1:00 PM | Stratospheric warming event may signal colder pattern ahead

Polar view of current stratospheric (10-millibars) temperature pattern and the 9-day forecast using the 00Z GFS computer forecast model; courtesy NOAA

Overview

One of the ways to monitor the potential for Arctic air outbreaks in the northern U.S. is to follow what is happening in the stratosphere over the polar region of the northern hemisphere. Sudden stratospheric warming (SSW) events are large, rapid temperature rises in the winter polar stratosphere occurring primarily in the Northern Hemisphere.  They have been found to often set off a chain of events in the atmosphere that ultimately leads to Arctic air outbreaks from northern Canada into the central and eastern U.S. Indeed, there appears to be a stratospheric warming event in the offing over the next week to ten days (above) centered near the North Pole that provides some reason to believe that the current warm temperature pattern in the eastern US could change during the month of January.   

Stratospheric warming

During the winter months in the lower polar stratosphere, temperatures are typically lower than minus 70° Celsius (purple area above). The cold temperatures are combined with strong westerly winds that form the southern boundary of the stratospheric polar vortex. The polar vortex plays a major role in determining how much Arctic air spills southward toward the mid-latitudes. This dominant structure is sometimes disrupted in some winters or even reversed. Under these circumstances, the temperatures in the lower stratosphere can rise by more than 50°K in just a few days. This sets off a reversal in the west-to-east winds and the collapse of the polar vortex. In response to the stratospheric warming at the high latitudes, the troposphere in turn cools down dramatically and this cold air displacement is then transported from the tropospheric high latitudes to the tropospheric middle latitudes. The entire process from the initial warming of the stratospheric at high latitudes to the cooling in the troposphere at middle latitudes can take weeks to unfold.

Comparison to last year’s stratospheric temperatures

The plot (below) shows the stratospheric (10-millibar) temperature pattern for 2014 and 2015 in the high latitude region of 90°N to 65°N. Interestingly, last winter season featured a sharp spike in stratospheric temperatures at just about this same time of year (i.e., mid-to-late December as indicated by the arrow below) and the following couple of months were indeed quite cold in the central and eastern US.  Some stratospheric warming events in recent decades have, in fact, been followed by widespread very cold air outbreaks across southern Canada and the US some two to three weeks after the initial upper atmosphere warming. For example, such an event occurred during December 1984 and this led to an extremely cold January 1985 in much of the central and eastern US. A detailed video discussion on “Stratospheric Warming” can be found on the “Meteorology 101” page of the Vencore weather web site.  We’ll continue to closely monitor temperature changes in the stratosphere over the North Pole during the next few weeks to see how this warming event unfolds. 

Stratospheric (10-millibars) temperature plot for 2014 and 2015 in high latitude region of 90°N to 65°N; courtesy NOAA

Meteorologist Paul Dorian

Vencore, Inc.

9:30 AM | Global sea ice fights its way back to near normal after sharp drop earlier this year

Overview

Strong El Nino conditions continue to exist in the tropical Pacific Ocean as we approach the end of 2015 and this natural oceanic phenomenon ranks as one of the three strongest during the past 50 years.  In fact, this El Nino (warmer-than-normal sea surface temperatures) is likely near its peak intensity level and it has already had major ramifications around the world.  For example, global temperatures have spiked to well above-normal levels and this temperature pattern is likely to continue well into 2016.  By late 2016, however, it is looking more and more likely that El Nino will completely reverse into La Nina (colder-than-normal) conditions and – if history is any guide - global temperatures are then likely to drop sharply. 

In addition to its impact on global temperatures, the on-going strong El Nino event in the tropical Pacific Ocean has no doubt played some role in the sharp drop of global sea ice extent in recent months despite the fact that it is far removed from the polar regions.  However, as we approach the end of the year - and despite the continuing strong El Nino event - global sea ice has actually fought its way back to near-normal levels – much as it was during the past couple of years.

red line represents global sea ice areal extent compared to the 1979-2008 normal as represented by the “zero” line; data courtesy University of Illinois "cryosphere"; NOAA/NCEP Snow and Ice Data Center

Southern Hemisphere

After soaring to near record high levels during the past couple of years, sea ice in the Southern Hemisphere dropped sharply recently in areal extent from way above-normal amounts to below-normal levels.  That drop to below-normal, however, has not lasted too long.  In the past several weeks, sea ice has actually increased in areas surrounding the continent of Antarctica relative-to-normal and it is now once again running at above-normal levels (plot below), but not nearly as high as it was about one year ago.  Southern hemisphere sea ice recently stayed above-normal for a period of more than three years, from 2012 to 2015. This is something that had never happened before during the satellite period dating back to 1979. 

Southern Hemisphere sea ice anomaly since 1979; data courtesy University of Illinois “cryosphere”

Northern Hemisphere

The northern hemisphere sea ice areal extent is still below-normal relative to all years going back to 1979 although it is well above the lowest point set during 2012 - and even above levels seen earlier this year. The northern hemisphere sea ice areal extent is currently 742,000 square kilometers below the mean using the base period of 1979-2008 for comparison (plot below). The northern hemisphere sea ice areal extent has leveled off in the past 10 years or so at below-normal levels after dropping consistently from the mid 1990’s to the middle of the last decade. In the time period before the mid 1990’s, the sea ice areal extent was generally above-normal dating back to 1979. 

Northern Hemisphere sea ice anomaly since 1979; data courtesy University of Illinois “cryosphere”

The directional shift in the sea ice areal extent trendline that developed during the mid-1990’s in the northern hemisphere correlates quite well with a northern Atlantic Ocean sea surface temperature cycle that is tracked by meteorologists through an index called the Atlantic Multidecadal Oscillation (AMO). Indeed, Atlantic Ocean sea surface temperature anomalies play a critical role in the overall northern hemisphere sea ice areal extent. The AMO index flipped in phase during the mid-1990’s from negative (cold) to positive (warm) and the sea ice areal extent trendline changed direction right around that point in time.

The Atlantic Ocean is just recently showing signs of a long-term temperature phase shift back to cold and if that trend continues over the next couple of years then the northern hemisphere sea ice areal extent is quite likely to return to the same above-normal levels that were seen prior to the mid 1990’s. (Click here for more information on the potential significant long-term temperature phase shift in the Atlantic Ocean: http://vencoreweather.com/2015/03/22/1230-pm-the-atlantic-ocean-is-showing-signs-of-a-possible-significant-long-term-shift-in-temperatures-from-warm-to-cold/). 

In fact, the northern hemisphere sea ice has actually shown great resiliency in recent years and has rebounded to the highest levels seen during the past ten years according to recent data from the Danish Meteorological Institute (plot below, http://ocean.dmi.dk/arctic/old_icecover.uk.php ).

Northern hemisphere sea ice extent for 2015 (black line); Note - the sea ice extent here is calculated with the coastal zones masked out.

Meteorologist Paul Dorian

Vencore, Inc.