Ocean Circulation At 50°N, you’ll find both polar bears and palm trees…. Polar Bear Provincial Park; Ontario, Canada Scilly Isles; Britain

50°N latitude www.polarhusky.com/logisticsmaps-and-route/circumpolar

Ocean Circulation Why the great difference in climate between interior Canada and the British Isles? Cold air flowing over Canada’s cold interior loses heat, but is warmed when it reaches the Atlantic Ocean and the Gulf Stream The Gulf Stream is a current, a moving mass of water driven by wind and differences in water density

Ocean Circulation There are 2 major types of currents: Surface currents are wind-driven movements of water at or near the ocean’s surface (uppermost 400 meters of ocean); involve ~10% of the world’s ocean water Thermohaline currents are slow, deep currents that arise from density differences caused by variations in water’s temperature and salinity

Surface currents Surface currents move water horizontally, and are primarily driven by winds Waves on the sea surface transfer energy from the moving air to the water by friction; the water flowing beneath the wind forms a surface current Only 2% of wind speed is transferred to ocean current (resulting current is 2% of wind speed) Water literally “piles up” in the direction of the wind and gravity pulls the water down slope

Surface currents When water moves down slope, the Coriolis effect intervenes! Because of the Coriolis effect, surface currents in the Northern Hemisphere flow to the right of the wind direction, and in the Southern Hemisphere flow to the left of the wind direction Additionally, continents and submarine topography frequently block or deflect flow into a circular pattern, called a gyre

(Winds driven by uneven solar heating) The formation of gyres  Clock-wise Counter clock-wise (Winds driven by uneven solar heating)

The North Atlantic Gyre

Surface currents The trade winds which blow from the southeast in the Southern Hemisphere, and from the northeast in the Northern Hemisphere set the current in motion between the tropics (equatorial currents) When equatorial currents reach the western boundary of the ocean basin, they must turn because they cannot cross land; Coriolis deflects these currents away from the equator (western boundary currents)

Western boundary currents Equatorial currents

The Gulf Stream is a western boundary current

Western boundary currents Western boundary currents are fast, narrow, and deep surface currents that carry warm water from the equator to the poles Eastern boundary currents, on the other hand, flow back across the ocean basin carrying cool water from the poles to the equator (also deflected by continents and the Coriolis effect)

Wind-driven surface currents

Ekman Spiral and Transport The collision of air molecules (in wind) with water molecules at the sea surface generates the water current Once this surface film of water molecules in set in motion, they exert a frictional drag on the water molecules immediately beneath them, getting these to move as well If the wind blows persistently, motion is transferred downward into the water column

Ekman Spiral and Transport As this wind-driven current deepens, its speed diminishes (decreases) because of the growing distance from the driving force (the wind) The current’s flow direction also changes with depth, the result of Coriolis deflection! In the Northern Hemisphere, surface current flows to the right of the wind direction When this topmost layer sets the underlying layer of water in motion, this deeper layer also moves to the right of the direction of flow with each successively deeper layer deflected to the right of the layer immediately above it

This spiraling flow pattern is called the Ekman spiral

Ekman Spiral and Transport Under the influence of a strong, persistent wind, the Ekman spiral may extend downward to a depth of 100-200 meters (~330-660 feet) The net transport* over this entire wind-driven spiral is 90° to the right of the wind direction in the Northern Hemisphere *net transport represents the average of all directions and speeds of the Ekman spiral

Ekman Transport *The immediate surface water moves in a direction of 45°; the overall transport of the water is 90°

Ekman Transport and Surface Currents The Ekman transport describes an ideal situation, but ideal conditions rarely exist in nature, so the actual movement of the surface currents deviate slightly from that expected from the Ekman spiral In shallow waters, for example, the Ekman transport can be very near the same direction as the wind

Ekman Transport and Surface Currents Because Ekman transport deflects surface water 90° to the right (in the Northern Hemisphere), as gyres rotate clockwise, a convergence of water occurs in the middle of the gyre Causes water to literally pile up in the center of the gyre Creates a hill of water that is up to 2 meters (6.6 feet) in height

Dome of water formed by Ekman Transport

Yes, Virginia, there really is a hill of water in the middle of the North Atlantic

Geostrophic Flow Gyres are in a constant balance between the pressure gradient formed by Ekman Transport and the Coriolis effect Coriolis wants to move water uphill – against the concentration gradient – and gravity wants to move the water downhill – against the Coriolis deflection Resulting current flows parallel to the slope

Geostrophic flow (currents) Water flows downhill but is deflected to the right by Coriolis effect!

Water flows downhill under the influence of gravity, but the Coriolis effect deflects it to the right (in the Northern Hemisphere) All currents in a gyre move as a result of geostrophic flow!

Western boundary currents are very fast and deep because there is a westward intensification of water piling up due to the eastward rotation of the Earth & Coriolis

North America Europe Equator

The Gulf Stream Remember, the Gulf Stream is a western boundary current Transports warm, tropical water northward Together with its eastward extension, the North Atlantic Current or Drift, keeps Ireland and the west coast of Great Britain warm, and parts of Norway ice- and snow-free year round

The Gulf Stream sometimes meanders, forming rings or eddies that trap cold or warm water in their centers Warm-water eddies bring coconuts and tropical fish to Long Island!

