Chapter 6 Flashcards
Monsoon comes from
“mausim,” which is Arabic for season
Ramage’s definition:
- January and July
- Prevailing wind directions shifts by at least 120°
- wind direction persists at least 40% of the time
- either January and July
- Mean wind speed exceeds 3 m/s
- Fewer than one cyclone-anticyclone alternation every 2 years
According to Ramage’s definition, ……………………have distinct monsoons.
only Asia, Australia, and Africa
A more relaxed definition commonly used:
- Wind must reverse in direction between summer and winter.
- Summer season must be very wet, and winter season very dry.
using the more relaxed definition ……….. have monsoon
North America also experiences a monsoon (in the Southwest U. S. and Northern Mexico.)
over 1⁄2 of the tropics and 1⁄4 of entire globe experience monsoon-type climates.
driving mechanisms for the monsoon:
- Differential heating of land and ocean
- Moist processes
- Rotation of the Earth
Differential Heating of Land and Ocean
Differential heating sets up a horizontal pressure gradient (similar to land/sea breeze only on much larger scale.)
The specific heat of water is much larger than that of dry soil.
- Ocean
- Effective heat capacity difference is even larger, as mass of ocean is much larger.
- In oceans, heat is effectively mixed downward tens of meters via turbulent mixing.
- Land
- Only the upper most few centimeters of land are heated, due to slow molecular transfer of heat vertically
- Saturated soil behaves more like “ocean” than land
Moist soil has higher specific heat than dry soil
Role of Moist Processes
Moisture acts as “stored energy” through latent heat release.
- Solar collector
- Evaporation occurs over the oceans, and then moisture is transported over the land, where it is released through condensation.
- This essentially “focuses” the effects of the solar heating collected over the ocean onto the land areas.
result of horizontal pressure gradient due to different heating capacities
Latent heating results in a more intense monsoon flow, and also a vertically deeper monsoon flow.
- A moist monsoon
- depth on the order of the troposphere.
- A dry monsoon
- much shallower, extending only to the mid-troposphere.
Moisture also changes the character of the
heating of the land. (This is one factor in monsoon variability and monsoon “breaks.” )
- If land is dry
- rising motion will occur closer to the coast, since the land will be very warm.
- Moist land acts more like ocean
- As land becomes wet from precipitation,
- the rising motion will move inland over drier land.
- Precipitation will progress inland, allowing coastal area to dry out. Cycle will then repeat itself.
- the rising motion will move inland over drier land.
Rotational and Frictional Effects
- The Coriolis effect causes the air to “swirl” into the monsoon rather then flow directly in. It results in
- cyclonic inflow at the surface
- anticyclonic outflow aloft
- longitudinal extent of low level is influenced by friction
- more corss isobaric flow at the surface
- less longitudinal extent than the upper level
- more corss isobaric flow at the surface
Land-Ocean Geometry
- Areas on or near Equator do not experience much rotation.
- Uniform surface (either all land or all water) result more in trade-wind trough ITCZ, and not monsoon (e.g., Central Pacific Ocean).
- Western boundary results in atmospheric jet (e.g., East African Low-level Jet).
Causes of Monsoons
- surface responds to the seasonal oscillation of solar heating
- positive net radiation in summer hemisphere
- the response is global scale summer monsoon
- positive net radiation in summer hemisphere
The evolution of the regional monsoons depends on
the distribution of land and ocean, SST gradients, and net ocean heat transport.
The evolution of the regional monsoons depends on the distribution of land and ocean, SST gradients, and net ocean heat transport.
- Difference in heat capacity of the land and ocean
- Horizontal temperature gradients
- lead to upper level pressure gradients and
- horizontal pressure gradients and
- a transverse circulation.
- Horizontal temperature gradients
- Over land, the only way to transport heat downward is through molecular diffusion with little storage while mixing in the ocean allows heat to be transported downward and stored.
