Module 5: The Surface Circulation

Ross Gyre

acts as a gatekeeper for warm-water exchanges between the Antarctic Circumpolar Current and the Ross Sea—a broad, shallow sea located at the "mouth" of the elephant head-shaped continent

Subpolar Gyres

adjacent to the Arctic and Antarctic Circles

Alaskan gyre

along whose eastern boundary flows the northbound, coastal-hugging Alaska Current

California Undercurrent

the first west coast astronomical observatory, and served for 17 years as president of the California Academy of Sciences (Wikipedia 2020). Underneath it all is another poleward-flowing current, the California Undercurrent, which flows at an average depth of about 656 feet (200 meters) underneath the California Current and may rise to the surface in fall and winter.

Ekman transport

, the net motion caused by blowing wind, is 90° to the right of the wind. The 90° net movement of the ocean by the wind-driven Ekman spiral is called Ekman transport

Norewegian Current

A northerly flow becomes the Norwegian Current, and a southerly flow moves along the Bay of Biscay toward Portugal

Northeast monsoon drift

A westward-flowing North Equatorial Current is present though it also goes by the name Northeast Monsoon Drift

Weddell Gyre

Across the "trunk" of the West Antarctic peninsula lies the Weddell Sea and the Weddell Gyre, a site of bottom water formation thought to be critically important to world ocean circulation and climate change

Western Intensification

As mentioned earlier, western boundary currents are faster, narrower, and more energetic than eastern boundary currents, which are slower, wider, and less energetic. The difference is explained by something called western intensification, which is caused by latitudinal differences in the Coriolis effect

North Equatorial Current

Back in the North Pacific, at the southern terminus of the California Current, we find the beginnings of the North Equatorial Current, a broad, westward-flowing surface current generally found between latitudes 10°N and 30°N. The North Equatorial Current forms the lower limb of the North Pacific Gyre but is strongly influenced by processes associated with the convergence of trade winds at the Intertropical Convergence Zone (ITCZ)

Monsoon

Be that as it may, there is one phenomenon of the Indian Ocean that we understand fairly well: the seasonally reversing wind pattern known as the mausin (alternatively mausam) in Arabic, the monsoon in English. Characterized by strong winds from the southwest in summer and strong northeast winds in winter, the monsoon brings seasonal changes in rainfall

Equatorial Currents move waters from what sides?

Equatorial currents move waters from the eastern sides of ocean basins to their western sides. Indeed, oceanographers use the terms eastern and western to distinguish the eastern and western halves of ocean basins

5 major eastern boundary currents

Five major eastern boundary currents can be found in the world ocean: the California Current (NPAC); the Peru-Chile Current, also known as the Peru Current or the Humboldt Current (SPAC); the Canary Current (NATL); the Benguela Current (SPAC); and the longest boundary current—at more than 3,400 miles (approximately 5,500 km) long—in the entire ocean, the poleward-flowing (instead of the usual equatorward-flowing) Leeuwin Current (SIND).

5 major subtropical gyres

Five major subtropical gyres occupy the three major ocean basins: the North Pacific Gyre (NPG), South Pacific Gyre (SPG), North Atlantic Gyre (NAG), South Atlantic Gyre (SAG), and the South Indian Gyre (SIG)—one for each major ocean basin except the North Indian. Squeezed and shortened by the presence of the Indian subcontinent, the North Indian Ocean basin lacks a true gyre, but in its place lives a fascinating system of currents that change direction seasonally.

Azores Current

From its starting point off the Grand Banks of Newfoundland, the North Atlantic Current immediately splits into two branches. One branch dips southward along the eastern edge of the Gulf Stream Recirculation Gyre, then swings eastward

Coastal Jets

In fact, if you study these images a bit, you'll notice cold water swirls, eddies with diameters from 10 to 100 kilometers. Some of the cold water projects in thin lines called coastal jets. These upwelled waters bring abundant, dissolved, biologically important elements to waters that were previously lacking

Deep Circulation

Movement of water created by the mixing of water masses of different densities. more important, albeit weaker and slower, set of currents churn in the abyssal ocean—the deep circulation

California Current

Near and dear to those of us living on the West Coast of the United States is the eastern boundary current

South Equatorial Current

South Equatorial Current, which you might think (and others have claimed) exists in the Southern Hemisphere. Due to the larger surface area of land in the Northern Hemisphere and larger surface area of ocean in the Southern Hemisphere, the trade winds and ITCZ shift northward, so that the westward-flowing South Equatorial Current flows from about 30°S to 5°N.

Overturning Circulation

Physical oceanographers also refer to the combined surface and deep circulation as the overturning circulation

South Pacific basin is home to what current?

