Transcript AMS Ocean Studies

Ocean Studies
Introduction to Oceanography
American Meteorological Society
Chapter 11
The Ocean, Atmosphere, and
Climate Variability
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Case in Point
– In 1982-83, the weather seemed to go wild in many parts
of the world.
– Just prior to these worldwide weather extremes, the ocean
circulation off the northwest coast of South America
changed drastically with dire implications for marine
production.
– Weather extremes were linked to large-scale
ocean/atmosphere circulation changes in the tropical
Pacific and soon a new scientific term was added to the
public’s vocabulary: El Niño.
– Spurred further research on ocean-atmosphere
interactions and the deployment of an array of in situ and
remote sensing instruments in the tropical Pacific to
provide early warning of the development of El Niño
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The Ocean, Atmosphere, and
Climate Variability
• Driving Question:
– How do interactions between the ocean and
atmosphere impact worldwide weather and
short-term climate variability?
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The Ocean, Atmosphere, and
Climate Variability
• In this chapter, we examine:
– Short-term climate fluctuations involving
interactions between the ocean and
atmosphere
• El Niño/La Niña
– Other examples of short-term climate
variability stemming from air/sea interactions
including the North Atlantic Oscillation, the
Arctic Oscillation, and the Pacific Decadal
Oscillation
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Earth’s Climate System
• CLIMATE CONTROLS
– Latitude, elevation, topography, proximity to large
bodies of water, Earth’s surface characteristics, net
incoming solar radiation, long-term average
atmospheric circulation, prevailing ocean circulation
– Atmospheric circulation encompasses the combined
influence of all weather systems operating at all
spatial and temporal scales ranging from sea breezes
to the prevailing winds that encircle the planet.
• Although strongly influenced by the other climate controls,
atmospheric circulation is considerably less regular and less
predictable than the others.
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Earth’s Climate System
• ROLE OF THE OCEAN
– The ocean influences radiational heating and cooling
of the planet.
– A primary control of how much solar radiation is
absorbed at the Earth’s surface
– Main source of the most important greenhouse gas
(water vapor) and is a major regulator of the
concentration of atmospheric carbon dioxide (CO2)
– At high latitudes highly reflective multi-year pack ice
greatly reduces the amount of solar radiation
absorbed by the ocean.
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Earth’s Climate System
• ROLE OF THE OCEAN
– Most water vapor, the principal greenhouse gas,
enters the atmosphere via evaporation of seawater.
– Carbon dioxide, a lesser greenhouse gas, cycles into
and out of the ocean depending on the sea surface
temperature, circulation patterns, and biological
activity in surface waters.
– The ocean influences the planetary energy budget not
only by affecting the radiational heating and
radiational cooling of the entire planet, but also by
contributing to the non-radiative latent heat and
sensible heat fluxes at the air-sea interface.
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Earth’s Climate System
• ROLE OF THE OCEAN
– Ocean currents strongly influence
climate.
– Cold surface currents, such as
the California Current, are heat
sinks; they chill and stabilize the
overlying air, thereby increasing
the frequency of sea fogs and
reducing the likelihood of
thunderstorms.
– Relatively warm surface currents,
such as the Gulf Stream, are heat
sources; they supply heat and
moisture to the overlying air,
destabilizing the air, thereby
energizing storm systems.
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Earth’s Climate System
• ROLE OF THE OCEAN
– Broad scale patterns of sea-surface
temperature (SST) strongly influence the
location of major features of the atmosphere’s
planetary scale circulation.
– When SST patterns change so too do the
locations of planetary-scale circulation
features.
• North-south shifts of the intertropical
convergence zone (ITCZ)
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Tropical Pacific Ocean/Atmosphere
• HISTORICAL PERSPECTIVE
– Originally, El Niño was the name given by fishermen to a period
of an unusually warm southward flowing ocean current and poor
fishing off the coast of Peru and Ecuador that often coincided
with the Christmas season.
– These warm water episodes are relatively brief, lasting perhaps
a month or two, before sea surface temperatures and the
fisheries return to normal.
– About every three to seven years, however, El Niño persists for
12 to 18 months or even longer and is accompanied by
significant changes in SST over vast stretches of the tropical
Pacific, major shifts in planetary-scale oceanic and atmospheric
circulations, and collapse of important South American fisheries.
