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

Lecture 17.?Seasons and Climate

https://oyc.yale.edu/geology-and-geophysics/gg-140/lecture-17

Climate Definition [00:00:00]

We spent quite a few weeks looking at physical processes in the atmosphere, like why do winds blow, the geostrophic law, hydrostatic law, thermodynamics of lifting air, how clouds form, how rain forms, how radiation causes the layering in the atmosphere.?Now we're moving into some other sections that are a little more descriptive?to see the big picture.

We looked at storms, various storm types. Now we're going to put those two things together and try to look at the distribution of climate around the globe.?We can describe climate pretty well if we know the monthly values of temperature and precipitation.?The way you would normally do that is to collect data for 10 or 20 or 30 years, and then you'd average together all the Januarys to get the average January, all the Februarys to get the average February. And you'd plot that up as the average seasonal cycle. And that's typically the definition of climate.

Latitudinal Climate Variations [00:02:55]

The factors that control climate are primarily these. Latitude, because of the way that the general circulation tends to give a latitudinal structure to climate--the Hadley cells, the belts of storms, the belts of deserts. So as you go from latitude to latitude, you're kind of moving through these different zones. But it's not everything, since mountains play a very big role, something called continentality?and ocean currents also play a role in determining what climate it is.?

So just to remind you about the general circulation, you've got the symmetry about the equator, with a Hadley cell rising near the equator sending air poleward in both hemispheres, descending near the tropics of Cancer and Capricorn, and then the trade winds under the influence of the Coriolis force--these winds don't move directly back towards the equator. Instead they get deflected to the right and to the left and end up being the northeast and the southeast trade winds. And then you've got the belts of storms, the belts of deserts.

If you took away the continents, you either had a uniform ocean-covered planet or a uniform land-covered planet, then you could probably pretty much end with the latitudinal control. That'd be really the dominant factor. But in fact, you've got mountain belts,?continents, oceans and?ocean currents, all of which give some east-west variations in climate. So it's not just latitude control, as this diagram would have you think, but it also depends where you are because of the mountains, the continents, and the ocean currents.?

This is annual precipitation for land only. You do see the latitudinal control from the general circulation. You see a belt of rainforest near the equator, expressed in South America, in Africa, and in the continent—in the so-called maritime continent off of southeast Asia.

You see the belt of deserts in both hemispheres about here, but it's best expressed in the Sahara desert and the Arabian peninsula. Then it tends to slide northward a little bit because of the mountains here. And we see it expressed a little bit in the desert southwest, but not so much in Florida. And then you see the belt of deserts especially on the west coast of South America, the west coast of Africa, and most of the interior of Australia.

The high latitudes appear as areas with very small precipitation. I hesitate to call them deserts,?because they're cold, the evaporation rate is very slow. And so what little precipitation you get could cause a very moist climate there. So precipitation is not the only factor. Evaporation plays an equally important role in determining the climate of these various regions. So you can see this latitudinal control, but it's also quite broken up by these other factors--the mountains, the continents, and the ocean currents.

Probably one of the best examples of that is in South America, where you've got a wet west coast and a dry east coast in these mid-latitudes. Well, that's because this is the belt of westerlies.?They have to climb over the mountains there. As the air rises, it cools adiabatically, and you get rain on the west coast. But then the air descends and you get dry conditions on the east coast.

This comes not from traditional land-based weather stations.?And rather dramatically, you find these east-west elongated structures, especially in the North Pacific. And that is an expression of the inter-tropical convergence zone, where the two trade winds are converging. Notice it's not at the equator. It's north of the equator. In the Western Pacific, you have a double structure, indicating that the inter-tropical convergence zone spends a few months a year here and a few months a year there. So it tends to move back and forth a little bit in the in the western tropical Pacific. Generally very dry conditions where you see the purples, and wetter conditions where you see the warmer colors there.

When you take a diagram like this and average it east to west, it's called a zonal average. A zonally averaged precipitation would be, at each latitude, just average around the globe to get the total precipitation. And that gives you a diagram like this. Now, this is broken up by season, or by month of the year rather, but they're not so terribly different. Basically, it's a three-humped profile. Three peaks. The biggest one is somewhere around the equator, although you see it moves around a little bit with month.

And that's the inter-tropical convergence zone. Winds converging at low altitudes. The air goes up, deep clouds, heavy precipitation. Then you get the belt of deserts. So even though you have all these interferences due to mountains and ocean currents and continents and so on, still you see that three-banded structure come through that's based on the general circulation. The ITCZ, the two belts of deserts where air is descending in the Hadley cells. And then these are the belts of mid-latitude storms. This is up in the westerlies, where you're getting all these comma clouds and cold fronts and warm fronts moving west to east, bringing you precipitation.?

