Climate Studies
introduction to climate science
Chapter 7
Atmosphere-Ocean Relationships
© American Meteorological Society
Background photo: Jean-Baptiste Dodane
Introduction
• The ocean is a major player in Earth’s climate
system, operating from days to millennia and
from local to global
– Covers about 71% of Earth’s surface
– Average albedo only 8%
– Influences radiational heating and cooling of
the planet
– Main source of water vapor
– Major regulator of the concentration of CO2
• Ocean and atmosphere work together in
governing climate
Mean State of the Ocean Circulation
• Surface ocean currents are wind-driven, so ocean
surface waters mirror long-term planetary-scale
atmospheric circulation.
• Gyres - large-scale systems of rotating currents
– Subtropical gyres driven by trade winds and the
westerlies, following clockwise movements in NH
– Subpolar gyres driven by counterclockwise surface
winds and sub-polar low pressure systems in NH
• Western Boundary Current - Currents that are
warm, deep and fast flowing on the west side of
ocean basins and carry water from the tropics
poleward, faster than the eastern counterpart
Mean State of the Ocean Circulation
Warm current: Gulf Stream, Kuroshio, Norwegian, E. Australia
Cold current: Canary, California, Peru, W. Australia
Ekman Spiral
• A steady wind causes surface waters to move at
an angle of 45° to the right of the wind in the
Northern Hemisphere (due to Coriolis effect)
• Each successively lower layer moves more
toward the right and at a slower speed
• Through a depth of about 100 to 150 m (330 to
500 ft.), the net water movement will be at 90°
to the right of the wind direction
Ekman Spiral
- A model of the
direction and speed
of the threedimensional current
pattern of surface
ocean caused by a
steady horizontal
surface wind.
Figure 7.2 The Ekman spiral
Upwelling and Downwelling
• Coastal upwelling - occurs
where Ekman transport
moves surface waters away
from the coast and surface
waters are replaced by
water that wells up from
below
• Coastal downwelling occurs where Ekman
transport moves surface
waters toward the coast,
the water piles up and sinks
• Upwelling or downwelling
depends on wind directions
Upwelling and Downwelling
Relatively cold,
upwelling waters along
the coast of central
and northern
California in response
to the Ekman transport
caused by winds
predominantly blowing
from the north.
Figure 7.7
Upwelling
• Upwelling waters originate below the pycnocline (the
water layer where the density gradient is greatest), and
brings colder water to the surface
• Coastal upwelling transports dissolved nutrients
(nitrogen and phosphorus compounds) from the ocean
depths into the photic zone
– Supports the growth of phytoplankton
• The world’s most productive fisheries are in areas of
coastal upwelling
– About half the world’s total fish catch comes from
upwelling zones
– Where ocean absorbs CO2 via the biological pump
Downwelling
• In zones of coastal downwelling, the
surface layer of warm, nutrient-deficient
water thickens as water sinks.
–Reduces biological productivity
–Transports heat, dissolved materials,
and dissolved CO2 and oxygen to
greater depths
– Where the ocean absorbs CO2 via the
‘dissolution’ pump
Thermocline
The deep ocean is dominated by
cold, high density, water.
Warm, low density water is
confined to the surface.
Surface water is separated from
deep water by a narrow zone in
which density changes rapidly
with depth – the pycnocline or
thermocline.
Thermocline is the
transition zone between
the surface water and
deep water below).
Figure 7.9 A temperature-depth ocean water
profile. The thermocline is a layer of water where
the temperature changes rapidly with depth. This
temperature-depth profile is what you might
expect to find in low to middle latitudes.
Thermohaline Circulation
• Thermohaline circulation - deep-ocean circulation
• It is driven by variations in density (thermo meaning
heat and haline referring to salinity)
• The density of sea water depends on a combination of
temperature and salinity.
• Stratification of the ocean controlled by temperature
difference between warm surface waters and cold
deep waters
– At high latitudes, salinity is control
• Deep circulation contributes to poleward heat
transport
– Varies from years to millennia, modulating climate.
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Thermohaline Circulation: NH
• Northern Hemisphere: Deep water forms in the
North Atlantic in late winter when the surface
water reaches its lowest temperature and
greatest density
• Open ocean convection - primary mechanism of
deep water formation in the Northern
Hemisphere where cold winds chill the surface
water to the extent that its density becomes
greater than that of the water beneath it,
creating an unstable water column and driving
overturning
Thermohaline Circulation: SH
• In the Southern Hemisphere, deep waters form at
several locations around the Antarctic continent,
primarily in the Weddell Sea
• Much of the deep water formation occurs
underneath floating sea ice. Water is cooled by
cold winds acting on openings (leads) in the sea ice
cover and becomes denser during formation of sea
ice
– Brine rejection - salt left behind during the freezing of
seawater increases the salinity of water immediately
below the freezing interface
• Water made denser by cooling and brine rejection
sinks along the continental slope of Antarctica into
the deep ocean.
