Tropical Circulation in a Global Context

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Tropical Circulation in a Global Context
Mean surface pressure and winds during DJF
Easterlies
Easterlies
Westerlies
Westerlies
NE trades
NE trades
NE trades
NE trades
SE trades
SE trades
Westerlies
Westerlies
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El Niño Southern OscillationENSO
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ENSO - Mode of Variability of Atmosphere-Ocean
System

• Climate System has many modes of variability
• Some modes are induced by external forcing from
  Sun (diurnal, seasonal, Milankovitch cycle
  relate to changes in orbital parameters of the
  Earth)
• Some modes are internal to the system and result
  from complex and non-linear interactions between
  the systems fluids (atmosphere and ocean) over a
  variety of time scales

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Internal Climate System Oscillations

• Climate system has its own internal oscillations
  independent of external forcing
• Modes of internal variability (Oscillations) vary
  over a variety of time scales
• Some of the strongest oscillations are
• 20 100 days tropical waves (hold your curiosity
  for a few more classes)
• El Nino Southern Oscillation - ENSO
• North Atlantic Oscillation NAO
• Pacific Decadal Oscillation PDO
• Arctic Oscillation AO
• Antarctic Oscillation AAO

High signal in high latitudes but can affect low
latitudes too
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What is El Niño Tropical Pacific Sea Surface
Temperature Anomalies
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History of El Niño

• Locally recognized in precolonial times
• Sir Gilbert Walker (1924)
• While working for the Indian Meteorological
  Service, he noticed pressure anomalies
• Attempted to relate anomalies to the failure of
  the Indian Monsoon, but failed
• H.P. Berlage (1950s) Develops Southern
  Oscillation Index (SOI)
• 1972-1973 Major El Niño causes collapse of
  Peruvian fishing
• 1983 Major El Niño causes flooding in Southern
  California
• 1998 Strongest El Niño of 20th century

Negative correlation
Sea Level at S Pacific High
Sea Level Pressure at Darwin
Pressure Anomalies From Caviedes, C., 2001, El
Niño in History Pg. 8
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Darwin
Tahiti
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The Southern Oscillation Index - SOI

• Calculated as the pressure differences between
  Tahiti and Darwin (Tahiti-Darwin)

D
T
Negative SOI is an indicator of El Niño
Sea Surface Temperature at equator and 110oW
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The Walker Circulation
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Typical Atmospheric Circulation Walker Cell
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Thermocline Depth at which the rate of decrease
of temperature with increase of depth is the
largest. It separates the zone between the mixed
layer above, much influenced by atmospheric
fluxes, and the deep ocean. In the tropics, the
thermocline is shallow on average, as in the
eastern Pacific (50m), or deeper as in the
western part (160-200m).
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Overview of Atmosphere-Ocean System during El Niño

• Increased SST in the Eastern Pacific
• Weakened Easterlies
• Humboldt Current is weakened
• Weakened upwelling
• Depression of the thermocline
• Decreased nutrient mixing
• Shifting rainfall patterns
• Drought in Australia
• Torrential floods in Peru in December
• Not every El Niño is the same
• They differ in timing, intensity extent

