In my last post I went into the idea of generic global climate patterns and the ecosystems/biomes they generate, and resulting “journeys” that might be taken across three particular types of patterns. Here I’m going to translate that approach to the actual globe.
First, to summarize those ideas from last time, below is a composite diagram of a generic Northern Hemisphere continent (picture a mirror image in the Southern Hemisphere). This time I’ve modified the zone labels a bit to merge their climatic (in bold) and ecological components. Also, to simplify things from here on I’ve merged the original three “West Coast” journey types into one.
So the three types are now:
East coast—a generally linear north-south gradient defined primarily by temperature.
West Coast—more complex and less linear than along East Coasts and defined more by precipitation than temperature, partly because temperatures in the three mid-latitude zones are moderated by oceanic influences. But there is a sharp contrast in both precipitation and (winter) temperatures, conveyed by the color contrast, between the Subpolar Rainforest and Polar Tundra zones. (I mentioned last time that this contrast hardly ever occurs in the real world without a mountain barrier, which would make it less relevant in this “long gradients” context, but it occurred to me that you can always find that contrast at sea level if you just stick to the coastline.)
Cross-Continental—a precipitation gradient from wet to dry, connecting the mid-latitude portions of the East and West Coasts. (The diagram shows the arrow beginning in the Subtropics, but it could begin in the Temperate zone too with a similar effect.)
Below is the generic pattern applied to the real continents, with the same biome types and color scheme. (Highland/mountain areas, which again I’m generally leaving out of the discussion, are shown in light grey. In this particular classification system they’re considered their own climate type even though they contain compressed versions—sound familiar?—of the large-scale patterns.) Overall, the generic pattern is still legible even though its distorted and sometimes disrupted by more localized conditions.
Global climate zones/biomes. (Adapted from https://kkh.ltrr.arizona.edu/kkh/climate/images/Trewartha.map.lrge.fix.jpg, with my own colors and some creative license with the eastern Black Sea coast—see below.)
Now I’ll focus on each of the three journey types, where they do occur and where they’d be expected to but don’t. You’ll see that many of my arrows join disconnected pieces of continents like Iceland and the UK, stretching the concept of “journey” a little bit, but that’s partly to emphasize that these patterns tend to be intact even across intervening water. Plus the fact is that, if we’re talking about native ecosystems, these patterns can only be experienced today as archipelagos anyways since those ecosystems have been broken up and mostly replaced by altered landscapes.
My main source for trying to understand the exceptions to these overall patterns is The Earth’s Problem Climates (by Glenn Trewartha, who also developed this particular climate classification system)—some of you won’t be surprised to hear me say that it’s become one of my favorite books ever. That’s despite that fact that, without a very deep knowledge of climatology or meteorology, it’s hard for me to get more than the overall gist of the explanations. They have a lot to do with seasonal variations in the strength, location, and orientation of winds and currents, adding much more complexity to the overall conditions I wrote about last time, interacting with topography and the shape of coastlines. In fact this complexity means that the primary causes of the anomalies rarely seem to be well understood. So I won’t try to give more than a basic idea of why the three typical patterns don’t always apply. (I still do get into the weeds a bit though, so just stick to the graphics if you’d like to skip the explanations.)
East Coast
Here’s the world map again showing only the zones making up the “East Coast” pattern/journey type. (From here on I’m going to ignore the Polar Ice Caps, since realistically if you were going to try and travel these routes you’d probably leave those out.)
“East Coast” climate zones/journeys. As with the generic continent a few of these zones do extend coast-to-coast.
