Download Circulation

Atmosphere/Ocean circulation
and Hurricane workshop
Chautauqua UWA-6, Dr. E.J. Zita
9-11 July 2007
Fire, Air, and Water:
Effects of the Sun, Atmosphere, and Oceans in
Climate Change and Global Warming
What causes atmospheric circulation?
Uneven insolation …
→ Warmer tropics …
→ Hot (equatorial) air rises …
→ Air flows to lower pressure
→ Convection cells drive weather
Low pressure → rising air cools →
condensation → precipitation
Q: Why do north-south convection flows
turn east or west?
Coriolis force turns winds to the right,
in the Northern hemisphere
Coriolis force animations
• http://www.baesi.org/TRG/coriolis/CORIOLIS.MOV
(Cor3)
• http://www.wiley.com/college/strahler/0471480533/animat
ions/ch07_animations/animation2.html (Cor2)
• http://www.classzone.com/books/earth_science/terc/conten
t/visualizations/es1904/es1904page01.cfm?chapter_no=vis
ualization (Cor1)
Thanks to Gerardo Chin-Leo for researching these animations.
Low pressure storms: CCW in N. hem
Heat transport and exchange
Your HYPOTHESES: What variables do
each of these depend on?
1. Heat required to raise temperature of
water?
2. Heat required to evaporate water?
3. Heat stored in water vapor?
4. Energy of hurricanes?
Q: How much heat is required to raise the
temperature of water?
To raise the temperature of a mass m(kg) of water by
DT (K) requires
Heat Q (joules) = m c DT, where
c = specific heat of water = 4186 J/kg.K
Ex: need Q1=4186 J to heat 1 kg of water by 1K
Q: How much heat is required to evaporate
water?
Evaporating a mass m(kg) of water requires
Heat Q (joules) = m L, where
L = the latent heat of vaporization of water = 22.6 x
105 J/kg
Ex: Need Qvap= 22.6 x 105 J/kg to evaporate 1 kg
of water! A million times more energy!
Q: How much heat is stored in water vapor?
This is the same as the heat it took to evaporate
the water.
Phase transitions absorb/release a LOT of heat –
with NO temperature change in the water!
Ex: why does it get warmer when it snows?
Q: How much energy is in a hurricane?
Warm sea + air →
evaporation →
stored latent heat →
http://www.futurasciences.com/comprendre/
d/images/573/cyclone_elena.jpg
Low pressure + Coriolis force + kinetic energy
→ tropical storms
How to quantify hurricane energy?
Hurricane workshop
Your HYPOTHESES: What variables does
hurricane destructiveness depend on?
• _______________________________
• _______________________________
• _______________________________
Power Dissipation Index
PDI ~ Kinetic energy * speed (let speed = v)
Kinetic energy = ½ mv2
PDI ~ speed to what power?
□
PDI = a v
(where a is some constant)
Hurricane speed and Temp
“Theoretically, the peak wind speed of tropical
cyclones should increase by about 5% for
every 1ºC in tropical ocean temperature.”
(Emanuel, p.687-688)
Tropical sea surface temperature has increased
about 0.5ºC in the last 30 years
How much should peak winds have increased?
Call this x%
Wind speed and PDI
If peak winds have increased by x%, then how
much should hurricane destructiveness (PDI)
increase?
Tropical storm data
Tropical storm data
Qualitative: Does there appear to be a correlation
between sea surface temperature (SST) and
power dissipation index (PDI)?
Quantitative? How much has the PDI risen in the
past 30 years?
Does this fit our predictions?
Testing & improving the model
What other factors
could contribute
to the huge
increase in PDI?
(Emanuel, p.688)
Significance and predictions
DISCUSS:
• Correlations?
• Causal relations?
• Strength of relations?
• Current trends?
• Relation to global
warming?
• Other?
Shall we take a break…
Then return to ocean circulation…
Dynamics of Marine Systems:
Ocean circulation and climate change
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Overview
Surface currents (geostrophic currents)
Deep currents (thermohaline circulation)
ENSO
Adapted from lectures by Dr. Gerardo Chin-Leo, TESC
Salinity and temperature gradients
drive flows and concentrate organisms.
Boundaries between strata are important because they are areas
where organisms accumulate. Boundaries also limit the exchange
of nutrients such as gases and other dissolved materials.
