Atmospheric Blocking in 2019: A Quiet Finish.

DISCUSSION: According to NASA and NOAA the year 2019 was the second warmest on record, coming in at a little more than 0.6o C above the 1981 - 2010 average. This year also began with El Niño conditions in place over the eastern Tropical Pacific Ocean. These conditions held well into the spring and early summer, and even into fall as the tropical Pacific was classified as warm neutral by the end of the year. In spite of the warm year globally, much of the central USA experienced cool conditions during February and March 2019. Also, a strong cold spell dominated much of the central USA in late October and early November. The result was an unusual Halloween snowfall over Missouri. This was associated with a moderate east Pacific blocking event.  
For the fourth consecutive year, we perform an overview the blocking occurrences in 2019 using the University of Missouri blocking event archive (http://weather.missouri.edu/gcc). We will examine the blocking occurrences for each region of the Northern Hemisphere (NH) and Southern Hemisphere (SH) separately, and discuss a few recent trends in blocking activity. While the NH was a bit active, the SH was very active continuing a trend toward more active blocking years globally (e.g., Kononova and Lupo 2020).
 
a.The Northern Hemisphere
During 2019, 45 blocking events occurred over the entire NH, which is lower than last year’s total (50) and within the typical range of the mean for the early 21st century occurrences (38). Since we typically expect +/- 8.5 events, 2019 was not unusual. The persistence of 2018 blocking events was similar to their climatological mean for early 21st century blocks (about 9 days), but the intensity was greater than the climatological mean strength. The year 2019 was dominated by a negative Arctic Oscillation Index (AO) (https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/month_ao_index.shtml). A negative AO indicates Northern Hemisphere flow that is strongly meridional or characterized by high amplitude waves in the jet stream. This may account for some very strong blocking events in 2019. As a consequence, we noted some unusually warm and cold periods regionally. However, what was noteworthy about the year was that from the last week of November 2019 into January 2020 featured only a few weak, short lived events.  

Over the Atlantic Region (80o W – 40o E longitude) in 2019, there were 19 blocking events that occurred which is slightly more than the regional mean. The Atlantic accounted for more than 40% of the NH total activity this year. During 2018, 13 of 22 Atlantic Region blocking events occurred over Eastern Europe and Western Russia. This year, the pattern reversed as only 6 of the 19 blocks occurred over the eastern part of the region. The Atlantic Region blocks persisted for about 9 days and these events were as strong on average as the typical regional event. However, in this region, there were very strong long-lived blocks early in the year, and weak shorter-lived ones later.  Two noteworthy events occurred over the United Kingdom in mid-to-late February. The second of the two was a very strong blocking event and associated with the warmest February day in UK history. Fig. 1a shows that most of Europe featured much above normal (+6o C) during this month. 

The Pacific Region (140o E- 100o W) was quite active (17 blocking events) was compared to the climatological normal (12) in number. The duration was a little less than typical (8-9 days versus about one more than this climatologically). These events overall were stronger than typical, but like the Atlantic Region featured strong events early in the year, and weaker events later. For the third consecutive year, most of these blocking events (14) occurred over the Northeast Pacific and distributed throughout the year. This resulted in very warm conditions as exemplified in Fig. 1a that saw +10 oC temperature anomalies over Northern Alaska. During February, two noteworthy blocks occurred over the Northeast Pacific. These also forced the very cold temperature anomalies over Canada and the Northern USA. The second of these events was a 15-day blocking event that lasted into March and was extremely strong. The same temperature anomalies persisted for March, but were not as strong. Nunes et al. (2017) and references therein show extreme cold over North America is typically associated with blocking in the eastern Pacific Region. Additionally, during late February 2019, North America was sandwiched in the middle of a very powerful NH simultaneous blocking episode.
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Another big story in the NH during this year was the very hot temperatures that led to “Baked Alaska” during late June and into July (Fig. 1b). Note that the temperature was 4-5 oC higher than normal over Southern Alaska resulting in a record high of 32 oC in early July. This was associated with a “heat dome” over Alaska. A blocking event from 27 June – 11 July persisted over the region. This is long-lived and the event was stronger than the typical summer season event. The conditions were similar to those of August 2004 that resulted in hot dry conditions and record fires (Hussain and Lupo, 2010).   

Figure 1. The Northern Hemisphere surface temperature anomaly (oC) for a) February 2019 (left), and b) July 2019 (right).