We can zoom in to a particular continent, like North America or our own country. Here is the annual average precipitation. The values are given here in inches. The dry values are the deep reds and the highest precipitations are the blues and the grays. And the highest precipitation in our country generally occurs in the west coast, especially the Pacific Northwest, where you get a lot of moist air masses and frontal storms moving in off the Pacific Ocean, hitting these mountains.

And so you get the combination of cyclonic frontal storms which are already precipitating, then the air has to run up over the mountains, and you get the extra precip there. Most of that water is removed that moves on from the Pacific Ocean by the mountain ranges of the west coast, leaving the interior rather dry. That's kind of a rain shadow effect.

And a fundamental question would be, if I've rained all the water out, or for whatever reason I'm not getting the precipitation downstream of the coastal mountains, how far does that effect extend eastward? And you see it here. By the time you get west of--into Colorado--and then eastward of that you're gradually starting to pick up your precipitation again. But that's because you now have got some new water sources. You've got moist air moving up in this region off the Gulf of Mexico, and along the east coast, you’ve got some, even though you're in the belt of westerlies, you occasionally have easterly flow that can bring moist air in off the Atlantic Ocean. So eventually this rain shadow reduces in its effect when you pick up some other water vapor sources.

Orographic Precipitation [00:14:10]

So just to zoom in on California, then?the blue colors are high precipitation and the reds are low. So there's a coastal mountain range that has precip. Then the central valley, which is quite dry, and then the Sierra Nevada range which lifts the air again. So this is a nice example of orographic precipitation. Orographic precipitation is defined as precipitation caused by the lifting of air by a mountain.

And the thermodynamics of it you are already quite familiar with. You lift the air. It's moving to lower pressure because pressure decreases as you go up in the atmosphere. The air expands, cools adiabatically. It drops the saturation vapor pressure. The relative humidity finally exceeds 100%. The excess water vapor condenses and, under the right conditions, you can get precipitation coming out of those clouds.

So orographic precipitation is really a big control. The water that falls in the mountains by this process pretty much gives California the water it needs for agriculture and for human consumption. It's a pretty dry state except for the influence of the mountains.

Oregon is a similar thing. Look at the contrast between the eastern and western part of the state. Precipitation in the west can be 140 to 160 inches. In the eastern part of the state where that air has climbed over the mountain range and now it descends again, having lost most of its water, the precipitation is less than this amount. So that's a big contrast. And if you drive across there, boy, you see it immediately in the vegetation. A kind of rainforest here on the west coast of Oregon. You begin to see ponderosa pine, then they get sparser and sparser. And by the time you get to eastern Oregon, there's mostly grasses and just a scattered pine tree or so.

Alaska is another good example of this. There's a coastal mountain range. Here's precipitation. The purples are the high precip. And you get that moist air coming off the Pacific Ocean, giving you heavy rainfalls in the mountains. And then north of that and east of that, the precipitation dies out quite quickly. In the north, you're getting very low precipitation, because the air that gets to those very high polar latitudes is cold. If it's cold, it means that it's gone through a cooling process as it moved from low latitudes. And the cooling has probably already removed most of its water vapor. So cold air can't hold very much water vapor. And therefore you can't wring out very much rain from cold air, because it doesn't have very much water to begin with. So finding these seemingly desert-like precipitations--200 millimeters, 20 centimeters--is a result of the air being too cold to provide much water vapor to that region.

Arizona is complicated because it's got mountains in certain areas and low-lying areas, but this is the mountainous area, and pretty much the high precipitation is controlled by the mountains. But generally, it's a pretty dry state. Notice the highest precipitation here. The highest precipitation would be 32 inches, about this much, up in the highest mountains.

Chapter 4: Continentality [00:20:18]

What is?continentality? It primarily has to do with the fact that oceans store heat very effectively, whereas land surfaces do not. So it's easy to change the temperature on land. You don't have to provide very much heat or remove very much heat to warm up or cool down a land surface, because remember, all you're doing is heating the first few centimeters of the soil. And that's not very much heat capacity. So you put in a little bit of heat, the temperature rises. You take out some heat, the temperature drops.

You try to do that over the ocean and two things get in the way. First of all, water has a very high heat capacity. That is to say, per kilogram, it's got a large ability to store heat. Second of all, water is a liquid, which means it's not just the first few centimeters you have got to cool down or heat up. It's probably the first several hundred meters, because that water is constantly being stirred. So you can't just heat or cool the top few centimeters. You've got to heat or cool the top few hundred meters of the ocean, which is an enormous heat capacity.

