Tropical Meteorology

Question 1

Describe the seasonal location of the equatorial trough in the Northern Hemisphere and explain the reason for the change in location?
SEE DIAGRAM

Answer 1

• Consistent with the meridional movement of the thermal equator north and south of latitude 0º each year.
• In the Northern summer (May to October), entire trough is located in Northern Hemisphere, but the latitudinal change in position and intensity of the trough varies greatly because of the differing nature of surface.

Question 2

How do oceanic regions and continental landmasses influence the equatorial trough?

Answer 2

• Over oceanic regions, the equatorial trough is constant in terms of latitude, but the influence of continental landmasses (their differing mountainous natures and local pressures) cause the trough to shift considerably and fracture in a few places.

Question 3

Describe the seasonal location of the equatorial trough in the Southern Hemisphere and explain the reason for the change in location?
SEE DIAGRAM

Answer 3

• In the Southern Summer (November to April), a large deflection takes the trough South through South Africa, while a smaller wave brushes the Northern parts of Australia (due to influence of variation in surface characteristics).
• The major difference is the position of the trough in the central Pacific and Atlantic where the equatorial trough remains in the Northern Hemisphere.
• This is because of the stronger trade wind strength in the North compared to the South, and also the influence of oceanic surface temperatures

Question 4

What is the key difference between the equatorial trough and the ITCZ?

Answer 4

The equatorial trough is a more or less permanent feature of convergence and convection, whereas the ITCZ is a erratic feature, the presence and intensity of which depends on the variable degree of trade wind convergence.

Question 5

What is the equatorial trough?

Answer 5

• The general term for the belt of comparatively low pressure around the globe in the tropics.
• The semi-continuous belt of low pressure lying between the sub-tropical high-pressure systems in the Northern and Southern Hemispheres.

Question 6

What is the ITCZ?

Answer 6

The ITCZ is a synoptic or weather feature within the equatorial trough. the ITCZ is where Northern and Southern hemisphere trade winds converge.

Question 7

Describe the origin of the SPCZ? Include where the converging airflows originate from

Answer 7

• Formed by the divergent easterlies of the high west of South America and the tradewinds in the Australasian region.
• The semi-stationary anticyclonic system in the eastern Pacific. The easterlies on the equator side of this system turn north-easterly on approaching the Western Pacific; and
• The anticyclones originating and travelling eastward from the Australia/New Zealand region. The southeasterlies preceding these systems extend to the region where the SPCZ forms.
• Thus when the North-easterlies meet, they form a convergence zone with weather characteristics very similar to the ITCZ.

Question 8

Where is the preferred location of the SPCZ?

Answer 8

• Identified from the eastern tip of Papua New Guinea, to slightly southeast of French Polynesia.
• Fluctuations in its intensity may bring cloudiness and rainfall down into Australia’s Northern Territory and Northern Queensland from time to time.

Question 9

Where does the position of the SPCZ stretch to? What are the latitudes of the SPCZ?

Answer 9

Its position also stretches from an area of warm tropical water near Papua New Guinea at approximately 140ºE into sub-tropics near 30ºS 120ºW in the East, sometimes as far as the Cook Islands.

Question 10

When is the SPCZ is more active, summer or winter?

Answer 10

Summer.

Question 11

What is the SPCZ associated with regards to air masses?

Answer 11

Associated with the northeast passage of mid-latitude cold fronts when the cloud band of the latter merges with that of the convergence zone.

Question 12

What weather is the SPCZ responsible for?

Answer 12

• Intense rainfall
• Large storms
• Tropical cyclones

Question 13

What are the latitudes of the trade winds?

Answer 13

General outflow from the high-pressure belts at average latitudes 30ºN and 30ºS towards the equatorial trough produces a steady wind in the lower troposphere.

Question 14

What is the vertical extent of the trade winds?

Answer 14

Approximately 8000 ft AMSL.

Question 15

What direction are the trade winds in the Northern and Southern Hemisphere respectively?