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Tropical Pacific Ocean/Atmosphere
• HISTORICAL PERSPECTIVE
– Southern oscillation: a seesaw variation in
air pressure across the tropical Indian and
Pacific Oceans
• When air pressure was low over the Indian Ocean
and the western tropical Pacific, it was high east of
the international dateline in the eastern tropical
Pacific.
• The Southern oscillation index (SOI) is based on
the difference in air pressure between Darwin and
Tahiti.
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Tropical Pacific Ocean/Atmosphere
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Variation in the southern oscillation index
based on monthly mean sea level pressure
anomalies at Darwin, Australia and Tahiti.
Strongly positive values of the index indicate
La Niña conditions and strongly negative
values of the index indicate El Niño conditions.
The thick black line is the 10-year running
mean (trend line)
Tropical Pacific Ocean/Atmosphere
• HISTORICAL PERSPECTIVE
– Relationship between El Niño and the
southern oscillation
• An El Niño episode begins when the air pressure
gradient across the tropical Pacific starts to
weaken, heralding the slackening of the trade
winds.
• Refer to this relationship between El Niño and the
Southern Oscillation by the acronym ENSO
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Tropical Pacific Ocean/Atmosphere
• HISTORICAL PERSPECTIVE
– What is unique about ENSO is the strong
coupling: changes in the ocean drive changes
in the atmosphere which then feedback and
further alter the ocean.
– La Niña: essentially opposite El Niño
– Some scientists refer to the warm El Niño and
cold La Niña as opposite extremes of the
ENSO cycle.
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Tropical Pacific Ocean/Atmosphere
• NEUTRAL CONDITIONS IN THE TROPICAL
PACIFIC
– Prevailing winds blow from the south or southeast
along the west coast of South America so that most of
the time Ekman transport drives warm surface water
to the left (westward), away from the coast.
• Causes cold, nutrient-rich water to well up from below the
thermocline and replace the warm, nutrient poor surface
water that is transported offshore
• Upwelling is also responsible for a tongue of relatively cool
surface waters along the equator in the eastern tropical
Pacific.
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Tropical Pacific Ocean/Atmosphere
• NEUTRAL CONDITIONS IN THE TROPICAL
PACIFIC
– Equatorial upwelling produces a strip of relatively low
sea surface temperatures along the equator from the
coast of South America westward to near the
international dateline.
– Trade winds drive a pool of relatively warm surface
waters westward toward Indonesia and northern
Australia.
– The contrast in sea-surface temperature between the
western and eastern tropical Pacific (averaging about
8 Celsius degrees or 14.4 Fahrenheit degrees) has
important implications for precipitation across the
tropical Pacific.
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Tropical Pacific Ocean/Atmosphere
Benchmark average rainfall in millimeters per day (mm/d) across the tropical
Pacific Ocean for the 10-year period 1998 through 2007. The heaviest
rainfall is in the western tropical Pacific where sea-surface temperatures are
highest. These data were obtained from the TRMM Microwave Imager and
IR sensors onboard geosynchronous satellites supplemented by
conventional rain gauge measurements.
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Tropical Pacific Ocean/Atmosphere
• NEUTRAL CONDITIONS IN THE TROPICAL
PACIFIC
– The contrast between relatively high air pressure over
the central and eastern tropical Pacific and relatively
low air pressure over the western tropical Pacific
ultimately drives the trade winds.
• Winds initially blow from regions where air pressure is
relatively high toward regions where air pressure is relatively
low.
• The greater the air pressure contrast the stronger are the
winds.
– High SST in the western tropical Pacific lower the
surface air pressure whereas low SST in the eastern
tropical Pacific raise the surface air pressure.
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Tropical Pacific Ocean/Atmosphere
• NEUTRAL CONDITIONS IN THE TROPICAL
PACIFIC
– In the western tropical Pacific warm humid air rises,
expands, and cools.
• Water vapor condenses into towering cumulonimbus
(thunderstorm) clouds that produce heavy rainfall.
• Aloft this air flows back eastward and sinks over the cooler
waters of the eastern tropical Pacific.
• Sinking air is compressed and warmed so that clouds
vaporize or fail to develop.
• This completes the large convective-type circulation known
as the Walker Circulation.
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Tropical Pacific Ocean/Atmosphere
Schematic block
diagram showing
ocean/atmosphere
conditions in the
tropical Pacific during
normal or neutral
episodes. Red
indicates areas of
highest sea-surface
temperatures
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Tropical Pacific Ocean/Atmosphere
• EL NIÑO, THE WARM PHASE
– With the onset of El Niño, air pressure falls over the
eastern tropical Pacific and rises over the western
tropical Pacific as part of the southern oscillation.
