As we just mentioned, atmospheric circulation is a key mechanism that
helps balance the distribution of solar radiation across Earth's surface. It
moves the heat from the equator toward the poles. Now a key part of this
mechanism involves rising atmospheric motion. The heating from the Sun migrates
north and south within the tropics over the course of the year, and so does the
tendency for rising atmospheric motion as the surface of Earth is heated in
different locations. Now warmer air is less dense than cold
air, and that creates what we call convective instability, the unstable
situation of having light air underneath heavy air. Where that instability exists
there's a tendency for rising motion. That warm air at the surface rises and
cools. Here we have a schematic that shows the large-scale circulation cells
in the atmosphere. The overall vertical circulation of the atmosphere on average
involves three circulation cells in each hemisphere, the first involves rising
near the equator descending in the subtropics then rising again in the
subpolar latitudes and then finally descending again near the poles, and we
can think of that as three circulating cells. Where we have rising motion, we
have low surface pressure. Where we have descending motion we have high surface
pressure. There's a tendency for rising air and rainfall in a zone of low
pressure, such as the ITCZ, the Intertropical Convergence zone, which is
that area in the tropics where the surface of the earth is most directly
heated by the Sun. Now the ITCZ is centered at the equator, but it shifts
north and south with the migration of the Sun over the seasons. This shifting
solar heating is more sluggish over the oceans, and more pronounced over large
land masses, which gives us dramatic wind patterns and rainfall patterns like the
Indian monsoon. Now the air rising in the tropics ultimately sinks in the
subtropics forming a band of high surface pressure which gives us the belt
of deserts. This pattern of circulation of rising air at the equator, and
rainfall near the equator, and descending dry air in the subtropics,
is known as the Hadley cell circulation. Air rises near the equator, spreads
towards the sub tropics, and sinks in the sub tropics. Now that you have that
sinking air in the subtropics but rising air in the mid latitudes, which forms the
next limb of the atmospheric circulation cells, and then finally you have that
rising air in mid latitudes descending near the polar regions, and that forms the
third cell of the atmospheric circulation. That rising takes place
within mid latitude storm systems, and those mid latitudes storm systems are
driven by the contrasts and temperature as warm subtropical air meets cold polar
air masses. Those differences in temperature actually drive storm systems
which provide lifting in the atmosphere and the rising motion in that sub polar
region. We call that location the polar front we're cold and warm air masses mix,
and where we have rising motion within those low pressure systems. As we go up
in the atmosphere, we find that the surface westerlies in the sub polar
regions actually get stronger and stronger as we go up in the troposphere,
until we hit the boundary between the troposphere and the stratosphere, what's
known as the tropopause. And at that location in the atmosphere, between 10
and 14 kilometers above the surface, and subpolar latitudes, we have what's known
as the jet stream, this band of very strong westerly winds. Winds that blow
from the west to the east. The jet stream moves mid latitude storm systems along,
and the properties of the jet stream are very closely tied to mid-latitude storm
systems, how they progress, how they intensify, where they are located. This is
a highly idealized depiction of the atmospheric circulation, it represents
long-term averages. In real time these motions are associated with storm
systems and convective thunderstorms in the tropics dynamical properties of the
atmosphere. All of which help Earth's climate system solve this problem of
relieving the unstable situation of an equator which is heated and a polar
region that is cooled. The atmosphere is trying to transport that excess of heat
from the equator towards the pole, and it does that in the form of these
atmospheric circulations. So wind patterns also transport heat poleward, the
lateral wind patterns are primarily governed by what's known as the pressure
gradient force and the Coriolis force, an effective force that exists due to the
fact that the earth is itself rotating. This balance between the pressure
gradient force and the Coriolis force is known as geostrophic balance. Now the
Coriolis force acts at right angles to the direction of motion, 90 degrees to
the right in the Northern Hemisphere, and 90 degrees to the left in the Southern
Hemisphere. The pressure gradient force is directed from regions of high surface
pressure to regions of low surface pressure. As a consequence geostrophic
balance leads to winds in the mid latitudes, between the subtropical high
pressure belt in the sub polar low pressure belt of the polar front blowing
from west to east. We call these westerly winds. The
westerly winds become stronger aloft, leading to the intense regions of high
wind known as the jet stream in the mid latitude upper troposphere. Conversely
winds in the tropics tend to blow from east to west, these are known as easterly
winds, or sometimes the trade winds. In the northern hemisphere geostrophic
balance implies counterclockwise rotation of winds about
low-pressure centers, and clockwise rotation of winds about high pressure
centers. The directions are opposite in the southern hemisphere. Due to the effect
of friction at Earth's surface, there is another additional component to the
winds, which blows out from high pressure centers, and in towards low pressure
centers. The result is spiraling in, or convergent motions towards low pressure
centers, and a spiraling out, or divergent motions about high pressure centers. The
convergence of the winds towards the low-pressure centers is associated with
the rising atmospheric motion that occurs within regions of low surface
pressure. The divergence of the winds away from the high pressure centers is
associated with the sinking atmospheric motion that occurs within regions of
high atmospheric surface pressure. Oceans also play a key role in relieving the
radiation imbalance by transporting heat from low to high latitudes. The first
type of ocean circulation is the horizontal circulation, the wind-driven
ocean gyres. Now the major surface currents are associated with the ocean
gyres, the warm poleward western boundary currents, such as the Gulf Stream which
is associated with the North Atlantic Ocean gyre, and the Kuroshio current
associated with the North Pacific Ocean gyre. These gyres also contain cold
equatorward eastern boundary currents, such as the canary current in the
Eastern North Atlantic and the California Current in the western North
Pacific. Similar current systems are found in the southern hemisphere. The
horizontal patterns of ocean circulation are driven by the alternating patterns
of wind as a function of latitude, and in particular by the tendency for westerly
winds in mid-latitudes and easterly winds in the tropics, as discussed
earlier. The second type of ocean circulation is the vertical circulation,
and in particular what we call the meridian 'el overturning circulation, or
sometimes simply the conveyor belt ocean circulation.
