Thermohaline Circulation SOEE 3410 Lecture 12 Thermohaline Circulation

Thermohaline Circulation SOEE 3410: 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 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Revision 90 Ferrel Cell 60 Polar Cell Heat Transport Net Radiation 30 0 30 60 90 Idealized model of atmospheric circulation. N. B. actual circulations are not continuous in space or time. SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

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 +ve -ve Air-sea interface Ocean +ve Qad Advection Net sensible heat Qsen -ve +ve -ve Qnet = Qsw+Qlat+Qsen+Qad SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics +ve

Radiation terms: short-wave & long-wave Qsw Qlw ECMWF 40 -year reanalysis. Units are W/m 2. +ve is into the surface. From Kallberg et al 2005. SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Latent & sensible heat terms Qlat Qsen ECMWF 40 -year reanalysis. Units are W/m 2. From Kallberg et al 2005. SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Net heating at surface Qsw +Qlw + Qlat + Qsen ECMWF 40 -year reanalysis. Units are W/m 2. From Kallberg et al 2005. SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Meridional Heat Transport Units of PW = 1015 W Heat transport north 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). SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Global surface current system From: Ocean Circulation - 2 nd ed. , The Open University SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

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 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Annual mean SST SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Annual mean global salinities SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Global thermohaline circulation - theory development • Gulf Stream region - well studied back to 19 th century • Deep measurements of T and S -> geostrophic flow calculations sea-surface east west Fpg Fc Gulf Stream - N Atlantic i. e. N hemisphere => Coriolis force acts to right of flow • 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 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Thermohaline circulation (Meridional overturning) SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Thermohaline circulation - North Atlantic Thermohaline forcings in North Atlantic: 1. 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 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Circulation in the Nordic Seas SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

NADW - deep convection SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

North Atlantic - Air-Sea interaction SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Deep convection SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Mean vertical structure At low and mid-latitudes, temperature primarily determines density. At high latitudes, T < 5 o. C - salinity is more important. SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

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? SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Modelling - capturing global heat transport World Ocean Circulation Experiment (WOCE) – 1990 s: • 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 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Modelling - capturing global heat transport Model: Theory: net heat transport from Atlantic Ocean into Indian Ocean net heat transport into Atlantic Ocean from Indian Ocean Why the difference? From “Ocean Circulation”, OU 2002 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Climate implications: Oceans heating up In the last half of the 20 th 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 Atmosphere 18. 2 x 1022 J 6. 6 x 1021 J Levitus et al. Science, 2000 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

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… SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Climate implications: salinity levels Salinity distributions have been changing in the last few decades Low latitude surface waters have become markedly more saline Water masses formed at high latitudes have become fresher SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

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 SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics

Summary: Climate implications • Climate change: - surface air temperatures SST surface salinities deep water formation thermohaline circulation feedback on climate SOEE 3410 : Coupled Ocean & Atmosphere Climate Dynamics