Lecture 4 : OCEANS (Outline) Basic Ocean Structures Warm up by sunlight! Basic Structures and Dynamics Upper Ocean (~100 m) Ekman transport Geostrophic currents Shallow, warm upper layer where light is Surface Ocean Circulation abundant and where most marine life can be found. Subtropicl gyre Boundary current Deep Ocean Deep Ocean Circulation Cold, dark, deep ocean where plenty supplies of Thermohaline conveyor belt nutrients and carbon exist. ESS200A ESS200A Prof. Jin-Yi Yu No sunlight! Prof. Jin-Yi Yu Basic Ocean Current Systems The State of Oceans Upper Ocean surface Temperature circulation warm on the upper ocean, cold in the deeper ocean. Salinity variations determined by evaporation, precipitation, Deep Ocean sea-ice formation and melt, and river runoff. Density deep ocean circulation small in the upper ocean, large in the deeper ocean. ESS200A ESS200A (from “Is The Temperature Rising?”) Prof. Jin-Yi Yu Prof. Jin-Yi Yu 1 Salinity E < P Sea-ice formation and melting Potential Temperature E > P Potential temperature is Salinity is the mass of very close to temperature in dissolved salts in a the ocean. kilogram of seawater. The average temperature Unit: ‰ (part per of the world ocean is about thousand; per mil). 3.6°C. The average salinity of the world ocean is 34.7‰. Four major factors that affect salinity: evaporation, precipitation, inflow of river water, and sea-ice formation and melting. ESS200A (from Global Physical Climatology ) ESS200A (from Global Physical Climatology ) Prof. Jin-Yi Yu Prof. Jin-Yi Yu Low density due to absorption of solar energy near the surface. Density and Temperature and Salinity Density Seawater is almost incompressible, so the density of seawater is always very close to 1000 kg/m 3. Potential density is the density that seawater with a particular salinity and temperature would have at zero water pressure (or at surface air pressure). Potential density = density – 1000 kg/m 3. (from Global Physical Climatology ) ESS200A ESS200A Prof. Jin-Yi Yu (Figure from Oceanography by Tom Garrison) Prof. Jin-Yi Yu 2 Vertical Structure of Ocean Mixed Layer Processes The depth of the mixed layer is determined by (1) the rate of Mixed Layer: T and S well mixed by winds buoyancy generation and (2) the rate of kinetic energy supply. Temperature Thermocline: large gradient of T and S The atmosphere can affect the Salinity mixed layer through three Deep Ocean: T and S independent of height processes: heating, wind forcing, cold and freshening (P-E). salty The global-average depth of high nutrient level the mixed layer is about 70 m. The heat capacity of the mixed layer is about 30 times the heat (from Global Physical Climatology ) capacity of the atmosphere. (from Climate System Modeling ) ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu Seasonal Variation of Mixed Layer Two Circulation Systems Summer: warm and thin. Winter: cold and deep (several hundred meters). density-driven wind-driven circulation (from Global Physical Climatology ) (Figure from The Earth System ) circulation ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu 3 Global Surface Currents Six Great Current Circuits in the World Ocean 5 of them are geostrophic gyres: North Pacific Gyre South Pacific Gyre North Atlantic Gyre South Atlantic Gyre Indian Ocean Gyre The 6 th and the largest current: Antarctic Circumpolr Current (also called West Wind Drift) (Figure from Oceanography by Tom Garrison) (from Climate System Modeling ) ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu Characteristics of the Gyres Major Current Names (Figure from Oceanography by Tom Garrison) Currents are in geostropic balance Each gyre includes 4 current components: Western Boundary Current Trade Wind-Driven Current two boundary currents: western and eastern Gulf Stream (in the North Atlantic) North Equatorial Current two transverse currents: easteward and westward Kuroshio Current (in the North Pacific) South Equatorial Current Western boundary current (jet stream of ocean) Brazil Current (in the South Atlantic) the fast, deep, and narrow current moves warm Eastern Australian Current (in the South Pacific) water polarward (transport ~50 Sv or greater) Agulhas Current (in the Indian Ocean) Eastern boundary current the slow, shallow, and broad current moves cold water equatorward (transport ~ 10-15 Sv) Eastern Boundary Current Westerly-Driven Current Trade wind-driven current Canary Current (in the North Atlantic) North Atlantic Current (in the North Atlantic) the moderately shallow and broad westward California Current (in the North Pacific) North Pacific Current (in the North Pacific) current (transport ~ 30 Sv) Benguela Current (in the South Atlantic) Westerly-driven current Peru Current (in the South Pacific) Western Australian Current (in the Indian Ocean) the wider and slower (than the trade wind-driven Volume transport unit: current) eastward current 1 sv = 1 Sverdrup = 1 million m 3/sec ESS200A ESS200A (the Amazon river has a transport of ~0.17 Sv) Prof. Jin-Yi Yu Prof. Jin-Yi Yu 4 Gulf Stream