GG 140: The Atmosphere, the Ocean, and Environmental Change

Lecture 18.?Seasons and Climate Classification

https://oyc.yale.edu/geology-and-geophysics/gg-140/lecture-18

Sunlight Seasonality in the Arctic Circle [00:00:00]

We've already talked about how the tilt of the Earth drives the seasons. But I wanted to be sure you were aware of the standard definitions regarding the tilt of the Earth. The tilt of the Earth's axis is 23 and 1/2 degrees in the current geologic era.?The Arctic Circle is 23 and 1/2 degrees off the North Pole, The tropic of Cancer—of Capricorn is--both tropical lines are 23 and 1/2 degrees off the Equator, and the Antarctic Circle is 23 and 1/2 degrees off the South Pole. So these really are all based on the tilt of the Earth.?One definition of the Arctic Circle would be that it's only points north of there that experience 24-hour sunlight in summer and 24-hour hour darkness in winter.

Seasonal Zone Shifts [00:05:03]

We're going to pick up on this seasonal theme. And how do the seasons work on our planet? There's the rotation axis, so this is the Northern Hemisphere and the Southern Hemisphere, June, July, and August compared to December, January, February.

And remember, June, July, and August is Northern Hemisphere summer. The Earth is tilted towards the--the North Pole is tilted towards the Sun, so the Northern Hemisphere receives much more heat than the Southern Hemisphere. And all of the belts and zones defining climate shift northwards, both those in the Southern Hemisphere and the Northern Hemisphere.

So the band of frontal cyclones weakens and moves northwards. The ITCZ moves north of the Equator. Frontal storms which were down here before move up there. Six months later--December, January, February--the Sun is hitting the Southern Hemisphere more, so all the belts and zones--which?are driven by the Sun, differential heating by the Sun, so it's not surprising that they can shift when the source of the heating shifts--they all shift southwards.

So let's look at these cities A, B, and C. So the thing that brings most of the rain to the world would be either the ITCZ in the tropics, which is that convergence of trade winds and lifting and deep convection, or the frontal storms, the comma clouds, the cold fronts, the warm fronts.

So in this season, city B will be getting rain from the ITCZ having shifted northward to its location, and city E in the Southern Hemisphere will be receiving rain from frontal storms that have shifted northward to its location. So rain in cities B and E in that season of the year.?Six months later, it's A and D that are receiving the rain, A because the frontal storms have shifted southward to its location and D because the ITCZ has shifted southward to its location.

It's going to be a little more complicated than this because of the continents and the mountains. So here's an actual depiction of the ITCZ shift. And it tends to shift more over the continents than it does over the oceans, at least if you look at South America and Africa.?Notice that the Equator comes right through here.?

In both the Eastern Pacific and the Atlantic Oceans, the ITCZ never actually shifts south of the Equator. There are no hurricanes in those two locations, and that's because these two continents stretch far enough into the Southern Hemisphere that they draw water, cold currents up into this region.?And by keeping that area cold, it's no longer possible for that area to be the center of convection. You have a lot of heat from the Sun,?but that's being counteracted by these cold currents coming up in the oceans in those locations.

In the Indian Ocean, however, you have an exaggerated shift in the Intertropical Convergence Zone. And this is called the monsoon effect or the monsoon region or just the monsoon. It is an exaggerated shift in the ITCZ, giving strong rainfall seasonality to that part of the world. The word "monsoon" comes from an Arabic term "mausam" or?"seasons." But it refers, in climatology, it refers to this exaggerated rainfall seasonality in southeast Asia and the Indian Ocean due to this big shift.

The ocean currents are playing some role in this. The continentality is playing some role in this. This will dominate the control of precipitation in the tropics, and I'll be going back over this.

For example, let's look at Rondonia in Brazil, which is south of the Equator. It's somewhere in here. And what's plotted here are the monthly precipitation values in millimeters. And notice that it has a January—or a November, December, January, February maximum.?This is Southern Hemisphere, so this would be a summertime rainfall maxima. And we would explain this by a shift of the Intertropical Convergence Zone coming down to that location in the Southern Hemisphere summer. And notice, it's a gigantic effect. June and July are very dry, whereas the summer months--be careful, these are the summer months--are quite wet.?

If you look in India, here's precip and temperature plotted here. Let's just look at the precip. There's a big maximum in June, July, and then it starts to decrease in August and September. We're in the Northern Hemisphere now, and this would be Northern Hemisphere summer. So this location also has a summertime maxima in precipitation due to the northward shift of the Intertropical Convergence Zone.?So on a typical day, you'd have deep convective clouds, heavy precipitation, sometimes flooding.?

Let's look at Yaounde, Cameroon. If you are close to the Equator, you are actually going to get two rainy seasons a year, one when the ITCZ is moving northward past you and the other six months later when the ITCZ is moving back southward past you. So these mark the extremes of the movement, but in between, you're going to get, in some cases, a double passage of the ITCZ. And that's what you see here, generally kind of a double rainfall peak due to the double passage of the ITCZ.

