https://link.springer.com/book/10.1007/978-3-031-32273-0?sap-outbound-id=14802A873BE175AE2B17913C3342C49392C8A725&utm_source=standard&utm_medium=email&utm_campaign=000_LAN36_0000019083_Book%20author%20congrats%20NEW&utm_content=EN_34155_20230927&mkt-key=8D09219F09111EDE96CA003253E6CC10

The chart below (from climate.gov) shows the average impacts of a La Niña.  In some places, conditions correlated well. For example, the Pacific Northwest, winter and spring conditions were cooler and wetter than normal.  For Seattle, temperatures from December 2021 through May 2022 were nearly 3ºF below normal. The nearly 36 inches of rain exceeded the normal by over five inches.

Looking at the weather from Los Angeles International Airport, we can see that December 2021, did not correlate to the warmer and drier conditions expected during a La Niña for southern California. Temperature was 3.3ºF below normal while the 8.24 inches of rain exceeded the normal by six inches. Starting in January, however, we saw the warmer dry conditions normally expected for Southern California during a La Niña. Temperatures averaged 1.4º F above normal and rainfall totaled 1.46 inches, well below the normal of 8.46 inches.

Unusual for a La Niña year, unprecidented heatwaves occurred in North America, China, the European continent and the UK where their hottest day on record with a temperature of 104º F was recorded. See  https://earthobservatory.nasa.gov/images/150152/a-july-of-extremes for more detail.

La Niña is a cooling of the equatorial Pacific and is based on water temperatures between 5°North latitude and 5°South latitude. A La Niña is characterized as weak (temperature 0.5°C to 0.9°C below normal) moderate (temperature 1.1°C to 1.4°C below normal), or strong (temperature 1.5°C to 1.9°C below normal). These anomalies must be persistent over several months. The NOAA depiction (above) gives a general idea of what may be expected. Statistically, La Niña results in cooler and wetter conditions during winter than normal in the Pacific Northwest and warmer and drier than normal conditions across the southern tier of the United States.

On the above chart for September 10, 2020, we can see very strong negative anomalies in parts of the Equatorial Pacific.

La Niña and snow in the Pacific Northwest (PNW)

Taking a look at snow in the Pacific Northwest (PNW) – we see that average snowfall at SeaTac Airport has decreased substantially over the past 70 years. We can examine two periods of 32 years each – the winter of 1949-50 through the winter of 1979-80 and the winter of 1980-81 through 2019-20. (Snowfall was not reported at SeaTac from the 1996-97 through the 2003-04 seasons). For the earlier period, average snowfall was nearly 15” per year. For the latter, it was less than 8”per year. The contrast is even more striking when we look at the seasons from 1949-50 through 1971-72. That 24 year period saw an average of over 17” of snow per year with eight seasons reporting over 20”. For the next 40 years of record there were only three seasons that exceeded 20”.

Of the years with snowfalls greater than 20”, five of them occurred during periods of weak or moderate La Niñas including the two winters when SeaTac reported over 60” of snow (1949-50 and 1968-69). During 13 other La Niñas, seasonal snowfall remained below 20”. So, we can see that, at best, La Niñas are only weakly associated with snowy winters.

Other factors that influence seasonal snowfall in the PNW include the rise of average winter temperature – about 3°Fahrenheit since the 1950s and the Pacific Decadal Oscillation which deserves a discussion of its own.

September 2020 ENSO update: La Niña is here! | NOAA Climate.gov

One important global impact of La Niña is its effect on the Atlantic hurricane season.La Niña reduces wind shear—the change in winds between the surface and the upper levels of the atmosphere—allowing hurricanes to grow.The likelihood of La Niña was factored into NOAA’s August outlook for the Atlantic hurricane season, which favored an “extremely active” season.

www.climate.gov

As we get ready for the cold and possible snow, it is interesting to review past winter weather in Seattle. Statistics from SeaTac Airport are the most comprehensive, so let’s take a look at those. Temperatures in Seattle have clearly moderated in recent years. The lowest temperature ever recorded at SeaTac Airport was 0⁰F degrees on 31 January 1950. In fact, 1950 had eight of the 11 coldest days in SeaTac history with low temperatures ranging from 0⁰F to 7⁰F. The last time SeaTac recorded a temperature below 20⁰F was a 19⁰F reading that occurred in December 2013. It is possible that the coming cold spell will see the coldest temperatures of the last decade.

