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Transcript
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