Executive summary: Well-known is the atmospheric warming of the past 2-3 decades due to rapid increase in CO2 emissions into the global atmosphere. A relatively new symptom of this warming has emerged with regularity in recent years. Frequency of climate extremes whether drought or flooding, hot or cold is increasing. The most notable catalyst is the emergence of vast areas of oceanic warming (or cooling) leading the atmosphere to produce long-duration regional climate extremes.
The first regional ocean thermal event occurring separate from El Nino southern oscillation (ENSO) occurred in late 2013 to 2014. A vast area of ocean warming in the northeast Pacific was given a rather un-scientific name by climate scientists: “The warm blob”. Across the vast warm waters high pressure ridging developed leading to an un-forecast historic drought across California. Since 2013-14 other oceanic warm and cool blobs have occurred causing the atmosphere above to create a stagnant and often hostile climate pattern.
The most recent “blob” evolved in 2018 south of Greenland as a widening vast area of much cooler than normal water. The cause of the cool blob is not well understood with opinions ranging from above normal fresh water melt from the Greenland Ice sheet to a possible change in the Atlantic Meridional Ocean Current (AMOC). Aloft the atmosphere over this large area of cooler than normal water surface also cooled and allowed the cold season polar vortex to linger over eastern Canada to Greenland into the summer season. The unusually strong polar vortex over or near Greenland was compensated for by the atmosphere with amplified warm ridge patterns upstream over the U.S. and downstream over Europe. The European ridge pattern brought historic drought and heat.
In this report a review of the cool SSTA pool evolution south of Greenland and the related attendant weather is reviewed beginning with the emergence of the cool pool last spring, its unusual intensity lasting into summer and northward retreating character as mid-summer arrived.
The lead response by the atmosphere to this cool blob of ocean water was a historic drought over Europe. In the U.S. both intense dryness and historic rainfall occurred due to this pattern. Of course, the precipitation regimes – particularly drought caused anomalous intense heat.
The sequence discussed in this report is speculated as the new normal in climate: Extreme regimes driven by regional SSTA warm and cool blobs of ocean water. The warm and cool blobs likely emerge as the atmosphere above slows down allowing regional oceanic (and atmosphere) regimes to develop. Monitoring for additional warm and cool blobs becomes as important as the conventional following of ENSO.
Discussion: The cool SSTA pool south of Greenland was initially evident by May 1, 2018 (Fig. 1). The temperature change of sea surface temperature anomalies (SSTA) in this region south of Greenland was -1.5C to -3.0C from early spring, a dramatic change. By July 1, 2018 the cool pool emerged and spread west-to-east across the north-central North Atlantic (Fig. 2). The cool pool blocked normal northeastward progression of the Gulf Stream causing water to warm and pile east of the New England/Mid-Atlantic States. The warm water in this region lead to both historic heat and wet weather in the Northeast States during summer – another “blob” climate event.
Fig. 1-2: The North Atlantic cool pool begins to emerge in April 2018 (left) intensifying as mid-summer 2018 arrives (right).
Warmest May on record: The evolving cool pool over the North Atlantic basin allowed a late season polar vortex over Eastern Canada to Greenland to strengthen in May (Fig. 3). To compensate for the deep cold pool of air in the upper atmosphere centered near Greenland an equally impressive but opposing upper warm ridge formed over the U.S. and Europe. As a residual effect of this pattern another less intense cool trough anchored farther downstream in Central Russia. The result was the warmest May on record for the U.S. and Europe while West-central/Central Russia was the coolest in 10 years (Fig. 4).
Fig. 3: The May 2018 North America to Central Russia 500 MB anomaly analysis to determine where the amplified trough/ridge pattern is located.
Fig. 4: North America and Europe thermal extremes are observed in May while Western Russia is cool.
The stage is set for Europe drought: While Europe was very dry during late spring the catalyst for accelerating drought was set by long-term dryness as supported by an anomalous 500 MB ridge pattern present much of the past 5 years (Fig. 5). An important factor in evolving drought intensity is the long-term climate pattern prior to the current climate dynamics that may cause drought. A flash drought scenario was indicated due to both long-term climate and short-term weather events early in the 2018 warm season across Europe.
Fig. 5: The 500 MB anomaly pattern for 2013-2017 across Europe and the Black Sea region supports persistent dryness increasing the risk of more dry extremes to follow as observed in 2018.
The June upper ridge shifts farther north: Entering meteorological summer a less amplified upper air pattern is normally expected. However, June 2018 brought an unusual climate scenario inspired by the cool blob. The upper trough over the North Atlantic cool pool of ocean water continued (Fig. 6). Upstream and downstream upper ridge patterns amplified over the U.S. and Europe. The West-central Russia cool trough persisted but farther downstream, record warmth occurred beneath another upper ridge pattern in North-central Russia.
