Transcript Krasting PowerPoint on Circulation

Atmospheric and Oceanic
General Circulation
Dr. John Krasting
NOAA/GFDL – Princeton, NJ
[email protected]
Rutgers Physical Climatology
October 18, 2012
Why is there circulation to begin with?
• The Earth has to maintain its
radiative balance!
• The goal is to redistribute
geographic variations in surface
heating caused by:
– Gradients of incoming solar
radiation
– Albedo variations
• To a first order, transport heat
away from the tropics to the poles.
In climate, it is useful to consider the
circulation averaged over a particular
latitude (zonal averages)
1
[ x] =
2p
ò
2p
0
x dl
(X can be any quantity –
i.e. temperature, moisture)
But typically we want an average over
some time period.
1
x=
Dt
ò
Dt
0
x dt
(X again can be any quantity –
i.e. temperature, moisture)
We can now define two different types
of eddies
x¢ = x - x
Eddies are defined as the deviation from the
time average
é x* ù = x - é xù
ë û
êë úû
Quasi-stationary eddies are the difference between
the time mean and the zonal mean
Let’s consider the northward transport
of temperature
* *ù
é
évT ù = [ v] [T ] + v T + év¢T ¢ù
ë û
êë
úû ë û
Mean Meridional Stationary
Circulation (MMC)
Eddies
Transient
Eddies
The choice of ΔT and Δλ matters
Typical Features
* *ù
é
évT ù = [ v] [T ] + v T + év¢T ¢ù
ë û
êë
úû ë û
Mean Meridional
Circulation (MMC)
• Hadley Cell
• Ferrel Cell
• Polar Cell
Stationary
Eddies
• Semi-permanent
highs and lows
• Planetary waves
Transient
Eddies
• Midlatitude
storms
Major components of the MMC
Neelin 2011
Major components of the MMC
• Hadley Cell
– Thermally-driven
– Rising air in the tropics from tropical convection
– Equator-ward surface air turns to the right and
gives rise to the easterly trade winds
• Ferrel Cell
– Residual from averaging many weather
disturbances
• Polar Cell
– Polar regions are typically areas of high pressure.
The rising branch of the Hadley Cell is
related to tropical convection and
carries moist warm air high into the
atmosphere
Consider Moist Static Energy (MSE) …
MSE = c pT + gz + Lq = sensible + potential + latent
The individual components of MSE are
larger than the net transport. MMC
transport of heat is not particularly
efficient!
The northward energy transport by
eddies is much larger than the MMC.
The Walker Circulation is the major
large-scale East-West feature of the
global atmospheric circulation.
Neelin 2011
La Niña
El Niño
Mean SLP Monthly Climatology
http://www.cpc.ncep.noaa.gov/products/precip/CWlin
k/climatology/Sea-Lvl-Pressure.shtml
Consider the oceans in addition to the
atmosphere …
¶Eao
= RTOA - Ñ· Fao
¶t
Time rate of change
of energy in the
atmosphere and
oceans
Radiative flux at the
top of the
atmosphere
Export of energy out
of the region
Key points about the oceans …
• All of the Earth’s oceans are connected
• Places where water sinks are called “mode
water formation” regions
• Tracing the path of mode waters (water with
similar properties) allows us to follow the
strength of the circulation
How is the ocean different from the
atmosphere?
• Ocean density is a function of temperature
and salinity
• Ocean heat storage is larger
• Ocean circulation time scales are longer
The rate of heat storage in the
atmosphere is negligible. Storage in the
ocean is a function of depth and time.
• The surface ocean
exchanges heat readily
with the atmosphere
(1-10 year time scales)
• The upper ocean
exchanges heat with
the deep ocean on 10100 year time scales
Neelin 2011
Two main types of ocean
circulation
• Wind-driven circulation
– Surface-based
– Examples include western boundary currents (i.e
the Gulf Stream, Kuroshio Current), and
subtropical gyres
• Thermohaline (or density-driven) circulation
– Involves the deep ocean
– Most notable feature is the Atlantic Meridional
Overturning Circulation (AMOC)
Neelin 2011
Global thermohaline circulation
Neelin 2011
AMOC
Ocean circulation is important for
carbon uptake
Takahashi