Angela's Blog

Winter Solstice: A Time to Celebrate

By: angelafritz, 7:23 PM GMT on December 21, 2011

Tomorrow, December 22nd, is the Winter Solstice: the shortest day of the year, and what we like to refer to as the first day of winter.

But what does that really mean? Hasn't winter already started? In the U.S., it came in with a bang but has since tapered off to a whimper. One week ago, on December 14th, the only place that fell below zero was Big Piney, Wyoming with a low of -2°F. That's pretty mild for December in the lower 48!

But in Europe, winter has already taken its toll. Last week, winter storm "Joachim" tore through western Europe, leaving as much as 5 feet of snow in its wake. Joachim's lowest central pressure got down to 963.8mb, and wind gusts up to 105mph (175 kph) were recorded.

From Christopher C. Burt's blog on Joachim:

Widespread wind damage in northern France brought down power lines resulting in 400,000 homes losing electricity. A large Maltese cargo ship, the TK Bremen, was washed ashore by 25-foot English Channel seas landing on the coast of Brittany. The crew was safe but some 200 tons of fuel oil leaked from the vessel.

So why do we wait so long to declare winter?

The first day of "meteorological winter" is December 1. Meteorologists like to break the year up into three-month chunks, which happen to coincide with the four seasons. Winter, for us, is December, January, and February. But traditionally, the rest of the world declares that winter begins on this, the shortest day of the year.

At 12:30am ET (5:30am GMT) on December 22, the sun is directly over the Tropic of Capricorn, which is 23.5 degrees south of the equator. For the Northern Hemisphere, the sun on this day is at the lowest point from in the sky for the entire year. The word "solstice" comes from the Latin word "solstitium," which literally means "the sun is standing."

During the Northern Hemisphere's Winter Solstice, the sun is centered over the Tropic of Capricorn—23.5° south. (Image source: Wikipedia)

For places in the Southern Hemisphere like Australia, Chile, Brazil, and southern Africa, it's the longest day of the year, and they're (usually) basking in warm summer weather.

But for us in the Northern Hemisphere, we're trying to get through a brutal winter that's only just started. Although the days will get longer from now until June, we won't see the temperatures and snow turn the corner until March. That's because temperature tends to lag the amount of sunlight we receive. The lowest temperatures are recorded about a month after the winter solstice, and the highest temperatures about a month after the summer solstice in the third week of June.

It's no surprise that many of our most important festivals and celebrations fall close to the winter solstice. It's the time when the nights get shorter and the days get longer, and culturally it symbolizes rebirth. Today begins the trend of longer days, shorter nights, and the migration of the Sun back to the Northern Hemisphere.

So, there's something to help you get through the rest of winter. Summer is just around the corner.


Winter Weather Atmospheric Phenomena

Updated: 12:47 AM GMT on December 22, 2011


Tales From AGU: 2011 Edition

By: angelafritz, 4:24 PM GMT on December 12, 2011

I attended the fall meeting of the American Geophysical Union last week—the world's largest gathering of earth scientists. This year there were 20,000 attendees, 12,000 poster presentations, 6,000 oral presentations, and 250 exhibitors, along with workshops, town halls, and social/networking events. It's always inspiring to be surrounded by so many brilliant minds, and I try to absorb as much as possible every year. This year was no exception: over 4 days I attended dozens of talks on climate change, hurricanes, cloud physics, and science communication. Those subjects don't even scratch the surface of all that the AGU Fall Meeting has to offer: ocean sciences, rock physics, bio-geoscience, seismology, and planetary science, to name just a few others. Here are a few of the talks I attended:

Evidence for an anthropogenic contribution to recent ocean-driven ice losses in West Antarctica
Dr. Eric Steig (Atmospheric Sciences, University of Washington)

Eric Steig (Earth and Space Sciences at U. Washington, as well as gave a talk on Tuesday that presented evidence for a manmade contribution to ice loss in West Antarctica. The ice sheet and glaciers in West Antarctica, like the Pine Island glacier and the Thwaites glacier, have been melting rapidly into the Amundsen Sea—an arm of the Southern Ocean, which surrounds Antarctica. Scientists are very concerned about this because of the potential sea level rise that will result from melting so much ice into the ocean. It's well-established that this process is speeding up due to the intrusion of warm water underneath the glaciers' ice shelves, which melts the glacier at a critical point where land ends and ocean begins. The ocean floor in this region is situated in such a way that it is particularly susceptible to the relatively warm Circumpolar Deep Water (CDW) that flows around the continent of Antarctica.

Previous research has suggested that changes in the winds that blow around Antarctica have allowed or forced the warm deep water into the Amundsen Sea, speeding up the melting process. However, Steig argued that the cause is a little farther away than that, in the tropical Pacific, which, in one way, is linked to the Southern Ocean via the atmosphere. He showed that there is a significant link between sea surface temperature in the central tropical Pacific and the winds that flow over the region around the Amundsen Sea. Since we're pretty sure that manmade warming is the reason water is getting warmer in the central Pacific, it's likely that we can also link the manmade global warming to the melting in the West Antarctic.

