Saturday, February 6, 2016

January Arctic Warmth

A blog post by Weather Underground's Bob Henson caught my eye on Thursday, as he discussed the extraordinary warmth that occurred in the Arctic basin during January.  To put this in context, I calculated the 60-90°N area-average mean temperature at 925mb (about 2200' elevation) for each January since 1950, according to the NCEP/NCAR reanalysis; the results are shown below.  The temperatures last month set a new record for January warmth in the reanalysis era, by a large margin.


It's interesting to see that a similar kind of outlier (for the time) occurred in January 1977, which is the winter that we've recently discussed as being very warm in Fairbanks.  January 1977 occurred just after the "great Pacific climate shift" and the onset of what turned out to be an enduring positive phase of the PDO.  One wonders if this means that the next decade or two will see a continuation of the positive PDO phase that we've observed in the past 2 years: have we just passed through another major Pacific climate shift?  From an Alaska-centric viewpoint, have we shifted into a new normal of winter warmth, as happened in the decades following 1976?  Let's hope not.

Looking at daily 925 mb temperatures since the beginning of 2015, the Arctic (and sub-Arctic) area has seen persistently above-normal temperatures, but since late December the area-average temperatures have been near or above previous record levels for the time of year.


On the chart below I've added the daily temperatures for the past two very strong El Niño events, 1997-98 and 1982-83, and we can see that neither of those winters produced unusually warm conditions over the Arctic, at least through January.  This suggests that we shouldn't pin the blame for the recent warmth on El Niño.  It has more to do with the transition to a strongly negative Arctic Oscillation at the beginning of January, which allowed cold air to spill south into the mid-latitudes and, conversely, warm air to invade the Arctic from the south.  I believe there is also a close connection to the stratospheric polar vortex, which was extremely intense in November and December, but began to undergo a weakening trend in January.


The following maps show the January temperature's departure from normal beginning at the surface, moving up through the troposphere (up to 300mb) and into the stratosphere.  There is a dramatic transition from extremely warm conditions in the lower-to-mid troposphere to unusually cold conditions in the stratosphere; note that I've doubled the range of the scale on the last two maps to accommodate the magnitude of the cold anomaly aloft.  The cold in the stratosphere was associated with the strength of the polar vortex, which remained much greater than normal in January.











Wednesday, February 3, 2016

Lack of Cold and Low Variance

There have been a few mentions recently of the lack of 30-below temperatures so far this winter in Fairbanks; the coldest temperature observed at the airport was -29°F on Christmas Day.  If this remains the coldest temperature of the winter, it would be almost unprecedented for lack of cold: in the past, only the winter of 1976-77 failed to reach -30°F (lowest temperature -28°F).  The lack of cold is similarly unusual if we look at the 850mb level aloft, where this winter's coldest temperature as measured by balloon sounding was only -14°F on November 18.  Only once before, in 2000-2001, did the Fairbanks soundings fail to measure a colder 850mb temperature at some point in the winter.

How likely is it that -30°F will be reached at the airport in the remainder of winter?  Based on the last 40 years of historical data, more than 70% of years reach -30°F on or after February 5, and the odds don't drop below 50% until February 22.  Even in March it's not too uncommon - in fact, 4 of the last 5 years have seen -30°F or colder in March.  However, the medium-range forecast is quite warm (see below), and of course the ongoing El Niño and positive PDO phase suggest that warmth will continue to dominate.

 



Despite the overall very warm pattern and lack of cold conditions this winter, the number of very warm days has not been particularly unusual.  Since November 1, the temperature has risen above freezing on only 5 days in Fairbanks, which is only slightly above the long-term normal of 4 days (in the period Nov 1 - Feb 2), and far below the record of 19 days in 1936-37 (Nov 1 - Feb 2).  In other words, the variance of temperature has been low, which is a characteristic of strong El Niño winters.

The chart below shows how the January-March standard deviation of daily mean temperature anomalies (departure from normal) varies with an index of El Niño/La Niña behavior.  The standard deviation is quite noticeably reduced when the ENSO index is above +1, so there's little doubt that the El Niño episode is contributing to the lack of variability in Fairbanks this winter.


