Typhoons and Potential Connections to Alaska

Source: Joint Typhoon Warning Center

That's super typhoon Jelewat, which presently has maximum sustained winds of 135 mph, with gusts to 165 mph.  The maximum significant wave height is 48 feet.  That will get your attention.

The current track forecast has it turning clockwise, brushing by Taiwan and then moving across Okinawa.  Little wonder why the US Navy and Pacific Air Forces are so concerned about typhoons.

Source: Joint Typhoon Warning Center

I've been looking at the long-range forecasts produced by the GFS and after undergoing what is known as extratropical transition, the remnants of Jelewat move over Nome, Alaska by October 4th.  The weak low center over western Alaska in the image below is it.

It is pretty common for tropical cyclone remnants to move across the North Pacific and undergo reintensification (a process known as extratropical transition).  Many move into the high latitudes.  What role do these events play in the climate of Alaska and the Arctic?  This strikes me as a potentially interesting avenue of research.  Some relevant discussion of major events from weather historian Christopher Burt is available here.  

Wild Times in the Gulf of Alaska

As we approach the cool-season, life in the midlatitudes is starting to get interesting.  Today we have an example of an explosively deepening cyclone or bomb over the Gulf of Alaska.  Watch in the loop below how the cyclone deepens from a central pressure of about 1002 mb to less than 960 mb 24 hours.

1200 UTC 25 Sep – 1200 UTC 26 Sep IR imagery and
GFS sea level pressure analysis

The term "bomb" was used in the classic paper by Sanders and Gyakum (1980), which used that label for extratropical cyclones with a central pressure that falls at least 24 mb in 24 hours.

Title and abstract of the seminal paper by
Sanders and Gyakum (1980)

Sanders and Gyakum (1980) showed that bombs in the Northern Hemisphere are most common over oceanic areas, especially along the Gulf Stream in the Atlantic and and from along the Kuroshio Current to the Gulf of Alaska over the Pacific Ocean.  Their analysis was based on a small sample, but more recent climatologies reflect this distribution.

Distribution of bomb events from Sanders and Gyakum (1980)

At the time, bombs were poorly forecast.  Growing up on the east coast, I experienced this first hand as forecasts of nor'easters (cyclones that frequently develop explosively and move up the US east coast) were notoriously bad.  Although annoying for landlubbers, these cyclones are particularly problematic for marine safety and shipping as they can produce hurricane-force winds and high seas.

Major field programs such as GALE (Genesis of Atlantic Lows Experiment) and ERICA (Experiment on Rapidly Intensifying Cyclones over the Atlantic) were held during the 1980s to better understand and forecast this type of cyclogenesis.

Today, bomb forecasts are much better, and this is largely a result of improved numerical model resolution and better assimilation of satellite data over the oceans.  Surprise events are rare and we typically have a good idea of the potential of an explosive cyclogenesis event several days in advance.  Below is the 72 hour GFS forecast valid for 1200 UTC 26 Sep (this morning) and it pretty much nails it.

Such a forecast would have been extremely rare in the 1980s, but is common place today.

Update@1115 MDT:

As shown in the 0000 UTC and recently released 1200 UTC manual surface analysis from the Ocean Prediction Center, the central pressure of this bomb appears to have dropped from about 991 mb to 958 mb in 12 h.  Very impressive!

Real Snow

Source: Alta.com

That's more like it.  Today you can believe your eyes.

Is this Snow?

Source: Alta.com

I'm not so sure.  I can't find snow in photos from other cameras (e.g., Snowbird Hidden Peak), I can't see it when I look at Lone Peak from my office, and the temperature at the top of the Collins lift has gotten no lower than 39ºF, which is quite marginal for snow to stick.  Rumor has it that the Sugarloaf camera image often suggests there's snow on the ground.  What do you think?

Climate Variability Paper

University of Utah Atmospheric Sciences Professor
Dr. Thomas Reichler

We interrupt today's blissfully cool and cloudy weather for a quick plug of a recent paper by University of Utah Atmospheric Sciences Professor Dr. Thomas Reichler, Atmospheric Sciences graduate student Junsu Kim, and two colleagues at other institutions, Elisa Manzina and Jürgen Kröger. The paper, which was published yesterday in Nature Geoscience and is available here (possibly paywalled for those outside the U or without a subscription to Nature), explores a possible link between the stratosphere and Atlantic climate variability.

To learn more, see the paper or this press release from the University of Utah.

A Fly in the Ointment

A high-amplitude upper-level ridge remains parked over western North America, but as shown in yesterday afternoon's 0000 UTC 23 Sep 500-mb height analysis, there is a potential fly in the ointment for Utah weather.

Source: NOAA/ESRL

The fly is the weak closed low that is sitting over the coast of Washington, which results from a weather feature that meteorologists call a coherent tropopause disturbance, or CTD.  A CTD is a localized cyclonic vortex, which is associated with counterclockwise rotation in the Northern Hemisphere and is strongest at the tropopause.  The tropopause separates the stratosphere from the troposphere.

Source: NASA

The CTD and closed low move into Nevada by tomorrow morning [1200 UTC (0600 MDT) 24 September].

A different perspective at this time is provided below by a 3-D colored representation of the tropopause, with cool colors indicating a lower tropopause and warm colors indicating a higher tropopause.  Note how the tropopause is locally depressed where the CTD and closed low are found over Nevada.

Next, we have a vertical slice taken through the CTD the line that bisects the image above.  In this slice, the tropopause is indicated by a red line.  Note how the tropopause is locally low just left of center where the vertical slice cuts through the CTD.  

The other lines are temperature contours.  Note how they dip down near and beneath the CTD, which is an indication of colder air aloft.  

Thus, as the CTD approaches Utah, temperatures in the upper-troposphere will fall.  This, combined with large-scale rising motion ahead of the CTD, will act to destabilize the atmosphere and initiate some showers and thunderstorms.  As a result, although we are still under the influence of a large-scale upper-level ridge, we have a chance of some isolated showers and thunderstorms today and scattered showers and thunderstorms tomorrow.  

Arctic Amplification and Midlatitude Weather

In the previous post, we discussed how the summer sea-ice extent is declining in the Arctic.  This sea-ice decline is simultaneously a result of and a contributor to enhanced warming of the Arctic relative to other regions, which is sometimes referred to as Arctic Amplification.  Arctic Amplification is produced by increased greenhouse gas concentrations combined with feedbacks such as increased absorption of solar radiation due to declining snow and ice cover.

An area of growing scientific interest concerns the implications of Arctic Amplification for midlatitude weather.  More rapid warming of the Arctic compared to the lower latitudes leads to a decrease in the average temperature contrast in the midlatitudes.  The strength of the upper-level flow is proportional to this temperature contrast.  Therefore, one would expect upper-level flow to weaken (on average) in response to Arctic Amplification.  

Recently, Francis and Vavrus (2012) proposed that this effect could contribute to more persistent weather patterns in the midlatitudes.  In particular, as the upper-level flow weakens, waves (i.e., troughs and ridges) tend to progress more slowly eastward.  They also suggest that the amplitude of these waves has increased due to Arctic Amplification, with upper-level ridges extending farther into the high latitudes, leading to higher amplitude upper-level waves that tend to move more slowly.

How the midlatitude flow will respond to global warming is a critical issue for projecting future regional climate change.  There is arctic amplification, but also other aspects of the climate system that are changing in ways that could affect the midlatitude flow.  This is an area of growing interest and research and fertile ground for motivated graduate students looking for a good thesis project.