Atmospheric and Oceanic General Circulation Dr. John Krasting NOAA/GFDL – Princeton, NJ 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) (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 Meridional Circulation (MMC) Stationary Eddies Transient Eddies

The choice of ΔT and Δλ matters

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

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 k/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 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