Saturday, January 15, 2011

Something a little different--a look at Australian weather

I thought I'd look at an area of the world that has been receiving a lot of attention as of late, though for a slightly different reason.  Much has been said about the devastating floods in Queensland and the Brisbane area of Australia over the past few weeks.  It has brought worldwide attention to the meteorology of Australia and the ongoing rains that have contributed to the flooding there.

One thing we have to remember about Australia is that it is in the southern hemisphere and consequently the "shapes" of its weather patterns are different from what we in the northern hemisphere are used to.  For instance, while we're in the middle of winter, it's the middle of summer there.  Consequently, temperatures are rather warm:
Fig 1 -- Surface temperature analysis at 00Z, Jan. 16, 2011.  From the Australian Bureau of Meteorology.
Most of the country is shaded in orange colors, representing anywhere from 20-30 degrees Celsius.  That's from 68-86 degrees Fahrenheit.  Much more summer-like than winter-like. Of course, Australia is also positioned slightly closer to the equator than the continental United States.  Take a look at the latitude lines on the map above.  The northern tip of Australia almost gets up to 10 degrees south latitude.  That's only 10 degrees away from the equator.  By comparison, Key West, Florida, is around 24 degrees north.  So northern Australia is 14 degrees closer to the equator than the southernmost part of the continental United States.  Down on the island of Tasmania off the southeastern coast of Australia, they get up to almost 45 degrees south. Mainland Australia only gets down to almost 40 degrees south.  In comparison, much of the US-Canada border is at 49 degrees north--so slightly further north than Australia is south.  Even with these slight differences in position, both Australia and the US span similar bands of latitude and consequently see somewhat similar types of weather systems.  They just look a little different to northern hemispheric eyes.  For instance, here's the latest surface analysis:
Fug 2 -- Surface MSLP and frontal analysis at 00Z, Jan 16, 2011.  From the Bureau of Meteorology.
Let's examine a few features on this map.  One is the low pressure center directly south of Australia.  That's a respectable 984 mb low sitting there, but notice the cold front associated with that low.  It seems oriented somewhat oddly compared to northern hemisphere cold fronts.  We're used to seeing the cold front drag to the south or southwest of a low pressure center because of counter-clockwise circulation around the low.  However, two things are different in the southern hemisphere:

  1. The cold air is located to the south instead of the north.  So any cold air being dragged in with low pressure centers will come from the south.
  2. The Coriolis effect, which turns winds to the right in the northern hemisphere and explains why we have counter-clockwise flow around low pressure centers, turns winds to the left in the southern hemisphere.  This means that winds flow clockwise around a low pressure center in the southern hemisphere.
This provides a sort of mirror-image effect for typical cold-core cyclone structure in the southern hemisphere. Remember our basic structure of a low pressure center and fronts in the northern hemisphere:
Fig 3 -- Simple diagram of the structure of a northern hemisphere extratropical cyclone.  From Paragliding Weather website.
The general flow of the winds is counter-clockwise around the low.  Warm air from the south (toward the equator) is advected northward and we see its leading edge as a warm front.  Cold air from the north is advected in, swinging around from the west on the back side of the low.  We see the leading edge of this air as a cold front.  What about in the southern hemisphere?
Fig 4 -- Simple diagram of the structure of a southern hemisphere extratropical cyclone at the surface.  From the South African Climate Systems Analysis group.
Here we see the winds flowing the opposite direction around the low--clockwise.  Warm air from the north (since the equator is now toward the north) is this time advected southward along the eastern side of the low. Cold air from the south is advected northward on the western side of the low.  The result is usually frontal boundaries that generally lie to the north of the low (as opposed to fronts that lie to the south of the low in the northern hemisphere).  So this is why we see all those cold fronts extending mostly to the north of the lows in the surface analysis above.

Note how I kept saying "clockwise" and "counter-clockwise" in terms of the winds instead of "cyclonic" or "anticyclonic".  The term "cyclonic" is defined as the way the air moves around a cyclone (a low-pressure center).  So in both the northern and southern hemisphere, the winds around the low pressure centers are technically "cyclonic".  They may be going opposite directions from a clockwise/counter-clockwise perspective, but the term "cyclonic" definitely applies to both.

Anyhow, now that we're oriented a bit, it's kind of fun to see how this spin affects different things.  You may have noticed in the surface analysis above that there's a feature called Severe TC "Zelia" to the northeast of Australia.  This is indeed a tropical cyclone-- a category three, as a mater of fact.  They are referred to as tropical cyclones in the southern hemisphere--"hurricane" is only used in the Atlantic basin.  So "Severe TC Zelia" would translate to "Major Hurricane Zelia" if it were up here.  Here's the forecast track from the Australian Bureau of Meteorology (the BOM):
Fig 5 -- Track and forecast track for severe TC Zelia as of 00Z, Jan 16, 2011.  From the BOM.
Note that the storm is forecast to stay well away from the mainland and weaken to a depression before it reaches New Zealand.  This is good news--as a tropical storm hitting Queensland with all their flooding is the last thing they need right now.

One thing that's NOT different about mid-latitude weather systems in both hemispheres is their direction of movement--the prevailing jet streams (and consequently storm motion) are still from west to east.  So just as hurricanes in the Atlantic move away from the equator and then get pushed around until they're moving to the east, so too do tropical cyclones in the southern hemisphere move away from the equator (south this time) and get pushed to the east the further they get from the equator.  Pretty fun.

This cyclone has plenty of energy to work with, too--here's the latest sea surface temperatures off of Australia:
Fig 6 -- Sea Surface temperatures off of Queensland and the Coral Sea from Jan 15, 2011.  From the BOM
All of that red is 29 degrees Celsius--some 84 degrees Fahrenheit.  Very, very warm water.  No wonder this storm strengthened to category three strength.  But you can see that it's going to be moving over much cooler waters the further south it goes.  This helps explain why the storm is forecast to weaken as it moves along.

And finally, here's the latest IR satellite image over Australia:
Fig 7 -- IR satellite image from 532Z.  From the BOM.
Some of these features we've been talking about show up rather nicely on this image.  TC Zelia is on the upper right side of the image--it's the cyclone with bright white cloud tops and a very dense core.  Note the orientation of the swirls of all those low pressure centers--if you spiral from the outside in it is a clockwise spiral!  That low pressure center to the south of Australia also shows up rather clearly.  Notice that where the cold front was indicated on the northeast side of the low that we see most of the high cloud tops (the darker gray shades compared to the surroundings), indicating some deeper convection there associated with the front.

Remember what I said, though, about Australia being closer to the equator than the US?  This explains why the main storm tracks (associated with the south polar jet) are well to the south of Australia--there's that parade of lows spinning around in the ocean south of Australia, rather far away from land.  While the northern US would be hit by storms at those latitudes during our summers, Australia is just enough closer to the equator to miss out on most of those storm tracks.  However, Australia extends close enough to the equator to get caught up in the tropical rain belts.  Note the string of "cloud blobs" across the northern edge of Australia in the IR image above?  Those are all  tropical waves--the kinds of storms we'd usually see in the Caribbean.  Australia is close enough to the equator that those impact the northern part of the country.  In fact, on the surface analysis in figure 2, you can see the long dotted line running from east to west across the northern part of the continent.  This persistent trough of low pressure brings lots of tropical rain to that belt during the summer.

So that was a quick look at something different--weather in the land down under.  It's summer there, and I thought we could use a little break from our winter.  Back to the US for my next post, though...

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