Surface Current – Geostrophic Gyre A river of current Mixed Layer Jet stream in the ocean Currents controlled by frictional force + Coriolis force wind-driven circulation Ekman transport (horizontal direction) convergence/divergence downwelling/upwelling at the bottom of mixed layer Speed = 2 m/sec Depth = 450 m Thermocline Width = 70 Km downwelling/upwelling in the mixed layer Color: clear and blue pressure gradient force + Coriolis force geostrophic current Sverdrup transport (horizontal) (Figure from Oceanography by Tom Garrison) ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu Step 1: Surface Winds Winds and Surface Currents Polar Cell Ferrel Cell Hadley Cell (Figure from The Earth System ) (Figure from Oceanography by Tom Garrison) ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu 5 Step 2: Ekman Layer Ekman Spiral – A Result of Coriolis Force (frictional force + Coriolis Force) (Figure from Oceanography by Tom Garrison) ESS200A (Figure from The Earth System ) ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu Formula for Ekman Transport How Deep is the Ekman Layer? D ∝ (ν/f)1/2 ν = vertical diffusivity of momentum f = Coriolis parameter = 2 Ωsin φ ESS200A (from Climate System Modeling ) ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu 6 Step 3: Geostrophic Current Ekman Transport (Pressure Gradient Force + Corioils Foce) NASA-TOPEX Observations of Sea-Level Hight (from Oceanography by Tom Garrison) (Figure from The Earth System ) ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu Ekman Transport Convergence/Divergence Geostrophic Current (Figure from The Earth System ) Surface wind + Coriolis Force Ekman Transport Forces Geostrophic Gyre Currents Convergence/divergence Thermocline (in the center of the gyre) Pressure Gradient Force Geostrophic Currents (Figure from The Earth System ) ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu 7 Step 4: Boundary Currents Boundary Currents Eastern boundary currents: broad and weak (Figure from Oceanography by Tom Garrison) ESS200A Western boundary currents: narrow and strong ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu Eastern Boundary Current Costal Upwelling/Downwelling Cold water from higher latitude ocean. Costal upwelling associated with subtropical A result of Ekman high pressure system. transport and mass Atmospheric subsidence continuity. produce persistent stratiform clouds, which further cool down SSTs by blocking (from Global Physical Climatology ) solar radiation. (Figure from Oceanography by Tom Garrison) ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu 8 Global Surface Currents Equatorial Current System The Equatorial Counter Current , which flows towards the east, is a partial return of water carried westward by the North and South Equatorial currents. ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu Equatorial Under Current Deep Ocean Circulation: Density-Driven The most prominent of all eastward flows is the Equatorial Undercurrent (EUC). It is a swift flowing ribbon of water extending over a distance of more than 14,000 km along the equator with a thickness of only 200 m and a width of at most 400 km. The current core is found at 200 m depth in the west, rises to 40 m or less in the east and shows typical speeds of up to 1.5 m s-1. Its existence remained unknown to oceanographers until 1952. ESS200A ESS200A Prof. Jin-Yi Yu (Figure from Oceanography by Tom Garrison) Prof. Jin-Yi Yu 9 Thermohaline Circulation Two Regions of Deep Water Formation Antarctic Bottom Water Salinity = 34.65‰ Temperature = -0.5°C Density = 1.0279 g/cm 3 Formed at Weddell Sea Related to ice formation During Winter North Atlantic Deep Water Due to winter cooling and evaporation. (Figure from Oceanography by Tom Garrison) (Figure from Oceanography by Tom Garrison) ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu Two Processes to Increase Ocean Water Mass Salinity in High Latitudes Surface Water to a depth of about 200 meters • Ocean water masses possess distinct, identifiable properties Central Water and don’t often mix easily Evaporation : Extremely cold, dry winter when they meet. to the bottom of the main thermocline air enhances evaporation from the relatively • In stead, they usually flow warm ocean increase salinity in the Intermediate Water above or below each other. to about 1500 meters • Ocean water mass can retain ocean. their identity for great distance Deep Water and long periods of time. Formation of Sea Ice : When sea ice forms, below intermediate water but not in • Oceanographers name water contact with the bottom masses according to their salts are left in the ocean increase relative position. salinity Bottom Water in contact with sea floor ESS200A ESS200A Prof. Jin-Yi Yu Prof. Jin-Yi Yu 10 Five Types of Air Masses Formation of Water Mass Theoretically, there should be 6 types of air masses (2 moisture Once a water parcel is removed from the surface layer its temperature and salinity do not change until it rises back up to the surface again, types x 3 temperature types).