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Tropical Pacific Sea Surface Temperature Anomalies
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El Niño and Water Temperature
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The importance of the westerly wind bursts
The question is What triggers an El Nino and why?
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The 1997-98 El Niño
Changes in the direction of the tropical easterly
westerly wind burst
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El Niño, Easterlies and Westerly Winds
Time
Westerly Anomalies
Westerlies
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What can cause the westerly wind bursts?
Several theories try to explain they onset of an
El Nino. Some say that an oscillation with period
of 30-60 days known as the Madden-Julian
oscillation could be important if occurring in
the right time. Sorry but we will see more on
the Madden-Julian oscillation in a few weeks But
lets examine some of the Advanced ENSO theories
to explain the consequences for ENSO
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Do we understand how and Why an ENSO episode
occurs?
There are several theories to explain ENSO and
none is considered absolute. There are many
unknown physical and dynamical processes and
interactions among scales that cannot be
represented in the state-of-the-art climate
models. Here we will explore one famous theory
The Delayed Oscillator
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Advanced ENSO Theory the delayed Oscillator
http//iri.columbia.edu/climate/ENSO/theory/index.
html
Imagine a very simplified model in which the
Pacific ocean is represented in blue and South
America and Australia in yellow
Temperature varies in depth with an idealized
thermocline ( a region of rapidly decreasing
temperature with depth that separates the warmer
near-surface ocean from the colder deep ocean).
There is no horizontal variations of temperature
(or salinity), there are also no horizontal
variations in density, and hence pressure there
are no mean currents. 
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Now we perturb the motionless ocean by turning on
an isolated patch of westerly (eastward) wind
stress (force per unit area that wind exerts on
the surface of the ocean) near the equator in the
central part of the ocean basin. This wind patch
is shown in Figure 2. The wind forcing is turned
on instantaneously and is kept constant for 30
days
Increases locally the depth of the warm water
layer near the equator, and decreases it locally
farther poleward in either hemisphere
Away from the equator, the near-surface ocean
(above the thermocline) Coriolis force results in
net transport (mass flow) to the right of the
direction of wind stress in the Northern
Hemisphere, and to the left of the wind-stress
forcing in the Southern Hemisphere
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Kelvin and Rossby waves in the Ocean
The mass surplus near the equator then begins to
disperse eastward as a so-called (downwelling)
Kelvin wave (deepens warm water layer)
Red colors height Blue colors - height
Note the differences in the structure of these
waves Kelvin is fast and equatorial and Rossby
is slower and affects higher latitudes
Rossby Wave
Kelvin wave
Rossby waves (upwelling waves) propagate westward
and shallow the warm water
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How does Kelvin and Rossby Waves evolve in the
Delayed Oscillator theory?
animation
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How does Kelvin and Rossby Waves evolve in the
Delayed Oscillator theory?
animation
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The Delayed Oscillator Theory Cross section
analysis of the ocean
Normal Conditions
Easterlies Pile up warm water in the western
Pacfic
Low Pressure
High Pressure
Depth
Warm
Cold
Positive Reinforcement Warm water in the
west low pressure, Cold water in the east
higher pressure Enhances pressure gradient
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El Nino Conditions weaker Easterlies (west-east
difference in atmospheric pressure decreases)
El Niño Conditions Pressure anomaly reverses
air flow
Weaker Easterlies
Difference in pressure decrease
Rossby Waves Kelving waves
Depth
Colder
Warmer
Kelvin Wave, Propagates east 2.9 m/sec (70
days) (positive)
Rosby Waves, Propagate west 0.93 m/sec (210
days) (negative)
Feedbacks Reinforce El Niño
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El Niño in Transition
Kelvin and Rossby waves reflect as they meet the
continents and return as Rossby and Kelvin waves,
respectively
Easterlies
Depth
Colder
Warmer
Kelvin Wave reflects, become Positive Rossby
Waves, propagate westward
Rossby waves reflect, become a negative Kelvin
Wave Propagate Eastward
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This simple scenario is different from ENSO in
the sense that it represents oscillations
deriving from a single wind impulse, whereas the
real ENSO involves a continuous wind-stress
forcing throughout the cycle also the time scale
of the ocean oscillations in the present example
are much shorter than those of ENSO. These issues
are addressed through one other critical element
of the theory the positive feedback, or coupled
instability mechanismHow to understand that???.
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Strong easterly winds piles up warm water in the
Pacific West. East-west gradients of pressure
increases, which increases the wind. Positive
feedback We have a condition for La Nina to
develop and sustain. The weakening of the trades
cause changes in the thermocline as we saw
before. Gradients decrease, convection is
displaced toward central Pacific positive
feedback and El Nino conditions
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• Animation El Nino 1998
• http//apollo.lsc.vsc.edu/classes/met130/notes/cha
  pter10/warmwater_anim.html

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In conclusion
The oscillatory mechanism inherent in the ocean
dynamics, by itself, would lead to rather weak
oscillations with periods of a few seasons. When
the two processes (atmosphere and ocean) are
allowed to operate together, as in the real
climate system (according to the theory), for a
range of conditions that includes those observed
in the tropical Pacific, coupled oscillations
with enhanced amplitude and interannual periods
(3-5 years) are favored
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The recent history of El Ninos/La Nina events
MEI is calculated as the first unrotated
Principal Component (PC) (statistical method
useful to characterize the co-variability of one
or more variables in space and/or time) of six
observed fields sea-level pressure (P), zonal
(U) and meridional (V) components of the surface
wind, sea surface temperature (S), surface air
temperature (A), and total cloudiness fraction of
the sky (C) combined.
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El Nino and the ocean heat budget
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• How can we measure and track changes in the
  oceanic heat-budget?
• We need to know both ocean currents and heat
  storage of the ocean. Like winds blowing around
  the highs and lows of atmospheric pressure, ocean
  currents flow around highs and lows of oceanic
  pressure that can be determined from the height
  of sea surface, called ocean surface topography.
• Ocean current velocity can be computed from the
  slope of the ocean surface
• As water heats up, it expands, and as it cools,
  it contracts, affecting the height of sea surface
  as well. Measuring the ocean surface topography
  gives the required information for studying
  global ocean circulation and the oceans heat
  budget.

