Meteorologist Sebastian Wache explains how climate change is impacting weather systems, and how ice melt in the Arctic can affect an Atlantic crossing, Northern European cruising season, and more

For bluewater cruisers one of the most significant impacts of climate change is the increased unpredictability of weather and disruption of established patterns. The Ocean Cruising Club commissioned a report into the effects climate change was having on ocean cruising, and asked several forecasting and routing experts to share their views on what seems to be happening, and what steps cruisers can take to ensure safe ocean passages and cruises. Among them was meteorologist Sebastian Wache: here he explains how ice melt in the Arctic can affect an Atlantic crossing, Northern European cruising season, and more.

Warming effect

The world is getting warmer. The greenhouse effect, which makes life on Earth possible in the first place, is intensified by the increase in greenhouse gases in the atmosphere. The fact that we have been burning materials for more than 150 years is changing the climate. But in the meantime, we are also interfering with day-to-day weather. But it’s important to understand why, and exactly what is happening.

Global warming affects the polar latitudes more severely than the tropics, so we see a continuous decrease of the ice surface in the Arctic. If there is less ice or snow, then the sun’s reflection is no longer as high. Dark surfaces absorb more heat. So, it gets warmer there.

Normally, we need cold poles and warm tropical regions. This is because these strong contrasts drive the jet stream, a strong wind band at about 60°N, which normally blows from west to east. It’s true that the wind band occasionally takes on wave patterns, transporting warm air masses to the north and cold air to the south. These waves (known as Rossby waves) regularly dissipate, and then we have the typical west-east flow again. But due to the warming of the poles, the temperature contrasts are weaker. Thus, the jet stream is not always necessarily as strong and tends more towards these wave patterns.

Searching for tradewinds: there have been disturbed trades at the start phase of recent ARC rallies, although they have filled in later in the crossing to provide classic downwind conditions

If a region is under a wave that tends to the north, then it lies in a ridge and warm air can accumulate underneath. A high-pressure area forms, which often remains very stable for a long time in one and the same place. It blocks low-pressure areas that have to move north and south. And we are seeing these so-called blocking phases more and more frequently, especially in the last five to 10 years.

Where the water goes

This also has an impact on regions that lie south of 60°N. Because lows, which are thus also deflected to the south, sometimes make it to 30°N and even further south with their zones of bad weather. And the further south the cold air from the north penetrates, the more intense the weather there becomes. This is because an unstable stratification forms in the atmosphere. Cold air at high altitude is transported with the low pressure to warmer regions. And this warm air is then below the cold air (eg warm water or land masses). This triggers bigger cumulus clouds with powerful showers and storms, often with severe weather potential. And this is where climate change comes into play again. As the temperature increases in the atmosphere, more water vapour can be absorbed into the air.

Water vapour is an energy carrier. If this energy is ‘let out’ through such weather patterns, there will be massive storms with heavy rain, flooding and so on. And as long as the high continues to lie stably in place, lows are produced on its edges at the same time.

So, we see that climate change is already having a massive impact on our weather and weather extremes (too dry in one place, too wet with storms in another). Since the oceans absorb a huge amount of excess heat from the atmosphere, but at the same time also interact with the atmosphere and influence the weather, there are changes here too. Warmer water evaporates more moisture. More water vapour and more energy enter the atmosphere.

Cold air at high altitude is transported with low pressure systems to warmer regions, triggering large cumulus clouds with thunder, rain and storms. Sebastian Wache/WeterWelt

At the same time, differences in salinity and temperature drive global ocean currents. These changes are not as immediate as we are seeing in weather patterns. Nevertheless, with the melt water from the cold Greenland Ice Sheet or the Antarctic, it must be assumed that, in addition to the temperature differences, an increase in fresh water also disturbs the salinity differences and thus exerts a considerable influence on the ocean currents. Calculations show, for example, a weakening of the North Atlantic Current.

As there are currently no major influences on the tropical regions, weather conditions here are likely to remain fairly normal despite the climate change. Therefore, the ‘Barefoot Route’ – (east to west along the equator) is still the least affected. But away from it, with each degree of latitude more, we are already seeing significant changes. Wind and weather patterns, as you may still know them from pilot charts, are subject to ever greater fluctuations.

Blocking highs

As long as the earth rotates, there will continue to be tradewinds and westerly winds in the higher latitudes. But it is precisely these long wave blocking phases that cause disturbances in the weather pattern. For example, at the ARC Atlantic Rally for Cruisers, we have been experiencing a frequently disturbed tradewind for years now. It may be a coincidence that this happens exactly at the start phase in the last few years because the tradewind usually regenerates again. But I also accompany countless yachts across the Atlantic away from the ARC and here, too, we have to pay more and more attention to suitable weather windows, because the tradewinds at the height of the Canaries are now being disturbed more often.

