Transcript AOSC200_summer_lect8a - Atmospheric and Oceanic Science

AOSC 200
Lesson 8
Oceanography
• The oceans play three important roles in
determining weather and climate
• (1) They are the major source of water
vapor needed to form precipitation.
• This drives the hydrological cycle.
• This water vapor is also the source of
the latent heat that, when released in
condensation, is the driving force
behind severe weather.
Energy gains and losses in the oceans
Fig. 8-1, p. 210
Oceanography
• (2) They exchange heat with the atmosphere.
• The previous slide showed the net energy
gains and losses in the oceans.
• On average the ocean gains energy in the
summer and loses energy in the winter.
• The oceans cool the atmosphere in the
summer and heat the atmosphere during the
winter
• The oceans act like a thermostat.
Fig. 8-2, p. 211
Oceanography
• (3) The oceans transport energy
poleward.
• The Pacific basin and the North Atlantic
basin have two major currents that
transport heat to the Pole.
• The Gulf Stream
• The Kuroshio Current
• These will be discussed later.
Fig. 8-3, p. 212
Fig. 8-4, p. 213
Ocean Temperature
• Can divide the ocean into three layers
• Top 100 meters is called the surface
zone, or mixed layer
• Wind driven waves and currents mix
this layer – uniform temperature
• Bottom layer, below about 1000 meters,
cold water -1 to 3 ºC
• Transition zone – Thermocline
• Sea Surface Temperature (SST)
Sea Surface Temperature
Fig. 8-5, p. 213
Major Ocean Currents
Fig. 8-6, p. 214
Fig. 8-8, p. 215
Ocean Currents
• You may note that the currents actually move
to the right of the wind direction
• Why?
• Because the Coriolis force also acts on the
oceans.
• At the surface of the ocean the wind exerts a
force on the water due to friction between the
waves and the wind.
• As soon as the ocean moves it acquires a
Coriolis force so that the resultant motion is to
the right of the wind direction in the Northern
hemisphere.
• But this is only true for the top layer of the
ocean
Ekman Spiral
• The second layer is dragged along by the top
layer by a frictional force. It in turn will also
acquire a Coriolis force and the resultant
motion of the second layer will be to the right
of the first layer.
• This process continues down through the
ocean until the direction of the ocean current
at a depth can be at 90 degrees to the
direction at the surface
• Known as the Ekman spiral
• Currents induced in the deeper parts of the
ocean are called Ekman transport.
Fig. 8-9, p. 215
Upwelling along the California Coast
• Los Angeles is at about 30 degrees latitude.
At most times of the year there is a high
pressure system in the Pacific and this can
produce Northerly winds along the coast.
• The resultant Ekman spiral produces a
current that flows away from the coast. Water
from deep in the ocean is brought up to
replace the water transported away.
• This is know as upwelling.
• This water is full of nutrients – large
population of plankton – bottom of food chain
– good fishing.
OCEAN CURRENTS
• MASSIVE PATTERN OF WATER FLOW
• WINDS BLOW STEADILY OVER THE OCEAN AND
FRICTION PUSHES THE OCEAN SURFACE IN
THE DIRECTION OF THE WINDS.
• OCEAN CURRENTS TEND TO FOLLOW THE
WIND PATTERNS
• HOWEVER THE CORIOLIS FORCE WILL MAKE
THE OCEAN CURRENT MOVE AT AN ANGLE TO
THE WIND DIRECTION
• EKMAN SPIRAL AND EKMAN TRANSPORT
• UPWELLING
Fig. 8-12, p. 218
EL NINO AND LA NINA
• .EL NINO IS THE PERIODIC WARMING OF THE
EQUATORIAL PACIFIC OCEAN BETWEEN SOUTH
AMERICA AND THE DATE LINE.
• USUALLY THE WINDS (TRADE WINDS) ARE NORTH
WESTERLY IN THE NORTHERN HEMISPHERE, SOUTH
WESTERLY IN THE SOUTHERN HEMISPHERE, WHICH
CAUSES THE OCEAN AT THE EQUATOR TO FLOW IN A
WESTERLY DIRECTION
• STRONG UPWELLING ALONG THE PERUVIAN COAST HIGH NUTRIENT LEVEL - LARGE SCHOOLS OF FISH
• IN EL NINO THE WINDS WEAKEN OR REVERSE- NO
UPWELLING - POOR FISHING
• LA NINA IS THE COUNTERPOINT OF EL NINO. WINDS
INTENSIFY AND THE UPWELLING INCREASES
Sea surface map shows the
position of the warm water
(red) in December 1997 (El
Nino) and the cold water
(blue) in December 1988 (La
Nina)
Fig. 8-14, p. 219
El Nino and La Nina
• How does El Nino affect global weather
patterns?
