Keith234
Storm Chaser
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Loc: 40.7N/73.3W Long Island
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I've noticed that when the trof moved off the east coast, a persistent system became established just east of the trof axis. How come a system like that does not become anchored on the east coast when the trof is not transient? Does the blocking need to more retrogressive?
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"I became insane with horrible periods of sanity"
Edgar Allan Poe
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Clark
Meteorologist
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Generally, these systems you have been seeing offshore in the Atlantic have been surface features with some upper-level support. There have been blocking scenarios mixed in at the mid- and upper-levels at times in association with these systems as well.
A mid-latitude weather system at the surface will develop when it is located ahead of a mid- to upper-level trough. This provides favorable dynamics in terms of rising/sinking motion (in association with the upper jet), vorticity (i.e. spin) generation via spin-up and advection processes, and temperature advection. This all works through something called the "action at a distance" principle, such that effects from an upper-level system are felt at the surface (and vice versa).
You see these systems form off of the east coast of the U.S. due in large part to the Gulf Stream. The warm waters from the current combined with cold air streaming across the U.S. in association with a front or the larger cyclone itself create very large temperature gradients. In meteorological jargon, this is called "baroclinicity." Needless to say, when you get a trough moving over such a region of baroclinicity, surface cyclone development is favored. This is one of the defining theories of development in meteorology and dates back about 45-50 years.
Note that it doesn't always have to be this baroclinicity that results in development; you often see a lot of systems form just east of the Rockies in association with a trough. This latter process occurs when flow rushes off of the mountains and stretches a column of air in the vertical; think like an ice skater starting with their arms way out and then gradually bringing them inward, spinning at an ever-faster rate while doing so. This results in development ahead of the trough at the surface. The Rockies are high enough above the surrounding regions to be favorable for this; the Appalachians are not.
In any case, I've gone off-topic for a bit there. The main point is that developing surface systems are always found in advance of a mid to upper-level trough for the reasons above as well as the fact that fronts slope back to the west (or north, for a warm front) with increasing height. For the proper stacking to occur, the trough must be located behind the front.
Usually, these cyclones develop along the Gulf stream and travel east and northeast as they strengthen. As they progress east, secondary waves tend to develop further to the south on the boundary while the primary system moves off and eventually weakens. This weakening process occurs as the upper-trough and surface system become located nearly over each other, as the evacuation of mass (outflow) can no longer occur. Midlatitude systems need shear to develop and strengthen, making this one of the big distinctions between them and hurricanes.
The secondary waves tend to be located sufficiently far enough off of the coast such that they are located where you mention. Due to the Gulf stream, it is very tough to see the pattern shift back towards the east, as any system approaching the coast is going to see the effects noted above. This tends to erode the western extent of the ridge and/or push the axis towards the east, resulting in blocking being much more common near Bermuda. Do note that in the summer, the above does not necessarily hold true -- the temperature gradient between the lands and oceans is much, much weaker. As the systems come off of the coast and approach the ridge/block, they tend to either ride over the block well to the north or become fractured, whereby the primary wave rides up to the north and the secondary wave gets caught up in no man's land in the ridge, setting up a Rex block (high-over-low) pattern. It should be noted too that systems sometimes just weaken upon approaching a blocking ridge; the evolution depends upon the strength of each system.
Over the past few weeks (until recently), this pattern had been reinforced commonly by troughs coming off of the east coast, lows developing along the Gulf stream, and the secondary waves getting trapped in the ridge. With time, the block broke down and the pattern has now become much more progressive.
How can this all relate to hurricanes? Blocking isn't as common -- particularly Rex blocking -- during the summer time. Typically, such as this year, heating processes in the upper atmosphere can strengthen the ridge above the storm as well as in the vicinity of the steering subtropical ridge. This is why a well-developed storm with good outflow can both strengthen a subtropical ridge as well as, in effect, "create its own environment." We saw that this year, as the models failed to capture this process well many times.
Pardon the length of this post, but to do the question justice, it took about this long. I added in the bit about hurricanes to keep us on topic, at least to some degree.
It sure is good to have you around here, Clark. You should start charging money. -HF
Edited by HanKFranK (Wed Feb 16 2005 06:30 AM)
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