Now this ocean circulation pattern is associated with a tendency for sinking
motion in the high latitudes of the North Atlantic and rising motion more
broadly in the tropics and subtropics of the Indian and Pacific Ocean. The
circulation pattern is driven by contrasts and density, which are in turn
largely due to variations in both temperature and salinity of seawater,
hence the term that sometimes used to describe this circulation, the
thermohaline circulation. The sinking motion is associated with relatively
cold salty surface waters of the sub polar North Atlantic, and the rising
motion within the relatively warm waters in the tropical and subtropical Pacific
and Indian Oceans. The northward extension of the thermohaline
circulation in the North Atlantic is referred to as the North Atlantic drift.
This current system represents a net transport of warm surface waters to
higher latitudes in the North Atlantic, and as an important means by which the
climate system transports heat poleward from lower latitudes to higher latitudes.
Changes in this current system are speculated as having played a key role
in past and potential future climate changes, as we will explore later in this
course. Just as the atmosphere exhibits natural internal variability that we
call weather, the climate system itself exhibits natural internal variability on
longer timescales, and the most important and most familiar of those modes of
natural internal variability in the climate system is the El Niño Southern
Oscillation or ENSO. Now in the case of ENSO what we see happening is an
interaction between the ocean and the atmosphere. The tropical Pacific Ocean
has current systems that are dynamic, the tropical atmosphere, the trade winds are
dynamic, and those two things can interact with each other because the
strength of the trade winds actually influences the ocean currents,
and how much cold water comes up from deep to the surface. When those winds die
down, when the trade winds weaken in the Eastern equatorial Pacific, the upwelling
of those cold waters to the surface weakens, and waters in that region warm
up dramatically. Typically when that happens, every few years at about
Christmas time and hence the name El Niño, which corresponds to the Christ
child, because the native South Americans saw that this phenomenon would tend to
happen around Christmas in the boreal winter. So those waters warm up in the
Eastern equatorial Pacific, and that changes the relative east-west pattern
of temperature along the equatorial Pacific. But that east-west pattern of
temperature drives a vertical circulation in the atmosphere, that's
called the Walker circulation, and the change in that Walker circulation
changes the strength of the trade winds. And so we've now come full circle
if you change the trade winds, you change the upwelling of cold water in the
tropical Pacific Ocean, but if you change the upwelling of cold water you change
the temperature patterns which influence the atmospheric circulation pattern,
which determines the trade winds in the first place. And so this is an example of
a coupled ocean atmosphere mode of variability you can't understand El Niño
by just understanding the atmosphere. You can't understand
the El Niño Southern Oscillation by understanding just the ocean. You have to
understand how they interact with each other to yield this mode of variability,
which tends to vary on timescales of three to seven years. Now when we look at
the global pattern of influence of El Niño, we see that what happens in the
tropical Pacific doesn't remain in the tropical
Pacific. Those changes in sea surface temperature in the eastern tropical
Pacific influence the jet streams in both hemispheres, and if you change the
northern hemisphere jet stream and you change the southern hemisphere jet
stream, you are going to change weather patterns around the entire world. So El
Niño events and their flipside La Niña events have a global impact on our
climate. By some measure they are the strongest signal in the climate record.
Much of the year to year variability in climate around the world
is influenced by El Niño. Now we suspect that climate change will likely modify
the effects of El Niño. So although it's a mode of natural internal variability
in the climate system, there's also the potential for climate change to change
those patterns, to change the behavior of El Niño, and if you change the behavior
of El Niño you're gonna influence drought patterns
in southwestern United States. You're going to influence hurricane seasons in
the tropical Atlantic and the tropical Pacific. So there are all sorts of
phenomena that are a great interest and concern to us that are influenced by El
Niño and if climate change changes El Niño it's going to impact these
phenomena. We still don't know exactly what those impacts will be, and hence
there is uncertainty in these regional impacts of climate change. But once again
uncertainty isn't our friend because you can't plan for or adapt to changes that
you don't know are coming.
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