The 6th hottest June on record was observed across North America while Europe was the hottest since 2013 (Fig. 7). Drought accelerated in Europe due to long-term dryness coupled with anomalous heat in June and very limited rainfall (Fig. 8). Dryness and attendant heat were emerging in Ukraine and the Black Sea region as high pressure expanded eastward.
Fig. 6: The June 2018 North America to Central Russia 500 MB anomaly analysis to determine where the amplified trough/ridge pattern is located.
Fig. 7: North America, Europe and Central Russia thermal extremes are observed in June along with accelerating drought.
Fig. 8: The catalyst to drought in Europe was a very dry month of June coupled with anomalous heat and long-term drying in this region.
Northwest flow thunderstorm pattern in the Great Plains: An amplifying ridge pattern coupled with anomalous heat in late spring to early summer implied risk of drought in the southern Great Plains (Fig. 9) expanding northward to the Corn Belt in June. However, northwest flow on the back side of the polar vortex over northeast Canada caused by the cool blob coupled with moisture from the Gulf of Mexico lead to an extreme wet scenario (Fig. 10) for much of the Midwest in June defeating an anticipated drought expansion. The northwest flow thunderstorm pattern was caused by the polar vortex in eastern Canada made stronger by the cool SSTA south of Greenland.
Fig. 9: At the end of May concern was that southern Great Plains drought would expand northward into the Corn Belt.
Fig. 10: Northwest flow on the back side of the eastern Canada polar vortex coupled with Gulf of Mexico moisture lead to excessive wet weather in June across the Midwest to Mid-Atlantic U.S.
Upper ridge is expansive in July: The polar vortex associated with the cool SSTA pattern south of Greenland shifted north due to seasonality during mid-summer (Fig. 11). To the south the middle latitude upper ridge expanded maintaining unusual heat in the U.S., Europe to Western Russia (Fig. 12). During July the strong character of the upper air pattern lead to more extreme precipitation regimes. While Central Europe drought continued a wet pattern emerged in Southeast Europe to the Black Sea region (Fig. 13). The super wet June prevented dryness observed in July in the Corn Belt from causing substantial issues (Fig. 14).
Fig. 11: The July 2018 North America to Central Russia 500 MB anomaly analysis to determine where the amplified trough/ridge pattern is located.
Fig. 12: In July 2018 historic heat continued in North America, Europe and into Western Russia.
Fig. 13: The July 2018 percent of normal rainfall in the Black Sea region.
Fig. 14: The July 2018 percent of normal rainfall in the U.S.
August ridge shift: Lowering sun angle cooling the polar region in late summer allowed the polar vortex to regain strength and shift south in August (Fig. 15). The farther south polar vortex caused upper wind speeds to increase and the persistent middle latitude ridge areas shifted east to the Northeast U.S. and Western Russia. After a wet July in Southwest Russia presence of the ridge pattern brought drier weather (Fig. 16). On the back side of the amplified upper ridge in the Northeast U.S. rainfall has become gradually excessive in the Central U.S. The current 7-day forecast (Fig. 17) adds to an already wet late summer pattern for the Great Plains.
Fig. 15: During August 2018 the polar vortex shifts farther south pushing the middle latitude ridge patterns to the Northeast U.S. and Western Russia.
Fig. 16: During August 2018 presence of an upper ridge brought a drier pattern change to Ukraine and Western Russia.
Fig. 17: The 7-day rainfall forecast valid Sep. 3-9, 2018 indicates 6-7 in. of rain from Omaha to Des Moines.
Conclusion: Extreme climate regimes are increasing. The cause of these episodes seems to be outside the ENSO regime. ENSO is the lead climate predictor therefore less involvement of ENSO in climate, especially extreme climate makes climate forecasts less certain. Extreme climate seems to be driven in-part by areas of ocean water which turn much warmer or cooler than the surrounding environment. The air aloft responds by warming (causing a persistent upper ridge) or cooling (causing a persistent upper trough). Ridge patterns cause long-lasting dry weather regimes which lead to drought while upper troughs can produce persistent extreme precipitation episodes in one area.
The most recent example of the scenario described are the climate patterns generated by the cool blob of ocean surface forming south of Greenland this past spring. The air aloft cooled as the attendant polar vortex lingered through summer. To compensate for the lingering cold polar vortex an amplified upper ridge pattern causing excessive heat and dryness formed affecting both the U.S. and Europe.
The upper ridge over the U.S. produced contrasting thermal regimes occasionally driving historic rainfall while at other times hot and dry weather was dominant. However, Europe was very dry and hot (except rainy Southern Europe). Farther downstream Russia experienced both unusually wet and hot/dry climate.
The influence warm and cool blobs of ocean water caused by a slowing atmosphere aloft related to climate change induced by the warming polar cap is becoming more frequent. The climate caused by these scenarios is frequently historic implying major burdens for the public, industry and certainly agriculture. Monitoring for developing cool and warm SSTA blobs becomes essential in climate prediction.