Answer 15

• Northern Hemisphere is north-easterly.

- Southern Hemisphere is south-easterly.

Question 16

Where do the trade winds converge?

Answer 16

Equatorial trough.

Question 17

What happens as the trade winds travel further from the highs towards to the equatorial trough with regards to anticyclonic subsidence?

Answer 17

As the trade winds penetrate into increasingly warm surface conditions, the effect of this subsidence decreases.

Question 18

What is the weather at the trade wind zone largely influenced by? What heights do these vary at?

Answer 18

Influenced by subsidence inversion, varying in height from 6000 to 8000 ft above sea level.

Question 19

What is the weather at trade wind zone like below a subsidence inversion?

Answer 19

• Good visibility with varying amounts of cumulus with bases between 2000 and 3000 ft, and tops at around 8000 ft.

Question 20

What will happen when trade winds cross the windward side of islands?

Answer 20

The amounts and limits of the varying amounts of cumulus with bases at 2000 and 3000 ft are prone to increase when trade winds cross the windward side of islands so that increased wind speeds and shower activity are likely in those locations.

Question 21

What is the moisture content below the subsidence inversion like with regards to the trade winds?
What is the weather like above 8000 ft?

Answer 21

• Moisture content greater than above.

- Good visibility and generally dry air and clear skies above 8000 ft.

Question 22

What happens to the subsidence inversion as they approach the vicinity of the equatorial trough.

Answer 22

• It weakens and rises in height.
• Increased instability from warm surfaces and, especially over ocean areas, great abundance of moisture, cause the formation and development of instability cloud which ultimately includes TCU’s with associated thunder, squall and shower activity.

Question 23

What are the typical wind velocity and direction found above the trade wind zone?

Answer 23

• Winds above the trade winds are usually from the west, often light.

Question 24

Describe the seasonal changes in the location of the trade winds and the effect seasons has on the wind direction.

Answer 24

• In the mid pacific, the trough never enters the southern hemisphere, meaning that the trade winds (south-easterlies of the southern hemisphere) cross the equator to flow to the trough.
• Similarly, during the southern winter, the equatorial trough is in the Northern Hemisphere north of Australia, so that trade winds in that region also cross the equator.
• In summertime in Aussie, the trough is south of the equator so that the Northern Hemisphere trade winds will have to do the crossing.
• When the south-east trade winds cross the equator en-route to the equatorial trough in the Northern Hemisphere, they will be subjected to the Coriolis force applicable to the Northern Hemisphere (veer to the right). Strength of the force is weak. Thus the trade winds blow initially towards the trough from the south and then to the southwest.
• The northeast trades from the Northern Hemisphere towards the trough near Northern Australia in summer will initially become northerly and ultimately north-west.

Question 25

Describe the typical wind strengths of the trade winds, including variation in strength during the summer and winter.

Answer 25

• Trade winds are quite steady and persistent in strength.
• In the Pacific region, they vary throughout the year from 10-20 kts with the occasional increase to 30 kts.
• Slightly stronger in winter than in summer (globally).

Question 26

Describe the effect of the trade winds on the weather experienced in northern Australia?

Answer 26

• During the southern summer, when the Northern Hemisphere trade winds come over the equator, they become north-westerly.
• These north-westerly winds arrive overland after travelling over very warm waters and can produce some intense cloud developments in coastal regions.
• This, coupled with the activity of the equatorial trough, a heat low and occasionally the SPCZ produces the wet season over the northern parts of the continent between December and March.

Question 27

Describe the effect of the trade winds on the weather experienced in island groups in the summertime?

Answer 27

• There is often a contrast between the surface temperatures of islands compared to ocean temperatures, which reverses seasonally.
• In the summertime, the land of major islands warms to a greater degree than the surrounding seas so that a sea breeze effect develops over land.
• Consistent with this, air pressure will be slightly lower over the land than over the sea and instability is increased.
• Thus, coastal regions exposed to the southeast experience an increase in cloud development and possibly heavy precipitation depending on the nature of the terrain.

