Download What`s up with the weather

What’s up with the weather
Earlier this month I noted that the then-approaching unseasonably cold air was the direct result of
a Pacific typhoon and promised to provide more info when I got the chance, so this is it. Actually,
what I will try to link is El Nino, Super Typhoon Nuri, a record-breaking Alaskan storm, the visit of
a strong jetstream, our cold spell, the Buffalo snow storms and, oh yeah, sprinkle in a little global
warming. The trail of linkages will take us from the waters off the coast of South America, across
the Pacific, up past Japan into the Bering Sea, and finally plunging down into the heart of North
America. My explanations and descriptions will rely on some personal previous knowledge of the
phenomena, some recent readings from sources such as the Weather Underground, and some
judicious speculation. Spoiler alert: the tale is quite a bit lengthier than the usual “What’s up”
post, so it will be presented as an attachment to this note.
The above introduction to the topic should give a hint that many of the apparently separate parts
of the planet’s ocean-atmosphere system are really connected. Messing with one part of the
system can affect others at considerable remove. Let’s start with some climate change general
background. There are deniers of the phenomenon who argue that global warming is not real,
because the planet’s atmosphere has not been warming in tandem with the increase in
greenhouse gases over the last 15 years or so. The answer to the objection is twofold. First, no
reliable prediction of temperature change has ever indicated that the warming would take the
form of a steady temperature climb, or in technical terms, “a monotonic increase in the secular
temperature trendline.” The general trend is upward, but with likely stops and starts over short
periods.
One reason for the stops may lead to a second part of the answer. Not all the energy that has
been added to the system by way of the effects of the greenhouse gases (GHG) is showing itself
as immediate air temperature change. Lot’s of the energy is apparently getting stored in other
mechanisms in other places, to be released later and only then appearing as measurable
atmospheric warming. The prediction models’ consensus indicates that, of all the warming that
should result from the increased GHG concentrations to date, only about a half has done so.
Again, reverting to the techspeak, “a certain amount of temperature change has been committed,
of which about half has been realized.” This is an important point, because it means that, should
we release not a single additional molecule of CO2 or methane anywhere into the atmosphere,
the earth’s temperature will still continue to coast upward for an additional increase about equal to
what we have already seen.
Now how does that relate to the recent cold snap and our Pacific typhoon? One might speculate
that some of the “missing” unrealized heat energy might be firing up a recent spate of monster
typhoons. In the past two years three such monster storms – “super typhoons” – have developed
in the western Pacific: Haiyan, Vongfong and Nuri. Each of these storms peaked out with steady
winds incredibly in the neighborhood of 180 mph, with gusts to well over 200 mph. What might
be the source of their great energy and how does that relate to our cold spell? This question
takes us to a discussion of El Nino and the current thermal state of the western Pacific Ocean.
Let’s start with “What is El Nino?” Actually, it is not just a single thing. It is one phase of a
sequence of conditions that influence the state of the ocean and atmosphere in the tropical
Pacific. It is a part of the more general phenomenon known as the El Nino Southern Oscillation
(ENSO), which includes an El Nino phase, a La Nina phase, and a neutral phase. Most generally
it is a phenomenon in the tropical Pacific that moves enormous volumes of water and
redistributes heat energy across the breadth of the ocean.
One part of the earth’s global circulation system is the equatorial trade winds – persistent easterly
surface winds that are found in the tropical latitudes over the open water of the three major
oceans. In the Pacific the easterly trades continuously push warm surface water to the west and
away from the west coast of South America. To replace the warm water leaving the coast, colder
waters are drawn up from the bottom. The colder water is rich in nutrients and so supports the
important fisheries that have traditionally been accessed by Peru and other nations in the area
since before the arrival of the Europeans.
The following chart illustrates the basic explanation…
Note the two circulations – water and air. The warm water moving west on the surface is
replaced by cooler water from below, while the easterly trades on the surface are replaced by a
return circulation aloft. This is the normal state of affairs, or the “neutral” phase. Note also that,
as the trades continue to blow, a large warm water pool continues to pile up in the western
Pacific. Note further that the return flow of air aloft is supported by prevalent upward convection
and thunderstorm activity above the warm water.
Now, as the warm water continues to pile up, the pool begins to backfill toward the east, dragging
its thunderstorms with it. As the storms move east, the long wind fetch across the Pacific
shortens and weakens, eventually weakening (or even shutting down) to the extent that the warm
water pool is no longer held back and begins to slosh back towards South America, eventually
reaching the coast and shutting down the cold upward flow, shutting down the fisheries as well.
This is normally a phenomenon with an early winter onset (December), so the natives named it
after the Christ Child, whose birthday is being celebrated about the time the fisheries begin to
shut down. On the other hand, should the easterlies strengthen and upwelling at the coast
increase, the condition can morph into the La Nina phase. We might speak then of El Nino and
La Nina as representing the warm phase and cold phase, respectively, of the overall ENSO
phenomenon.
Next step… How does the ENSO relate to Typhoon Nuri?