Now, the polar front also moves. Everything moves together. And so in the Northern Hemisphere summertime, the polar front is weak, and it moves northward. Weak, of course, because the pole and the Equator are getting about the same amount of sunlight. Therefore, you don't have very much of a north-south temperature gradient, and that's what defines the word "front."

However, in the winter season, polar latitudes are getting no radiation from the Sun, the Equator is still getting a lot, and therefore you build up a strong polar front, and it moves southward to roughly 30 degrees north. But you see it meanders a little bit around there.

So if you looked?at Los Angeles and plotted up its precipitation, it'll get precipitation starting in November, then December, January, February, March, decreasing into April. This is Northern Hemisphere, so we would call this a wintertime precipitation. Here's the temperature plotted up here, so that rain is coming in the cool season, when the temperature is down a little bit. So that is a mark of this kind of a phenomenon, where it's getting its rain from frontal cyclones that have moved equatorward in the cool season.

Here is Rome in Italy at 42 degrees north, and you have a similar story. There's the temperature in red, and it's in the cool season of the year. We're still in the Northern Hemisphere, so the cool season gives the precipitation due to frontal cyclones.

And here is Perth, Australia, 32 south. We’ve got to flip everything around here. So the rainy season is in June, July, and August. That is wintertime, as you can tell, because the temperatures are minimum at that time of the year. And so that is a, in this case, a northward shift or an equatorward shift in the polar front, bringing frontal cyclones into Perth in the wintertime.

Precipitation Seasonality [00:16:57]

Some of this is nicely summarized in a diagram that appeared in a book by Wallace and Hobbs. They took rainfall data from the Northern Hemisphere and plotted it with little clock arrows, little clock hands. The length of each arrow is the degree of seasonal change in precipitation. So the longer the arrow, the bigger the difference there is between the dry season and the wet season. And the orientation of the arrow tells you in what season of the year do you get the rainfall maximum.

So a poleward-oriented arrow, such as these here, would have a July 1st maximum. That is a summertime maximum. This is Northern Hemisphere, so I can use the word "summer."?That would be the shift of the ITCZ northward to bring you rainfall in the summertime. And if we look at India, Mumbai here or many other places in southeast Asia, we find those northward-pointing arrows. So every time you see the northward-pointing arrows, including some over here, that is the northward shift of the ITCZ reaching that location in the summertime.

But let's look at some other locations. For example, in the Mediterranean region, all the arrows are towards the south, which means a wintertime maxima. And that would be, of course,?the frontal cyclones. The polar front strengthens and moves southwards, and then throughout the winter, you have storm after storm after storm, frontal activity that'll bring rain to this area.

And if we go around to California, the arrows are a little bit tilted. They're generally tilted aiming towards the Equator, with some lag.?California is often described as having a Mediterranean climate, because it has the same kind of wintertime precipitation that does the Mediterranean.

Why does the Nile behave the opposite to other rivers??In the summer months it’s getting rain from the shift of the ITCZ, whereas in the north,?the frontal cyclones were contributing more.?Other rivers?are?generally the rivers coming off from the Mediterranean region. And that's going to be mostly wintertime due to the southward shift of the polar front. The Nile, on the other hand, actually, the water doesn't originate from here. The water originates further south.?Herodotus spending his life looking at rivers that have a wintertime or a springtime maxima, and now he's down in Egypt looking at water that actually fell down in this region. The source of the Nile?is up in the Horn of Africa?and that has a summertime precipitation. It's the fact that the Nile is controlled by the water that falls here, which is controlled by the northward shift of the Intertropical Convergence Zone.?

Being raised around here or in North America, these latitudes, probably think that seasonality has a lot to do with temperature. And you'd be right, but that's a narrow, parochial attitude from where you were raised. If you were raised in the tropics, you would think that seasonality has everything to do with rainfall.?If?you take the 12 monthly precip and 12 monthly temperature points and plot them and connect them with lines that link them up in chronological order, they'll form some kind of a figure on this temperature, precipitation map.

And in the tropics, for sites in the tropics, that figure looks like this. You get a big precipitation seasonality, but not very much change in temperature. On the other hand, in mid-latitudes, you get a big temperature change but not much change in precipitation.?

This wouldn't apply for every mid-latitude site and every tropical site, but generally this is the rule, that seasonality in the tropics is referring to a precipitation seasonality, a wet season and a dry season, whereas seasonality in mid and high latitudes is referring primarily to a temperature seasonality. So seasons mean different things depending where you were brought up.?

Climate Classification [00:24:49]

Now we turn to this question of climate classification. We'd like to codify all of this. And the way forward on this was over 100 years ago by Koppen, and then later others have modified this slightly. So there are a few different versions of this scheme around, but generally it's called the Koppen climate classification scheme. It uses the 12 monthly temperature and precipitation values with thresholds to assign each point on the Earth to a small set of climate zones.

And the way those thresholds are designed is that it tries to capture those aspects of climatology that would control the vegetation that grows. So ideally, if the Koppen scheme works, in a particular climate zone defined by temperature and precipitation, you would find a particular kind of natural vegetation, because it's adapted to that particular climate zone.

There are six broad categories--tropical, dry, temperate, continental, polar, and highland climates--and there are subcategories under those. And well they’re a little bit--let's face it, they're a little bit messy to work with.

