Global scale circulation

Single cell circulation: hypotheses

Single cell circulation

Three cell circulation

Jet Streams Jets are due to Coriolis force. They are expected in convergence zones. At the ground level they are expected along the equator (westward) and along the polar circles (eastward) On altitude they are expected along the tropics (eastward).

Climate

The role of zonal gradients (ocean/continents) Summer (July) Winter (January)

Monsoons

Rain distribution Seasonal rain variation is associated to the earth rotation axis inclination. In summer it inclines left and the dry area moves north, while in winter it inclines left and the dry area moves south, as well as the subpolar lows, bringing rains southward. Lisbon is dry in summer and wet in winter.

Air Sea Interaction: wind forcing and geostrophic equilibrium Summer (July)

Upwelling: Ekman Spiral

Upwelling off California

Surface Temperature (in upwelling and downwelling conditions) Agosto 98

https://www. learner. org/courses/envsci/visual/visual. php https://www.learner.org/courses/envsci/visual/visual.php?shortname=sea_surface_temperature

El Niño Happens when easterly winds are weak and is stronger if they reverse and become westerly winds. This happens cyclically with the southern oscillation. This is why sometimes is called ENSO – El Niño Southern Oscillation

La Niña Happens when easterlies are so intense that force upwelling along the equator line.

North Atlantic Oscillation (NAO)

Summary Global circulation is explained by (a) the 3 cells model, modified by the zonal gradients created by continents and (b) by the Coriolis force that forces the jets, associated with the polar front, in case of the polar jet. Seasonal oscillation is explained by the oscillation of the earth inclination associated to the 3 cells model. Ocean circulation is closely linked to atmospheric circulation. The exception are the coastal/shallow areas where the tide plays the major role.