Atmospheric Blocking in 2018: A Very Active Year

DISCUSSION: According to NASA and NOAA the year 2018 was the fourth warmest on record, coming in at around 0.8 degrees C above the 20th century average. This year also began with La Niña conditions in place over the eastern Tropical Pacific Ocean. These conditions held well into the spring and early summer, then during the fall weak El Niño conditions developed. In spite of the warm year globally, much of the central USA experienced cool conditions in March and April 2018, followed by a warm May and June. Also, a strong cold spell dominated much of the central USA from mid-October through November.

The phenomenon described last year, the pool of warm oceanic water in the Northeast Pacific known colloquially as “the blob” (e.g. Bond et al. 2015, Pinhero et al. 2019), was present again this year. But, the “blob” did not make much news in 2018, see last year’s publication for more on this event.

Here, we perform an overview the blocking occurrences in 2018 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.  

a.The Northern Hemisphere

As noted in last year’s installment, the number of blocking events that occur annually has been higher since about 2000 than the previous 30-year period (1970- 1999), and a new publication (Lupo et al. 2019) will highlight these trends.  During 2018, 50 blocking events occurred over the entire NH, which is higher than last year’s total (40) and quite a bit higher than the mean early 21st century occurrences (38). Since we typically expect +/- 8.5 events, 2018 was a “blocky” year. The persistence of 2018 blocking events was similar to their climatological mean for early 21st century blocks (about 9 days), and their intensity was close to the climatological mean strength as well.

Over the Atlantic Region (80 degrees W – 40 degrees E longitude) in 2018, there were 22 blocking events that occurred and this is almost 40% more than the regional mean. We have stated that the occurrence of blocking can be episodic, and during 2018, 13 of these Atlantic Region blocking events occurred over Eastern Europe and Western Russia. Three of these occurred during October and November in particular. The first one during mid-October caused a strong warm spell across much of Eastern Europe and Western Russia (Fig. 1a), and in some places the warmth was record setting. The latter two blocking events occurred during November and were about two weeks in duration each. The first of these was a moderately strong event, while the second was classified as strong. These events led to warmer than normal conditions over northeastern Europe and cooler than normal conditions from Ukraine to the Urals (Fig. 1b) during the month of November.

Within the Pacific Region (140 degrees E- 100 degrees W), the 2018 blocking occurrence (13) was close to the climatological normal (12) in number and duration (9-10 days). For the second straight year, most of these blocking events (11) occurred over the Northeast Pacific, but unlike last year, these were distributed throughout the year. One event occurred during mid-October (9-19 October), and combined with the Atlantic Region event described above, resulted in a very cold month for the western 2/3 of North America (Fig. 1a). Thus, North America was caught in the middle of a NH simultaneous blocking episode, which is not exactly rare. However, when the impact North America tend to anchor in persistent cool conditions. Also, Nunes et al. (2017) and references therein show extreme cold over North America is typically associated with blocking in the eastern Pacific Region. As we stated last year, the re-emergence of the Pacific Region “ridiculously resilient ridge” provided the impetus for more Pacific blocking. This also caused more blocking to occur throughout 2018 in the east Pacific. This prevalence for blocking over the eastern Pacific in 2018 led to Alaska experiencing a very warm year as seen in Fig. 1. The early winter saw very little snow over the interior of Alaska.  

​Figure 1. The Northern Hemisphere surface temperature anomaly (oC) for a) mid-October, 2018 (left), and b) November 2018 (right).
In 2018, it was the Continental Region (Weidenmann et al. 2002) that experienced more than double the number of events that during the previous year (2017). This was nearly 50% more than typical as well. These 15 blocking events were sprinkled over the Asian Continent throughout the year, and for the second year in a row, none occurred over North America in 2018. But, as shown in many studies, the occurrence of blocking over North America is comparatively rare.  
 
a.The Southern Hemisphere
                In the SH, there were 27 events during 2018 which is the most since record began to be kept in 1970. This breaks the record previously set in 2013 (24), and this is about a 65% greater frequency of occurrence over the annual climatological value (16.5). Weidenmann et al. (2002) demonstrated that most blocking events occur in the South Pacific and during the months of May and June. The record setting year was paced by the occurrence of 19 events over the South Pacific, and seven over the Indian Ocean sector. Normal for these two regions is 12 and three events, respectively. Like last year, the normal peak time only involved five SH block occurrences (late fall - May and June). Also, following 2017, the spring period from October to December saw six block occurrences. This time of the year is very quiet normally in the SH with respect to blocking activity. Most of the blocking events (16) occurred over the southwest Pacific from Australia to New Zealand and near the dateline throughout the year. This resulted in very warm temperatures over the western Pacific in 2018 (Fig. 2a), and Australian heat was often in the news in late 2018 into early 2019. Additionally Argentina and Brazil were cooler than normal.
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The SH blocking of 2018 was a little less persistent than typical, the mean event lasting for seven days (compared to eight typically). During the year, only three events persisted for more than 10 days. These were a 17-day event near Australia in October, and two events (10 and 12 days) during the month of May. One of these May events occurred over the western Pacific and the other over the eastern Pacific. This double blocking event resulted in a temperature pattern for a 12 day period that mimicked the year overall in general (Fig. 2). Note than much of South America experienced cooler winter season temperatures at this time. In spite of the increased occurrence of SH blocking in 2018, the intensity of these events was very close to the climatological mean. 

Figure 2: The Southern Hemisphere surface temperature anomaly (oC) for a) all of 2018 (left), and b) 24 May – 5 June 2018 (right).
                In summary, for the third consecutive year, the number of blocking events globally was up (77 events). During 2018, there were 26% more blocking events globally than in 2017 (61) and this difference was accounted for by positive anomalies in both hemispheres. Only the NH Pacific and SH Atlantic showed blocking occurrences near the climatological norm, all other regions discussed were greater than normal. This year there were not any blocking events occurring in either hemisphere that made it onto the list of the top 20 strongest or persistent blocking events on record.  Also, 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 Eastern Europe (especially the fall), the northeast Pacific and Alaska, and the entire western Pacific from Australia to New Zealand during 2018. Blocking also brought cooler conditions to the central USA and South America during their respective fall seasons, and over western Russia up to the Urals during the fall.
 
References:
Bond, N.A., Cronin, M.F., Freeland H, and Mantua, N., 2015: Causes and impacts of the 2014 warm anomaly in the NE Pacific. Geophysical Research Letters, 42, 3414-3420. DOI: 10.1002/2105GL063306, 2015.
 
Lupo, A.R., A.D. Jensen, I.I. Mokhov, A.V. Timazhev, T. Eichler, and B. Efe, 2019: Changes in global blocking character during the most recent decades, Under Review, Atmosphere, January, 2019. 
 
Pinheiro, M.C., Ullrich, P.A., and Grotjahn, R., 2018” Atmospheric blocking and intercomparison of objective detection methods: Flow field characteristics. Under Review, Climate Dynamics, January, 2019.
 
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.

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© 2019 Meteorologist Anthony Lupo