Blocking high pressure systems are becoming more frequent, such as this system over Europe in October 2022

Based on the aforementioned blocking phases with a huge stationary high, this also has an impact on other regions. In autumn 2022, such a very stable situation over Central Europe ensured that lows repeatedly penetrated the Bay of Biscay for weeks and caused high waves and storms there. Sailors could not leave their marinas for weeks to get to the Canaries.

It can even impact in the summer. If such a high pressure lies over the North and Baltic Seas in July or August, for example, there is often no wind for a long time. Thermals barely factor then either, because they also need warm land and cold water, and that becomes difficult when the water is already over 20° and often as warm as the air. The years 2018 and 2022 were characterised by too little wind in these regions.

At the same time the low pressure systems elsewhere provide a much higher potential for destruction, such as in Corsica in 2022 or Mallorca in 2023. This shows what higher global temperatures bring with them in terms of energy.

Tropical storms are also part of this. They are also low pressure systems that get their energy from the warmth of the air and especially the water; Hurricane Ian in 2022 showed us what such systems were capable of in Florida. Although the number of tropical cyclones is not increasing, the intensity of the systems can now be far greater.

A blocking high pressure system over Europe in October 2022 saw a light wind start to the Rolex Middle Sea Race. Photo: Kurt Arrigo/Rolex

Adapt to change

This all sounds very dramatic, and in a sense it is. We weather forecasters and sailors just have to deal with it: even the weather models have not necessarily fully adjusted to the new intensities. Take the severe weather in Corsica in 2022, for example. The strength of the gusts with over 80 knots was underestimated by all models. One came closest to reality, forecasting 50-60 knots.

It’s important to monitor weather data regularly, even during stable blocking phases. If possible, you should update the data twice a day. During the ‘medicane’ (the Mediterranean Hurricane in the Ionian Sea in 2020), I received a lot of requests, especially from charter sailors, about where to go to avoid the storm. I sent a lot of crews to Corfu, as they were safe there. The development of this storm, however, could be seen quite early on. You can still rely on weather models – you just have to look at them very often, maybe even more often than before.

If you’re considering a longer ocean crossing, then the planning should start the same way as before: familiarise yourself with the global wind and current circulation.

The 2020 ‘medicane’ (Mediterranean Hurricane) was well forecast but still rendered many ports in the Ionian unusable

First of all you should know how winds develop and where they normally blow. You can use pilot charts for this purpose, but you shouldn’t rely too much on wind statistics because in the end it always depends on the weather situation at that specific time and location. It’s one reason why weather routers’ services are requested more and more often (I also see it in the increased popularity of webinars): skippers are increasingly uncertain about the weather, and that uncertainty increases when they can’t find weather statistics for the spot they’re in.

This may not be so concerning for a crew that has a lot of time: they can simply wait for the next weather window, but if the yacht has been chartered or is on a delivery where there are deadlines to be met then changing weather patterns can present a big problem.

Increased awareness

I do see positive trends among sailors and weather forecasting to contend with the increased changeability. Whether it’s Ventusky, Windy, WXcharts or others, there are increasing numbers of websites that display weather in a very user-friendly way without requiring a degree in meteorology. Understanding and watching weather has become much easier, and more and more people are becoming enthusiastic about it. Consequently, we weather routers are now asked more detailed questions and at seminars in sailing clubs,

Agia Efimia in Greece after the 2020 ‘medicane’. Photo: dpa picture alliance/Alamy

Due to the change in global weather patterns, cruisers will have to do more short-term planning in the future. I see a trend towards more last-minute departures and charter bookings, based on looking at the weather development for the next 10-14 days.

For long cruises, such as across the Atlantic, the classic seasons remain. The hurricane season continues to steer the sailing season, but here too, the increasing influence of global warming has become apparent in recent years.

The years 2017-2020 (2017 Hurricane Ophelia, 2018 Tropical Storm Pablo, 2020 Tropical Storm Theta) showed what overly warm water in autumn can lead to. Cold air masses from the north met water that was still too warm on the Atlantic, so lows could form into tropical storms and even hurricanes in regions where they normally do not form at all. Near the Azores, Madeira and the Canary Islands, eddies of enormous strength suddenly appeared, which were completely untypical for the season and region. Even Portugal, Spain and Ireland were threatened by the force of these storms. Fortunately, they weakened considerably or did not make landfall at all.

These are new developments. Such storms do appear on the statistics, but if they only appear on one day of the year in one of the quadrants of the pilot charts and then move on to the next, they do not obviously stand out. It’s therefore even more important to know what can theoretically happen in a given season and then regularly look at the actual weather data to identify or eliminate the risk.

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