But what about the possibility of significant snow this coming week? That is a tricky question.  Historically, there have been some very snowy months in Seattle history. The snowiest month ever was January, 1950 when 57.2 inches of snow fell. In the period December 1968 to January 1969, a total of 67.5 inches fell. Just last February 20.2 inches of snow accumulated, making February, 2019 snowiest month since January, 1969.  

Although the recent forecast models that have been consistent in predicting very cold temperatures, the timing and amounts of any moisture  are unclear. For snow, not only do we need cold air, but we also need a moisture source. Frequently, an intense but short-lived snow event accompanies the initial outbreak of cold air from Canada’s Fraser River Valley. This is a possibility, for at least part of the region, Sunday night. The forecast models are indicating the possibility of moisture coming into Puget Sound by late Wednesday (15 January). We shall wait and see.

We have heard a lot about global warming in the past few years. Then, in mid-November 2019, we saw cold temperature records shattered in much of the Central and Southern United States.  Concurrently, parts of Asia and much of Eastern Europe were experiencing abnormally high temperatures.  How can we put this all in perspective with respect to global climate change? Here is a little background.

Late in the year 406 A.D. over a frozen Rhine River, many accounts state that Germanic tribes crossed into Gaul. This was the initial invasion into the Roman Empire, that would eventually lead to the Empire’s fall. If the Germanic tribes had waited 100 years, it would have been more difficult as the Rhine then rarely froze solid. Climate had changed and temperatures were rising.

By the year 800 A.D. what is known as the Medieval Warm Period had started. Mild temperatures prevailed until about 1300A.D. In Europe, crops thrived and populations increased. The Vikings colonized Iceland and Greenland. Then things again began to change. Beginning around 1400 and lasting until the middle of the 19^th century, a cool period set in. New crops were devised to survive the cold and the Dutch began reclaiming land from the sea to provide more land for raising crops. This period was known as the Little Ice Age. It reached its peak from the years 1645 until 1715.  Glaciers advanced and destroyed alpine villages in France and Switzerland. The extreme cold and advancing glaciers were associated with what is known as the Maunder Minimum in sunspot activity.

Solar temperature is directly related to sunspot activity. With a warmer sun, we see more sunspots. Most scientists believe both the Medieval Warm Period and the Little Ice Age resulted from changes in the sun’s radiation, associated with the number of sunspots. Other contributions may have come from the level of volcanic activity. When a strong volcanic eruption occurs, sulfur dioxide expelled into the stratosphere may form droplets of sulfuric acid. These droplets help block incoming solar radiation. During the Little Ice Age, for example, the sun was colder than normal and volcanic activity was higher.

We must consider that the climate is always changing. To reflect this, the National Climatic Data Center recalculates precipitation and temperature averages every ten years using the previous 30 years’ data. For example, in 2011, climatic averages were calculated using data from 1981 through 2010.  In 2021, they will be recalculated again using data from 1991 through 2020.

Many scientists believe the current warming is due to an increase in greenhouse gases such as carbon dioxide and methane.  However, the sun reached a peak in warmth during the 20^th century that was comparable to the warm sun of the Medieval Warm Period.  For the past four solar cycles, each lasting about 11 years, the sun has been cooling, and today the sun is as cool as it was over 200 years ago. Some scientists think this solar cooling will offset any anthropogenic (human-caused) effect we are currently seeing and may put at least a temporary halt to global warming.  

So what does the future hold? We know the effects of a cooler sun are delayed, but we should be seeing them soon. Will it be enough to halt, or even reverse, global warming? The answer remains to be seen, but any effect will last only as long as the sun remains cool.

Sunspots picture (https://www.nasa.gov/mission_pages/sdo/images/index.html)

https://eos.org/features/how-sudden-stratospheric-warming-affects-the-whole-atmosphere

The authors  (Pedatella et. al.) describe how an SSW can move down from the stratosphere and affect the troposphere – depressing the tracks of extra tropical cyclones southward in the Northern Hemisphere. This can result in very cold temperatures and heavy snow in Eastern North America. The interaction of a SSW with the troposphere is not completely understood.  These SSW events occur about six times in a decade.

We have learned a lot in recent years about atmosphere-ocean interactions and about stratosphere-troposphere interactions. However, there remains much more to learn and understand about how the weather on Earth responds to these various phenomena.