Motion of the Ocean Ch. 9 – Currents

Ocean Currents The steady flow of water in a prevailing direction Basically, an area where most of the water is flowing together in the same direction

Causes of Currents Three major factors drive ocean currents ▫1. Wind ▫2. Changes in sea level (gravity) ▫3. Variations in water density Wind driven currents are also affected by the Coriolis Effect ▫This deflects currents clockwise in the North and counterclockwise in the South

Ekman Transport The wind and the Coriolis effect influence water below the surface As water on the surface is pushed by wind and curved by the Coriolis effect, it drags deeper water along due to friction This deeper water also curves (spirals) and drags even deeper water along with it.

Ekman Transport This spiraling effect pushes water slightly to the right of water above and is called the Ekman spiral The net motion of the water is called the Ekman transport ▫90 degrees to the right of the wind Ekman transport causes water to flow in a different direction from the wind and other ocean currents

Geostrophic Currents Geostrophic means “related to Earth’s rotation” Geostrophic currents are formed due to Earth’s rotation

Geostrophic Currents Earth’s rotation causes a ‘bulge’ of water to form in the western portion of each ocean basin Gravity causes water to flow from this high elevation to a lower elevation creating a current

Area of high sea level

Boundary Currents Currents that flow around the edge of ocean basins Created by differences in sea surface elevation and are a type of geostrophic current Two types ▫Western ▫Eastern

WesternEastern Found on the east coast of continents Found on the west coast of continents Warm waterCold water Narrow, fast, & deep waterWide, slow, & shallow water

Countercurrents Ekman transport is not the only way that water flows in directions different from the major currents Countercurrents run beside and opposite to adjacent currents ▫Causes are unknown ▫It is hypothesized that countercurrents are created when the Trade Winds calm ▫This causes water to flow back eastwards alongside the major currents

Undercurrents Flow beneath the adjacent current and are found beneath most major currents The cause of undercurrents is unknown Have significant effect on local climates

Gyres A gyre is the large circular flow of water in each ocean basin Are the result of three processes ▫Collision of ocean currents with continents ▫Movement of the Trade Winds and Westerlies ▫The Coriolis Effect

There are five major gyres... One in each ocean basin: ▫North Atlantic Gyre ▫South Atlantic Gyre ▫North Pacific Gyre ▫South Pacific Gyre ▫Indian Ocean Gyre

Upwelling and Downwelling So far, all currents we have discussed flow horizontally Blowing wind can cause vertical movement of water as well Ekman transport can cause surface water to move towards or away from the coast When this happens, an upwelling or downwelling is created

How Upwellings are Born

Upwellings Upward vertical currents that bring deep water to the surface Occur when Ekman transport blow water away from the coast, allowing deeper water to rise Bring large amounts of nutrients to the surface and increase biological productivity Exampes: West coast of Peru

Downwellings Downward vertical currents that push surface water to the bottom Downwellings occur when Ekman transport pushes surface water into the coast and down Downwellings bring nutrients to the ocean floor ecosystems ▫They do not have major impacts on biological productivity  Ex. east Coast of Peru

How Downwellings are Born

El Nino Southern Oscillation (ENSO) El Nino Southern Oscillation is an example of an ocean current system with significant impacts on global weather Under non-ENSO conditions, the Trade Winds blow to the west.

The Trade Winds

ENSO This creates an upwelling on the west coast of South America and a downwelling on the east coast of Indonesia This results in dry conditions in South America and heavy rains in Indonesia

Non-ENSO (normal) Conditions

Upwelling Downwelling

ENSO Events During an ENSO event the trade winds weaken or even reverse This causes downwellings near South America and upwellings near Indonesia ▫This kills or drives off the anchovies that Peruvian fisherfolk depend on This results in rain over the Pacific and dry conditions over South America and Indonesia

ENSO Conditions

ENSO (El Nino) Conditions UpwellingDownwelling

ENSO Events (cont.) El Nino events occurs at 3-7 year intervals and typically last 5 years They have global effects on weather ENSO events cause: ▫Tornados and drought in the US ▫Drought in India and China ▫Drought and wildfires in Australia and South America ▫Flooding in Africa

Heat Transport and Climate Currents play a critical role by transporting heat Equatorial currents transport heat and warm up higher latitude regions When they enter cold Arctic waters these currents sink, becoming part of thermohaline circulation ▫Water movement driven by differences in temperature and salinity (density)

Thermohaline Circulation (Deep Currents) Creates slowly moving water masses These deep water masses form in the North Atlantic Deep water rises to join surface currents in the Indian and North Pacific Oceans In this way surface currents and deep currents form an interconnected flowing mass of water throughout the oceans.

The Ocean Conveyor Belt The interconnected flow of currents that redistribute heat is called the ocean conveyor belt The ocean conveyor belt is important because it moderates the world’s climate ▫This circulation carries heat away from the tropics and keeps the tropics from getting too hot ▫Some scientists hypothesize that some Ice Ages may have resulted from a disruption of the conveyor belt.

Studying Currents Two main approaches to study currents: ▫1. The “float” method  Studying the current by tracking a drifting object  Examples: drifter buoys, rubber duckies, satellites ▫2. The “flow” method  Studying the currents by staying in one place and measuring changes in the velocity of water  Examples: Doppler Acoustic Current Meter