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The 1997- 98 El Niño
Sea Surface Height
Animation of TOPEX http//sealevel.jpl.nasa.gov/g
allery/tiffs/videos/pacific96-05.mpg
Southern Oscillation Index
SLP_animation
http//topex-www.jpl.nasa.gov/mission/jason-1.html

SOI from ftp//ftp.bom.gov.au/anon/home/ncc/www/sc
o/soi/soiplaintext.html
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El Nino global Impacts
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1997-98 wind anomalies and OLR anomalies

• Question for group discussion
• Interpret where and when the largest anomalous
  precipitation and convection is observed
• Where suppression is observed
• How does it relate to wind anomalies?

Colors OLR anomalies
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Impacts of El Niño Teleconnections
Upper Tropospheric pressure anomalies
High Pressure Anomalies in the western
Pacific create pressure anomalies at higher
latitudes These anomalies change the position
of the jet stream
Teleconnections a change in one part of the
globe has an impact in another part . El Niño has
many teleconnections
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NH Winter
El Nino
Seasonal Changes in the Impact of ENSO

• Impacts Temperature
• Impacts Rainfall
• The strength of a teleconnection varies
• Some areas are very predictable
• Some areas are not

La Nina
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NH Summer
Seasonal Changes in the Impact of La Niña events
NH Summer
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El Niño Impacts in North America Or why does it
rain like cats and dogs in Santa Barbara?
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Enhanced Precipitation in Santa Barbara The
Pineapple Express
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Enhanced Hurricane Activity in the North Pacific
ExTyphoon Lupit, Oct 21/09
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Connections to Climate Change

• Hypothesis In a warmer world , it is suggested
  that El Niño will occur more often and will be
  more powerful than present ones.
• Models diverge on whether this hypothesis is
  correct or not
• Some data sets (e.g, corals) suggest that strong
  El Niño have occurred in the past before
  anthropogenic warming

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Typical US Temperature, Precipitation and Jet
Stream Patterns during El Niño and La Niña Winters
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How do we know so much?

• New measurement techniques in the ocean TAO_array
  , TAO_mooring , atlas_mooring_instruments
• New observations systems (satellite)
• New computer model and methods to integrate the
  data and make forecasts

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U.S. Precipitation Departures (mm) and Frequency
of Occurrence () for El Niño during Dec - Feb
FREQUENCY (right panel) indicates the percentage
of El Niño years that the indicated departure
(left panel) occurred. For example, above-average
seasonal precipitation over eastern Texas gt70 of
the El Niño years.
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U.S. Temp. Departures (C) and Frequency of
Occurrence () for El Niño during Dec - Feb
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Tsonis et al. 2003 Periods with large rate of
change of global temperature (not the actual
temperature) are related to an increase in the
frequency of El Nino (opposed for La Nina) El
Nino is activated to reverse positive global
temperaturetrends, and La Nina to reverse
negative trends
dT/dt 0 1 EN/4 yrs
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A Little Bit of Rationale (Tsonis et al. 2003)

• An increase in global temperatures leads to rapid
  (and greater) heat storage in the tropical
  Pacific (evidence in the past century 0.2 C
  western Equatorial Pacific).
• Increased heat storage leads to stronger trade
  winds and this induces an increase in trade winds
  fluctuations.
• With rapid storage of energy, the thermal and
  physical inertia of the regions of heat storage
  in the western Pacific should increase more
  rapidly, with the tendency for enhanced sea
  surface elevation gradients.
• Large sea surface elevation gradients suppress
  the trigger of La Nina (remember the explanation
  of the Delayed Oscillator), even when typical
  trade winds are strong and fluctuations are large.