Question 28

Describe the effect of the trade winds on the weather experienced in island groups in the winter?

Answer 28

• The land is colder, so pressures are higher over land and stability is enhanced, reducing cloud developments and generally more stable.
• Southeast coasts may still experience cloudiness from exposure to the trade winds.

Question 29

With regard to the trade winds, describe what is meant by an individual cumulus disturbance?
- Include its lifespan, size, how they are formed etc

Answer 29

• Smallest type of disturbance
• Measured in a lifespan of only a few hours and a dimension 5km across, resulting from turbulence experienced in the trade wind friction layer.
• These individual clouds often align themselves in cloud streets more or less parallel to the wind direction, or arranged in honeycomb patterns.
• Degree of turbulence is small enough to retain the mixing layer at and below the height of the cloud base in conditions of high instability when more interaction results between the lower and higher levels of air.

Question 30

With regard to the trade winds, describe what is meant by mesoscale convective areas (MCAs)?
- Include its lifespan, size, how they are formed, what they are characterised by

Answer 30

• Mesoscale Convective Areas (MCAs) consist of grouped clusters of cumulus and CB developments within areas from 100 km to 1000 km across.
• MCAs are characterised by instability through deep layers - often part of the equatorial trough.
• MCAs last no longer than one or two days, others can persist for extended periods of time.

Question 31

With regard to the trade winds, describe what is meant by wave disturbances/closed circulation disturbances?
- Include its origin, likelihood of turning into a tropical depression then into a tropical cyclone.

Answer 31

• Rare for closed circulation disturbances to appear in tropical latitudes. Instead, disturbances demonstrate themselves in wave-form in the equatorial easterlies where waves travel from the East to the West but not necessarily at the same speed as the trade winds.
• The formation of a wave originates 2º to 4º of latitudes on the pole-ward side of the equatorial trough.
• Approximately 25% of wave disturbances that form in the tropics in a year develop to the tropical depression stage while some 10% become tropical cyclones.

Question 32

With regards to wave disturbances/closed circulation disturbances, where are eastern waves more frequent? What is their lifespan, size and speed at which they travel at?

Answer 32

• Easterly waves are more frequent in the Northern Hemisphere than in the south Pacific.
• Easterly waves normally have a lifespan of about 10 days, have an average wavelength of around 3000 km, travel approximately 6º in longitude per day and slope eastward with height.

Question 33

With regard to the trade winds, describe its seasonal location and direction?

Answer 33

• During the southern winter, the equatorial trough is in the Northern Hemisphere north of Australia so that trade winds in that region also cross the equator.
• In summertime in Australia, the trough is south of the equator so that the Northern Hemisphere trade winds will have to do the cross (shown in the above diagrams).
• When the south-east trade winds cross the equator en-route to the equatorial trough in the Northern Hemisphere, they will be subjected to Coriolis force, thus veering to the right. The strength of the force is weak, thus after crossing the equator, the trade winds blow initially towards the trough from the south, and ultimately from the southwest.
• The North-east trades from the North Hemisphere crossing the equator towards the trough near northern Australia in summer will initially become northern and ultimately north-west.

Question 34

Describe any topographical influences on the trade winds? (summer and winter influences).

Answer 34

• There’s a contrast between the surface temperatures of islands compared to ocean temperatures which reverse seasonally.
• E.g. in the summertime, the land of major islands warm to a greater degree than the surrounding areas so that sea breeze effects develop over the land.
• Air pressure will be slightly lower over the land than over the sea and instability is increased. Thus coastal regions exposed to the southeast experience an increase in cloud development and possible heavy precipitation depending on the nature of the terrain. This effect will be felt even on the lee side of the islands.
• In winter, the vice versa occurs. The land is colder than the surrounding seas so that pressures are somewhat higher inland and stability is enhanced.
• Some cloudiness may still occur at the southeast coasts from exposure to the trade winds, general conditions are drier and more stable.