So here is a plot of the annual range of temperature. It's the typical maximum temperature versus the minimum temperature. First of all, you don't see much of a range in the tropics anyway, over land or sea, because there's not that much difference in how the sun hits the tropics between winter and summer.?

So let's shift our attention more to mid-latitudes, especially to the northern latitudes where you have a couple of very large continents and then oceans in between. Look, the oceans barely change their temperature between winter and summer, but the continents change their temperature by a great deal. For example, up in the northern part of eastern Eurasia, you could have an annual range of 55--I think that's in Celsius--of 55 degrees between winter and summer. That's an enormous temperature difference. That's sweltering in the summer and so cold it's hard to describe how cold it is in the wintertime. So in the middle of continents, you get this enormous temperature range.

Continentality is a big player in climate, as you can imagine. You see it in the southern hemisphere too, but not so much becasue?the big difference between northern and southern hemisphere in this regard is that the northern hemisphere is the hemisphere of continents. The southern hemisphere is more the hemisphere of oceans. The percent land mass is much greater in the northern hemisphere that it is in the southern hemisphere, especially if you're excluding the equatorial region. So there's this asymmetry between the northern and southern part of our planet, climate-wise, because of the way continentality works differently in the two hemispheres.

You notice, if you did a zonal average of this quantity, you'd find a much bigger annual range in the northern hemisphere than in the southern hemisphere. Well that has nothing to do with how the sun hits the Earth. Over a year that's pretty much symmetric between the northern and southern hemispheres. But it has everything to do with the fact that there's more continents in the northern hemisphere than there are in the southern hemisphere.

Here it is over the United States, the annual range of temperature. It's largest in this interior region of the west, where it's dry and high. If you have higher elevation, you tend to have a larger annual range, because there's less air above you. You can cool down more at night. And it's drier. There are fewer clouds, more sunlight. It can heat up more during the day.

Alaska has a very small annual range near the ocean. Of course that's because it's close to the ocean, which is buffering the temperature. And then up in the interior, especially in high latitudes where there's a great deal of difference between summer and winter insolation--solar radiation--you get a very big annual range in temperature.

Ocean Currents and Climate [00:26:39]

And finally, I'll mention ocean currents.?Generally in each of the ocean basins, you have a gyre, a wind-driven gyre. But it carries, generally, cold water?towards the equator and warm water away from the equator, or just?poleward.?

So for example, if you're a Californian, you've got the cold California current coming down along the coast. In southern California,?be prepared for a shock when you jump in the water, because the water's always cold there, because you've got the California current. Whereas when I go vacationing down on Cape Hatteras, for example, in the summertime, you can walk in the water and it's very, very pleasant and warm, because that is warm water being brought northwards in the Gulf Stream. So that's a real difference. And notice that's going to work oppositely, typically, on the two sides of a continent. You're going to have cold water or warm water, depending where you fit into that current scheme.

Biome [00:28:39]

What can you do with just the annual average values? This is put together by a couple of authors that have tried to show the relationship between climate and vegetation based on an annual average values only. It's kind of useful, but I'm going to argue it's not complete, because it doesn't include the seasonality.?

So if you take the annual average temperature plotted on this axis, the annual average precipitation plotted on the x axis--come along, and then they have tried to illustrate what kind of vegetation you would get for each of those climate zones. It doesn't include the seasons in there. And the seasons really are needed to do a good job of this.?The concept is called a biome. The relationship between climate and vegetation is called the biome concept.

Effects of the Earth’s Rotation around the Sun [00:31:46]

So just five bullet points concerning seasons. What causes the seasons on Earth is the tilt of the Earth's axis, 23 and a half degrees currently. The?tilt is?measured from the plane of the ecliptic. The plane of the ecliptic is that plane in which the Earth moves in its orbit. That is our reference plane, and relative to that plane, the tilt is measured, the tilt of the Earth spinning on its axis.

For this reason, because it's a tilt effect, during part of the year the northern hemisphere is tilted towards the sun, and part of the Earth is tilted away from the sun. So the seasons are opposite in the two hemispheres. When it's winter in the north, it's summer in the south, and vice versa. That's the primary effect of it being a tilt effect, is you get those opposite seasonalities.

Now there is another effect. The Earth's orbit is slightly elliptical, which means for a few months a year, we're a little bit closer to the sun. For a few months of the year we're a little bit further away from the sun. That in and of itself would cause a slight seasonality on planet Earth. It'd be warmer when we're closer to the sun, a little cooler when we're further away. (A) that's a pretty small effect. And (b) note that if that were the dominant effect, that would cause simultaneous seasons in both hemispheres. It'd be summer in both hemispheres at the same time and winter in both hemispheres at the same time. So this does not explain the basic thing we know about seasons, in that it's primarily reversed in the two hemispheres.

