5. Geostrophic flow and western
intensification
Geostrophic flow
causes a hill to form
in subtropical gyres
The center of the
gyre is shifted to the
west because of
Earth’s rotation
Western boundary
currents are
intensified Figure 7-7
7. Factors Influencing Nature and Movement of Ocean
Currents:
1. Factors related to the earth’s rotation:
Gravitational force and force of deflection.
2. Factors originating within the sea:
Atmospheric pressure, winds, precipitation, evaporation
and insolation.
3. Factors originating within the sea:
Pressure gradient, temperature difference, salinity, density
and melting of ice.
4. Factors modifying the ocean currents:
Direction and shape of the coast, seasonal variations and
bottom topography.
15. Weddel Sea (Flichner ice shelf) and Ross Sea (Ross Ice
Shelf)
Weddel Sea – partially isolated embayment -greatest
contributor
There is less entrainment than with NADW so AABW is
densest water in ocean.
16. Cold wind blows ice offshore (polyna) allowing ice to
continually form.
During freezing, salts are left behind (brine formation)
resulting in water that is more saline.
Surface waters are chilled to temperature of ~ -1.9°C,
salinity is 34.6 psu.
This cold dense water collects on the Antarctic shelf and
sinks to the bottom of the adjacent deep-ocean basin.
In the process of mixing, mixes with other waters and is
warmed.
Resulting water is ~ -0.4-1°C and 34.6 to 34.8 psu.
25. South Atlantic – upper water gyre – extends from
surface to a depth of 200 m near the equator to 800m
southern limits of gyre at Subtropical convergence
Wind stress of South East trade winds between equator
and 10-15° S – main driving force
Acts on sea and forms South Equatorial current (SEC)
– greatest strength just below equator – flows west
towards American side of South Atlantic
Spills by topographic interference by eastern
prominence of Brazil. Part of SEC moves off
northeastern coast of South America towards
Caribbean and North Atlantic, rest is turns southwards
as brazil current
26. Brazil current coming from the tropics is warm and
saline, turns east and continues across Atlantic as
Antarctic Circumpolar current (WWD) and moves
eastward. The Brazil current is much smaller than the
Northern Hemisphere counterpart i.e. the Gulf stream
due to the splitting of SEC
WWD than turns north up on African side as the
Benguela Current which flows equatorward along
Africa’s western coast
Benguela current is slow drifting cold current because
of the contribution of Subantartic water and of
upwelling along the African coast
Falkland current
27. Falkland current – is outside the South Atlantic gyre,
but is a significant north bound flow of cold water.
Current flows from Drake passage and moves along
the western margin of South Atlantic up the coast of
South America. Falkland current impart cold current
that moves along the coast of Argentina as far as
north as 30°S thus separating Brazil current from
coast at this point.
South Atlantic circulation is bounded on south by
Subtropical Convergence.
39. Upwelling and downwelling
Vertical movement of water
Upwelling = movement of deep water to surface
Hoists cold, nutrient-rich water to surface
Produces high productivities and abundant marine life
Downwelling = movement of surface water down
Moves warm, nutrient-depleted surface water down
Not associated with high productivities or abundant
marine life
42. El Nino and La Nina
El Nino is a change in water temperature in the
Pacific ocean that produces a warm current.
La Nina is a change in temperature in the Eastern
Pacific that causes surface water temperature to
be much colder than usual
43. BOTH El nino and La Nina can cause flooding (too
much rain) and drought (too little rain) in different
places on Earth. Upwelling does not occur where it
normally would and this affects fish and sealife.
44. El Niño-Southern Oscillation
(ENSO)
El Niño = warm surface current in equatorial eastern
Pacific that occurs periodically around December
Southern Oscillation = change in atmospheric
pressure over Pacific Ocean accompanying El Niño
ENSO describes a combined oceanic-atmospheric
disturbance
45. • Oceanic and atmospheric
phenomenon in the Pacific Ocean
• Occurs during December
• 2 to 7 year cycle
Sea Surface Temperature
Atmospheric Winds
Upwelling
54. Coriolis Effect
Because of the coriolis effect, winds appear to deflected
to the east or west depending on the direction winds
are traveling.
55. A buoy records data about surface ocean temperature
and transmits (sends) the information to a satellite in
space that then transmits(sends) the information to
scientists.
57. Water has a much higher heat capacity (absorbs and lets go
of heat more) slowly than land, water temperature will
increase and decrease less than land temperature.
e.g. during daytime, land temperatures might change by
tens of degrees,
water temperature change by less than half a degree.
58. i.e. coastal land temperatures don’t fluctuate (go up and
down) extremely (a lot) because the ocean water nearby
doesn’t fluctuate much.
