Friday, September 11, 2015

Waviness in the tropics

Despite this being one of the most significant El Nino years we've had in a while (with the potential to become one of the strongest on record), we've still managed to have some tropical storm activity in the Atlantic.  So far there have been eight named storms, only two of which have actually reached hurricane strength.  Depressed tropical cyclone activity in the Atlantic is expected in strong El Nino years, so the small number of hurricanes so far isn't too surprising. (There's still a ways to go in the hurricane season yet, though...)

However, every few days it seems we get a new "feature" in the far eastern Atlantic that the National Hurricane Center  identifies and keeps an eye on as it moves to the west across the ocean.  These features are called "tropical waves" and they happen all the time.  The NHC forecast discussions have an entire section devoted to describing the current "tropical waves".  So just what is a "tropical wave"?

When I was much younger, I would hear them talking about tropical waves on the Weather Channel and picture something like this:

And, admittedly for a very long time, I thought that hurricanes ultimately came from these giant waves on the surface of the ocean that somehow triggered hurricanes above them.  Let's just be clear here...while a warm ocean surface plays a tremendous role in supporting hurricane development, ocean waves are not what we mean by "tropical waves".  Let's look a little higher up in the atmosphere.

To understand what makes a tropical wave, we have to understand the air pressure patterns on the earth's surface.  Some of you may be familiar with what's called the "Hadley Circulation": the large scale motions of the atmosphere that dominate the tropics.
In short, near the equator it is very warm and there is a lot of convective (thunderstorm) activity.  All this warm air rising in the thunderstorms migrates away from the equator at higher altitudes before sinking again in the "subtropics"--about 30 degrees north and south of the equator.  When all that air rises near the equator, it creates lower pressure near the surface (think of all that air lifting up and away from the surface...the pressure you feel pushing down on you is lower).  Oppositely, the sinking air in the subtropics tends to create higher pressure in those regions.  Pressure again decreases as you get towards the northern mid-latitudes.  The result is a series of pressure "belts" that circle the earth in a mean sense:
These belts are an "average" condition of the atmosphere.  When we're looking at weather (particularly stormy weather), we're interested in the deviations from this average.  We have "troughs" where the pressure is lower than normal and "ridges" where the pressure is higher than normal.  This are where the idea of "waves" in the atmosphere come from---they are perturbations along these average, static belts.

Since storms are typically associated with lower pressure, we tend to look for "troughs" in the flow when we care about storms.  In the mid-latitudes (some 30-60 degrees N or S; the latitude band of the continental US), we live in a zone where there is typically higher pressure to the south and lower pressure to the north.  Therefore, our troughs tend to show up as perturbations extending down from north.  They "perturb" the average pressure pattern in a way that looks (and behaves) like a wave.  Here's an old Weather Channel map showing what this looks like in an idealized sense:
Note the "waviness" of the flow shown by the jet stream as it navigates around these areas of higher and lower pressure.  These are the "waves" we are talking about when we're talking about the atmosphere.  Not ocean waves, but pressure waves.

As I mentioned above, when we look at maps like this in the northern hemisphere mid-latitudes, our terminology makes sense.  A high pressure "ridge" looks like a "ridge" on this map, reaching up toward the north.  And a low pressure "trough", looks like a deep "trough" being dug down into the flow.  That's where this terminology comes from.

But near the tropics, remember that the pressure pattern is different.  There is lower pressure near the equator on average with higher pressure to the north.  We still look for troughs of lower pressure to cause storms---the same mechanics are at work there.  But the way troughs look is different: with low pressure to the south and high pressure to the north, a low pressure "trough" would actually be digging UP from the low pressure near the equator into the higher pressure to the north.  Here's a Wikipedia image that shows what these sorts of troughs look like:

The troughs are "upside down", but we still call them troughs.  Sometimes you'll hear them referred to as "inverted troughs".  This is one of the hardest things to explain to our Weather 101 students---a trough of low pressure doesn't always look like a "trough" on the map.  In the tropics they are upside down.

However, regardless of how they are oriented, it still makes the flow "wavy".  And that's where we get our "tropical waves".  They're still troughs of lower pressure, but they're just oriented in the opposite sense of what we are used to.  They also move from east to west, giving them another name: "easterly waves".   These areas of lower pressure extending north into the subtropics provide the seeds for eventual development of tropical storms and hurricanes, and they can also bring stormy squalls just on their own.  For weather forecasting in the tropics and subtropics, monitoring these tropical waves is essential.

On a side note, it actually is also possible for us to get "inverted" pressure troughs here in the mid-latitudes.  In fact, it's a common occurrence on the west coast in the summer, where you may have heard of the "thermal trough".  But that's an entirely different blog post...

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