Oceanography and Climate Change
• Physical
– Storage and redistribution of heat
– Albedo (ocean and ice reflectivity)
• Chemical
– Source and sink of greenhouse gases
• Biological
– Source and sink of greenhouse gases
– Source of cloud condensation nuclei
Ocean Currents
• Surface wind-driven circulation. The motion
and pattern of surface currents are a result of
the presence of continents and the balance
between the Wind Stress and the Coriolis
effect. Geostrophic (geo=earth,
strophio=turn) currents.
– Short term storage (days-years) and transfer of
heat to higher latitudes
• Thermohaline circulation. These deep
currents result from the sinking of dense
water in the North Atlantic and Antarctic
Oceans, forcing horizontal circulation of
water in the deep-sea
– Long term storage (~1000 y) storage and transfer
of heat, nutrients and CO2
Surface currents
How are surface currents measured?
• Directly
– Stationary current meters
– Drifters
• Indirectly from the determination of
– wind patterns and knowledge of how water
responds to wind stress
– the density structure in the oceans
(currents flow from low to high density areas)
Geostrophic current
A horizontal current of water that is the result of the balance
between flow down a slope due to gravity and the Coriolis
effect.
Ekman transport
Ekman transport → convergence
Deep ocean conveyor belt
How are deep sea currents measured?
• Inferred from the density structure of the oceans
• Radiotracers
– H3 (tritium) from Atomic Bomb testing can be used as a
tracer for deep water movements
– C14. The ratio of C14:C12 in the surface ocean reflects
the concentration in the atmosphere. Deep ocean water
is removed from atmospheric exchange and the ratio
changes as the C14 decays
Production of C14 by cosmic rays
CALCULATIONS
The rate of radionuclide decay is directly proportional to the
number of radionuclides in a given sample and can be
expressed using the equation N = No e-kt which can be
rewritten as:
ln (N / No) = - k t , Where:
ln = natural logarithm function
N = radioactivity at present. 14C radioactivity of the sample
being analyzed.
No = radioactivity at time of formation. Typically the 14C
radioactivity of a standard material.
k = decay constant. This is unique for each radionuclide and
equals 0.693 / , where  is the half-life of the isotope. 0.693
is the ln(2). The decay constant is negative because the rate
decreases over time.
EXERCISE: In a deep-sea sediment core, at 20 cm depth foraminifera
(CaCO3 containing organisms) have a 14C radioactivity of 5.5 cpms g-1. At
40 cm depth the foraminifera have a 14C radioactivity of 1.64 cpms g-1.
Determine the sedimentation rate in cms per 1000 yrs.
At 20 cm the age of the foram is:
ln (5.5 / 13.6) = - (0.693 / 5730) t
ln (0.40) = - (1.22 X 10-4) t
-0.9163 / - 1.22 X 10-4 = t = 7,421 yrs
At 40 cm the age of the foram is:
ln (1.64/ 13.6) = - (0.693 / 5730) t
ln (0.12) = - (1.22 X 10-4) t
-2.1154 / - 1.22 X 10-4 = t = 17,339 yrs
The difference in age of the 2 layers is 17,339 - 7,421 = 9,918 years
Thus it took 9,918 years to deposit 20 cm of sediment
The rate is 20 cm / 9,918 year = 2.02 cm per 1000 years
How are mean global temperatures
estimated?
El Niño – Southern Oscillation
El Niño / Southern Oscillation
Interannual climate disturbance characterized by warming of the
equatorial Pacific. This is caused by oscillations in the atmospheric
pressure systems in the Southern Hemisphere which results in:
-Weakening of southwest trades wind leading to weak upwelling off the
coast of Peru. This causes the presence of warm water, reversal of local
climate, low primary production, low production of anchovies and
seabirds (and drier winters in the Pacific Northwest).
-Warming of the Equatorial Pacific, disruption of global weather patterns
ENSO demonstrates the tight coupling between the ocean and the
atmosphere. This close relationship makes it difficult to determine what
triggers an ENSO event.
Time scale: 2-7 years
Southern Oscillation (pressure)
El Niño / Southern Oscillation
Changes atmospheric CO2
Changes wind patterns and air temperatures
Changes clouds, water vapor content, and hurricanes
Predicted to increase (IPCC, 2007)
El Niño and Hurricanes
El Niño ↔ warmer Pacific waters
→ more Pacific storms
El Niño ↔ cooler Atlantic waters
→ fewer Atlantic tropical
cyclones
NOAA – AP – July 2007