Download Where`s the Low Pressure Area?

Day
San Francisco in the summer:
Leave your shorts at home!
¥Land heats up- air rises
¥Onshore winds bring warm, moist air over cool
water (more on this cool water later)
¥This creates Fog
¥Fog drawn inland by the winds
Night
Seasonal changes in wind patterns and
pressure zones over continents and oceans
Ocean
Land
Ocean
Summer- Where’s the Low Pressure Area?
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Seasonal changes in wind patterns and
pressure zones over continents and oceans
Semi-permanent seasonal zones of
different pressure
Summer: In general....
High Pressure areas are over the oceans
Low Pressure areas are over the continents
Ocean
Land
Ocean
Winter: In general....
Low Pressure areas are over the oceans
High Pressure areas are over the continents
Winter- Where’s the High Pressure Area
Semi-permanent seasonal zones
Seasonal zones
of high and low pressure- Summer
of high and low pressure- Winter
Winds (air flow) around pressure zones are deflected
due to Coriolis effect:
•Clockwise around (and out of) High-Pressure cells
Seasonal zones
of high and low pressure- Winter
– (Northern Hemisphere)
•Counterclockwise around (and into) Low-Pressure
cells
– (Northern Hemisphere)
•These are superimposed on the Trades, Westerlies,
etc.
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Hurricanes
•Hot, humid air rising-- LOW PRESSURE
•Coriolis effect creates spiraling winds
•Storm tides- sea surface “dome” in low-P
center
•Plus wind-driven surface currents
•Coincidence of storm tide and high tide cause
the worst flooding of coastal areas
2. Seasonal monsoons
(India, Southeast Asia)
Summer: Hot continent - rising air above it
Creates onshore winds (and rain)
Winter: Cold continent, offshore winds
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Monsoon Weather
Summer (Wet)
Winter (Dry)
El Niño
Lecture 23 -- The Oceans
Preview: El Ni o / La Ni a
OCEAN CIRCULATION -- BASIC CONCEPTS
Wind-driven surface currents
Density-driven deep circulation
•Named for wind pattern that can set in just after
Christmas (literally, “The Christ Child”)
•Trade wind system breaks down- global changes
•Cause: Complex interaction between winds and
ocean currents
•La Niña - Trade winds increase
Fig. 6.25
Average
Conditions
Your Classic Chicken and Egg Situation...
•Atmospheric winds determine ocean surface
currents patterns
•Ocean surface currents influence ocean surface
temperature patterns
•Surface temperatures influence winds
El Ni o
This is a feedback loop.
•Two processes affect each other
•Feedback from each process to the other
•Can lead to oscillations or “chaos”
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Fig. 6.25
Average
Conditions
El Ni o
OCEAN CIRCULATION -- INTRODUCTION
1. Surface Currents
•Horizontal currents in upper few 100 meters;
–v = about 1 m/s
•Driving force: prevailing winds initiate currents
•Modifying factors: deflection by ...
1) Coriolis effect- right deflection in Northern
Hemis.
2) Continents
General pattern -- rotary circulation (gyres);
major gyres centered in sub-tropics
Deep Circulation in the Oceans
The Coriolis Effect causes water to push
inward toward the center of the gyre
Driving force:
Sinking of dense water (e.g., N. Atlantic)
Cooling, sea-ice, and evaporation
General pattern -•Sinking, spreading, eventual upwelling
Modifying factors: deflection by ...
1) Coriolis effect
2) Continents and mid-ocean ridges
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Review: What controls the density of
the water?
• Temperature
• Salinity (evap. and sea ice)
• Pressure (???)- not important
Surface and deep circulation are
coupled in the "Global Conveyor Belt"
• Transport of surface waters to high latitudes
• Sinking and flow at depth
• Upwelling - return to surface
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