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Role of the oceans in the climate system • heat exchange and transport • hydrological cycle and air-sea exchange of moisture • wind, currents, and upwelling • gas exchange and carbon cycle
Role of the oceans in the climate system
• heat exchange and transport
• hydrological cycle and air-sea exchange of moisture • wind, currents, and upwelling
• gas exchange and carbon cycle
Heat transport
1. Wind Driven Circulation - surface 2. Thermohaline Circulation (density) - deep
Two Primary types of Ocean Circulation
Forces acting on ocean currents
1. Primary forces - initiate water movement a) wind stress b) density differences
2. Secondary forces - factors that influence the direction and nature of flow
a) coriolis force b) gravity c) friction d) shape of the ocean basins
Seasonal pressure and wind patterns
Surface ocean currents
Major ocean surface currents of the world
Wind Driven Circulation and Upwelling in the Ocean
• Ocean gyre circulation • Western and eastern boundary currents • Surface wind and Ekman transport • Coastal upwelling system • Surface wind and open ocean upwelling
Stream function for flow in a basin as calculated by Stommel (1948). Left: Flow for non-rotating basin or flow for a basin with constant rotation. Right: Flow when rotation varies linearly with latitude.
Henry Stommel showed that intensified western boundary currents are required for flow to circulate around an ocean basin when the Coriolis parameter varies with latitude.
Western intensification
dynamic topography
surface ocean currents
Sea Surface Heightsea surface height
wind driven surface ocean currentsUNITS - 1 sverdrup (sv) - 1 million m3/sec
for comparison, Amazon flow ~ 0.1 sv
WESTERN BOUNDARY CURRENTS narrow, deep, warm, and fast current
Gulf stream (N. Atlantic, 55 sv)
Kuroshio current (N. Pacific)
Brazil current (S. Atlantic)
Agulhas current (Indian)
East Australian current (S. Pacific)
heat transport - Western boundary currents export a heat to high latitudes (in mid-latitudes - 10 billion calories/sec)
western boundary currents
Gulf Stream an intensified western boundary current
Gulf Stream
wind driven surface ocean currents
EASTERN BOUNDARY CURRENTS broad, shallow, cold, and slow, currents (15 -20 sv)
Canary current (N. Atlantic)
California current (N. Pacific)
Benguela current (S. Atlantic)
Peru current (S. Pacific)
West Australian curent (Indian)
eastern boundary currents
wind driven surface ocean currents
TRANSVERSE CURRENTS flow east and west linking the boundary currents
equatorial currents (broad, shallow, Westward Flowing) driven by the trades, move east to west
West Wind Drift - Antarctic Circumpolar Current (ACC) Greatest of all currents - 100 sv west to east , driven by the persistent westerlies, isolates Antarctica from warm western boundary currents
Counter currents - equatorial counter currents - return flow (west to east) along the equator undercurrents - counter currents beneath the surface ( 100 to 200m)
Antarctic Circumpolar
Current
ACC
Wind Driven Circulation and Upwelling in the Ocean
• Surface wind and Ekman transport
• Coastal and open ocean upwelling systems
An Important Observation in Oceanography
The voyage of the Fram was led by Fridtjof Nansen (1861-1930). The Fram drifted a total of 1030 miles during her 3-year entrapment. The drift of the Fram proved that no continent existed in the Artic Sea, and ice that coved the polar area throughout the year was not of glacial origin. Nansen noticed that surface ice drifts at a direction about 45o to the right relative to the wind direction.
V. Walfrid Ekman (1874-1954), a Scandinavian physicist developed the mathematical model and explanation for the relationship of wind to the ocean currents it drives.
Current in the Upper Ocean Layer and and Ekman Spiral Wind drives surface water in
a direction 45o to the right of the wind in the Northern Hemisphere. Deeper water continues to deflect to the right and moves at a slower speed with increase depth. Ekman transport, which is the net water movement, thus, is at right angles to the wind direction. This illustrates the principle, but in reality, the angles usually are somewhat less. The surface layer of water in which this spiraling occurs is called the Ekman layer.
Coastal upwelling
A Coastal Upwelling System
Coastal upwelling
Equatorial upwelling
Equatorial upwelling
Antarctic divergence
upwelling
Convergence and Divergence Zones of surface convergence and divergence mark regions of sinking
and rising water; driving force is Coriolis effect
Convergence = two water masses flow towards each other, water is subducted: downwelling
Divergence = two water masses flow apart from each other, water from depth replaces surface water: upwelling
Upwelling brings oxygen and nutrients into surface layer
Surface convergence occurs at: •0° in the tropical convergence, •30°–40°N & S in the subtropical convergence •50°N&S in the Arctic & Antarctic convergence
Surface divergence occurs in three areas: •on either side of the tropical convergence •in the Antarctic divergence
