Lesson 5 - ENSO Cycles

Question 1

Describe the ENSO cycle in a normal year?

Answer 1

• Trade winds blow equator wide and westwards
• The winds blow towards the warm water of the Western Pacific
• Convectional uplift occurs as the water heats the atmosphere
• The trade winds push the warm air westwards. Along the coast of Peru, the shallow position of the thermocline allows winds to pull up water from below
• This causes upwelling of nutrient-rich, cold water, leading to optimum fishing conditions
• The pressure of trade winds results in sea levels in Australasia being 50cm higher than Peru and sea temperatures being 8 degrees warmer
• A Walker loop returns the air

Question 2

During a normal year, which ocean is affected?

Answer 2

The Pacific Ocean

Question 3

Describe the action of trade winds during a normal year?

Answer 3

Trade winds are westerly blowing winds, they blow warm water towards the west.

Question 4

During a normal year, what impact does the warmer rising air in the west have?

Answer 4

It stimulates atmospheric circulation; air rises, cools and condenses which causes the formation of clouds and stimulates rainfall.

Question 5

Describe what happens during an El Nino year?

Answer 5

• The trade winds in the Western Pacific weaken and die
• There may even be a reverse direction and flow
• The piled up water which would normally occur in the west moves back east, leading to a 30cm sea level rise in Peru
• The region of rising air moves east along with the associated convectional uplift. Upper air disturbances can lead to teleconnections all around the world
• The Eastern Pacific Ocean becomes 6-8 degrees warmer. The El Nino Effect overrides the cold northbound Humboldt Current, thus breaking the food chain. Lack of phytoplankton results in a reduction in fish numbers, which in turn affects fish-eating birds on the Galapagos Islands
• Conditions are calmer across the whole Pacific

Question 6

How often does El Nino usually occur?

Answer 6

Every 7 years

Question 7

What happens to trade winds during an El Nino year?

Answer 7

Trade winds are weakened or reversed

Question 8

During an El Nino year, where is the wet and unsettled weather located?

Answer 8

During an El Nino year, the wet and unsettled weather is located in the centre of the Pacific Ocean.

Question 9

During an El Nino year, where is there an increased risk of flooding?

Answer 9

• Peru

Question 10

During an El Nino year, where is there an increased risk of drought?

Answer 10

• India
• Brazil
• Indonesia

Question 11

Describe what happens during a La Nina year?

Answer 11

• This is an exaggerated version of a normal year. There are extremely strong trade winds
• The trade winds push warm water westwards, giving a sea level up to 1 meter higher in Indonesia and the Philippines
• Low pressure develops with very strong convectional uplift as very warm water heats the atmosphere. This leads to heavy rain in Southeast Asia
• An increase in Equatorial undercurrent and very strong upwelling of cold water off Peru results in strong high pressure and extreme drought. This can be a major problem in the already semi-arid areas of Northern Chile and Peru.

Question 12

Describe the movement of warm water in a La Nina year?

Answer 12

More of the warmest water is pushed towards the west of the Pacific.

Question 13

How does La Nina impact global temperatures?

Answer 13

Global temperatures become cooler.