Mika Rantanen Profile picture
Apr 25 7 tweets 3 min read Twitter logo Read on Twitter
Summer is approaching in northern extratropics and with summer comes the heatwaves.

Although it's still April, a major summer-like heatwave will hit Spain this week. Temperatures are forecast to rise close to 40°C which would be unprecedented for April. #AprilHeatwave Image
The dynamic development of the heatwave is worth highlighting.

Blocking high acts like a stone in the river by causing a split of the jet stream and the occurrence of a double jet situation. Image
As a side note, the synoptic situation reminds me of the recent paper by @syn_efou et al. who found that the increasing trends of western European heatwaves are to linked more persistent double jet situations:

nature.com/articles/s4146…
However, the most worrying aspect of the heatwave is that it comes when a large part of Spain is already suffering from drought.

You can easily see how the terrain is much browner this year than a year ago in April.

Imagery from NASA Worldview.
So the incoming heatwave will dry the soils even more, forming a well-known positive soil moisture feedback. Thus, a larger fraction of solar radiation is consumed to warm the air which exaggerates the heatwave magnitude.

nyaspubs.onlinelibrary.wiley.com/doi/10.1111/ny… Image
The latest extended range forecast by ECMWF indicates that warm and dry conditions would continue in SW-Europe in the coming weeks.

This is bad news for agriculture which is already in a trouble with the drought.

charts.ecmwf.int/products/exten… Image
Finally, every heatwave in the current climate is of course worsened by human-induced climate change.

This does not mean that the heatwave is caused by climate change: it means that temperatures will rise higher than they would have risen without climate change.

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More from @mikarantane

Dec 14, 2022
Runsas lumitilanne ja viileä alkutalvi Etelä-Suomessa puhuttaa.



Onko #ilmastonmuutos-ennusteet pielessä vai mistä tämä johtuu?

Ketju aiheesta 🧵 👇
Ensinnäkin pitää muistaa, että talvi-ilmasto on Suomessa luonnostaan hyvin vaihteleva, eikä vaihtelun odoteta loppuvan tulevaisuudessa.

Kylmät talvet ovat tulevaisuudessakin mahdollisia, vaikkakin niiden todennäköisyys hiljalleen pienenee ilmaston lämmetessä.
Myös lumensyvyydessa on luonnostaan suurta vuosien välistä vaihtelua. Tästä esimerkinä neljä runsaslumista talvea 2010-2013, joista voit lukea lisää täältä: rmets.onlinelibrary.wiley.com/doi/10.1002/we…

Pitkän ajan trendi lumensyvyyssummassa on kuitenkin laskeva, varsinkin Etelä-Suomessa.
Read 9 tweets
Aug 11, 2022
🚨New paper alert 🚨

I'm excited to announce that our open-access paper "The Arctic has warmed nearly four times faster than the globe since 1979" is now out in @CommsEarth: nature.com/articles/s4324…

#ArcticAmplification #Arctic

Thread of the main results:

1/13
We report that the Arctic has warmed four times faster than the globe in the last 43 years.

The four-fold Arctic amplification (AA) ratio is higher than what is generally reported in literature and media.

2/13
One reason for the higher ratio is that we defined the Arctic properly (with the Arctic Circle, 66.5°N), and calculated the trends since 1979 when satellite data became available.

With these parameters, the observed AA ratio is 3.7-4.1, depending slightly on the dataset.

3/13
Read 13 tweets
Jun 24, 2022
Miksi viimeaikaisissa helleaalloissa on ollut niin kuuma? Mitkä ilmakehän tekijät selittävät korkeita lämpötiloja? #helle #helteet

KETJU 🔗aiheesta (1/7):
Ensimmäinen, melko selvä syy on lämpimien ilmamassojen virtaus eteläisimmiltä leveysasteilta.

Ranskaan virtasi kuumaa, Saharan ilmamassaa Afrikasta, ja meidän juhannushelteet selittyvät osaltaan lämpimällä ilmavirtauksella Etelä-Euroopasta (kuvan nuoli).

(2/7)
Toinen syy liittyy korkeapaineiden dynamiikkaan. Olet varmaan huomannut, että pumpatessa ilmaa renkaaseen pumppu kuumenee. Samaa tapahtuu korkeapaineissa!

Laskeutuvan ilmapaketin paine kasvaa, ja fysiikan lakien mukaan sen lämpötila nousee.

(3/7)
Read 7 tweets
Jun 13, 2022
A major heatwave will hit this week to western Europe.
Temperatures over 40°C are expected in France and widely in Spain.

And this all will happen after a record-warm May in France and the 2nd warmest May in Spain.

Some thoughts in a thread 👇
The heatwave forms when an upper-level ridge strengthens in the downstream of the Azorean cut-off low.

The ridge involves hot airmass from Africa, and perhaps more importantly, descending air parcels adiabatically warming when reaching surface.
The evolution resembles 2019 western European heatwave, but this time the downstream ridge responsible for the heatwave does not grow as strong in amplitude, and also the wave breaking looks more cyclonic (or less anticyclonic) than in 2019.
Read 6 tweets

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