SEASONAL VARIATIONS IN REGIONAL CIRCULATION SYSTEMS: THE MONSOONS

The word “Monsoon” • The word "monsoon" is derived from the

Arabic word "mausim" which means season. Ancient traders sailing in the Indian Ocean and adjoining Arabian Sea used it to describe a system of alternating winds which blow persistently from the northeast during the northern winter and from the opposite direction, the southwest, during the northern summer. Thus, the term monsoon actually refers solely to a seasonal wind shift, and not to precipitation

Outline• Definition of Monsoon

The Monsoon MakersAnnual Monsoon CycleVariability of the monsoons

Regional Monsoon SystemsThe Indian MonsoonThe East Asia MonsoonThe Australian Monsoon / East Asia Winter

MonsoonThe African MonsoonThe American Monsoon

Summary

Let’s examine first the Indian Monsoon

DEFINITION

• Prevailing wind direction shifts by at least 120º betwen

January and July

• Average frequency of prevailing directions in January and

July exceeds 40%

The Monsoon Makers

1) Seasonal Heating

2) Moisture Processes

3) The Earth’s Rotation

The Monsoon Makers: 1) Seasonal HeatingSeasonal contrasts in land surface temperatures produce atmospheric pressure changes.

~ 30C

~ 86F

H

Seasonal changes in sea level pressure

H H

L

L

The Monsoon Makers 1) Seasonal Heating

As a result, there are major seasonal wind reversals referred to as “the monsoons”

The Monsoon Makers 2) Moisture Processes

http://www.geo.arizona.edu/Antevs/ecol438/monsoon.gif

Energy released in the form of latent heat of condensation raises summer land-ocean pressure differences to a point higher than they would be in the absence of moisture in the atmosphere.

Moisture Monsoon Magnitude

Heat

Heat

The Monsoon Makers 3) The Earth’s Rotation

The air in monsoon currents moves in curved paths

Winds change direction as they cross the equator because of changes in the Coriolis Force

Equator

Fc

Fc

The Monsoon Makers

1) Seasonal Heating

2) Moisture Processes

3) The Earth’s Rotation

Moiture releases energy (latent heat) that intensifies the monsoon

Seasonal temperature and pressure changes produce seasonal wind reversals

Air moves in curved paths and winds change direction as they cross the equator

The Annual Monsoon Cycle

Mean annual cycle of precipitation over several major monsoon areas:[North America Monsoon (NAMS, 20°–37°N, 248°–257°E); South America Monsoon (SAMS, 20°–37°N,40°–60°W); India (6°–37°N, 68°–98°E); Sahel (10°–20°N, 15°– 15°W)]. For comparison, one non monsoon region with a large annual cycle is also shown [Pacific Northwest (PNW: 42°–50°N, 112°–124°W)].

FIG. 1. from Vera et al (2006)

Apr – northern hemisphere tropical-subtropical latitudes are beginning to warm upMay to Jun – heating of northern tropical lands intensifies.

Jun to Jul – sensible heat input at the surface is close to a maximumSep – cessation of the northern hemisphere monsoon

Weak Vertical Motion

Maximum Vertical

Motion and Moisture

Monsoon Maximum Intensity

Vertical Motion

Intensifies Predomina

nt wind direction

is onshore Onset of

the monsoon

Weak Vertical Motion

Variability of the MonsoonsThere is considerable variability in the onset, duration and magnitude of the monsoons

Mechanisms:

forcingBoundary

dynamics Internal

Internal Dynamics = variations in the atmospheric circulation(e.g. travelling disturbances, thermal and orographic forcing, and tropical-extratropical interactions)

Boundary Forcing = changes in surface conditions(e.g. extent of snow cover, soil moisture, and sea surface temperature) energy balance geographical distribution of heat and moisture

Internal Dynamics + Boundary Forcing = Monsoon variations

Internal Dynamics: Thermal and Orographic forcing

Boundary Forcing: What is going on?

Boundary Forcing: What’s going on?