• Trade winds slacken in the western and central equatorial
Pacific.
• Trade winds west of the international dateline may reverse
direction and blow toward the east.
– With relaxation of the trade winds, the westward flow
of the equatorial currents weakens and at times
reverses direction.
• The thick layer of warm surface water normally in the west
drifts slowly eastward across the tropical Pacific.
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Tropical Pacific Ocean/Atmosphere
Schematic block
diagram showing
ocean/atmosphere
conditions in the
tropical Pacific
during El Niño
conditions. Red
indicates areas of
highest sea-surface
temperatures
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Tropical Pacific Ocean/Atmosphere
• EL NIÑO, THE WARM PHASE
– In the western tropical Pacific, SST drops, sea-level
falls, and the thermocline rises.
– In the eastern tropical Pacific, SST rises, sea-level
climbs, and the thermocline deepens.
– Arrival of warm surface waters in the eastern tropical
Pacific reduces upwelling of nutrient-rich waters along
the coast of Ecuador and Peru.
• The commercial fish harvest plummets.
• Warmer surface waters can also severely stress coral reefs
living in shallow tropical waters.
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Tropical Pacific Ocean/Atmosphere
Sea level record at
Galápagos in the
eastern tropical Pacific
based on tide gauge
records and
expressed in cm as
departure from the
long-term average.
Relatively high sea
levels correspond to
El Niño episodes.
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Tropical Pacific Ocean/Atmosphere
• EL NIÑO, THE WARM PHASE
– During El Niño, lower than usual SST in the western tropical
Pacific and higher than usual SST in the central and eastern
tropical Pacific coupled with the change in trade wind circulation
give rise to anomalous weather patterns in the tropics and
subtropics.
• El Niño also influences the intensity, frequency, and spatial
distribution of tropical cyclones.
– Has a ripple effect on the weather and climate of middle
latitudes, especially in winter
– Linkage between changes in atmospheric circulation occurring in
widely separated regions of the globe, often over distances of
thousands of kilometers, is known as a teleconnection.
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Tropical Pacific Ocean/Atmosphere
• THE 1997-98 EL NIÑO
– Rivaled its 1982-83 predecessor as the most intense
of the 20th century
– Developed rapidly with the trade winds weakening
and eventually reversing direction in the western
tropical Pacific in early 1997
– Equatorial upwelling ceased during the Northern
Hemisphere summer of 1997.
– Warming was rapid in the eastern tropical Pacific with
SST setting new record highs each successive month
from June through December.
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Evolution of the 1997-98 El Niño as derived from changes in ocean surface height
as measured by altimeter sensors onboard the TOPEX/Poseidon satellite.
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Tropical Pacific Ocean/Atmosphere
• LA NIÑA,THE COLD PHASE
– La Niña is a period of unusually strong trade winds
and exceptionally vigorous upwelling in the eastern
tropical Pacific.
– Tends to persist for 12 to 18 months
– Surface waters are colder than usual over the central
and eastern tropical Pacific and somewhat warmer
than usual over the western tropical Pacific.
– Accompanying La Niña are worldwide weather
extremes that are often opposite those observed
during El Niño.
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Tropical Pacific Ocean/Atmosphere
Schematic block
diagram showing
ocean/atmosphere
conditions in the
tropical Pacific
during La Niña
conditions. Red
indicates areas of
highest sea-surface
temperatures
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Tropical Pacific Ocean/Atmosphere
• PREDICTING AND MONITORING EL NIÑO
AND LA NIÑA
– Forecasters rely on two basic types of numerical
models to predict the onset, evolution, and decay of
El Niño or La Niña: empirical (or statistical) models
and dynamical models.
– An empirical model compares current and evolving
oceanic and atmospheric conditions with comparable
observational data for periods preceding El Niño (or
La Niña) episodes over the prior 40 years.
– A dynamical model consists of a series of
mathematical equations that simulates interactions or
coupling among atmosphere, ocean, and land.
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Tropical Pacific Ocean/Atmosphere
• PREDICTING AND MONITORING EL
NIÑO AND LA NIÑA
– Reliable observational data from the tropical
Pacific Ocean and atmosphere are essential
for detecting a developing El Niño or La Niña,
and for initializing numerical models.