So as it goes around the sun, the orientation of that tilt remains fixed. So as it goes around its orbit, notice the orientation of my pencil stays fixed. That arises from the principle of conservation of angular momentum. Angular momentum is the quantity that the Earth has when it spins. It's a vector quantity pointing in a certain direction. And that doesn't change. It's that constant orientation of the rotation axis that is important in understanding the seasons.?

Now, September 21st, on that day, that was the autumnal equinox. That's when the Earth had this orientation. That tilt was, in a way perpendicular to a line between the sun and the Earth, so that as the Earth spun on its axis, it had equal day and night everywhere around the globe.

Now we are moving towards December 21st, which will be the winter solstice, and on that day the orientation of the Earth will be like that relative to the sun. So when it spins on its axis again towards the east, the southern hemisphere is receiving a lot of heat, the northern hemisphere is receiving less, and the north pole is getting none. Notice this is completely in shadow back here, because of that direction.

Three months after that at the spring equinox, it'll be like this again. And then on June 21st, the summer solstice, that will be the orientation of the Earth relative to the sun. And the north pole will have sunlight 24 hours a day. The south pole will have none. A lot of radiation in the northern hemisphere. Less in the southern hemisphere, because it's tilted away. And that is the cycle we go through. Remember, it all rises again, because as the Earth goes around in its orbit, that orientation of the axis stays fixed as it goes around the sun. That's the key ingredient there.

This shift of the radiation from hemisphere to hemisphere moves all of these belts and zones along with them. So we've talked about the ITCZ, the belt of deserts to the north to south of that, the belts of storms to the north and south of that. Imagine that whole set of systems shifting northwards in the summertime for the northern hemisphere, and shifting southwards for the summer in the southern hemisphere.

That's probably the easiest way to understand how climate works on our planet, is to envision that the general circulation is driven by the sun, and therefore if, because of the tilt of the Earth's axis, you move most of the radiation into the northern hemisphere, everything's going to move with it. If you move it into the southern hemisphere, everything's going to move southward into it.?

If you're on the Earth, in kind of an Earth-fixed coordinate system, it seems to you like the sun is actually moving. In the summertime it seems to be up high in the sky. In the winter time it's lower in the sky. It's just the fact that the axis of the Earth is tilted.

For example, if you're at 41 degrees north latitude, as we are here in New Haven, in the summertime at the summer solstice, the sun at noon would be 23 and a half degrees above the equator, which means it's never overhead here in New Haven. But that solar zenith angle is not very great. You'd have to subtract 23 from 41 to get the solar zenith angle at noon on the 21st of June. On the 21st of December at noon, the sun is down here. We're still in New Haven. The solar zenith angle would then be 41 degrees plus 23, which?is?64 degrees. The sun at noon is 64 degrees below your zenith point.

And that has big influences?on the amount of radiation received because of that cosine factor. It's the cosine of the zenith angle that determines how much radiation is being received per unit area on the Earth. So when the sun is above, you get the full solar radiance. When it's at some solar zenith angle, it's the solar constant times the cosine of the solar zenith angle. So the greater that off-zenith angle is, the smaller is the cosine and the less radiation you receive. So that cosine effect is another important way to understand how the seasons work.

I did mention, however, this other effect, this non-circular aspect to the Earth's orbit. And it's shown a little bit exaggerated in this diagram. But at the present time, the perihelion--the day of the year when we're closest to the sun--is the third of January. The aphelion--when we're furthest away from the sun--is the fourth of July.?

So if you're at a particular latitude and you go through the months, how does the insolation vary? So what's shown here are four different latitudes--the equator, and then three latitudes in the northern hemisphere. And what's plotted is the solar insolation averaged over a day in watts per square meter.

Seasonal Zone Shifts [00:46:32]

For June, July, and August, when the northern hemisphere is tilted towards the sun, first of all, the polar front is very weak. And that's because the amount of radiation is almost the same between the pole and the equator. So there's not very much differential heating in the summer hemisphere. And in June, July, and August, the northern hemisphere is the summer hemisphere. The ITCZ, which we've been talking about being located at the equator, actually shifts northwards several degrees. And the frontal storms in the southern hemisphere become very strong. And the polar front moves a little bit northward as well.

Six months later--December, January, February--when the southern hemisphere is receiving most of the radiation, the frontal storms in the northern hemisphere are very strong, because there's no radiation reaching the north pole, and a lot of radiation reaching the equator. So a lot of differential heating. In the southern hemisphere, the ITCZ shifts southwards, and you have weak frontal storms in the southern hemisphere, because there's very little difference between the solar insolation here and there.