Patterns and observations? Patterns and observations? Observations? Patterns? Observations? Patterns? Geometry of the ocean Actual bathymetry (with vertical exaggeration) Continental Continental Basin Mid-ocean Seamount Trench shelf slope rise (Abyssal plain) ridge Abyssal plain flatness ~ 1:1000 Exaggeration: 1000:1 1
Geometry of the ocean Summary Seafloor is created at spreading centers and is destroyed at subduction zones Processes at plate boundaries lead to the creation of mountains, trenches, abyssal plains, and earthquakes Gravity (slab pull, ridge push) is the primary driver of plate tectonics and generates convection cells within the mantle The Atlantic ocean is growing, the Pacific Ocean is shrinking, and the shape of the ocean is changing over very long time scales. The shapes of the ocean basins are the result of plate tectonics and the deposition and erosion of sediment Biological significance of ocean currents Biological significance of ocean currents Blue: Low chlorophyll Green: Higher chlorophyll Yellow/Red: Really high chlorophyll Satellite image of chlorophyll-a in surface water. This is a measure of phytoplankton biomass 2
Pressure 1/17/2018 Ocean currents Learning objectives Understand the forces that drive ocean currents Learn how these forces are related to winds, tides, density and sea-surface slope Relate these processes to major ocean currents around the globe Primary forces Pressure gradients Coriolis Wind and friction Gravity Pressure gradient Formation: Sea-surface slope or variation in seawater density Direction: From high to low pressure Magnitude: Equal to the (-) spatial variation in pressure (the slope) Force Variation in the Coriolis effect with latitude Coriolis effect Coriolis effect as a function of latitude 1 0.8 0.6 0.4 Equator Pole 0.2 Coriolis effect (acceleration) earth s rotation velocity latitude C F = 2 w v sin f Distance 0 0 15 30 45 60 75 90 Latitude 3
What direction does the water flow? Imagine flow due to pressure gradient + Coriolis Direction? To the right of the pressure gradient in the Northern Hemisphere To the left of the pressure gradient in the Southern Hemisphere In Northern Hemisphere: Pressure gradient Wind Formation: Variation in atmospheric temperature creates atmospheric pressure gradients. Pressure gradients + Coriolis create surface wind patterns Friction between wind and surface water (wind stress) creates currents Direction is determined by wind stress, Coriolis, and friction In Northern Hemisphere: Friction Velocity Wind stress Coriolis Coriolis Velocity Pressure gradients, Coriolis, and wind-driven circulation I. Geostrophic flow: (Pressure-gradient, coriolis balance) Winds Polar Easterlies II. Wind-driven circulation Westerlies Northern Hemisphere Ekman spiral Trade Winds Westerlies Ekman depth (opposite of surface current direction not 100 m) Polar Easterlies 4
Surface Dynamic Topography Surface Currents WIND Red = high Blue = low WIND WIND WIND Surface Dynamic Topography (red is high, blue is low) Surface Currents 5
Surface Dynamic Topography (red is high, blue is low) Upwelling of deep water around Antarctica Surface Dynamic Topography (red is high, blue is low) Great ocean conveyor Blue arrows: Deep currents (~ 3000 meters depth) Red Arrows: Surface current 6
Top 1000 m 1/17/2018 Atlantic Ocean Deep water oxygen T S Ocean viewed at 4000m water depth Oxygen drops from Atlantic to Pacific Dissolved oxygen traces the path of deep ocean water Neutral density γ n O 2 south north south north Significance of deep-water formation and the ocean conveyor Supplies oxygen to the deep ocean Removes CO 2 from the atmosphere (approximately 30% of anthropogenic CO 2 has entered the ocean) Strongly influences earth s climate (by removing cold surface water) Affects nearly all aspects of deep-water chemistry Surface Currents 7
Equatorial currents Biological significance of ocean currents E-W Sea surface slope Asia N. America Satellite image of chlorophyll-a in surface water. This is a measure of phytoplankton biomass Significance of upwelling Important locations of upwelling Antarctica Equatorial waters Eastern sides of ocean basins (Peru, California, West Africa, etc.) North Pacific (end of the Ocean Conveyor) Affects Brings cold, deep water to sea surface Influences weather by changing air temperatures Brings nutrients to the surface to stimulate phytoplankton growth Other types of mixing: N. Atlantic mixing in winter delivers nutrients to surface waters, initiates great ocean conveyor Summary Flow moves 90 to the right of pressure gradient in the N. hemisphere Flow moves 90 to the left of pressure gradient in S. hemisphere Surface water moves 45 to right of the wind in N.H. and to the left of the wind in the S.H. Average flow moves 90 to right of wind in N.H. and to the left in S.H. Oceanic gyres circulate around mounds indicating high pressure Gravity is important for the formation of deep water Phytoplankton abundance is highest in areas of upwelling and mixing 8