The enormous pool of warm water that the trades pile up in the western Pacific serves as a
breeding ground for tropical storms, which feed and grow off the heat energy of the ocean waters.
During the past two years this pool has become extremely deep, the result being the
development and growth of the three powerful super typhoons mentioned above. Why?
In the normal world of tropical storms their strength is often limited by the seeds they carry within
themselves. The storms feed off the ocean surface heat, but as they grow and their winds
increase, the strengthening winds roil up the warm ocean waters, mixing colder water from below,
which lowers the surface water temperatures and limits further growth. Infrared imagery of a
strong storm’s track across the ocean often reveals the shadow of cooler temperatures left in the
wake. Recently, however, the western Pacific warm pool has deepened to such an extent that
the winds of even a powerful storm have not mixed the ocean deep enough to draw up the cooler
waters from the depths. The result has been an absence of an important growth-limiting factor.
(Although not part of this discussion, one might ask why the larger volume of warm ocean waters
exists in the Pacific. One could speculate that that this might be one of the places where the heat
to support unrealized climate change temperatures are hiding, here in the form of heat energy of
warm water. If true, we can expect that the excess warm water will eventually slosh back in the
form of a more powerful than normal El Nino, which will then show itself as a noticeable uptick in
the global average temperatures, as happened with the powerful 1998 El Nino.)
One of the functions of tropical storms globally is to redistribute energy, primarily thermal energy,
from the tropics, where it is in surplus, to the poles, where there is a deficit. There are several
different ways that a tropical storm can move energy from one place to another. One form of
energy is kinetic energy – the movement of air and water in the form of high winds, waves, storm
surges. Another form of energy that is transported is the sensible heat of warm tropical air and
water that is dumped into the high latitudes. Another significant source of energy that is
transported is that of the latent heat of condensation. Water vapor contains much more energy
than liquid water. Whenever a mass of water vapor condenses from gas to liquid (or solid) this
extra heat is released into the atmosphere. Putting all the energy sources together, the amount
of energy transported by tropical storms is enormous. Most times it is quietly dumped into the
polar regions with little ado. This time Nuri made the process quite dramatic.
Here are the tracks of the three storms in question.
Haiyan
Vongfong
Nuri
Haiyan unfortunately expended much of its energy on the Philippines, killing up to 6000 people
and doing enormous damage due to its high wind which hit the coast at maximum intensity, but
mostly due to the enormous accompanying storm surge. Vongfong weakened before passing
over Japan and dropping heavy rains. Thereafter it wound down gracefully in the North Pacific.
Nuri did something quite different, however. It entered the Bering Sea and merged with an
already existing subpolar storm in the area, greatly strengthening it. Added to the mix was an
extremely energetic jet stream that drove in from the west with peak winds of over 200 mph.
Now things get a little bit more complicated in the explanation, so I’ll take an aside here to explain
what is going on when the jet enters the picture…
The jetstream contribution to the deepening storm was in two forms, both of which contributed to
the increase in upward convective motion in the atmosphere. The two mechanisms were
“divergence aloft” and “positive vorticity advection.” As to the former, when high velocity air (a
jetstream) comes down the back side, toward the base, and then around, a deep trough it
experiences a relaxation of the gradient of the height lines that define the trough. It is said that
the gradient lessens. (The lines are farther apart.) When that happens the air that comprises the
jet slows down and spreads out laterally. It experiences divergence. To make up for the air that
is spreading out and away, additional air from below rises to fill the loss. The result is upward
convection.
Next there is the “vorticity” that is carried by the jet. Vorticity involves the “twistiness” of a large
parcel or volume of air. The twistiness is really the angular momentum of the air that is
transported with the flow – cyclonic twist is called “positive” vorticity, while anticyclonic is termed
“negative” vorticity. The visual indication of the positive and negative vorticity that is carried by a
stream of air is the “ripples” in the flow – troughs and ridges, moving along with the moving air, as
is depicted following…
An idealized schematic showing a 500-mb ridge (left) and a 500-mb trough (right).
500-mb height lines are the thin, dark contours, and the capsule-shaped features
represent air parcels. Note how an air parcel in the ridge shows anticyclonic spin
(negative relative vorticity) and an air parcel in the trough shows cyclonic spin
(positive relative vorticity).
When positive vorticity is transported into an area, it is said that the region is experiencing
“positive vorticity advection” (PVA). In other words, the region is experiencing an enhancement in
cyclonic rotation of the air that it contains. If you look at the lines on an upper air chart that
represent the flow of the air (the height lines on a constant pressure chart, e.g.), vorticity can be
defined as the product of the curvature of the lines (the steepness or shallowness of the ripples -the ridges and troughs -- that are moving along with the flow) and the speed of the flow. Shallow
ripples have low values of curvature, while steep ripples have high values of curvature.