In 2019, the Continental Region (40o – 140o E) experienced nine blocking events which close to the mean of eight events. These events all occurred from May – November over various parts of Eurasia. Also, this means the Continental Region was less active than the previous two years. These events were typical in duration and strength. The warm conditions over Southern Russia and Afghanistan during July (see Fig. 1b) may have resulted from a strong long-lived summer event in this region (12-24 July). For the third consecutive year, no blocking occurred over North America in 2019. As shown in many studies, the occurrence of blocking over North America is comparatively rare.  
 
a.The Southern Hemisphere
               In 2019, the SH was nearly as active (26 blocking events) as 2018 (27 events), the second most since 1970. Last year, blocking was quite active in the Indian Ocean Region (30o E – 130o E). This year, there were only two events in both the Atlantic and Indian Ocean regions. This is more typical of SH blocking. Most of the blocking (22 events) occurred over the Pacific Region, which is the highest in the 50-year period (1970 – present). Wiedenmann et al. (2002) demonstrated that most blocking events occur in the South Pacific and during the months of May and June. In the SH overall, the blocking events were close to climatological mean strength (2.94 vs. 2.85), but about one day shorter than the mean duration (under 7 days vs under 8 days).

The record setting year in the South Pacific was paced by the occurrence of 22 events over the South Pacific, and their mean duration and strength was similar to the hemispheric means. The long-term anomalies in duration and strength were identical to the SH overall, which is not surprising since the Pacific region was so dominant. This year, the peak occurrence of the blocking events (10) was April to July, which is normal. For the third consecutive year, spring was fairly active (Oct- Dec – seven events) which is unusual. Last year most Pacific Region events occurred over the southwest Pacific, but this year more blocking events (14 of 22 Pacific Region events) occurred over the southeastern Pacific. These events were typical in duration, but weaker than normal. Blocking in the southeast Pacific will lead to colder conditions over southern South America. Fig. 2a shows the impact of four late winter events that occurred over a 37-day period from 15 August – 21 September 2019. Two of these events were quite strong for the SH leading to the cool September over South America. The early September event in the southeast Pacific was the 9th strongest in the 1970-2019 period.   

Figure 2: The Southern Hemisphere surface temperature anomaly (oC) for a) September 2019 (left), and b) surface soil moisture anomaly for December 2019 (right).

Australian heat was once again in the news from late 2019 into early 2020. Fig. 2b shows the composite soil moisture for the SH, and it is noteworthy that the entire Australian continent was dry. The months leading up to December were not unusually warm, but these months were also dry. The result was catastrophic fires continent-wide which devastated forests, degraded human health over large areas, and caused the deaths of people and scores of wildlife. There did not seem to be a direct link to the occurrence of blocking, but this weather was noteworthy nonetheless. 
  
In summary, the number of blocking events globally was 71 events compared to the global normal of 55 events. The active year was driven mainly by the occurrences of blocking in the SH. However, the year finished relatively quietly in December. In both the NH and SH, the Pacific Regions were more active than normal with record activity in the SH Pacific. The other regions in each hemisphere were closer to their climatological mean values. Only one event, a southeast Pacific event in September cracked the top 10 strongest events in the climatological data. This event came in at number 9 (BI=4.75) all-time.  For the most part, the duration and intensity of blocking in both hemispheres were very consistent with those which have occurred since 2000. Finally, blocking episodes were at least partly responsible for anomalous warm temperature conditions over Europe during February and March. The central part of North America was quite cold over this same time period due to blocking in the Pacific and Atlantic region. An active northeast Pacific kept Alaska and far east Russia warm for most of the year. The active southeast Pacific resulted in a cold late winter over South America.  
 
References:

Hussain, A., and A.R. Lupo, 2010: Scale and stability analysis of blocking events from 2002-2004: A case study of an unusually persistent blocking event leading to a heat wave in the Gulf of Alaska during August 2004. Advances in Meteorology, 2010, Article ID 610263, 15 pages doi:10.1155/2010/610263.
 
Kononova, N.K., and A.R. Lupo, 2020: Dynamics of the global atmospheric circulation and climate change. Atmosphere, 11(2), under review.
 
Nunes, M.J., A.R. Lupo, M.G. Lebedeva, Y.G. Chendev, and A.B. Solovyov, 2017: The occurrence of extreme monthly temperatures and precipitation in two global regions. Papers in Applied Geography, DOI: 10.1080/23754931.2017.1286253.
 
Wiedenmann, J.M., A.R. Lupo, I.I. Mokhov, and E. Tikhonova, 2002: The Climatology of Blocking Anticyclones for the Northern and Southern Hemisphere: Block Intensity as a Diagnostic. Journal of Climate, 15, 3459-3473.
 
Anthony R Lupo is a professor of Atmospheric Science specializing in the study of blocking anticyclone and jet stream dynamics at the University of Missouri and contributor to The Global Climate and Weather Center.