Fairbanks winter temperature variance is similarly affected by the PDO phase (which is of course correlated with ENSO), and if we create a combined index of PDO+ENSO behavior, the overall correlation with the variance is slightly greater than for ENSO alone.

Here's a chart showing how the ENSO/PDO effects on temperature variance change through the year; each column represents the correlation coefficient for a three-month period.  Interestingly the variance reduction for El Niño is slightly greater in late winter than early winter, which probably reflects the fact that El Niño's impacts on the atmospheric circulation reach their peak in late winter.  The opposite effect is observed in late summer and autumn, with temperature variance being somewhat enhanced during El Niño conditions and reduced during La Niña.


Saturday, January 30, 2016

Exceptionally Dry

Ever since the persistent snows of November ended about 2 months ago, the weather has been very dry indeed in Fairbanks.  Through yesterday with its meager 0.2" of snow, the December-January period has seen only 1.9" of snowfall and 0.08" of liquid-equivalent precipitation.  If this stands, it will be the driest December-January period on record (1930-present); normal snowfall and precipitation are 19.3" and 0.98" respectively for this period.  For comparison, the last 2 winters saw 16.8" and 19.8" of snow in December through January.

However, if we look beyond just calendar months, there have been a couple of occasions when even less snow fell over the same length of time during winter; mid-winter 1952-1953 and early winter 1953-1954 both had 62-day spells with only 1.4" of snow.

I thought it would be interesting to look at the upper-level flow patterns that typically accompany very dry or very wet conditions on a monthly time scale in Fairbanks.  This is a similar analysis to the one I did for extreme daily temperature anomalies, see here.

The maps below show the average 500mb height anomalies that accompanied the 8 driest (left) and wettest (right) instances of each calendar month in Fairbanks since 1950.  In winter the driest conditions in Fairbanks tend to be associated with unusually low pressure to the south of Alaska, which reduces the strength of the westerly flow aloft and prevents weather disturbances (fronts and lows) from reaching the interior.  But it's very interesting to note that the height anomalies accompanying dry weather are less consistent and less pronounced than those accompanying wet weather.  My interpretation of this is that there is more than one way to get dry weather: very warm weather, with chinook flow, is dry, but very cold weather is dry as well.  Higher precipitation amounts tend to occur with near-normal temperatures, as we saw in this post.  Very wet weather, in contrast, tends to occur only one way in winter: when westerly flow is enhanced around an anomalous ridge to the southwest.


























So does the flow pattern in the past 2 months match the historical pattern for dry weather?  The map below shows the 500mb height anomaly since December 1; it closely resembles the dry weather pattern for December.

Unsurprisingly, the dry weather pattern is also quite similar to the typical pattern associated with El Niño.  The map below shows the mean height anomaly during 10 significant El Niño's, indicating that low pressure aloft in the Gulf of Alaska is the preferred circulation state.


Tuesday, January 26, 2016

More on Temperature Extremes

As a follow-up to Saturday's post, I put together an experimental graphic in another attempt to visualize how the frequency of warm and cold extremes in Fairbanks depends on the phases of ENSO, the PDO, and the North Pacific Mode (NPM).  The results provide an additional perspective and are quite informative, I think.

The charts below show the number of days each winter (65 winters, 1950-2014) in which warm (red) or cold (blue) extremes were observed in Fairbanks, with the positions of the markers indicating the winter mean phases of the oceanic temperature patterns.  As in the previous post, I used the 2nd and 98th percentiles of the daily temperature distribution as the thresholds for defining "extremes".  Note that 12 of the winters had no warm extremes, and 23 of the winters had no cold extremes; these winters are not plotted on the charts.



Some interesting aspects of the PDO-ENSO charts are that La Niña favors warm extremes (more than El Niño) when the PDO is negative, as I noted before; this continues to be a surprise to me.  Note the high overall frequency of extremes (cold or warm) when La Niña accompanies a negative PDO.  It's also interesting to see just how rare cold extremes are when the PDO is positive.