Imagine our jetsream, carrying its positive and negative vorticity (trough and ridge ripples),
entering into the region of our growing storm in the Bering Sea. With an incoming tough in the jet,
the growing storm is now experiencing PVA. An interesting result of solving the atmospheric
equations of motion is the finding that areas that experience PVA also experience enhanced
upward vertical motion. Upward motion enhances the convection, which cools the air, which in
turn then allows for the increased condensation of water vapor to liquid, which then results in the
addition of latent heat, which then further increases vertical convection. The bottom line here is
that the entry of the jet into the mix adds two mechanisms that make the Bering Sea storm all the
stronger.
The sum total of the mix of Nuri, an existing storm in place, and a powerful Jetstream generated
what is known in the weather lingo as a “bomb” – a rapidly intensifying major storm of the middle
and high latitudes. Eventually, the Bering Sea bomb looked like this…
The storm bottomed out at a central pressure of 924 mb, an incredible 27.29” of surface pressure,
very nearly the pressure of Hurricane Andrew at landfall in Florida. To further put this Bering Sea
storm in context, the size of the storm eventually reached nearly 3 million square miles – about
the size of the entire CONUS. It had a large area of 80 mph hurricane force winds that churned
the seas off the Aleutian Peninsula and island chain to wave heights of 50-75 feet. It is believed
to have been perhaps the strongest non-tropical storm ever observed in the Northern
Hemisphere.
We now move to the next and nearly final phase of the story – the impact of the strengthening of
the storm on the shape of the trough. Here we need to know two things, one is our previous
description of the constituents of vorticity (curvature and speed of the flow) and the other is the
physics principle that angular momentum (and hence vorticity) is conserved.
We need now to get back to our jetstream, entering the storm, carrying its positive and negative
vorticity. As the jet enters the actual storm, it will follow the contours of the upper air flow – in this
case the large upper air trough that is associated with the low pressure storm on the surface. As
it enters the trough, recall that the pressure gradient relaxes and the speed of the jet is lessened.
At the same time the vorticity should remain relatively unchanged. Vorticity is composed of
curvature multiplied by speed, so if the speed lessens, the curvature must increase proportionally.
The effect of increasing the curvature in the existing trough is to enhance the loopiness of the
major trough – to make it deeper and more pronounced.
In the sequence of troughs and ridges in the upper air, a trough or a ridge does not generally
exist in isolation. Downstream of a major trough there is generally induced a major ridge, and
beyond the ridge another major trough, and so on for a few cycles.
The following picture illustrates this condition, with a major induced ridge immediately
downstream and over Alaska and another major trough beyond it over the western part of the
continent. The upper air flow over North America now shows a flow of Arctic air almost from the
North Pole directly into the United States, creating a southward displacement of the Polar Vortex
similar to that of last winter. When that cold northerly flow broke into the west central part of the
country it produced mid-November temperatures that would be considered cold even in January.
Casper, WY, dropped to -27 and Denver to -14.
The cold air reached all the way to the Gulf, generating temperatures 20, 25 or even 30 degrees
below normal. The following shows the temperatures in the River City for 12-20 November, the
peak of the cold period here.
TEMPERATURE IN F:
:PCPN:
SNOW: WIND
:SUNSHINE: SKY
:PK WND
================================================================================
1
2
3
4
5 6A 6B
7
8
9
10 11 12 13
14 15
16
17 18
12Z AVG MX 2MIN
DY MAX MIN AVG DEP HDD CDD WTR SNW DPTH SPD SPD DIR MIN PSBL S-S WX
SPD DR
================================================================================
12
13
14
15
16
17
18
19
20
52
44
46
49
56
54
56
67
66
39
35
35
40
37
31
34
38
47
46
40
41
45
47
43
45
53
57
-16
-22
-21
-16
-14
-17
-15
-7
-2
19
25
24
20
18
22
20
12
8
0
0
0
0
0
0
0
0
0
0.00
0.00
0.00
0.04
0.03
0.00
0.00
0.00
0.17
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
M
0 14.3 22 20
0 14.0 22 350
0 6.5 13 50
0 5.2 10 120
0 9.4 24 360
0 10.4 21 10
0 4.9 13 170
0 6.7 18 220
0 2.7 10 170
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
6
10
10
10 1
10 1
3
5
2
9 1
29
29
16
12
32
27
17
23
13
20
360
40
120
350
340
160
220
160
The final chapter of the story played out last week, during 17-21 November. During that period a
second surge of Arctic area plunged south, but this time a bit farther east. The result was another
example of the deep polar vortex trough, bringing air from Greenland and the pole down into the
Midwest. The front portion of the trough, as seen below, exhibited an intensely cold flow oriented
in a SW-NE direction, nearly parallel to the axis of the two Lower Great Lakes – Erie and Ontario.
For five days cold air made use of the full fetch of those two lakes to pick up moisture from their
still warm surfaces and drop it in the form of one of the strongest “lake effect” storms ever
observed in the Great Lakes. Snow totals as high as 88 inches were observed in South Buffalo.
Now you know the entire story – a tale of moving heat around on a global basis, how one thing is
connected to another and thence to still another again, or…
“When we try to pick out anything by itself, we find it hitched to
everything else in the universe.”
-- John Muir