Southern Annular Mode and Westerly Index

Question 1

What do the Westerly Index and Zonal Wind refer to?

Answer 1

Strength and stability of the westerly wave pattern.

Question 2

What does a high zonal index indicate?

Answer 2

Stable, westerly flow, regular patterns of longs, minimal north/south fluctuations.

Question 3

What does a low zonal index indicate? What is its other name?

Answer 3

• Meridional flow
• Unstable north/south fluctuations, frequent or short-lived intense short wave disruptions to the normal flow.
• Wave has a higher amplitude, i.e. troughs and ridges are deeper.
• If amplitudes increase sufficiently, waves can break up, leaving pools of cold and warm air behind.

Question 4

What is meant by the zonal index?

Answer 4

Stability of the Rossby Wave system and the presence of long and short waves are measured by the zonal index.

Question 5

What does a high zonal index situation represent? What happens to surface weather conditions in a high zonal index?

Answer 5

• Periods where stable conditions and permanent long-wave patterns dominate with the general direction of the atmosphere being west to east.
• The result is more stable surface conditions and fewer fluctuations in surface weather.

Question 6

What does a low zonal index situation represent? What happens to surface weather conditions in a low zonal index?

Answer 6

• Westerly flow maintained however, there are greater shifts of the atmosphere north and south.
• This results in creating larger surface weather conditions.

Question 7

What is meant by the term ‘zonal wind’?

Answer 7

Those winds that flow within the zone of the mid-latitudes.

Question 8

What is the Southern Annular Mode defined as?

Answer 8

The zonal index for the Southern Hemisphere.

Question 9

What does a positive S.A.M. represent?

Answer 9

• Westerly winds contracting toward Antarctica.
• Weaker westerles
• More settled weather
• Higher average pressures
• Fewer/weaker storms in New Zealand

Question 10

What does a negative S.A.M. represent?

Answer 10

• Expansion of belt toward the equator.
• Stronger westerlies
• Unsettled weather
• Storms in New Zealand

Question 11

What are the other two climate oscillations that affect New Zealand called? How long do they last?

Answer 11

• Interdecadal Pacific Oscillation (IPO) which are changes in sea surface temperatures in the Pacific Ocean. Last 20-30 years.
• El Nino Southern Oscillation (ENSO) which occurs every 2-7 years and lasts on average 12 months.

Question 12

Why is the Southern Annular Mode important to NZ climatology?

Answer 12

Controls where and how strong westerly winds in the mid-latitudes blow and because it affects climate/meteorological conditions across Australasia.

Question 13

Define the Westerly Index over NZ?

Answer 13

• Refers to the strength and stability of the Rossby Wave system over/near New Zealand.
• Measured largely through changes in the cycle of the Southern Annular Mode.

Question 14

What do high indices on weather maps indicate?

Answer 14

• High Zonal Index

- Presence of a more stable system dominated by long-wave patterns

Question 15

What do low indices on weather maps indicate?

Answer 15

• Low Zonal Index

- Presence of a less stable system dominated by shorter wave patterns

Question 16

How is the SAM index itself measured?

Answer 16

• variations in zonal mean sea-level pressures between the latitudes 40°S and 65°S.
• These allow phase changes to be predicted several weeks in advance which is important as changes can happen suddenly.

Question 17

Explain the weather across New Zealand in a positive SAM event

Answer 17

• Associated with higher than normal air pressure in NZ.
• Tends to bring light winds and tranquil weather conditions.
• Fewer cold fronts and drier conditions for most of NZ.
• Higher temperatures particularly in the West Coasts of both islands.
• Weaker westerly winds.

Question 18

Describe the weather in the Antarctic in a positive SAM event

Answer 18

Windiness and storm activity increases over the Southern Ocean and Antarctica.

Question 19

Explain the weather across New Zealand in a negative SAM event

Answer 19

• Lower than normal air pressure in NZ.
• Higher pressure well to the south of the country.
• Westerlies increase around NZ, increasing the risk of unsettled weather conditions.
• Allows more cold weather around NZ.
• Can bring snow to the Alps.
• Cooler and wetter conditions in the West Coasts of both islands.

Question 20

Describe the weather in the southern ocean and the Antarctic in a negative SAM event

Answer 20

• Calmer weather in the southern ocean, where westerlies are weaker than normal.
• Better weather in the Antarctic