The Circulation of the Oceans

Key Questions Why do ocean currents form? How can the circulations of both the surface ocean and the deep ocean basins be driven by solar radiation and be closely linked, yet operate at very different scales? What role do ocean circulation play in the global climate system?

Types of Ocean Circulation Surface ocean circulation Moved by winds Upper 50-100 m of ocean Thermohaline circulation Moved by density differences Both contribute to Redistribution of energy in the Earth System Distribution of nutrients in the ocean

SURFACE-OCEAN CIRCULATION Energy the driving force Available solar energy drives circulation of atmosphere and ocean Circulation of ocean linked to circulation of atmosphere by friction Imbalance in latitudinal distribution of energy creates wind Currents move due to friction as blowing winds drag the surface along Force known as wind stress

Solar Energy and Oceans Solar energy warms top few hundred meters of ocean 90% of radiation absorbed in top 100 m of ocean Unlike atmosphere, oceans do not move directly by temperature gradient Warm on top, stable Ocean is stratified wrt temperature (and density and salinity) Oceans have high heat capacity Ocean surface temperature influences atmospheric circulation Resulting winds determines circulation of upper ocean

SURFACE CURRENTS

CORIOLIS EFFECT Coriolis affects ocean circulation To the right in Northern Hemisphere To the left in Southern Hemisphere Deflection 20-25 from wind direction

GYRES Large circular ocean circulation patterns Combination of Coriolis Effect and continent obstacles

EKMAN SPIRAL & TRANSPORT

CONVERGENCE & DOWNWELLING

DIVERGENCE & UPWELLING Ekman transport due to the easterly trade winds causes upwelling along the equator.

COASTAL DIVERGENCE/UPWELLLING & CONVERGENCE/DOWNWELLING Coastal upwelling areas across the globe

The Ekman Spiral & Upwellng and Downwelling Bombeamento de Ekman

Geostrophic Flow Ekman transport piles up water within subtropical gyres Surface water flows downhill and to the right (Northern Hemisphere) Geostrophic flow – balance of Coriolis Effect and gravitational forces Ideal geostrophic flow would make water flow in circles within the gyre Friction generates actual geostrophic flow, slowing the water down so it falls to the outside of the gyre © 2011 Pearson Education, Inc.

GEOSTROPHIC CURRENT

BOUNDARY CURRENTS

BOUNDARY CURRENTS Western Boundary Currents Eastern Boundary Currents Narrow Fast Eastern Boundary Currents Diffuse Slow Divergent Upwelling

Western Intensification Top of hill of water displaced toward west due to Earth’s rotation Western boundary currents intensified in both hemispheres Faster (100s mi/day) Narrower (60 mi) Deeper (up to 1.2 mi) Warm Coriolis Effect contributes to western intensification © 2011 Pearson Education, Inc.

THE GULF STREAM – Western Boundary Narrow – 50-75 km wide Warm – 20° C + Deep – 1 km Fast – 3-10 km/hr

Eastern Boundary Currents Eastern side of ocean basins Tend to have the opposite properties of Western Boundary Currents Cold Slow (10s of mi/day) Shallow (0.3 mi) Wide (600 mi) © 2011 Pearson Education, Inc.

CANARY CURRENT– Eastern Boundary Slow Shallow – 500m Broad – 1000 km

SARGASSO SEA The Sargasso Sea in the North Atlantic is bounded by the Gulf Stream on the west, the North Atlantic Current on the north, the Canary Current on the east, and the North Equatorial Current on the south

GREAT PACIFIC GARBAGE PATCH

GREAT PACIFIC GARBAGE PATCH Areas with a higher concentration of plastic Much of the debris found in these areas are small bits of plastic (microplastics) that are suspended throughout the water column The debris is more like flecks of pepper floating throughout a bowl of soup, rather than a skim of fat that accumulates (or sits) on the surface

VORTICITY CCW = + CW = -

Ocean Circulation & Sea Surface Temperatures

Ocean Circulation & Sea Surface Temperatures Water moving toward poles warmer that polar ocean Water moving toward equator colder than tropical ocean Ocean currents add to latitudinal redistribution of energy Warmer water moved toward poles, cooler water moved toward equator

Ocean Circulation & Sea Surface Temperatures

SNEAKER OCEANOGRAPHY

Ocean Memory Ocean absorb & store large amounts of heat & release slowly Oceans memory of temperature change longer than atmosphere Oceans most likely to have processes that cause climate anomalies for long periods of time Where to look? Tropics Poles

