Extremely cold weather events caused by arctic air mass and its synoptic situation in Finland from the year 1950 onwards

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Doris Audrey Johns
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1 Extremely cold weather events caused by arctic air mass and its synoptic situation in Finland from the year 1950 onwards Senior meteorologist Henri Nyman Finnish Meteorological Institute, Weather and Safety Centre

2 Motivation and research method Motivation To find tools to make longer winter weather forecasts through synoptic understanding. To help society manage the possible problems (like the energy sufficiency) that extreme coldness could cause. Research method The re-analyzed European weather maps made by NCEP (National Centers for Environmental Prediction) were used to find the extremely cold weather events

3 Extremely cold weather event - definition Situations were estimated subjectively from weather maps. The events started when the temperature at the height of 850 hpa dropped below -18 C (so called arctic air mass) for most part of the country. The events ended when the temperature at that level remained warmer than -15 C for at least two days for most part of the country. To be included as an extremely cold weather event, this situation should last at least 6 days. No surface observations were included!

4 Extremely cold weather events in Finland from the year 1950 Year Days Length (days) Year Days Length (days) Year Days Length (days) Overall: 26 periods avarage length 9,3 days Blue marked is a Ridge-type and red marked is a Blocking-type extremely cold weather event (these types are introduced later)

5 Total number Total number Extremely cold weather events in Finland from the year 1950 Extremely cold weather events, monthly distribution Extremely cold weather events, distribution by decade December January February March

6 Synoptic situation in extremely cold weather events The beginning and the end of cold weather events were separately examined. Classification was done subjectively by comparing separately first and last days of synoptic weather maps. (Source of maps: Main focus was in the upper field (500 hpa) and its upper lows and ridges. After classification average maps of 500 hpa geopotential height, 850 hpa temperature and surface pressure for the different period types with NOAAs Reanalysis Data Composites were drawn (

7 Formation of extremely cold weather event Classification identified two different types: Blocking- and Ridge-type. Four cases could not be identified (evidence of both types). Ridge-type events: A strong ridge of high over Western Europe. A wide area of low east or southeast of Finland. Overall 13 cases. Blocking-type events: A strong blocking high over Greenland. An area of low east of Finland towards Central Europe and Atlantic. Overall 9 cases

8 Ridge-type formation

9 Blocking-type formation

from Jyväskylä, when the arctic air mass arrived.

10 The origin of the arctic air mass In the left backward calculated (5 days) trajectories at the level of 850 hpa from Jyväskylä, when the arctic air mass arrived. In the right corresponding trajectories at the level of 500 hpa

11 End of extremely cold weather event Classification identified also two different types: Cut-off lowtype and Westerly-type. Five cases could not be identified. Cut-off low-type warming: Cold air moves to Central Europe and makes a cut-off low. A ridge of high with slightly milder air from southeast develops over Finland. Overall 11 cases. Westerly-type warming: Westerly flow intensifies over Northern Atlantic and cold air moves towards Siberia. Warming over Finland can be extremely rapid. Overall 10 cases

12 Cut-off low-type warming

13 Westerly-type warming

14 Implications for research And the future of extremely cold weather events in Europe

15 Global warming Is expected particularly during the winter and in the northern latitudes (IPCC). Arctic sea ice has already diminished rapidly -> decreases the formation of arctic air mass. Probability of extremely cold weather events seems to be decreasing. Source: IPCC Source:

16 Natural variability The trend of NAO/AO-index is decreasing -> weaker westerly flow causes generally colder winters in Europe. Here is a quick example of the correlations of AO-index and mean temperature in Jyväskylä (central Finland). In this case, the correlation was clearly higher at the beginning of the winter (December) Mean AO-index and mean temperature in Jyväskylä (December) Mean T Mean AO

recent (10-20 years) decrease in Arctic sea ice.

17 Natural variability There is evidence of a multidecadal oscillation in arctic sea ice drifting (Mysak 2001), which could explain the recent (10-20 years) decrease in Arctic sea ice. And the correlation of sea ice drifting with AO-index seems to be quite high

18 Conclusions There are still many open questions about the future arctic climate. In my opinion: Studies should concentrate more on the possible changes in global (northern hemisphere) synoptic weather situations in different future scenarios (for example, changes in the arctic sea ice extent). Most studies are made using computer modeling, but it can t describe the synoptic situations as good as human reasoning and interpretation. My own interest is to study the correlation between the Arctic sea ice extent in September and the formation of polar vortex. If a correlation can be found, could it be possible to make a better forecast about upcoming winter?

19 Thank you! Senior meteorologist Henri Nyman Finnish Meteorological Institute, Weather and Safety Centre Address: Erik Palménin aukio 1, FI HELSINKI Phone:

Special blog on winter 2016/2017 retrospective can be found here -

March 4, 2019 Special blog on winter 2016/2017 retrospective can be found here - http://www.aer.com/winter2017 Special blog on winter 2015/2016 retrospective can be found here - http://www.aer.com/winter2016