A New Estimate of the Earth’s Land Surface Temperature History (Berkeley BEST)
Dr. Richard Muller (Physics, University of California at Berkeley) / Dr. Robert Rohde (Novim Group)

Berkeley's BEST temperature project was represented at the meeting by Dr. Robert Rohde, who is the lead scientist on the project under the founder and director, Dr. Richard Muller. Rohde was the lead author on the paper describing the project's averaging process.

This talk presented the results of the BEST study and the method, which I covered in a blog last month. But at the end of the presentation, Rohde showed this simple and visually pleasing animation of the BEST temperature reconstruction. Watch as temperature stations come online and the temperature anomalies (difference from average) increase with time:

Assessing hurricane vulnerability changes arising from climate variability and change
Dr. Greg Holland (NESL, NCAR)

Dr. Holland spoke about defining and quantifying hurricane risk, and considered whether this risk would change for some communities in a warming environment. A community that is susceptible to natural disasters, like hurricanes, needs to understand its vulnerability. Dr. Holland argued that vulnerability isn't the right word—they should consider "survivability." For example: which conveys the message you're trying to relay? "Are you vulnerable to this hurricane?" or "Can you survive this hurricane?" The next step is to define the hazard. In the case of hurricanes, the hazard is wind, waves, surge, flood, etc. Most people say hurricane intensity is the main cause of the hazard, when in reality this is not the case. How large the hurricane is (its radius, for example) and how fast the hurricane is moving (its translational speed) are actually more valuable indicators of hazard than intensity. Larger hurricanes tend to have bigger storm surges and produce more rain. Hurricanes that move faster tend to do less damage than the ones that are slow and "linger" over an area. Most people in the hurricane world have understood this for a while, but the question Holland raised in his talk is "how will these things change in the future, and how does that change the risk?" If we're assuming there will be a 5mph increase in hurricane intensity, on average, how does that change the risk? According to Holland, it doesn't, because intensity is not where the risk lies. He argues that if, in a warming world, hurricanes shrink in size and move faster, the risk is actually reduced. However, the question of what will actually happen to hurricanes in a warming world is still up for debate.

Hurricanes in a Warming Climate
Dr. Kerry Emanuel (Earth, Atmospheric, and Planetary Science, MIT)

Dr. Kerry Emanuel gave a good talk on tropical cyclones in a warming climate. We know that power dissipation (the potential destructiveness of a hurricane) and sea surface temperature are well-linked. Power dissipation has doubled in the last 30 years, and it's also linked to northern hemisphere air temperature on a decadal time scale. Potential intensity, or the maximum potential wind speed a hurricane can achieve given its environment, has also increased in the last 30 years, by 10% at least. Dr. Emanuel did an interesting analysis using the potential intensity theory where he shows that if Katrina had happened in 1980, it would have peaked at a category 4, not a category 5. An interesting question: would that have been enough to prevent the levees from breaking? Outflow temperature of hurricanes, the "exhaust" of the hurricane at the upper levels of the atmosphere, has cooled in the last 30 years, as well. In order to strengthen, a hurricane needs a cold upper atmosphere just as much as it needs a warm lower atmosphere. These "cold upper atmosphere" trends have not been forecasted in the global climate models, so arguably, they cannot forecast a future trend in hurricane strength.

Hurricane Katrina on August 28, 2005. Source: NOAA

Evaluating tropical cyclone genesis in four global models
Daniel Halperin (Earth, Ocean, and Atmospheric Science, Florida State University)

Daniel Halperin from Florida State University presented an evaluation of tropical cyclone genesis in four models from 2004 to 2010: the CMC, GFS, UKMET and NOGAPS. Unfortunately he did not include the ECMWF in the study, though we know that it would have out-performed the four included here. He used common evaluation metrics: hits and false alarms. The idea here is that you want the model to have a low false alarm rate (forecasting a hurricane when it's not going to happen) and a high hit rate (correctly forecasting a hurricane).

The Canadian CMC has improved since 2004, but still has a ~55% false alarm rate as of 2010. Its worst year was 2007, when nearly 80% of its forecasts were false alarms.

The GFS has improved markedly, especially since 2007 when it had almost a 90% false alarm rate.
In 2010, that was down to ~38%, and their "correctly predicted" rate is up to almost 50% from 10%. However, spatially, Halperin shows that the GFS misses almost all tropical cyclones that form in the Gulf of Mexico. As is expected, it performs best in the central Atlantic, where tropical cyclones develop from waves that track across the ocean from Africa and are relatively easy to pick up on.

