Mid-Latitude Meteorology

What are the key characteristics of weather in the mid-latitudes?

- Large temperature contrasts

- Sustained westerly flow (upper and lower troposphere)

- Closed circular pressure systems

- Advection of warm and cold air

- Frontal systems

- Greater variation in weather

What is the zonal index?

Propensity for westerly winds to blow across NZ (dependent on where and how strong the highs and lows are)

What is a high index and its impacts on NZ?

- Strong and stable westerly flow

- Highs to the north

- Lows to the south

- Repeated every 4-7 days

- Exists 85% of the time

- Warm air from Australia with the wind, affects west coast

What is a low index and how will it affect NZ?

- Weak westerly or easterly

- Lows to the north

- Highs to the south

- Rare

- Moist low-level air from sub tropics onto east coast

What is the Southern Annular Mode (SAM)?

- Measures movement and variability in westerly winds around the south pole

- North-south shift of westerly winds from their normal position

- Driven by changes in ocean temperatures, currents and relative pressures

What is a positive SAM and its affects on NZ?

- Winds strengthen

- Moves further south

- Lower anomalous pressure over Antarctica

- Higher pressure in the mid-latitudes

- Moves away from NZ

- Light winds

What is a negative SAM and its affects on NZ?

- Westerly wind belt moves northward

- Higher pressure over Antarctica

- Lower pressure in mid-latitudes

- Moves towards NZ

- Westerlies increase

- More unsettled weather

- More frontal systems (with polar front)

What is the air mass concept?

- Air mass is homogenous horizontally with temperature, stability and moisture

- Conditions of the source region must be stable and homogenous which sitting air masses with develop the properties of due to vertical mixing

- Must sit for 3-7 days

- Moves then due to PG

What conditions is maritime tropical?

- Warm

- Wet

- Unstable

What conditions is maritime polar?

- Cold

- Wet

- Stable

What conditions is continental tropical?

- Warm

- Dry

- Unstable

What conditions is continental polar?

- Cold

- Dry

- Stable

What is air mass modification?

Movement of an air mass from one region to another will start to develop characteristics of its new region through stability, temperature and moisture content

What is warm advection?

Warm are mass travels to a colder region

What is cold advection?

Cold air mass travels to a warmer region

What is frontogenesis?

- An air mass advecting finds another air mass already there

- A front forms along the boundary

What factors affect air mass modification?

Air masses begin to change as they move away from their source regions. Depends on

- Trajectory of air

- Degree of heat and moisture exchanges between air mass and contrasting underlying surfaces

- Extend of mechanical mixing occurring

What are there meteorological conditions associated with cold advection?

- Low-level instability

- Cu, TCu, Cb

- Possibly heavy precipitation

What are there meteorological conditions associated with warm advection?

- Low-level stability

- St, Sc, As

- Intermittent showers

What is a MLC?

Large, synoptic scale low pressure storms with attached frontal systems (forms out of polar front) lasting 3 - 10 days

What are the requirements for a mid-latitude cyclone (MLC) and why?

- Large surface temperature gradients

- Different air mass types (forms fronts - key source of energy)

- Strong jet stream (pushes and moves it, strengthens, more divergence)

- Strong cyclonic flow at surface

- Upper level divergence exceeding surface convergence (low pressure system)

How is the MLC and polar jet related?

- Strengthens westerly flow acting to drag storm into mid-latitudes

- Upper level divergence (more cyclonic turning)

- Strengthens frontal uplift

- Maintains MLC once it departs from polar front (when over system core)

What is the initial stage of an MLC?

- Disturbance/trigger upsets balance along stationary front

- Cold air advects into warmer air forming cold front

- Warm air forced up, pressure begins to fall

- Region begins to turn cyclonically

- Warm advection forms warm front

- Pressure dalls further

What is the development stage of an MLC?

- Pressure continues to reduce

- More cyclonic turning

- More cold advection

- More cold fronts

- Increased frontal uplift

- Size increases

- Begins interacting with polar jet

- Deeper low

What is the maturity stage of an MLC?

- Increased cyclonic turning, advection and frontal movement have destabilised the polar front

- Most of storm extends into westerly wind belt

- Polar jet and westerly winds steer storm as it breaks away from polar front (due to large amounts of cyclonic motion)

- Takes with it the fronts which have been created

- Frontal uplift provides the energy

What is the decay stage of an MLC?

- System is fuelled by frontal uplift

- Relies on frontal energy it brought with it

- Cold fronts move faster than warm fronts

- Creates an occluded front

- Decreases amount of frontal uplift

- Uplift zone gets pushed away from low pressure centre

- System turns into a cold core low as warm air is cut off

- Eventually dissipates