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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! - PowerPoint PPT Presentation
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Atmospheric and Oceanic General Circulation
Dr. John KrastingNOAA/GFDL – Princeton, NJ
Rutgers Physical ClimatologyOctober 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)
(X can be any quantity – i.e. temperature, moisture)
But typically we want an average over some time period.
(X again can be any quantity – i.e. temperature, moisture)
We can now define two different types of eddies
Quasi-stationary eddies are the difference between the time mean and the zonal mean
Eddies are defined as the deviation from the time average
Let’s consider the northward transport of temperature
Mean MeridionalCirculation (MMC)
StationaryEddies
TransientEddies
The choice of ΔT and Δλ matters
Typical Features
Mean MeridionalCirculation (MMC)
StationaryEddies
TransientEddies
• Hadley Cell• Ferrel Cell• Polar Cell
• Semi-permanenthighs and lows
• Planetary waves
• Midlatitudestorms
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) …
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/CWlink/climatology/Sea-Lvl-Pressure.shtml
Consider the oceans in addition to the atmosphere …
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 10-100 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