• Western Pacific experiences less rainfall as
warm water moves east.
• This shift in rain patterns moves the
subtropical jet stream from its normal path.
• This change in the path of the sub-tropical Jet
allows El Nino to affect the weather and
climate of the mid-latitudes as well as the
tropics.
• The commodities markets use the NOAA El
Nino forecasts to influence their buying and
• http://www.youtube.com/watch?v=pOhqzx
LrS0g
Computer generated image of Hurricane Mitch –
October 26, 1998. Near Honduras
Note the distinct ‘eye’ and the large extent of the hurricane
Fig. 8-18, p. 223
Damage from hurricane Andrew, August 1992
Fig. 8-19, p. 224
Fig. 8-23, p. 228
Fig. 8.29
TROPICAL CYLONES
• THESE INTENSE TROPICAL STORMS ARE
KNOWN BY DIFFERENT NAMES IN VARIOUS
PARTS OF THE GLOBE:
•
HURRICANES - ATLANTIC
•
TYPHOONS - WESTERN PACIFIC
•
CYCLONES - INDIAN OCEAN
•
• MOST FORM BETWEEN THE LATITUDES OF 5
TO 20 DEGREES.
• LESS THAN 5 DEGREES, THE CORIOLIS FORCE
IS TOO SMALL
• MORE THAN 20 DEGREES, THE TEMPERATURE
OF THE OCEAN IS TOO COLD
• MUST HAVE WIND SPEEDS OF MORE THAN 119
KM PER HOUR AND HAVE A ROTARY
CIRCULATION TO BE A HURRICANE/TYPHOON
TROPICAL CYCLONE FORMATION
• HURRICANES DEVELOP MOST OFTEN WHEN
OCEAN WATERS HAVE REACHED MAXIMUM
TEMPERATURES - 25 C OR HIGHER.
• INITIAL STAGE IS SOME FORM OF
DISTURBANCE - NAMED BY THE WEATHER
SERVICE AS TROPICAL DISTURBANCES.
• ONLY A FEW TROPICAL DISTURBANCES
DEVELOP 119 KM PER HOUR WINDS
• IF WINDS <119 BUT > 61 - TROPICAL STORM
• NAMES GIVEN WHEN STORM REACHES
TROPICAL STORM STATUS
TROPICAL CYCLONES
• MATURE TROPICAL CYCLONES AVERAGE
ABOUT 600 KM ACROSS
• BAROMETRIC PRESSURE CAN DROP ACROSS
CYCLONE FROM 1010 TO 950 MILLIBARS
• THIS GENERATES RAPID, INWARD SPIRALING
WINDS.
• AS AIR MOVES CLOSER TO CENTER ITS
VELOCITY INCREASES
• CONSERVATION OF ANGULAR MOMENTUM.
• IN THE CENTER OF THE EYE THE AIR IS
DESCENDING – HENCE IT IS FREE OF CLOUDS.
TROPICAL CYCLONES
• MOIST SURFACE AIR IS TURNED UPWARD AND
ASCENDS.
• AS MOIST AIR MOVES UP IT COOLS AND WATER
VAPOR CONDENSES.
• THIS PROVIDES LATENT HEAT TO INCREASE
BUOYANCY OF THE RISING AIR.
• NEAR THE TOP OF THE HURRICANE THE
AIRFLOW IS OUTWARD.
• AIR MUST BE PULLED AWAY FROM HURRICANE
AT THE TOP – REINFORCEMENT FROM ABOVE.
• THE CENTRAL DOUGHNUT AREA HAS
DOWNWARD MOTION, IS CALLED THE EYE OF
THE STORM
• EYE WALL CONTAINS STRONGEST WINDS.
Fig. 8-24, p. 229
Schematic of a Hurricane
TROPICAL CYCLONES
• TROPICAL CYCLONES DIMINISH WHEN
• THEY MOVE OVER COLDER OCEAN WATERS
• MOVE ONTO LAND
• REACH A LOCATION WHEN LARGE-SCALE FLOW ALOFT IS
UNFAVORABLE
Fig. 8.27
STORM SURGE
Fig. 8.35
TROPICAL CYCLONE DAMAGE
• WIND DAMAGE
• STORM SURGE
• INLAND FLOODING