Shocking scenes in Venice have made headlines far and wide this week. The legendary Italian city experienced its highest water levels in more than 50 years. Reaching 1.87 m (6 ft) above mean water at their peak, they submerged over 80% of the city and even flooded its historic basilica. Italy is reportedly set to declare a national emergency.
This compact blog piece examines the how and the why of this tragic event.
The event can be put down to a combination of extreme weather and long-term climate change.
Weather-wise, the culprit was an unusually deep low pressure system crossing Italy. It travelled from just west of Sicilly as of noon GMT Tue 12th Nov to the middle of the Adriatic Sea as of 6 am GMT Wed 13th Nov (Figure 1).
Figure 1: Charts showing, for Italy and its surrounds, mean sea-level pressure (MSLP), for noon GMT Tue 12th Nov (left) and 6 am GMT Wed 13th Nov (right). Resulting net movement of sea water is indicated by blue arrows in the left image. Adapted from charts sourced at https://www.tropicaltidbits.com/.
This intensity and movement of low pressure delivered two key impacts:
- The strong anticlockwise flow of air around the low moved water from the Meditteranean and Ionian Seas northward. That water was then ‘funelled’ into the Adriatic, causing it to pile up. As the mean sea floor elevation increased on approach to Venice, this building of a storm surge was increased even further.
- The low sea-level pressure (SLP) added to the sea level rise. Low SLP means less atmosphere pressing down on the surface. A reduction of just 1 mb (a.k.a. hPa) in SLP increases sea level by 1 cm on average. The 992 mb central pressure shown for 6 am GMT Wed 13th is over 12 hPa below the long-term average for the northern Adriatic Sea. So, the deep low added at least 12 cm to the observed peak water level. That’s a contribution of over 6%.
The second of those is also linked to the first; the deeper a low, the stronger winds around it tend to be. The analysis data (Figure 2) shows a wide swathe of 35-45 knot sustained winds. That’s equivalent to a tropical storm – impressive for a non-tropical low.
Figure 2: Charts showing, for Italy and its surrounds, mean wind speed (knots) at 10 m elevation, for noon GMT Tue 12th Nov (left) and 6 am GMT Wed 13th Nov (right). Adapted from charts sourced at https://www.tropicaltidbits.com/.
Climate-wise, the finger points at a long-term rising trend in mean sea level (MSL).
“Climate-wise, the finger points at a long-term rising trend in mean sea level (MSL).”
The global mean has increased by approx. 95 mm since 1993. That’s an average of just over 3.5 mm per year. The change is far from uniform across the world, however. This regional map shows that the rate is much higher around Venice – between 5 and 10 mm per year. It may not sound like much, but where water piles up, that change effectively gets applied over and over.
Of the top ten Acqua Alta (high water) events in recorded history, half occurred in the past five years. This year’s was the second highest, while last year saw what was then the highest in a decade.
Although a single event can never truly be attributed to climate change, statistics like these paint a picture of greatly increased probability of severe high water events in a warmer world.
That conclusion is backed up by downward trends in observed global land ice mass. As glaciers, for example, melt away, they release vast amounts of freshwater – enough to significantly raise global MSL.
Thermodynamics also provides reason; the warmer water is, the less dense it is. This means that a certain mass of water takes up more space than it did before. The seabed holds fast, so the entirety of the change goes into sea level.
Finally, a less certain contributor to this event is a possible increase in large jet stream meanders, caused by the Arctic region warming faster than the non-Arctic (Arctic amplification). Such an event occurred during the 2019 Venice high water (Figure 3). It’s not often that the jet stream travels as far south as the north African coastline, let alone some way beyond that!
“It’s not often that the jet stream travels as far south as the north African coastline, let alone some way beyond that!”
Figure 3: Chart showing, for Europe, mean wind speed (knots) at jet stream altitude, for 6 pm GMT Tue 12th Nov. Chart sourced at https://www.tropicaltidbits.com/.
The jet stream consists of fast winds occurring at about 8-15 km (5-9 miles) altitude. It acts as a steering force on weather systems beneath it. So, when it meanders a long way north or south, weather systems deviate their paths in the same way. An unusually large meander leads to atypical weather system paths that bring very anomalous weather events where they travel.
A Very Concerning Future
All these climatic changes are still ongoing. As mean sea level continues to rise, the extremity of weather required to bring damaging floods is reduced. So, a greater number of weather systems will meet the criteria, leading to even more frequent floods. Meanwhile, the largest floods will be able to reach higher still.
Venice is far from defenceless, however. The Mose flood defence project is expected to be operational by the end of 2021. I sincerely hope that the troubled past of this project can be overcome, so that this target can be met.
James Peacock MSc
Head Meteorologist at MetSwift