Atmospheric and Oceanic General Circulation

Dr. John KrastingNOAA/GFDL – Princeton, NJ

John.Krasting@noaa.gov

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