Dr John Krasting NOAAGFDL Princeton NJ JohnKrastingnoaagov Rutgers Physical Climatology October 18 2012 Why is there circulation to begin with The Earth has to maintain its radiative balance ID: 351835
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Slide1
Atmospheric and Oceanic General Circulation
Dr. John Krasting
NOAA/GFDL – Princeton, NJ
John.Krasting@noaa.gov
Rutgers Physical Climatology
October 18, 2012Slide2
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.Slide3
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)Slide4
But typically we want an average over some time period.
(X again can be any quantity –
i.e. temperature, moisture)Slide5
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 averageSlide6Slide7
Let’s consider the northward transport of temperature
Mean
Meridional
Circulation (MMC)
Stationary
Eddies
Transient
EddiesSlide8
The choice of ΔT and Δλ mattersSlide9
Typical Features
Mean
Meridional
Circulation (MMC)
Stationary
Eddies
Transient
Eddies
Hadley Cell
Ferrel
Cell
Polar Cell
Semi-permanent
highs and lows
Planetary waves
Midlatitude
storms Slide10
Major components of the MMC
Neelin
2011Slide11
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 disturbancesPolar CellPolar regions are typically areas of high pressure.Slide12
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) …Slide13
The individual components of MSE are larger than the net transport. MMC transport of heat is not particularly efficient!Slide14
The northward energy transport by eddies is much larger than the MMC.Slide15
The Walker Circulation is the major large-scale East-West feature of the global atmospheric circulation.
Neelin
2011Slide16
La Niña
El NiñoSlide17
Mean SLP Monthly Climatology
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/climatology/Sea-Lvl-Pressure.shtmlSlide18
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 regionSlide19Slide20
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 circulationSlide21
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 longerSlide22Slide23
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
2011Slide24
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) circulationInvolves the deep oceanMost notable feature is the Atlantic
Meridional
Overturning Circulation (AMOC)Slide25
Neelin
2011Slide26
Global thermohaline circulation
Neelin
2011Slide27
AMOCSlide28
Ocean circulation is important for carbon uptake
Takahashi