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Thermohaline Circulation

Thermohaline Circulation. SOEE3410: Lecture 12. Thermohaline Circulation. Global heat redistribution - oceans’ role Surface currents Density-driven deep currents Thermohaline ‘conveyor belt’ North Atlantic Deep Water (NADW) Climate implications.

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Thermohaline Circulation

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  1. Thermohaline Circulation SOEE3410: Lecture 12

  2. Thermohaline Circulation • Global heat redistribution - oceans’ role • Surface currents • Density-driven deep currents • Thermohaline ‘conveyor belt’ • North Atlantic Deep Water (NADW) • Climate implications SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  3. Revision 90 60 30 0 30 60 90 Ferrel Cell Polar Cell Net Radiation Heat Transport Idealized model of atmospheric circulation.N.B. actual circulations are not continuous in space or time. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  4. Heat budget equation Heat fluxes into a region of ocean: Incoming Short wave radiation Qsw Net Long-wave radiation Qlw Turbulent Fluxes Atmosphere -ve Net latent heat Qlat Net sensible heat Qsen +ve -ve -ve Air-sea interface +ve +ve +ve -ve Qad Advection Ocean Qnet = Qsw+Qlw+Qlat+Qsen+Qad SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  5. Radiation terms: short-wave & long-wave Qsw Qlw ECMWF 40-year reanalysis. Units are W/m2. +ve is into the surface. From Kallberg et al 2005. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  6. Latent & sensible heat terms Qlat Qsen ECMWF 40-year reanalysis. Units are W/m2. From Kallberg et al 2005. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  7. Net heating at surface Qsw +Qlw + Qlat + Qsen ECMWF 40-year reanalysis. Units are W/m2. From Kallberg et al 2005. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  8. Meridional Heat Transport Heat transport north Units of PW = 1015 W Heat transport south Northward heat transport for 1988 in each ocean and the total transport summed over all oceans calculated by the residual method using atmospheric heat transport from ECMWF and top of the atmosphere heat fluxes from the Earth Radiation Budget Experiment satellite. From Houghton et al., (1996: 212), using data from Trenberth and Solomon (1994). SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  9. Global surface current system From: Ocean Circulation - 2nd ed., The Open University SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  10. Deep ocean circulation • Deep ocean i.e. the other 80 % of the ocean is driven by: • Poleward transport of heat • Air-sea interaction at the poles • Results in the formation of water masses • i.e. waters with specific core temperature and salinity signatures • Spatial differences in heat/salt (density) drive a much slower process • than the surface wind-driven circulation called: • Thermohaline Circulation Thermo haline (heat) (salinity) Density SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  11. Annual mean SST SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  12. Annual mean global salinities SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  13. Global thermohaline circulation - theory development • Gulf Stream region - well studied back to 19th century • Deep measurements of T and S -> geostrophic flow calculations sea-surface east Gulf Stream - N Atlantic i.e. N hemisphere => Coriolis force acts to right of flow west Fpg Fc • Calculations: northward (NE) surface flow - Gulf Stream • Also evidence of deep counter-currents i.e. southward (SW) flow • Stommel (1965) developed theory of global thermohaline circulation SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  14. Thermohaline circulation (Meridional overturning) SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  15. Thermohaline circulation - North Atlantic • Thermohaline forcings in North Atlantic: • Thermal forcing • High-latitude cooling; low-latitude heating • => northward surface flow • 2. Haline forcing • Net high-latitude freshwater gain at surface from melting ice in summer and precipitation and continental run-off; • low-latitude evaporation increases surface salinity • => southward surface flow • High latitude increase in density (brine rejection from freezing ice + cooling) causes sinking • => southward deepwater flow • At present, thermal forcing dominates North Atlantic, • =>flow of upper current is northward SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  16. Circulation in the Nordic Seas SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  17. NADW - deep convection SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  18. North Atlantic - Air-Sea interaction SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  19. Deep convection SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  20. Mean vertical structure At low and mid-latitudes, temperature primarily determines density. At high latitudes, T < 5oC - salinity is more important. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  21. SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  22. Summary - thermohaline circulation • Generally, at high latitudes, oceans: • - lose heat • - gain fresh water (precipitation and continental runoff), • which have opposite effects on the density of ocean water • Density of high latitude water is influenced by warm, salty water from the low latitudes. • This constitutes the positive feedback maintaining the Atlantic Thermohaline circulation • This intricate balance is influenced by: • - surface heat fluxes, and • - fresh water mass • i.e. precipitation, evaporation, continental runoff, sea-ice formation • Are these processes likely to change in the future. • What are the climate implications? SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  23. Modelling - capturing global heat transport • World Ocean Circulation Experiment (WOCE) – 1990s: • establish role of oceans in global climate system • obtain suitable basline dataset for assessing future climate • develop means to predict climate change i.e. models From “Ocean Circulation”, OU 2002 SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  24. Modelling - capturing global heat transport Model: net heat transport from Atlantic Ocean into Indian Ocean Theory: net heat transport into Atlantic Ocean from Indian Ocean Why the difference? From “Ocean Circulation”, OU 2002 SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  25. Climate implications: Oceans heating up In the last half of the 20th century, it is clear that the world oceans are heating up. The oceans have absorbed about 30 times more heat than the atmosphere since 1955 Oceans 18.2 x 1022 J Atmosphere 6.6 x 1021 J Levitus et al. Science, 2000 SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  26. Climate implications: not just about temperature It is expected that the planetary water cycle – Evaporation/Condensation/Precipitation and Freezing/Melting – will be altered as a result of global warming. There is ample evidence of it already happening… SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  27. Climate implications: salinity levels Salinity distributions have been changingin the last few decades Low latitude surface waters have become markedly more saline Water masses formed at high latitudes have become fresher SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  28. Climate implications: precipitation Increased precipitation is perhaps the dominant factor: - elevating continental run-off into the Arctic basins, - contributing to salinity decreases in N Atlantic and N Pacific SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

  29. Summary: Climate implications • Climate change: • - surface air temperatures • - SST • - surface salinities • - deep water formation • - thermohaline circulation • - feedback on climate SOEE3410 : Coupled Ocean & Atmosphere Climate Dynamics

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