Lecture 9 (13 February) – Tropical Climates and Life Zones

· Within the lowland tropics, annual variation in temperature is no greater than diurnal variation, therefore seasonality is reflected in differences in rainfall. Monthly temperatures tend to peak during dry seasons because of greater insolation when days are not cloudy. Dry season days have greater diurnal temperature variation than do wet season days (because there is high heat loss on clear nights).

· Warm air comprises atmospheric regions of low pressure with high moisture-holding ability. Warm air tends to rise and to be associated with convective storms. Cool air forms high pressure regions, tends to descend, and to have a drying effect. The "thermal equator" occurs where the sun is directly overhead. At the thermal equator, warm moist air rises, expands and cools as it rises (i.e. “adiabatic cooling” = decreased temperature without heat loss to the surrounding environment), and rain falls. In the upper atmosphere the now dry air further cools and eventually descends to the surface at about 30 degrees North and South latitude producing deserts at these latitudes. Air rushing back to the thermal equator along the surface produces the intertropical convergence zone (ITCZ) approximately at the thermal equator.

· One of the subtle concerns associated with global warming is that because of increased temperature high in the Earth’s atmosphere, rising air masses may not cool as quickly as at present and so instead of cool dry air descending at about 30 degrees N & S latitude, it may not descend until ca. 40 degrees latitude. That could bring desert-like conditions to what are now the “breadbasket of the world” – the major grain growing areas of the northern hemisphere.

· Tropical seasons are "wet" and "dry" (instead of a warm summer and cold winter), and are controlled by the latitudinal seasonal movement of the thermal equator and the ITCZ. A second short dry season can result (the "veranillo") near the Equator when the thermal equator has moved to its most northward or southward position. (Note: consider the "A" diagram.)

· Two main questions concerning tropical climates are: 1) what controls the occurrence of wet and dry seasons, and 2) what influences regional differences in the severity of dry seasons which impose moisture stress on vegetation. The answer to the first question is the movement of the thermal equator and intertropical convergence zone. The answer to the second to a great extent involves ocean currents and relative temperature differences between air masses above water and adjacent land.

· At the intertropical convergence zone (i.e., where the winds from the north and the south converge) winds deflect to the west because of the coriolis effect. Hence, they come from the east and are called "Easterlies" or "Trade winds". These reliable winds drag ocean surface water to produce major westward moving ocean currents along the geographic equator. Major ocean currents have a clockwise rotation in Northern Hemisphere ocean basins, and a counter-clockwise rotation in the Southern Hemisphere. Ocean currents moving towards the Equator from high latitudes along the western sides of continents tend to be colder than adjacent lands within the tropics. Therefore air above them, which they cool, holds little moisture, and when it moves ashore it tends to be drying. This is why the Atacama Desert in northern Chile is the driest place on Earth. East sides of continents within the tropics experience warm, moist air above warm equatorial currents, and when the land is relatively cooler than the ocean, they receive rain, hence they tend to be wet.

· Two major exceptions to the seasonal and geographic patterns described above are the El Niño - Southern Oscillation and the Southeast Asian “monsoon”.

· The "El Niño - Southern Oscillation” (ENSO) occurs when a warm countercurrent to the cold, upwelling Humboldt current moves down the west coast of South America from the Equator. This can make winds from the ocean warmer and wetter than the land and bring unusual rainfall to normally xeric lands (e.g., rain in Chile’s Atacama desert).

· In the Asian tropics, a huge low pressure zone that develops over the Mongolian plateau in the summer tends to suppress the drying northeast Trade Winds, hence causing onshore air flow from over the ocean, and consequent very heavy rains called the "monsoon".

· Natural tropical vegetation may be classified based upon climate, physiognomy, or species composition. Classification based upon species composition (one approach popular in Europe is called “ordination” of plant communities) may not group vegetations that are similar in physiognomy and function. For example, in Kalimantan where different species of Dipterocarpaceae may dominate adjacent hilltops, a species-based classification would distinguish vegetation that all could be considered “Mixed Dipterocarp Forest”. Physiognomic classifications based on “how the vegetation looks”, e.g., how many canopy strata are there, are trees buttressed, are lianas present, tend to be strongly correlated with climatic classifications, but are laborious to compile because they require physically surveying the vegetation to be classified. Climatic classifications can be based upon readily available temperature and rainfall data. One of the latter is the “Tropical Life Zone” classification of Holdridge.

· Precipitation falling on forests (in the lowland tropics, “precipitation” = rain, because snow, sleet, and hail do not fall) can return to the atmosphere as water vapor through the processes of evaporation or plant transpiration, or it can reach the water table directly as surface run-off into streams or by percolation downwards through the soil. The latter process causes "leaching", the removal of mineral nutrients in solution from the upper levels of soil. How much water could leave the forest by "evapotranspiration" depends principally upon how much solar energy is input to the forest, and this relationship is the basis for Holdridge’s classification.

· The Holdridge Life Zone classification system has independent axes of total annual precipitation and biotemperature, and a dependent axis of potential evapotranspiration ratio (PET ratio). Areas with a PET ratio greater than 1.00 tend to have insufficient precipitation to meet plant needs; areas with a PET ratio less than 1.00 have a surfeit of water, and consequently have soils subject to leaching of mineral nutrients. The Holdridge Life Zone system relates water availability to vegetation to the amount of water that can be lost from ecosystems because of the thermal energy in the environment. Because potential evapotranspiration decreases as biotemperature decreases, the same amount of rainfall at lower and lower biotemperatures (for example, as when going up a mountain) is effectively “wetter” as far as plants’ water needs are concerned (i.e., PET ratio decreases). The Holdridge system does not explicitly consider thermoperiodicity or seasonality of precipitation, nor does it consider soil differences.