Atmospheric and Oceanic General Circulation

1. Atmospheric and Oceanic General Circulation Dr. John KrastingNOAA/GFDL – Princeton, NJ John.Krasting@noaa.gov Rutgers Physical Climatology October 18, 2012

2. 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.

3. 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)

4. But typically we want an average over some time period. (X again can be any quantity – i.e. temperature, moisture)

5. We can now define two different types of eddies Eddies are defined as the deviation from the time average Quasi-stationary eddies are the difference between the time mean and the zonal mean

7. Let’s consider the northward transport of temperature Mean MeridionalCirculation (MMC) StationaryEddies TransientEddies

8. The choice of ΔT and Δλ matters

9. Typical Features Mean MeridionalCirculation (MMC) StationaryEddies TransientEddies • Hadley Cell • Ferrel Cell • Polar Cell • Midlatitudestorms • Semi-permanenthighs and lows • Planetary waves

10. Major components of the MMC Neelin 2011

11. 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.

12. 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) …

13. The individual components of MSE are larger than the net transport. MMC transport of heat is not particularly efficient!

14. The northward energy transport by eddies is much larger than the MMC.

15. The Walker Circulation is the major large-scale East-West feature of the global atmospheric circulation. Neelin 2011

16. La Niña El Niño

17. Mean SLP Monthly Climatology http://www.cpc.ncep.noaa.gov/products/precip/CWlink/climatology/Sea-Lvl-Pressure.shtml

18. Consider the oceans in addition to the atmosphere … Time rate of change of energy in the atmosphere and oceans Export of energy out of the region Radiative flux at the top of the atmosphere

20. 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

21. 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

23. 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

24. 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)

25. Neelin 2011

26. Global thermohaline circulation Neelin 2011

27. AMOC

28. Ocean circulation is important for carbon uptake Takahashi