: The atmosphere is heated from the bottom, air column becomes unstable and rises. • However, the oceans are heated from the top therefore become more stable. So how the energy received from the sun keeps the ocean circulation going? Figure from: The Earth System. Kump,Casting and Crane, 2004 the Atmosphere holds the key for that. 4 What drives the Ocean circulation? Atmosphere Oceans Wind stress momentum Wind driven (Surface) circulation Heating and cooling Sun Temp. density Thermohaline (deep) Radiation from the Evaporation and Salinity precipitation circulation 5 Two kinds of circulation Surface ocean: Wind-driven circulation, at the mixed The pycnocline separates the surface zone (aka mixed area between the surface and layer) from the deep ocean the thermocline (300-1000 m depth). Ocean currents also subject to Coriolis force, which deflects them 20-25 degrees from wind direction Deep ocean: Thermohaline circulation driven by differences in density (temperature and salinity). Figure from: The Earth System. Kump,Casting and Crane, 2004 6 The Wind Driven Ocean Circulation Subtropical Gyre Antarctic Circumpolar Current Equatorial currents (ACC) 7 Ekman Transport – How winds effect currents? • Considered the effect of wind and friction on multiple levels in the ocean. • Coriolis effects all layers because they’re moving • current velocity decreased exponentially with depth • Surface current moved at 450 to the wind. • Deviation is increased with depth (Ekman Spiral) • The mean current moves at right angle to the wind direction (to the right in the NH, and to the left in the SH) 8 How to build a subtropical gyre 1. Start from the winds: prevailing winds move surface waters toward subtropical regions both from tropical areas and from high- latitudes. 2. Add continent boundaries the subtropical gyres are zones of convergence where surface water piles up and forming slopes. 9 Gyre Dynamics (how to build II) 4. Consider changes in planetary vorticity to get an asymmetric gyre. (Find more about that in Ch. 5 of ‘The Earth System’, or in section 4.2 of the Open University ‘Ocean Circulation’). 3. get geostrophic: Water responds to the slopes of the ocean surface as it would on land -- it runs downhill, but because of earth's rotation it is deflect and flows parallel to the slope. •The balance between gravitational forces and the Coriolis effect, gives rise to geostrophic currents. Their velocity is proportional to the pressure 10 gradient. The subtropical gyres The asymmetrical subtropical gyres are the main mechanism that carry heat poleward and cold water towards the equator. western boundary currents are narrow, fast and deep Gulf Stream (western North Atlantic): 1.5-2.5 m/s (5-9 km/hr) Kuroshio Current (western North Pacific) eastern boundary currents are wide, shallow, and sluggish. Canary Current (eastern North Atlantic), California Current (eastern North Pacific): 0.03-0.07 m/s (< 2 km/hr) 11 Equatorial Currents •Best developed in the Pacific •Consists of: • currents flowing east-to-west (e.g. north equatorial current, south equatorial current), directly driven by the prevailing Trade winds. •The equatorial countercurrent flows down the slope (caused by the trade winds) towards the east, in zone of small westward Costal upwelling wind stress (the Doldrums). 12 The Antarctic Circumpolar Current (ACC) • The only current that flows around the globe (~24,000 km). • “mighty Current” – transports more water than any other current, est. at 100-150 Sverdrups (1 Sv=106 m3 s-1). • extend from the sea surface to depths of 2000- 4000m deep and can be as wide as 2000 km. • typical velocity ~0.1m/s (Much slower than the Gulf Stream ~1-2.5 m/s). •The ACC is in approximately geostrophic equilibrium (PGF=CF) Try to figure out: Drake Passage If the ACC is in geostrophic balance and flows from west to east. • In which direction the Coriolis force is acting? • Could you say will the sea surface be higher to the north or south of the ACC? Would you expect the prevailing winds over Figure from: The Earth System. Kump,Casting and Crane, 2004 the Southern Ocean to be westerly (flowing at the same direction as the ACC) or easterly? (see slide #4 in lec. 3) For more information about the ACC (and other currents) see the ’Ocean Surface Currents’ page from the University13 of Miami at: http://oceancurrents.rsmas.miami.edu/index.html. Thermohaline circulation Another mechanism to drive ocean circulation is through fluxes of heat and freshwater • Seawater are made denser by cooling and/or increasing salinity. •Deep water is formed in localised areas. when the water column then becomes unstable, leading to large-scale deep overturning of the oceans. It is estimated that the turnover time required to replace the deep water through deep water formation is on the order of 1000 -5000 years. 