– Accuracy of dynamical models depends on
• How well their component equations simulate the
coupled ocean/atmosphere/land system
• Reliable observational data
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Tropical Pacific Ocean/Atmosphere
• PREDICTING AND MONITORING EL NIÑO
AND LA NIÑA
– ENSO Observing System: consists of an array of
moored and drifting instrumented buoys, island and
coastal tide gauges, ship-based measurements, and
satellites
• TAO (Tropical Atmosphere /Ocean) array of moored buoys in
the tropical Pacific Ocean
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– Renamed TAO/TRITON in 2000
– Presently consists of approximately 70 deep-sea moorings that
measure several atmospheric variables (air temperature, wind,
relative humidity) as well as oceanic parameters (sea-surface
and subsurface temperatures at 10 depths in the upper 500 m
or 1640 ft)
Tropical Pacific Ocean/Atmosphere
Components of the ENSO Observing
System provide advance warning and
monitor the development and decay of El
Niño and La Niña events
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An instrumented TAO
moored buoy
Tropical Pacific Ocean/Atmosphere
• PREDICTING AND
MONITORING EL NIÑO
AND LA NIÑA
– Remote sensing by satellite
plays an important role in
providing early warning of
an evolving El Niño or La
Niña.
• Sensors onboard NOAA
and NASA satellites
monitor cloud cover and
map sea surface
temperatures.
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Sea level record at a location along the
equator in the eastern tropical Pacific
derived from measurements made by
the TOPEX/Poseidon satellite. Sea level
is expressed in cm as departure from the
long-term average. Relatively high sea
levels correspond to El Niño episodes.
Tropical Pacific Ocean/Atmosphere
• FREQUENCY OF EL NIÑO AND LA NIÑA
– In 2003, NOAA scientists created an Index
that forms the basis for operational definitions
of El Niño and La Niña.
• Based on six variables measured in the tropical
Pacific: sea-level air pressure, zonal (east-west)
component of surface wind, meridional component
of surface wind, surface air temperature, sky cloud
cover, and sea-surface temperature
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Tropical Pacific Ocean/Atmosphere
Variations in the Multivariate ENSO Index showing the
sequence of El Niño and La Niña events since 1950
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North Atlantic Oscillation
– The North Atlantic Oscillation (NAO) refers
to a seesaw variation in air pressure between
Iceland and the Azores.
– Influences precipitation and temperatures
primarily in winter (December to March) over
eastern North America and much of Europe
and North Africa
• NAO Index is directly proportional to the strength
of the North Atlantic air pressure gradient
– Varies significantly from one year to the next and from
decade to decade and is much less regular than the
ENSO cycle
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North Atlantic Oscillation
Record of the North Atlantic Oscillation (NAO) during winter
(December to March) through 2006-2007, based on the
difference between the normalized sea-level air pressure at
Gibraltar and the normalized sea-level air pressure over
southwest Iceland. Solid black line is a running mean.
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Arctic Oscillation
– Seesaw variation in air pressure between the North
Pole and the margins of the polar region
– Shifts between negative and positive phases
• During its negative phase, the air pressure gradient is weaker
and the polar vortex circulation is not as strong as usual
– Allows bitterly cold Arctic air masses to more frequently move
out of their source regions in the far north and plunge
southeastward into middle latitudes
• When the Arctic Oscillation is in its positive phase, the air
pressure gradient is greater and winds encircling the Arctic
are stronger
– Stronger winds act as a dam to impede the southeastward flow
of Arctic air
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Pacific Decadal Oscillation
– Long-lived variation in climate over the North Pacific and North
America
– Sea-surface temperatures fluctuate between the north central
Pacific and the west coast of North America.
– During a PDO warm phase, SST are lower than usual over the
broad central interior of the North Pacific and above average in a
narrow strip along the coasts of Alaska, western Canada, and
the Pacific Northwest.
– During a PDO cold phase, SST are higher in the North Pacific
interior and lower along the coast.
– Key to the climatic impact of PDO is the strength of the subpolar
Aleutian low.
• During a PDO warm phase, the Aleutian low is well developed and
its strong counterclockwise winds steer mild and relatively dry air
masses into the Pacific Northwest.
• During a PDO cold phase, the Aleutian cyclone is weaker so that
cold, moist air masses more frequently invade the Pacific
Northwest.
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Conclusions
– El Niño and La Niña involve interactions
between the tropical Pacific Ocean and
atmosphere.
– Short term fluctuations in climate induced by
El Niño and La Niña as well as the longerterm NAO, AO, and PDO are superimposed
on much longer period variations in climate
that have longer lasting impacts on the Earth
system.
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