5/12/17 EPSS 15 Spring 2017 Introduction to Oceanography Laboratory #6 Ocean and Atmosphere Circulation Atmospheric Circulation • The amount of heat per unit area at the equator is greater than at higher latitudes. • This creates a heat differential/ imbalance. • The winds are generated to restore equilibrium. 1 5/12/17 If the Earth did not rotate we would observe a single circulation cell in each hemisphere that would bring hot air from the equator to the cooler polar regions. Warm air rises near the Equator and travels toward the poles This is deflected by the Coriolis effect and sinks at 30° where it flows as surface winds toward the Equator (Trade winds) and toward 60°(Westerlies) 2 5/12/17 Ocean Circulation • Surface currents - driven by winds • Deep ocean circulation - caused by buoyancy forces (temperature and salinity differences; driving force is gravity) Ekman Transport Winds push the ocean surface and set it in motion, but due to the Coriolis effect transport is deflected. Deflection is to the right in the northern hemisphere and left in the southern hemisphere. Under ideal conditions, the surface layer is deflected 45°and net transport is 90°. 3 5/12/17 Ocean Circulation • Surface circulation is driven by the winds. Transport is deflected by the North subtropical Coriolis effect and gyre by continents forming the subtropical gyres. These subtropical gyres are the main South subtropical component of gyre surface circulation Subtropical Gyres There are five subtropical gyres; each has four component currents 4 5/12/17 Subtropical Gyres have four components: 1. Equatorial Current - driven by Tradewinds 2. Western Boundary Current (HOT) - deflected by Coriolis and continents 3. West wind drift - driven by Westerlies 4. Eastern Boundary Current (COLD) - deflected by Coriolis and continents Eastern Western BC BC Deep Currents • Driven by buoyancy (density) forces - Gravity is the driving force. • Referred to as thermohaline circulation: - thermo=heat, haline=salt. • Both of these affect density. • Flow is much slower than surface currents: ~1000 years for deep water to circulate. • Formation of deep water is primarily due to evaporation and the formation of sea ice: - both these processes make the water more dense and thus sink to the bottom. 5 5/12/17 NADW - North Atlantic Deep Water – forms near Greenland; evaporation and cooling causes surface water to sink and flow south AABW - Antarctic Bottom Water – Densest deep water – Cold, dense water from sea ice formation in the Weddell Sea sinks and mixes with NADW to form AABW Young deep mixing Old deep water here here water here Deep Currents 6 5/12/17 Oceans have a stable density stratification, with less dense layers over deeper, more dense layers Pycnocline is a region of rapid density change, caused by steep gradients in temperature and salinity Water masses can be distinguished by their temperatures and salinities 7 5/12/17 EPSS 15 Fluid Dynamics Demo 8 5/12/17 9 5/12/17 10 5/12/17 11 .