The pattern exists in its complete form in Northern Hemisphere where significant lengths of east coast exist (so that leaves out Europe and northern Africa). But that isn’t the case in the Southern Hemisphere, as I’m suggesting with the thinner arrows. All of this is shown more clearly and diagrammatically below. I’ve lined up all the “East Coast” zones and, corresponding with the map above, colored them in (where that zone is present) or left them blank (where it’s absent) for each of the earth’s three main north-south “axes”:
The main reason for all the white space in the Southern Hemisphere, where the pattern doesn’t extend further south than the subtropics, is of course that there’s less land toward the south pole. But that doesn’t explain everything:
South America reaches nearly to Antarctica, but you can see in the map of global climate types that, south of the Subtropical Forest zone, instead of the continued forest-to-tundra progression is the Patagonian Desert. That’s partly due to the rain shadow effect of the Andes, stripping moisture from winds blowing off the Pacific, but it’s also because of the narrowing of the continent at that latitude. The temperate zones of eastern North America and Eastern Asia are wet much less because of the oceans to the east (there the winds generally blow toward those oceans, not off of them) than because of rainfall from other sources (in North America that’s mostly the Gulf of Mexico). But in South America there isn’t enough land to the east for those sources to take effect. The Patagonian desert is somewhat analogous to the semi-arid Great Plains in North America, though the Andes make it even drier.
(There’s another anomaly that’s less relevant because it sits near the equator between the two arrows but you may have noticed it—a piece of semi-desert on the northwest coast of Brazil where there would otherwise be rainforest. It isn’t because of topography; apparently the Southern Hemisphere high pressure zone (normally around Patagonia) shifts northward during the southern winter, and then for complicated reasons a piece of it gets “stuck” there for the remainder of the year.)
In Africa the anomaly is toward the equator, where the Somali Desert and the East African savannas replace the tropical rainforest zone (except at high altitudes). Here the winds blow mostly from the east, which ought to bring moisture, but the winds and ocean interact in complicated ways that bring less rain than you’d expect.
Australia extends a bit further south than Africa and seems like it would have a Temperate Forest zone. But since the ocean wraps around to the east, the winds from the west bring an oceanic influence (part of the “West Coast” pattern) to the southeast coast—hence the white space. (This happens to a much more limited extent at the southern tips of South America and Africa, probably because of subtle differences in wind direction combined with the shape of the land.)
West Coast
With the ”West Coast” pattern, the three north-south axes incorporate all six non-polar continents and the pattern overall is more filled-out than the “East Coast” type. It extends from end-to-end along the west coast of the Americas; in Africa again it’s cut off on the south end but Europe fills it out completely on the north end. (In case you’re wondering, Iceland does have some tiny pockets of Subpolar Rainforest, even if the trees are only a few meters high, and once had a lot more. The “Celtic Rainforests” of Ireland and the UK aren’t much more widespread, but many so-called “sacred groves” fit into that category.)
The third axis is shorter and also a bit more convoluted. Asia doesn’t have much of an obvious west coast—it’s stretched over a wide longitudinal distance, so the pattern runs more diagonally. The Southern Hemisphere half is again cut off at the bottom in Australia, and (as I mentioned above) the southernmost part of it wraps over to the east coast.
“West Coast” climate zones/journeys. (Note the Eastern Black Sea coast is classified as Subtropical Forest—part of the “East Coast” type—in the Trewartha system but as Temperate Rainforest on other maps. Since it is in fact a rainforest and is on the west coast of a landmass, I’ve gone with the latter designation.)
Cross-Continental
This third type connects the wet temperate or subtropical zones of the “East Coast” pattern to the desert zones of the “West Coast” pattern, and it’s mostly predictable where those other two patterns occur predictably. It’s absent in northern Africa because there’s no “east coast” at that latitude; but, since the Arabian/Iranian/Central Asian deserts represent an eastward extension of the Sahara, the transition to wet is essentially pushed eastward to China (and also northward by the Tibetan Plateau). You might notice there’s a reversal of the pattern (becoming dry-to-wet) between Central Asia and Europe; more on that in a later post.
I’ve shown Australia with an empty rather than solid arrow because though the right progression is there, technically the Great Dividing Range (the grey dashed line) disqualifies it from my list. I’m focusing on journeys where elevation change is imperceptible or has an insignificant effect on the pattern, and the rain shadow effect of The Great Dividing Range is at least part of the reason the forest-steppe boundary occurs where it does. But I’m making an exception since those mountains are relatively low, and Australia does round out the map nicely….
“Cross-Continental” climate zones/journeys.
I was originally going to end this post by zooming in even further on a few of these regions, bringing in some more evocative imagery than just maps and diagrams, but I think this is enough for now. More next time!
Darren