14 Key features of the Thermohaline circulation 1) Deep water formation: in a few localized areas: North Atlantic: in the Greenland-Iceland- Norwegian (GIN) Seas, the Labrador Sea Soutern Ocean: in the Weddell Sea, the Ross Sea. The Mediterranean 3) Upwelling of deep waters not as localized as convection Sea. and difficult to observe (ACC?). 2) Spreading of 4) Near-surface currents: these are required to close the flow deep waters ( ~20% of the Gulf stream, ~20Sv). 15 Spreading of deep waters Water Masses basics: Most of the heat and salt exchange between the atmosphere and the oceans occurs in the upper 150m . Once a parcel of water is removed from the surface, its properties (T,S) will not change until it rises again, many years later. Spreading of deep waters North Atlantic Deep Water (NADW), Antarctic Bottom Water (AABW) Movement of water masses is slowly – and it is unrealistic to measure directly. We deduce it by measuring the age of the water (through carbon- 14 dating, for example). or from the distribution of the water properties themselves. 16 Role of Oceans in Climate variations On time-scale of months or years Oceans are vast reservoir of heat and will regulate the climate by heating or cooling the atmosphere (hurricanes / El Niño). On longer time-scale it is the large heat transport (1PW) of the deep circulation that could change the climate. 3-D Schematic of Thermohaline Circulation by William Schmitz17 The THC and Climate change Change in surface air temperature during years 20-30 after One way to estimate the the collapse of the THC. (from Vellinga and Wood, 2002). effect of the THC is to switch it off in coupled climate models (by adding a lot of freshwater to the northern Atlantic), and compare the surface climate before and -60C after switching it off. Unfortunately, the magnitude and location of the cooling is model dependent. most tend to affect temperatures over land in north-western Europe (Scandinavia, Britain) by several degrees, others show strong cooling further west 18 [This figure]. But have the THC ever been shut? Both Palaeo-data and model simulations suggest that the THC have been weakened or shut during the last glacial. •Would future global warming effect the THC? •Surface warming and surface freshening both reduce density of high latitudes. •Most models predict a significant weakening of the NADW These are the sort of model experiments that we perform in Schematic of three circulation modes of the glacial Atlantic: the prevailing cold mode (center), the warm mode associated with Dansgaard-Oeschger events (lower) and the ‘off’ mode occurring after Heinrich events (upper The role of weakening the Gulf Stream is globe). From: S. Rahmstorf: Thermohaline Ocean Circulation. In: debateable, see: realclimate.org Encyclopedia of Quaternary http://www.realclimate.org/index.php/archives/2005/05/gulf- Sciences, Edited by S. A. Elias. Elsevier, Amsterdam 2006. stream-slowdown/ 19 Summary • The circulations of the oceans and atmosphere are driven by energy from the sun and modified by the earth rotation. Both help to reduce equator-to-pole temperature gradients. • Oceans and atmosphere are a single system of two interacting components, coupled by the air-sea interactions. Oceans can absorb heat in one region and restore it to the atmosphere (perhaps decades or centuries later) at a quite different place. • Thermohaline and wind-driven currents cannot be separated by oceanographic measurements. • two distinct physical forcing • very different time scales • but not two uniquely separable circulations. • A THC collapse is seen to have happen in the past. It is discussed as one of "low probability - high impact“ risks associated with global warming. 20 Reading List Most of the material covered here is in: Kump, L.R., Kasting, J.F and Robert, G. Crane: The Earth System (Ch. 5) Other sources: Open University: Ocean Circulation Tomczak, M., Godfrey, S.J: Regional Oceanography (Ch. 1-5). This is an online book that can be downloaded from: http://www.es.flinders.edu.au/~mattom/regoc/pdfversion.html Bigg, GR.: The Oceans and Climate Dennis Hartmann: Global Physical Climatology Good online sources for the thermohaline circulation: Rahmstorf, S: A brief fact sheet http://www.pik-potsdam.de/~stefan/thc_fact_sheet.html Rahmstorf, S: Thermohaline Ocean Circulation. In: Encyclopedia of Quaternary Sciences, http://www.pik-potsdam.de/~stefan/Publications/Book_chapters/Rahmstorf_EQS_2006.pdf 21 Extra Slides • Why there’s no deep water formation in the Pacific.
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