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 Makers Annual Monsoon Cycle Variability of the monsoons Regional Monsoon Systems The Indian Monsoon The East Asia Monsoon The Australian Monsoon / East Asia Winter Monsoon The African Monsoon The 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 Heating Seasonal contrasts in land surface temperatures produce atmospheric pressure changes. ~ 30C ~ 86F

H Seasonal changes in sea level pressure HH 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 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 up May to Jun – heating of northern tropical lands intensifies. Jun to Jul – sensible heat input at the surface is close to a maximum Sep – cessation of the northern hemisphere monsoon Weak Vertical Motion Maximum Vertical Motion and Moisture Maximum Vertical Motion and Moisture Monsoon Maximum Intensity Monsoon Maximum Intensity Vertical Motion Intensifies Vertical Motion Intensifies Predominant wind direction is onshore Predominant wind direction is onshore Onset of the monsoon Onset of the monsoon Weak Vertical Motion

Variability of the Monsoons There is considerable variability in the onset, duration and magnitude of the monsoons Mechanisms: Internal Dynamics Internal Dynamics = variations in the atmospheric circulation (e.g. travelling disturbances, thermal and orographic forcing, and tropical-extratropical interactions) Boundary Forcing 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 Internal Dynamics + Boundary Forcing = Monsoon variations

Internal Dynamics: Thermal and Orographic forcing

Boundary Forcing: What is going on?

Boundary Forcing: What’s going on?