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Thermohaline Circulation
• Currents are weak but the volume of deep waters is
much greater so the magnitude transported is similar
to surface water
• The large scale overturning is driven by sinking of cold,
dense, water at high latitudes.
• Time scale of the overturning: ~ 1000 years
Atlantic meridional overturning circulation
• The Atlantic meridional overturning
circulation (AMOC) is characterized by a northward
flow of warm, salty water in the upper layers of the
Atlantic, and a southward flow of colder, deep
waters that are part of the thermohaline
circulation.
• The AMOC is an important component of the
Earth’s climate system, and is a result of both
atmospheric and thermohaline drivers.
• AMOC carries up to 25% of the northward global
atmosphere-ocean heat transport in the northern
hemisphere
• AMOC is the largest carbon sink in the Northern
Hemisphere.
“They infer that the slowdown in
the AMOC was probably a
response to warming caused by
anthropogenic greenhouse-gas
emissions. A possible mechanism
could be enhanced melting of
the Greenland Ice Sheet, which
adds fresh water to the surface
ocean and reduces the density of
the water that drives deep
convection.”
https://www.nature.com/articles/d41586-018-04086-4
El Niño, La Niña
and the Southern Oscillation
• For more than a century, scientists have been aware of
short-term (inter-annual) variations in climate at many
locations worldwide.
• El Niño and La Niña
– Readily apparent in the tropical Pacific Ocean with
variations on a quasi-periodic basis
– Cause weather extremes in many other parts of the
world.
• ENSO: relationship between El Niño and the
Southern Oscillation
• ENSO is coupled atmosphere-ocean changes: changes in
the ocean drive changes in the atmosphere that then
feedback and further alter the ocean
Historical Perspective
• Originally, local fishermen in Peru and Ecuador
named annual, wind-driven warm ocean current,
accompanied by poor fishing, El Niño because it
coincided with Christmas
– Typically brief, El Niño persists for 12 to 18
months every 3 to 7 years
– Scientist us the term “El Niño” for only
anomalies
Neutral Conditions in the Tropical Pacific
Figure 7.13 Schematic block
diagram showing
atmosphere-ocean
conditions in the tropical
Pacific during normal
(neutral) ENSO episodes. Red
indicates areas of highest
SSTs.
Neutral Conditions in the Tropical Pacific
• Prevailing winds blow from the south or southeast
along the west coast of South America
– Most of the time Ekman transport drives warm surface
waters westward
– Upwelling of cold, nutrient-rich waters along the west
coast, replacing the warm, nutrient-poor surface
waters that are transported offshore, supporting a
diverse marine ecosystem and highly productive
fishery.
• Equatorial upwelling produces a strip of relatively
low SSTs along the equator – “cold tongue”
Neutral Conditions in the Tropical Pacific
• Trade winds drive warm surface waters
westward toward Indonesia and northern
Australia.
– Increases the depth of the thermocline from
50 m (165 ft.) in the eastern tropical Pacific to
150 m (490 ft.) in the western
– Raises sea level in the western tropical Pacific
to about 60 cm (2 ft.) higher than in the east
El Niño
(Warm Phase)
• El Niño - air pressure gradient across the tropical Pacific
weakens
– In the western Pacific the trade winds slacken, SSTs drop, sea level
falls and the thermocline rise
– In the eastern tropical Pacific, SSTs rise, sea level climbs and the
thermocline deepens
El Niño (Warm Phase)
• Relaxing trade winds, weaken the westward
flow of the equatorial currents, even reversing
direction
• The thick layer of warm surface water drifts
eastward, until deflected toward the north
and south by the continental landmasses
– May take several months for higher SSTs to
reach the west coasts of North and South
America
El Niño (Warm Phase)
• Warm surface waters in the eastern tropical Pacific
reduces upwelling of nutrient-rich waters along the coast
of Ecuador and Peru.
– Phytoplankton populations decline and the commercial fish
harvest plummets
– In the 1970s, Peru’s fishing industry account for about 20% of
the total global catch of anchovies
– 1972-73 El Niño, and over-fishing, the Peruvian fishery failed
and has not recovered
• Warmer surface waters stress coral reefs living in shallow
tropical waters.