Atmosphere & Ocean Interaction

ENSO El Nino-Southern Oscillation event EL Nino – Southern Oscillation Major shift in tropical Pacific ocean circulation that occurs every 2-10 years Large changes in circulation of tropical atmosphere Significant climate anomalies in tropics and midlatitudes Southern Oscillation When pressures are low in the western Pacific, they tend to be high in the east When pressures are high in the western Pacific, they tend to be lower the east

Western Pacific – intense convection Strong easterly winds Upwelling coastal SA Heavy precipitation in Australia and Indonesia La Nina – when normal pattern intensifies Trade winds weaken/reverse Western Pacific – build up of warm water Kelvin wave moves back across ocean (time = 60 days) Central Pacific warms (convection intense) Upwelling coastal SA stops (reduces biologic productivity) Heavy precipitation in central Pacific & western Americas

Southern Oscillation Index (SOI)

CLLIMATIC IMPACTS OF ENSO

Deep-Ocean Currents Thermohaline Circulation – deep ocean circulation driven by temperature and density differences in water Below the pycnocline 90% of all ocean water Slow velocity Takes about 1,000 years to complete one loop © 2011 Pearson Education, Inc.

SALINITY Total dissolved inorganic solids A measure of the quantity of elements (Na + Cl, most common) dissolved in a given mass of seawater g salt / kg seawater average is 35 PSU (Practical Salinity Units – based on conductivity) or 35 ppt (parts per thousand, or ‰) or per mil

SOURCE OF IONS IN SEAWATER Weathering of crustal rocks (Na) 4 x 106 tons annually Outgassing from volcanoes (Cl) Recirculating in the mantle (at mid ocean ridges)

Salinity

PRINCIPLE OF CONSTANT PROPORTIONS Dissolved ions in seawater found in nearly constant proportions worldwide Exception – ions used by marine organisms Calcium Silicon

WHY ISN’T THE OCEAN GETTING SALTIER? Salt is removed: Evaporation – evaporate deposits Biological processes – shells Chemical reactions – with volcanic rock Sea spray – removed Na + Cl RESULT - removal rate = input rate

T (Thermocline) + H (Halocline) = P (Pycnocline) Low Latitudes

Western Atlantic

Pacific Ocean

Thermohaline Circulation Originates in high latitude surface ocean Cooled, now dense surface water sinks and changes little. Deep-water masses identified on temperature–salinity (T–S) diagram Identifies deep water masses based on temperature, salinity, and resulting density © 2011 Pearson Education, Inc.

Thermohaline Circulation © 2011 Pearson Education, Inc.

T–S Diagram © 2011 Pearson Education, Inc.

Thermohaline Circulation Some bottom water masses Antarctic Bottom Water (AABW) North Atlantic Deep Water (NADW) Antarctic Intermediate Water (AAIW) Cold surface seawater sinks at polar regions and moves equatorward © 2011 Pearson Education, Inc.

AABW FORAMTION

NADW FORMATION

Circulation of the Atlantic Ocean

AGE OF OCEAN WATER A parcel of AABW will reemerge at the surface in the Indian Ocean in ~335 yrs and in the Pacific in ~595 yrs Average residence time in deep ocean is ~500 yrs

Conveyor Belt Circulation © 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

THERMOHALINE CIRCULATION Water sinking at high latitudes rises somewhere else Upwelling returns bottom waters to surface Former deep water moved to poles through surface circulation Works like giant conveyor belt Recycles nutrients Impacts climate

Putting it all together …… © 2011 Pearson Education, Inc.

RECYLING NUTRIENTS Plants and animals living at or near the surface deplete nutrients Organisms die, sink to the bottom and decompose Nutrients released Deep ocean rich in nutrients Thermohaline circulation distributes nutrients around globe Upwelling (along continental margins) returns nutrients to surface Great concentrations of marine life along margins

PRODUCTIVITY OF THE OCEAN

OCEAN CIRCULATION & CLIMATE Transport of warm surface water toward pole transfers excess solar energy poleward Provides almost as much poleward heat transfer as the atmosphere Transports more heat at low latitudes (Atmosphere transports more at mid- and high latitudes)

OCEAN CIRCULATION & CLIMATE

OCEAN CIRCULATION & CLIMATE Oceans are a huge reservoir of heat Pools of warmer than normal or cooler than normal will cool or warm the atmosphere Months Seasons Years Based on the rates of bottom water formation, it would take ~1000 yrs to recycle all the water in the oceans Thermohaline circulation could moderate the climate over time periods of ~ 1000 yrs