AOSC 200Lesson 14

Oceanography• The oceans plat 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.

Fig. 8-1, p. 210

Energy gains and losses in the oceans

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)

Fig. 8-5, p. 213

Sea Surface Temperature

Fig. 8-6, p. 214

Major Ocean Currents

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 an 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 BLOW TOWARDS THE EQUATOR - WHICH CAUSES THE OCEAN TO FLOW IN A WESTERLY DIRECTION (REMEMBER THE CORIOLIS FORCE ACTS IN A WESTERLY DIRECTION IN THE SOUTHERN HEMISPHERE

• 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

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 selling.

Fig. 8-14, p. 219