Global oceanic and atmospheric circulation

HON 305VDr. Andersen

Atmospheric circulation: Simplistic model

● Assume:● Earth is not rotating● Earth's surface is

uniform

● Global influx of solar radiation and loss of longwave radiation creates a gradient of decreasing temperature from the equator to the poles

Atmospheric circulation: Spin the planet

● If you account for Earth's rotation, Coriolis forces cause the circulation pattern to break up into three cells (Hadley, Ferrel, and polar) and two jet streams (subtropical and polar) in each hemisphere

● Notice the weirdness at the north edge of the Ferrel cell; this collision of warm and cold air masses produces weather fronts and cyclonic storms

● Intertropical convergence zone (ITCZ) follows equator exactly

Actual global circulation

● Doesn't match up with 3-cell model for a couple of reasons● Air heats and cools differently over land and water● Elevation influences air flow, higher elevation often intensifies

pressure gradients

● Effects can be seen in global climate animations from the University of Oregon, and also in these animations from the National Center for Atmospheric Research (NCAR).● Atmospheric circulation● Vertical velocities● Temperatures

Actual circulation: January

● Southern hemisphere tilted toward sun

● ITCZ south of the equator

● Big dips of ITCZ toward the South Pole over major land masses because land heats up faster than ocean

Actual circulation: July

● ITCZ shifts to the north by as much as 40 degrees latitude as northern hemisphere tilts toward the sun

● Land areas of North Africa and Asia warm rapidly to form an Asiatic low-pressure zone (contrast with high pressure in that area in January) that becomes part of the ITCZ, drives Asian monsoon

● Southern polar zone remains cold even during southern hemisphere summer because Antarctic ice sheet reflects most solar radiation

Oceanic circulation

● Generated by:● Winds above surface waters (thus tied to atmospheric circulation)

● Evaporation

● Sinking of cold water at high latitudes

● Coriolis forces

● Flow patterns constrained by continents bordering the three ocean basins (Atlantic, Pacific, and Indian), forming large-scale gyres

● Each gyre has two east-west currents (at top and bottom) and two north-south currents along edges of continents

● Flow directions determined by prevailing winds and by Coriolis forces

● Sometimes the Pacific Equatorial Counter Current intensifies, driving El Nino conditions

● Western boundary currents flow from equator to high latitudes, tend to be narrow and fast-moving; the deepest oceanic surface currents

● Eastern boundary currents flow from high latitudes to the equator, tend to be broad, slow-moving, and shallow

Subsurface circulation

● Subsurface circulation much slower than surface currents

● Driven by variation in the density of sea water● Temperature● Salinity

● Cold saline water sinks in the North Atlantic, flows to (and then around) Antarctica, then northward into Indian and Pacific Oceans, where it rises and joins surface flow

● One complete circuit estimated to take about 1000 years

Video here

GCMs: Climate modeling

● Components● Atmosphere● Ocean● Land surfaces● Cryosphere (ice and snow)● Associated biological and chemical processes

● Simplest GCMs consider either only atmosphere (AGCM) or only ocean (OGCM)

● Newer more sophisticated GCMs include coupled dynamics of most major systems (AOGCM), additional links and components added frequently

GCMs

● U.N. Intergovernmental Panel on Climate Change (IPCC) predictions based on coupled models (AOGCM)

● Overall uncertainty depends on uncertainty of component models

● Based on Navier-Stokes equations for fluid flow with thermodynamic terms added

● Solved numerically on computer by constructing a 3D grid over the Earth's surface

GCMs and climate change

● AOGCMs used to predict future climate under various emissions scenarios

● Atmospheric greenhouse gas concentrations typically simply an input to the model, although it is possible to couple an AOGCM with a global carbon-cycle model (and possibly an economic model of global energy portfolios)

GCMs: Uncertainty

● Interaction with global carbon cycle one source of uncertainty

● Another is the role of convective processes, which generally take place at scales smaller than the resolution of the model grid

● Major source of uncertainty is the role of clouds; cloud formation and response to changed climate not as well known as you might expect

GCMs: Confidence

● Models are based both on physical law (conservation of mass, Navier-Stokes equations) and massive amounts of data

● Ability to simulate current climate based on first principles

● Ability to provide insight into past climates and climatic events (such as the climate impacts of historic volcanic eruptions)

● What are the main similarities between atmospheric and oceanic circulation?

● Are any of the results of large-scale atmospheric circulation apparent to the casual observer?

● Why does it rain so much in the tropics?