9 Climates of the Tropics

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(9)Climates of the Tropics

• Characteristics of tropical weather
• Causes of tropical variability
• A climates
• B climates
• GEOG 31064 Principles of Climatology
• (Dr. Scott Sheridan)

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Definitions of the tropics

• A climate all months warmer than 18C
• Places where it never freezes
• No temperature distinction between winter and
  summer
• 23.5N to 23.5S
• Tropical rain forests

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Hadley Cell cross profile
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No large pressure gradient
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Generally weak upper-level winds
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Minimal year-to-year variability
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High incoming radiation throughout year
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Outgoing longwave radiation varies
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Solar control of temperature

• Primary cycle is diurnal
• Day-to-day variability dominated by sun/cloud

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Temperature in the tropics

• Where no dry season, 1-3 mean difference
  between winter and summer
• Where there is one, somewhat larger
• Diurnal temperature range exceeds annual
  temperature range

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Tropical precipitation

• Significant spatial variability, relative
  position between subtropical high and Hadley Cell
• Virtually all precipitation is convective
• Time of day important

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Some causes of tropical weather variability

• ITCZ seasonality and variability
• El Nino Southern Oscillation
• Tropical cyclones
• Madden Julian Oscillation
• Quasi-Biennial Oscillation

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ITCZ position and intensity
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Tropical cyclones

• Necessary ingredients
• Warm water
• Coriolis force
• Weak vertical wind shear
• Initial convergence at surface

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Tropical cyclones and disturbances

• Easterly waves

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Madden Julian Oscillation

• 40-50 (30-60) day cycle
• Occurs to the tropics, though effects extend
  outward
• Coupled ocean-atmosphere energy pulse that moves
  eastward
• Most significantly observed in Indian Ocean and
  Western Pacific

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A (Tropical) climates

• Mean temperature of coldest month at least 18C
  (64F)
• Enough precipitation to not be B climate
• Three subdivisions
• Af (Tropical rainforest) no dry season
• Am (Tropical monsoon) short dry season
• Aw (Tropical wet-and-dry) short wet season
• Subdivisions grade into one another

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Af, Am, and Aw
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Af (Tropical rainy) Am (Monsoon)

• Temperature
• Uniformly high year-round
• Mean temperature near 80
• Temperatures outside of 60 - 95 rare
• Annual temperature range less than 5 (Af) to 10
  (Am)
• No to short dry season (1-4 months)
• Cloud Cover very high
• Winds
• Trade winds dominate (usu. Easterlies)
• Weak, except areas affected by sea breeze

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Af (Tropical rainy) Am (Monsoon)

• Precipitation
• Annual totals 60 to 150 inches
• Not always higher in Af vs. Am
• South America greater than Africa
• Oceania greatest
• Enhanced by orography Mt. Waialeale (450)
• Entirely convectional
• Many locations have 100 thunderstorms / year
• Diurnal cycle strong

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Af climographs
TRUK, Micronesia 3 sunny days a year!
BELEM, Brazil
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Am climograph
HO CHI MINH CITY, Viet Nam
Note small seasonality in temperature Mean
diurnal range 11 in August, 18 in February
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Af Am

• Tropical rain forest biome
• Very high stream drainage density
• Droughts uncommon

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Aw (Tropical wet-and-dry)

• Temperature
• Greater annual range and diurnal range
• Less precipitation overall
• Distinct, longer dry season (4-8 months)
• Seasons not defined by temperature so much as
  rainfall
• Winter Dominated by subtropical high
• Summer Dominated by the ITCZ
• Tropical rainforest grades into savanna

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Aw climographs
CALCUTTA, India Ganges climate warmest before
solstice
DAKAR, Senegal
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Tropical highland climates (H)

• Share many similarities to their lowland
  counterparts
• Small annual temperature range
• Precipitation pattern related to ITCZ influence,
  as well as orography
• Not technically A climates due to temperatures
  being too cold
• Vegetation changes as one increases elevation,
  similar to changes as one goes poleward

QUITO, Ecuador 0N, 8,600 feet elevation Smallest
annual temperature range in the world 0.8F
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B (Dry) climates

• Similarities
• Severe temperature ranges for particular latitude
• Many temperature extremes
• Large daily ranges, clear skies, low humidity
• Meager, unreliable precipitation
• Windy
• Except West coast deserts

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B (Dry) climates

• Evaporation exceeds precipitation
• Calculations are complicated because of different
  levels of evaporation
• Paris, France and the Texas Panhandle both
  receive the same amount of precipitation a year
• Four main subcategories
• BWh (Low-latitude desert)
• BWn (West Coast desert)
• BSh (Low-latitude steppe)
• BSk and BWk (Cold desert / steppe)

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Precipitation variability
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BWh
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BWh (Low latitude deserts)

• Temperature
• Annual range larger than other tropical areas
  (15-25)
• Very large diurnal ranges, upwards of 60 in
  extreme cases
• Record 100 I-n-salah, Algeria
• Very hot summer temperatures, highs in 90s and
  100s
• Solar control in summer, some synoptic control in
  winter, esp. on poleward edges (e.g. Arizona)
• Freezes occur even in some tropical locations

• Precipitation and moisture
• Under 10 inches year-round
• In some places, years pass with no precipitation
• Yuma, Arizona, 2002 0.03
• Violent convection precipitation, frequently
  evaporates before reaching ground
• Largest moisture deficits on planet
• Yuma, summer average 1 rain, 55 evaporation
• Sunniest climates 75 percent of possible
  sunshine
• Yuma, sunniest city in US 88 of possible sun,
  97 in June

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BWh climograph
LUXOR, Egypt Mean annual precipitation 0.05
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BWn (West Coast deserts)

• Baja California, Peru, Ecuador, Chile, Namibia,
  Morocco
• Cool currents run adjacent
• Stable conditions persist year-round rainfall
  inhibited
• Muted temperature ranges
• With ocean present, frequent fog
• Driest place on Earth Arica, Chile 0.60 of
  rain in 50 years
• Some places get more than 10 times their rainfall
  in collected dew

Namib Desert (NASA)
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BWn climograph
WALVIS BAY, Namibia
Mean annual precipitation 0.4 (5 rain days a
year) Diurnal temperature range averages 6 180
days of fog a year Reaches 90 only 1 time a year
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BSh
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BSh (Low-latitude steppe)

• Precipitation more than half of evaporation
  needs, but still less.
• On poleward side of BWh
• Mid-latitude precipitation (wintertime peak)
• Some synoptic control, as mid-latitude cyclones
  approach region
• On equatorward side of BWh
• ITCZ precipitation (summertime peak)
• Little synoptic control
• Precipitation still very variable, more
  dangerously so, since in wet years the climate
  may appear hospitable.

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BSh climographs
MERREDIN, Western Australia
TENNANT CREEK, Northern Territory, Australia
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BSk, BWk
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BSk,BWk (Cold desert / steppe)

• Different genesis from low-latitude desert /
  steppe
• Moisture not available, due to circulation and/or
  topography
• Driest places ringed by mountains (e.g. Nevada)
• Temperature
• Large diurnal cycle, as most low-latitude deserts
• Much greater annual range due to latitude
• Very low temperatures in wintertime
• Synoptic controls on weather conditions

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BSk, BSk climograph
ULAN BATOR, Mongolia (BSk) 48N, 4100 feet
elevation