The PDO-NPM charts below show that a negative PDO with positive NPM is very favorable for cold extremes, but some of these winters also have a good number of warm extremes.  The most favorable phases for warmth are positive PDO with negative NPM; this differs from the positive PDO - positive NPM setup of the past couple of winters, which suggests that the PDO phase is a better explanation for the recent unusual warmth than the NPM.



Saturday, January 23, 2016

PDO, ENSO, and Temperature Extremes

I've recently been doing a bit of research to see how the phases of the PDO and El Niño/La Niña affect temperature extremes during winter in Alaska.  We've previously looked at changes in the distribution of seasonal mean temperatures, but the frequency of extremes is a different question that is very important for some applications.

I'll start with a map showing the frequency of warm and cold extremes during El Niño winters, regardless of the PDO phase - see the first image below.  I've defined the extreme thresholds as the 2nd and 98th percentiles of the daily historical distribution of daily mean temperature, so the thresholds are different for every day and are equally likely to be exceeded at any time of year.  The columns on the map show the frequency of exceedance for 22 winters with the highest November-March mean Oceanic Niño Index, i.e. the warmest one-third of winters since 1950 in the Niño3.4 region of the equatorial Pacific Ocean.  The heights of the columns show the exceedance frequency, with the horizontal dotted line indicating the long-term normal frequency of 2% (i.e. 3 days per winter).

As expected, warm extremes are more common than normal, and more common than cold extremes, during El Niño winters in south-central and southeastern Alaska.  This matches the sign of the seasonal mean temperature signal.  However, in western Alaska - and in Fairbanks - cold extremes are nearly as common as warm extremes, and neither is more common than normal, despite the fact that El Niño winters are more often warmer than normal overall.


If we look only at El Niño winters when the PDO is also in the top one-third of the historical range, the warm signal is much more widespread and is overwhelming in the south-central and southeast; cold extremes are almost unheard of when the oceanic temperature patterns show these anomalies.


In sharp contrast, when the PDO is either negative or near-neutral (i.e. bottom two-thirds of the historical range), El Niño winters generally fail to bring warm extremes, and cold extremes are quite heavily favored in most locations.  We see again that El Niño winters look vastly different depending on the PDO phase, and the difference is at least as dramatic for extremes as it is for seasonal mean temperatures.


Looking at the same analysis for La Niña winters, the results show that cold extremes are more common than normal and more common than warm extremes at every location; the cold signal includes western Alaska, unlike El Niño's warm signal.  However, it's interesting to note that the frequency of warm extremes is not reduced compared to normal from Anchorage to Barrow and throughout the interior; this is a result of increased variance during La Niña winters.


A negative PDO phase boosts the chances of very unusual cold conditions during La Niña, but the frequency of warm extremes still remains near-normal in the central part of the state; this is quite surprising, as I would expect these winters to be overwhelmingly cold.  It seems that, at least for the interior, a negative PDO is nowhere near as effective in amplifying La Niña cold as a positive PDO is in boosting El Niño warmth.


When La Niña is not accompanied by a negative PDO phase, the cold signal goes away and most locations see slightly higher chances of warm extremes than cold extremes.  Again it's notable that the PDO signal does not overwhelm the La Niña signal as it does for El Niño.  We might conclude that the PDO phase is less significant for Alaska's winter temperatures when La Niña is in play, and more significant during El Niño conditions; or alternatively we could say that La Niña is more reliably connected to unusual temperatures than El Niño.


Here is a corresponding set of maps conditioned on a positive or negative PDO phase, with or without El Niño and La Niña.  A noteworthy point here is that, during positive PDO winters, warm extremes are more common in the southwest (Bethel, St Paul, Cold Bay) when El Niño is not observed.  Similarly, during negative PDO winters, warm extremes are actually more common from Anchorage to Kotzebue and in the central interior during La Niña than during non-La Niña winters.








And finally, two maps showing the frequency of extremes for neutral PDO winters and neutral ENSO winters.  Notice that cold extremes are less common than warm extremes in many locations when the ENSO phase is near neutral.