This is a table I pulled off of a recent published paper on the Koppen classes. The point?is, you define the broad category, like the A climates, in terms of whether the minimum temperature is higher than a certain value, and then you break it down into subcategories based, in this case, on precipitation.

The arid climates' precipitation annual total, less than 10 PTH. I don't know what that is. But then it's subdivided into two categories. Desert would be the very dry areas, and the steppe would get a little more precipitation, and so on. So using the temperature and precipitation values, you go through and you classify each area by its seasonal cycle and the mean values.

And you end up with?a diagram like this in your textbook, where the color schemes represent the different climate categories. For example, the moist tropical climates are the A climates, Aw, Af, Am. The dry climates, you find they're the B climates. You find some of them in the desert areas here and here. And in the desert Southwest, you find some B climates.

This is the Western Hemisphere, North and South America, and then we do something similar to that for Asia, Africa, and Australia as well using the same scheme. So it's not zonal, but you can see zonal aspects to it. But all of those factors that I mentioned the other day are working, not only the zonal aspects of the general circulation, but also the mountains, the continentality, and the cool and warm ocean currents that are bringing up water to the coastlines of these continents.?

Examples of Seasonality [00:33:44]

Let's look at a couple little additional examples of seasonality.?

Here's a couple of satellite images taken from North America—from South America, where the Maranon and the Ucayali join to form the Amazon.?And at the end of the rainy season, those rivers are pretty swollen. Every place you see blue, that's water. But at the end of the dry season, the flow in those rivers is very much reduced. And what the purple color here is are those dry sand banks on the side of the river. Because the river is now lower and moving much more slowly, you have barren, unvegetated areas.?That's been meandering back and forth over geologic time, leaving all these little remnants of meanders in the river. And most of that erosion takes place in the wet season, when the flow of the rivers is high.

Here in New England, the Dfa climate,?it's a temperature seasonality, rather than a precipitation one. And so the summertimes look like this, with the deciduous trees being out. Then they drop their leaves, and it gets very cold and the snow falls.?Looking down from a satellite, you can map out the distribution of vegetation using something called NDVI. It's the Normalized Difference Vegetation Index.?It's the ratio of the reflectivity in the near-infrared to the red. It turns out that vegetation has a very strong signature if you look at those two bands.

What I'm mostly going to be seeing is the distribution of deciduous trees--oaks, maples, and so on--and that's what we see. So the seasonality in New England, while it is--at its root it's a temperature seasonality, it also has a big impact on the vegetation. Leaf-on, leaf-off controlled by that temperature, and you can see it from space.

Now, in areas where there hasn't been much change, those might be conifers. They look about the same in the winter and summer. Or they might be barren areas, where there's no vegetation in either season.?I've subtracted the NDVI of April--June minus April to get the difference in vegetation. And in this case, it's going to show me where the deciduous trees have leafed out because of the seasonality.?I'm going to do the same trick now for August minus June to see how vegetation has changed over the summer. And you see these little pockets of red are where there's a big positive change, where there's much more vegetation in August than in June.

And I'm calling that the growing season, because what that's going to do is pick out the agriculture. Farming in this part of the world is usually a summer activity. You plow and plant in the spring, and then your crops grow--your corn or your wheat or whatever grow over the summer months, and you harvest in August, September, October. So this difference between those two months is largely going to be a measure of where agriculture remains.

If I look at the daily discharge data from the Quinnipiac or most other rivers,?there is a seasonal cycle.?More discharge in the winter, less in the summer, June, July, and August. Winter, less in the summer, June, July, August.?This is an order of magnitude difference. Look, this is 200 to 300 to 400 cubic feet per second down to, well, 60 or 70 cubic feet per second down there. So a gigantic seasonality in the river flow and yet not the rain.?

Evaporation is--(a), it's very strongly controlled by temperature, and since New England has a temperature seasonality, you're going to get a big seasonality in evaporation. Also, from the trees. The deciduous trees evaporate water from their leaves. As soon as the leaves fall off, you don't get that so-called evapotranspiration. So that's a big seasonal cycle.?So during the wintertime, most of the rain that falls ends up in the rivers. During the summertime, most of the rain that falls evaporates, and much less flows down the river to the sea.

Does New England have a wet-dry season? That depends on how you define it, doesn't it? Because it doesn't have much of a seasonal variation in precipitation, and yet it's got a strong seasonality in the amount of water in the rivers because of the evaporation effect.

Seasonally Controlled Events [00:44:06]

And then just to remind you that it's not just this smooth cycle of the seasons that matters, but it's the events that can also matter and would be normally considered to be part of the climatology. For example, hurricanes, which are discrete events, and you may not even get one in any particular year, but still, they occur in the late summer or fall in either hemisphere.

Severe Oklahoma thunderstorms are a springtime phenomenon. Nor'easters in New England are wintertime. California fires are primarily in the fall. Antarctic ozone hole?occurs in October. And El Ni?o when it starts up?usually starts up in December. So it's not just a smooth cycle of the seasons, but it's the way the seasons control events that can be quite important as well.