NOGAPS false alarms are down to around 50% from nearly 100% in 2004-2006, but their rate of correct forecasts is still pretty low, just 12% in 2010.

The UKMET has been consistent since 2004, with false alarms hovering around 50%, and correct forecasts steadily increasing from 20% in 2004 to 45% in 2010. However, like the GFS, the UKMET misses almost all tropical cyclones that form in the Gulf of Mexico.

In summary, overall, the GFS and the UKMET are performing best out of the four models tested in this study.


Another big theme at this year's conference was science communication. I'll be back some time this week with a post on that, which will include the 2011 update to the "six Americas."


Hurricane Climate Change Extreme Weather


Global Warming Has Pushed the Arctic into a “New Normal,” Report Says

By: angelafritz, 5:35 PM GMT on December 02, 2011

By Andrew Freeman
Climate Central

Since 2006, the Arctic has been less Arctic — warmer, and with less snow and ice than the region used to have — according to the latest comprehensive analysis of the Arctic environment released today by the National Oceanic and Atmospheric Administration (NOAA).

This doesn't come as a huge surprise. Scientists have long expected that greenhouse gases would warm the Far North up faster than other parts of the globe since various feedback cycles unique to the Arctic can magnify relatively small temperature changes (melting ice and snow, for example, let exposed land and water absorb more of the Sun's heat, which melts more ice and snow, and so on). This "Arctic amplification" is one reason why the polar bear, which relies on sea ice for survival, has been the enduring symbol of global warming activism.

arctic warming
This past year (October 2010-September 2011), surface temperatures in the Arctic were 1.5 degrees Celsius warmer than average. The image above shows where average air temperatures were up to 3 degrees Celsius above (red) or below (blue) the long-term average (1981-2010). Credit: NOAA

Cute as they are, though, polar bears don't have much to do with the lives of most Americans, so using them as a global-warming mascot sends the message that it's happening far, far away.

But as the new NOAA report makes clear, it isn't. More and more, what happens in the Arctic isn't staying in the Arctic. One of the more intriguing findings of the 2011 Arctic Report Card concerns what some scientists are referring to as the "warm Arctic/cold continents" climate pattern, featuring winds that drive warmer air into the Arctic, while displacing frigid Arctic air masses to the south, into the US and Europe.

When this pattern occurred during December 2010, it contributed to freak snowstorms in the eastern US and western Europe. In western Greenland, meanwhile, and in other parts of the Arctic, temperatures were above average. At Climate Central we like to refer to this pattern as the "Arctic Paradox." We even made a nifty graphic to describe it.This climate pattern may be triggered in part by the loss of summer sea ice in the Arctic, since this alters the exchange of heat and moisture between the Arctic Ocean and the atmosphere.

According to the Report Card, the five deepest summer meltbacks in sea ice in the satellite record, which extends back to 1979, all occurred during the past five years. In 2011, sea ice extent at the end of the melt season was the second-lowest on record, while sea ice volume (that is the ice's extent times its average thickness) set a new record low.

arctic paradox

Jim Overland, an oceanographer at NOAA's Pacific Marine Environmental Laboratory, notes that during the 2011 melt season, 35 percent of the Arctic sea ice area was lost, and this is only expected to get worse in coming decades as the climate continues to warm. "We're starting to see modifications of weather patterns... it's a major research question over the next few years of how the climate will change overall [in response to sea ice loss]."

According to the report, the warm Arctic-cold continent climate pattern existed in late fall 2010 and part of last winter. During both periods, the report says, "an increased linkage between Arctic climate and mid-latitude severe weather occurred."

The report warns that it's very tough for the Arctic climate system to reverse course from the accelerated warming and melting path that it is on right now. "Once multi-year sea ice and glacial mass is gone," it says, "it is difficult to return to previous conditions."

The peer-reviewed report, which is the result of an international collaboration among 121 researchers from 14 countries, contains many other findings that point to the rapid pace of climate change in the region. Here are just a few of them:

  • In 2011, the average annual near-surface air temperatures over much of the Arctic Ocean were approximately 2.5°F greater than they were in the period from 1981-2010.

  • Starting on June 30, Barrow, Alaska had a record 86 consecutive days above freezing. The previous record was set in 2009, with 68 days.
  • For the sixth straight year, Fairbanks in central Alaska had its first freeze more than two weeks later than the long-term average.

  • The mass loss from Greenland's ice sheet  in 2011 was 70 percent larger than the 2003 to 2009 average of 250 gigatons per year. (Melting of the Greenland ice sheet contributes to sea level rise, whereas melting of sea ice, which is already floating, does not.)

Climate Change Sea Ice Extreme Weather


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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Angela's Blog

About angelafritz

Atmospheric Scientist here at Weather Underground, with serious nerd love for tropical cyclones and climate change. Twitter: @WunderAngela

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