103. Surface currents closely follow
global wind belt pattern
Trade winds at 0-30º
blow surface currents to
the east
Prevailing westerlies at
30-60º blow currents to
the west
Figure 7-3
105. Current gyres
Gyres are large circular-moving loops of
water
Subtropical gyres
Five main gyres (one in each ocean basin):
North Pacific
South Pacific
North Atlantic
South Atlantic
Indian
Generally 4 currents in each gyre
Centered at about 30º north or south latitude
106. Current gyres
Gyres (continued)
Subpolar gyres
Smaller and fewer than subtropical gyres
Generally 2 currents in each gyre
Centered at about 60º north or south latitude
Rotate in the opposite direction of adjoining subtropical
gyres
107. Western intensification of
subtropical gyres
The western boundary currents of all subtropical
gyres are:
Fast
Narrow
Deep
Western boundary currents are also warm
Eastern boundary currents of subtropical gyres have
opposite characteristics
108. Currents and climate
Warm current warms
air high water vapor
humid coastal climate
Cool current cools air
low water vapor dry
coastal climate
Figure 7-8a
109. Upwelling and downwelling
Vertical movement of water ( )
Upwelling = movement of deep water to surface
Hoists cold, nutrient-rich water to surface
Produces high productivities and abundant marine life
Downwelling = movement of surface water down
Moves warm, nutrient-depleted surface water down
Not associated with high productivities or abundant
marine life
110. Coastal upwelling and
downwelling
Ekman transport moves surface water away from
shore, producing upwelling
Ekman transport moves surface water towards shore,
producing downwelling
Figure 7-11
111. Other types of upwelling
Equatorial upwelling
Offshore wind
Sea floor obstruction
Sharp bend in coastal
geometry
Figure 7-9
Equatorial upwelling
115. The Gulf Stream and sea
surface temperatures
The Gulf Stream is a
warm, western
intensified current
Meanders as it moves
into the North Atlantic
Creates warm and cold
core rings
Figure 7-16
117. El Niño-Southern Oscillation
(ENSO)
El Niño = warm surface current in equatorial eastern
Pacific that occurs periodically around Christmastime
Southern Oscillation = change in atmospheric
pressure over Pacific Ocean accompanying El Niño
ENSO describes a combined oceanic-atmospheric
disturbance
121. The 1997-98 El Niño
Sea surface
temperature
anomaly map shows
warming during
severe 1997-98 El
Niño
Internet site for El
Niño visualizations
Current state of the
tropical Pacific
Figure 7-19a
122. El Niño recurrence interval
Typical recurrence interval for El Niños = 2-12
years
Pacific has alternated between El Niño and La
Niña events since 1950
Figure 7-20
125. Deep currents
Deep currents:
Form in subpolar regions at the surface
Are created when high density surface water sinks
Factors affecting density of surface water:
Temperature (most important factor)
Salinity
Deep currents are also known as thermohaline
circulation
127. Identification of deep currents
Deep currents are
identified by
measuring
temperature (T)
and salinity (S),
from which
density can be
determined
Figure 7-24
The global ocean circulation system, often called the Ocean Conveyor, transports heat throughout the planet. White sections represent warm surface currents. Purple sections represent deep cold currents. (Illustration by Jayne Doucette, WHOI Graphic Services).
Floods
El Niño Sea-Level Rise Wreaks Havoc in California's San Francisco Bay Region(31-Jan-2000)
1998 California Floods (11-Mar-1998)
The Spring Runoff Pulse from the Sierra Nevada (14-Jan-1998)
Effects of El Niño on Streamflow, Lake Level, and Landslide Potential (16-Dec-1997)
Climate and Floods in the Southwestern U.S. (10-Jul-1997)
Real-time flows on rivers and streams
More USGS information on Floods
Landslides
Recent landslide events--News and Information (updates regularly)
Landslide publications and reports (14-Oct-2003)
USGS Circular 1244 (26-Sep-2003)"National Landslide Hazards Mitigation Strategy—A Framework for Loss Reduction"
USGS Landslide Hazards web site
More USGS information on Landslides
Information on Landslides during the 1997-98 El Niño:
Map Showing Locations of Damaging Landslides in Alameda County, California, Resulting From 1997-98 El Niño Rainstorms (10-Jan-2000)
El Niño and 1998 California Landslides (20-Mar-1998)
Geologic mapping and El Niño: Landslide and debris-flow susceptibility maps, including southern California, Mojave Desert, and San Francisco Bay Area (02-Feb-1998)
Landslide Recognition and Safety Guidelines (29-Jan-1998)
USGS Producing Landslide Hazard Maps for Emergency Services in San Francisco Bay Area (16-Dec-1997)
Potential San Francisco Bay Landslides During El Niño (16-Dec-1997)
El Niño and the National Landslide Hazard Outlook for 1997-1998 (16-Dec-1997)
Effects of El Niño on Streamflow, Lake Level, and Landslide Potential(16-Dec-1997)
Coastal hazards
El Niño Sea-Level Rise Wreaks Havoc (31-Jan-2000)in California's San Francisco Bay Region
Coastal Erosion Along the U.S. West Coast During 1997-98 El Niño (12-August-99)
Coastal Erosion From El Niño Winter Storms (31-Aug-1998)with before and after air photos from Southern Washington, Northern Oregon, Central California, and Southern California
1982-1983 El Niño Coastal Erosion, San Mateo County, California (6-May-1998)
1997-98 El Niño Coastal Monitoring Program (31-Mar-1998)with before and after photos of Santa Cruz County, California beach erosion.
Hydroclimatology of San Francisco Bay Freshwater Inflows and Salinity, with weather and salinity movies (14-Jan-1998)
El Niño Effects on Sea-Level Near San Francisco Bay (16-Dec-1997)
Coastal Impacts of an El Niño Season (3-Nov-1997)
More USGS information on Coastal hazards
Climate
Long-term climate variation in the Mojave Desert (15-Jan-1998)
The Spring Runoff Pulse from the Sierra Nevada (14-Jan-1998)
El Niño Effects on Sea-Level Near San Francisco Bay (16-Dec-1997)
Effects of El Niño on Streamflow, Lake Level, and Landslide Potential (revised 16-Dec-1997)
Climate and Floods in the Southwestern U.S. (10-Jul-1997)