– During unusually high SSTs, corals expel their colorful
zooxanthellae and appear white, known as coral bleaching
– Extended bleaching can kill coral polyps and corals,
destroying habitats for a wide variety of marine organisms
El Niño (Warm Phase)
• El Niño influences the intensity, frequency and spatial
distribution of tropical cyclones
– Because of the extensive area of warmer water over the
eastern tropical Pacific, their hurricanes can travel farther
north and west
• Teleconnection - linkage between changes in
atmospheric circulation occurring in widely separated
regions of the globe, often over thousands of
kilometers
– Higher than usual SST over the central and eastern tropical
Pacific heats and destabilizes the troposphere
– Deep convection generates towering thunderstorms that
help drive atmospheric circulation, governing the course of
jet streams, storm tracks and moisture transport by winds
at higher latitudes.
La Niña
(Cold Phase)
• La Niña - period of unusually strong trade winds
– Colder than usual surface waters over the central and eastern tropical
Pacific, with exceptionally vigorous upwelling
– Somewhat warmer than usual surface waters over the western
tropical Pacific
– SST anomalies are essentially opposite those observed during El Niño
La Niña (Cold Phase)
• Across middle latitudes of the Northern Hemisphere,
winds tend to be more meridional during La Niña
– Steering cold air masses toward the south and warm air
masses toward the north.
• An extreme meridional flow pattern can cause
weather hazard
– If a cutoff high persists over the same area, the weather
remains dry and the probability of drought increases.
– Caused the severe summer drought in central U.S. during
the La Niña of 1988.
ENSO Index
• SSTs are drawn from an area of the tropical Pacific that includes the
equatorial cold tongue, bounded by longitude 120 °W and 170 °W, latitude
5 °N and 5 °S
• El Niño is characterized by a positive SST departure from normal greater
than or equal to 0.5 Celsius degree, averaged over three consecutive
months.
• La Niña is characterized similarly, but by a negative SST departure from
ENSO Index
• ENSO Index - based on six variables measured in the
tropical Pacific: sea-level air pressure, zonal (eastwest) component of surface wind, meridional (northsouth) component of surface wind, surface air
temperature, sky cloud cover and sea-surface
temperatures. Can also be referred to as the
Multivariate ENSO Index.
Event
Occurrence
El Niño
Weak
1952, 1953, 1958, 1969, 1976, 1977, 2004, 2006
Moderate
1951, 1963, 1968, 1986, 1987, 1991, 1994, 2002, 2009
Strong
1957, 1965, 1972, 1982, 1997
La Niña
Weak
1950, 1954, 1956, 1964, 1967, 1971, 1974, 1983, 1984, 1995, 2000, 2005,
2008, 2011
Moderate
1955, 1970, 1998, 2007
Strong
1973, 1975, 1988, 1999, 2010
• ENSO Alert System - launched by NOAA’s Climate Prediction Center
– watch when conditions in the equatorial Pacific are favorable for the
development of El Niño or La Niña within the next three months
– advisory when El Niño or La Niña conditions have developed and are
expected to continue
• During the second half of the 20th century, El Niño conditions
prevailed 31% of the time and La Niña occurred 23%
Ocean as a Carbon Sink: biological pump
• Biological component of the carbon cycle in the ocean
– Through photosynthesis algae fix carbon in the ocean from CO2 and
produce oxygen
– Other organisms construct shells and exoskeletons from carbon then
die and shells precipitate to the ocean bottom and form sedimentary
rock
CO2 dissolution in the ocean:
Ocean Acidification
• CO2 + H2O ⇌ H2CO3
• H2CO3 is a weak acid
• Ocean acidification change in the ocean’s
chemistry due to absorption
of excessive amounts of
anthropogenic CO2
H2O ⇌ H+1 + OH-1
pH - measure of the hydrogen
and hydroxyl ion
concentration, ranging from 014 with 7 as neutral, above 7
as basic and below 7 as acidic
Ocean
Acidification
• For 300 million years, ocean pH has been marginally basic,
about 8.2
– It’s now 8.1, for a 25% to 30% increase in acidity
– Removes the essential “building blocks” for shell formation, and
organisms suffer
Sea-Level Rise
Two mechanisms:
• Thermal expansion of
seawater: Water always
contracts when its
temperature drops and
expands when
temperature rises.
• Global warming trend
causes melting of landbased polar ice sheets
and mountain glaciers
The latest IPCC report (AR5) noted
with high confidence that the rate
of sea level rise since the mid-19th
century has been larger than the
mean rate during the previous two
millennia
Sea-Level Rise
• The glass on the left is half filled with water
• Adding ice cubes (center), simulates glacier moving from land
to sea
– The water level immediately rises to near the top of the glass
– When glacial ice leaves the land and enters the ocean, causing a rise
in sea level
• The floating ice cubes melt (right), simulates floating ice
shelves
– The water level is unchanged
– Melting of floating ice shelves do not alter sea level