Early last year I wrote a series of posts on “long gradients,” inspired by the dry-wet transition on a drive from California to Maine and back; this next series will pick back up on that theme, zooming out to the entire globe.
First, a quick recap of the “long gradient” concept. Having grown up in the relatively climatically uniform Midwest I think of these large-scale transitions, extending over many hundreds of miles, as more “typical” than the more rapid and more easily experienced kind generated by topography. My worldviews focus on the latter, “compressed” type, because by definition they’re better at eliciting that “world-at-my-fingertips” feeling that I go on and on about. They also lend themselves better to being captured and accentuated on paper. But the large-scale kind can have a similar impact if you travel them quickly enough. (A car works if you don’t stop very frequently, but for me this would also be a side benefit of having more widespread high-speed rail.) And if you are maximizing speed, in a sense that impact can be even greater than for the small-scale kind because you’re covering a distance that’s still continental or global in scale. Time is being compressed without compressing space.
Global Climates: The Typical Pattern
Needless to say plenty of these long gradients exist across the globe, and they can be grouped into some broad categories based on whether they’re created by variations in temperature, precipitation, or usually some combination of both. But they can also be broken down further, and classified, into particular patterns that repeat across the different continents. It’s probably somewhat common knowledge that the same types of climates can be found in multiple places around the world—e.g. “Mediterranean” climates aren’t just found around the Mediterranean, and tropical rainforest climates are found in multiple places along the equator. But that also holds true for groupings or progressions of climate types, beyond the obvious transition from polar to tropical. You can start to see these patterns in the climatic map below, of a generic “continent” spanning both hemispheres. (I’ve chosen the colors to reflect degree of similarity.)
(This post will stay on this generic global level. It does get into the weeds a bit, but as a framework for looking at the specific places I’ll get into later on, the main points should be enough.)
There are different climate classification systems out there; this is based on the Trewartha system, though I’ve changed some of the names because I find the original terminology to be a little confusing/inconsistent. I’ve probably sacrificed some accuracy as a result—I think clarity’s more important here, but those of you who are into this stuff, don’t hesitate to call me out on it! (Diagram based on https://www.geocurrents.info/place/australia-and-pacific/australias-climatic-anomalies/attachment/hypothetical-continent-climate-map. )
Next, below is the typical ecosystem/biome corresponding to each climate type.
Driving Factors
Besides the expected north-south gradient, in the map you’re probably picking up on a differentiation between east and west that wouldn’t be explained by latitude—again likely familiar, but the more detailed distinctions and the reasons might not be. In a nutshell that differentiation is the result of variation in prevailing wind direction combined with ocean currents, related to the continental/oceanic distinction that I’ll explain below. Here is a quick (and simplified) overview of those and other major factors generating the pattern:
Solar angle (varies directly with latitude). This is the most straightforward one—highest angle at the equator (hence generally warmer air, which also holds more moisture), lowest angle at the poles (lower temperatures and less moisture).
Wind direction relative to ocean position. Having an ocean nearby always has some moderating effect, but more important is whether the prevailing winds blow off the ocean toward land (magnifying the moderating effect) or vice-versa. Wind direction (and atmospheric pressure, below) are largely determined by something called “atmospheric circulation cells”—themselves a function of solar angle, i.e. uneven heating of the atmosphere—combined with the rotation of the earth. Those two phenomena explain why prevailing wind direction varies by latitude.
Ocean currents. Just as uneven heating of the atmosphere determines air flow patterns, uneven heating of the oceans generates analogous flows underwater. They’re warm or cold based on the latitude where the water originates. The strength of their effect on air temperature (and also precipitation, since warm air holds less moisture than cold air) over adjacent land is, again, largely dependent on wind direction.
Atmospheric pressure. The winds in those atmospheric circulation cells create alternating zones of high and low pressure roughly around the equator and 30° and 60° N/S. Low pressure (rising and condensing air) tends to increase precipitation, while high pressure decreases it.
Journey Types
Of course the generic map ignores the wildly varied shapes of continents and oceans, adding complexity to all of the driving factors above. And it ignores topography and elevation, which impact air circulation and temperature. So there are plenty of exceptions to the generic climate/biogeographic pattern, which themselves are interesting and I’ll get to some of them later on. For now though, the above factors generate what I classify into five typical “sub-patterns” or gradients that can theoretically be journeyed across, as indicated by the arrows below. (Again I’ve simplified the driving factors for the sake of clarity, making some assumptions in the process—if anyone wants to correct me, that only adds to the “intrigue”!)
“East Coast” (along the east coasts of continents, all the way from the poles to the equator). As is clear from the map, the overall east coast progression of climates/biomes is more straightforward than along the west coast—temperature and precipitation both generally increase all the way from the poles to the equator in a relatively linear and consistent way. Basically this is because, due to prevailing wind direction, the ocean has relatively little influence at the latitudes where it matters.
The prevailing winds blow in the landward direction at high and low latitudes where ocean currents are cold and warm respectively, doing little to moderate the cooling/drying and warming/moistening effects of latitude. At middle latitudes where the oceanic effect would be more “disruptive,” the prevailing winds blow off the land instead, and so the climate of that temperate zone is considered continental rather than oceanic. (There are complicated reasons why the mid-latitude high- and low-pressure zones seem to have much less influence on precipitation here than on the west side, generally specific to the shapes of the different continents.)
“Temperate West Coast” (along the west coasts of continents, toward the equator from subpolar rainforest to desert). This progression of generally oceanic (also called maritime) climates contrasts directly with the continental progression on the east side. The prevailing winds, blowing off the ocean, strengthen the impacts of the warm current toward the poles and the cold current toward the equator, setting up a wet-to-dry precipitation gradient and moderating what would otherwise be a (latitude-generated) cold-to-warm gradient like along the east coast. So while average temperatures do increase moving toward the equator, the rainfall gradient is much more significant. The precipitation extremes are also magnified by the low-pressure zone at the pole-ward end and the high-pressure zone at the equator-ward end.
“Tropical West Coast” (along west coasts, toward the equator from desert to tropical rainforest). This transition, almost entirely within the tropics, is defined by increasing precipitation from the high-pressure zone at the pole-ward end to the low-pressure zone at the equator. Since the prevailing winds blow off the land here, the cold ocean current doesn’t have the effect of counteracting the high rainfall generated by that low-pressure zone.
“Polar West Coast” (along west coasts, from subpolar rainforest to the poles). At I’ve tried to indicate with the color choices, this transition is, theoretically, very abrupt in terms of both temperature and precipitation. Moving toward the poles, the wind direction shifts seaward, magnifying the effects of increasing latitude by mostly eliminating the temperature-moderating and moisture-providing influence of the ocean. I say “theoretically” because I don’t think there’s actually anywhere in the world where this pattern exists without the complicating influence of mountains (more on this later).
“Cross-Continental” (from eastern forests to western deserts). As I said above the reasons the western arid zone doesn’t extend all the way to the east coast are partly continent-specific, having to do with the weakening of that high-pressure zone. Another explanation, though, is the drying influence of the cold current along the west coast, especially where the prevailing winds blow from ocean to land.
Next time I’ll bring these journeys more “down-to-earth,” tying them to some specific places.
Darren