Wind can cause vertical movement of water Wind-driven water is usually horizontal in nature, but can sometimes induce vertical movement in the surface water Upwelling is the vertical movement of cold, deep, nutrient-rich water to the surface Enhances productivity, which can support incredible numbers of large marine life Downwelling is the vertical movement of surface water to deeper parts of the ocean Decreases productivity, but transports necessary dissolved oxygen to organisms on the deep-sea floor

Upwelling When surface waters move away (or diverge) from an area on the ocean’s surface, upwelling occurs Upwelling commonly occurs along the equator (equatorial upwelling) and along the west Coast of the United States (coastal upwelling) Creates areas of high productivity that are some of the most prolific fishing grounds in the world

Equatorial Upwelling

Equatorial Upwelling As the SE trade winds pass over the equator, they cause water in the northern hemisphere to veer to the right, and water in the southern hemisphere to veer to the left (Coriolis deflection). Surface water diverges, resulting in equatorial upwelling

Coastal Upwelling

Coastal Upwelling (Global)

Coastal Downwelling

El Niño Under ‘normal’ circumstances, upwelling brings cold, nutrient waters to the coasts of Peru, driving an important anchovy fishery Pacific Warm Pool

Normal conditions Pacific Warm Pool The Pacific Warm Pool contains some of the warmest water on Earth; very low in nutrients

El Niño Every few years, a current of warm water occurs off the coast of Peru, reducing the commercial catch of anchovies Sea birds and seals that depend upon the anchovies for food suffered as well The warm current brought with it increased rainfall (good for Peru; usually arid) Usually occurred around Christmas and was named “El Niño” (“the child”); today known as the El Niño – Southern Oscillation (ENSO)

El Niño During an El Niño, the high pressure zone along the coast of South America weakens, reducing the pressure gradient difference (high to low) Causes southeast trade winds to diminish (or in extreme cases, to blow in the opposite direction!) Without the trade winds, the warm water pool on the western side of the Pacific begins to flow towards South America

El Niño The Pacific Warm Pool creates a band of warm water that stretches across the equatorial Pacific Ocean Begins moving in Sept and reaches the coast of Peru ~Dec or Jan; temperature of seawater off Peru can increase <10°C (18°F) during this time! Sea level can increase as much as 8 inches, simply due to thermal expansion of water along the coast!

El Niño (underway)

El Niño (established)

El Niño

El Niño As the warm water increases sea surface temperature, corals become bleached Near Peru, coastal upwelling brings warmer, nutrient-poor water to the surface, instead of cool, nutrient-rich water limiting productivity During a strong El Niño, the low pressure zone migrates (and remains) over South America, while high pressure moves towards Indonesia, bringing very dry conditions to Indonesia and Australia

Normal conditions

Normal conditions

La Niña Occasionally, conditions opposite of El Niño occur; these are known as ENSO cool phase, or La Niña La Niña is characterized by normal conditions intensified, resulting in stronger trade winds, causing more upwelling, and a band of cooler water stretching across the equatorial Pacific Usually occurs after an El Niño event; enhanced productivity (good)

El Niño and La Niña

El Niño and La Niña El Niño conditions occur on average every 2 to 10 years, but on a highly irregular basis Increased global warming may be promoting, and/or enhancing El Niño events; increased sea surface temperatures can trigger more frequent and more severe events Strong El Niño events can alter global weather patterns, resulting in flooding, droughts, fires, tropical storms, and erosion

Deep Ocean (Thermohaline) Circulation Subsurface, or deep ocean currents, arise from density differences between water masses produced by variations in temperature (thermal effect) and salinity (haline effect) For this reason, they are collectively referred to as thermohaline circulation As you saw in lab, when 2 water masses with different densities come into contact, the denser water mass slips beneath the less dense water mass

Deep Ocean (Thermohaline) Circulation Deep water currents move large volumes of water and are much slower than surface currents It takes a deep water current an entire year to travel the same distance as a western intensified surface current can move in one hour! Although temperature and salinity both affect the density of seawater, temperature has a greater influence on density

Deep Ocean (Thermohaline) Circulation Most water involved in deep-ocean currents originated at the surface in high latitudes There, the surface water cooled and its salinity increased as sea ice formed (both increasing its density) When this surface water becomes dense enough, it sinks, initiating deep ocean currents

Deep Ocean (Thermohaline) Circulation Once the water sinks, it is removed from the physical processes that increased its density in the first place, so its temperature and salinity remain largely unchanged Thus, a temperature-salinity (T-S) diagram can be used to identify deep-water masses based on their characteristic temperature, salinity, and resulting density

Sources of deep water Huge masses of deep water form beneath sea ice off Antarctica Here, rapid freezing produces very cold, high density water that sinks and becomes Antarctic bottom water, the densest water of the open ocean Large masses of deep water also forms in the North Atlantic during sea ice formation; this dense mass is known as North Atlantic Deep Water

Sources of deep water

Conveyer-Belt Circulation For every liter that sinks from the surface into the deep ocean, a liter of deep water must return to the surface somewhere else Beginning in the North Atlantic, surface water carries heat to high latitudes via the Gulf Stream; cooling in the North Atlantic during winter increases the density of this surface water until it sinks and flows southward towards the equator

Conveyor-Belt Circulation

Conveyor-Belt Circulation In 2005, researchers noticed that the flow of the northern Gulf Stream had decreased by ~30% since 1957 Researchers also noted that the water of the North Atlantic was becoming fresher (less saline) as the Earth continues to warm (increasing precipitation and polar ice melting)

A Global Warning? Freshening of the North Atlantic from global warming could slow or stop the sinking of cold, salty water, which drives the conveyor belt , and brings warm water via the Gulf Stream to the North Atlantic This warm water gives up heat to the atmosphere and moderates temperature in many parts of the globe

The Great Pacific Garbage Patch, and other interesting tidbits http://www.youtube.com/watch?v=uLrVCI4N67M