The South Pacific basin is home to the famous Peru Current—also known as the Humboldt Current, named after Prussian explorer and biogeographer Alexander von Humboldt

Subtropical Gyres

The centers of the oceanic gyres can be found in two general locations in the world ocean: the subtropical gyres, straddling the Tropic of Cancer in the northern hemisphere and the Tropic of Capricorn in the southern hemisphere

Boundary Currents

The continental borders of the eastern and western halves of ocean basins define a set of important surface currents known as the boundary currents

West wind drift

The currents that flow along the northern boundary (in the Northern Hemisphere) or southern boundary (in the Southern Hemisphere) of the subtropical gyres move water left to right—that is, from west to east, or zonally, just like the equatorial currents. Some oceanographers refer to these eastward-flowing currents as the west wind drift, after the winds that generate them—the westerlies

Recirculation gyre

The energy in eddies can also contribute to a recirculation of water within the main flow

Meridional currents

The equatorial currents and the west wind drift currents represent examples of zonal currents in a gyre. Currents that move along lines of longitude—toward the poles or the equator

Ekman Suction

The movement of surface water offshore and sucks cold, nutrient-rich waters from 100 to 300 meters to the surface. Physical oceanographers refer to this process as Ekman suction

Equatorial Currents

The principal equatorial currents include the North Equatorial Current (NPG, NAG), the South Equatorial Current (SPG, SAG, SIG), the North Equatorial Countercurrent (NPG, NAG), the South Equatorial Countercurrent (SPG, SAG), and the Equatorial Undercurrent in the Pacific, Atlantic, and Indian Oceans. Note that in the Pacific Ocean, the Equatorial Countercurrent is also known as the , named after the man who discovered it, Scripps oceanographer Townsend Cromwell (1922-1958), who died in a plane crash en route to a research expedition

Mindanao Current

The southward flow forms the lesser-known Mindanao Current, which originates from Mindanao, the second-largest island in the Philippines, alternatively known as the Land of Promise

Gyres

The surface circulation crosses the ocean like a vast network of oceanic rivers. However, unlike rivers, the currents that make up the surface circulation in each basin form an ensemble of currents known as oceanic gyres

Winter monsoon

The winter monsoon, or northwest monsoon, brings dry weather to the Indian subcontinent and beyond.

Geostrophic Current

This balanced current is called a geostrophic current, one that is under the influence of Earth's rotation. Geostrophic flows happen in the atmosphere too; geostrophic winds blow at right angles to atmospheric pressure gradients and rotate around centers of high or low pressure.

Somali Current

This strong eastward-flowing current sends branches into the Arabian Sea and Bay of Bengal as well. The South Equatorial Current crawls up the northern coast of Africa and southeastern Arabia, where it's called the Somali Current.

North Pacific Current and North Atlantic Current

Though complicated, their flows—highly dependent on the boundary currents to which they connect—exert a great influence on weather patterns in the US and Europe.

Surface Circulation

Though oceanographers and sailors give names to the different surface currents that flow beside coastlines and along the equator—known as the surface circulation

World Ocean Circulation

Traditionally, oceanographers have treated surface and deep circulation as two independent systems. However, in recent decades, they have increasingly recognized that these are part of one global ocean circulation system—the world ocean circulation

Western boundary currents

Western boundary currents flow along the east coasts of continents

Famous western boundary current

While the Kuroshio and Gulf Stream are the poster children of western boundary currents for oceanographers (at least in the Northern Hemisphere), among the movie-watching public, the most famous western boundary current has to be the East Australian Current,

Zonal Currents

While we're defining parts of the surface circulation, I should also note that oceanographers refer to currents that move along lines of latitude—east to west or west to east—as zonal currents

Bengula Current

an eastern boundary current that traces its northerly path along the western shores of South Africa and Namibia. Shipwrecks and the dead bodies of whales and seals that litter the northern section of coastal Namibia have earned it the nickname of Skeleton Coast, a testament to its high productivity and dangerous fogs

Kuroshio Current

called the "black current," presumably for its dark blue waters

Ekman Depth

defined as the depth where the current flow is 37% of the surface current flow and in a direction 180° opposite the direction of the wind—the Ekman spiral runs out of steam, so to speak, and the motion ceases.

Pressure Gradients

differences in pressure between two locations—in the ocean cause currents, just like pressure gradients in the atmosphere cause winds. Where elevations exist, water begins to flow downhill

Upwelling

eastern boundary currents draw their energy from seasonal winds that cause upwelling, the movement of subsurface waters towards the surface

Easter boundary Currents

eastern boundary currents flow along the west coasts

Western Subartic Gyre

fed by the East Kamchatka and Oyashio Currents

Southwest monsoon drift

hardly a drift with current speeds in excess of 1 mph

California Countercurrent

northerly coastal flow

Indonesian Throughflow (ITF)

that acts as a kind of control valve for exchanges of waters between the Pacific, Indian, and even the Atlantic Oceans. And scientists now recognize that the Indian Ocean spawns the Madden-Julian Oscillation, an eastward-traveling, globe-trotting atmospheric "wave" that enhances or suppresses rainfall. As described by Gottschalck (2014),

Cold-core rings

that trap cold oceanic water inside a circle of warm boundary current water

California Current System

the California Current is best described as a system of currents, and, indeed, most authors refer to the boundary current of the eastern North Pacific as the California Current System

Leeuwin Current

the Leeuwin Current—the only poleward flowing eastern boundary current in the world—joins a complex of other currents along the western and southern shores of Australia. Unlike other eastern boundary currents, the Leeuwin Current is generally nutrient-poor and lacks upwelling, though the potential exists for enhanced productivity due to eddies that spin off the current

Portugal-Canary Current system

the Portugal-Canary Current System bathes the coastlines of Portugal and North Africa and displays a temporal and spatial complexity similar to the California Current System

Sea Surface Height

the changes in elevation of the sea surface

Summer monsoon

vapor. So as it rises above the subcontinent and cools, that water vapor is released as torrential rain. The combination of southwest winds and heavy rains characterizes the summer monsoon, also known as the southwest monsoon.

Warm-core rings

which are the opposite—warm water trapped inside a ring of cold water