Record temperatures: What impact on seas and coastal areas?
A Superheated Sea: More Than Just a Rising Thermometer
Terrestrial heatwaves are now increasingly accompanied by marine heatwaves. These prolonged periods of abnormally high sea surface temperatures can last for weeks or even months, and their effects go far beyond simple surface warming.
The ocean’s surface temperature is rising, and this heat eventually penetrates deeper. As a result, the entire water column warms, permanently altering the stratification of water masses. This phenomenon, combined with thermal expansion – the expansion of water volume due to heat – contributes to an acceleration of sea-level rise, even in areas that do not experience direct glacial melt.
Another consequence is the disruption of exchanges between the air and the sea. Normally, the oceans absorb over 90% of the excess heat linked to human activities. However, when the surface temperature remains abnormally high, this absorption capacity slows down, modifying the flows of humidity, energy, and CO₂ at the air-sea interface. The weather itself is affected.
An Atmosphere Influenced on a Large Scale
Oceans act as planetary thermal regulators. When they overheat, the atmospheric balance shifts. On a synoptic scale, certain high-pressure zones can strengthen abnormally over warm waters or, conversely, become blocked in a stationary position. These blocking situations, typical of European heatwave episodes, often originate in ocean temperature anomalies.
The consequences are also felt locally. In the days following a marine heatwave, air instability can cause violent storms, especially on coastlines. Hot, moisture-saturated air, combined with more dynamic atmospheric currents, creates conditions conducive to intense storm episodes, sometimes even tornadic in temperate zones.
On a larger scale, the overheating of certain ocean areas can disrupt the usual paths of tropical cyclones. Warmer waters modify the genesis and intensity of these systems while influencing their path. Some hurricanes are now moving further north than in the past, while others accelerate sharply as they approach continents.
Disrupted Currents, a Threatened Climate Balance
The repetition of marine heatwaves also affects the dynamics of currents. When an ocean region experiences a prolonged rise in temperature, surface currents can slow down or divert, altering the general circulation of water masses.
Even more worrying is that the thermohaline circulation, the vast oceanic conveyor belt that redistributes heat around the planet, is becoming vulnerable. Based on differences in temperature and salinity, this complex mechanism is essential to the global climate. A marine heatwave, by homogenizing the water column and modifying density gradients, can disrupt this system, with consequences extending far beyond the ocean basin concerned.
These disturbances also impact atmospheric jet streams and precipitation patterns. By altering the distribution of heat between oceans and continents, a superheated sea influences the trajectories of disturbances and the stability of air masses. The result is more extreme, less predictable weather, even thousands of kilometers from the initial anomaly.
The Coastline, on the Front Line
These upheavals directly impact coastal areas, which bear the brunt of the combined consequences of terrestrial and marine heatwaves. Erosion is accelerating: between rising sea levels and more frequent storms, beaches are receding, and cliffs are collapsing more rapidly.
Prolonged drought episodes linked to blocked anticyclones weaken coastal soils, making them more susceptible to salinization. The intrusion of seawater into groundwater threatens coastal agriculture and compromises the availability of fresh water.
Port infrastructure must also adapt. From seawalls to pontoons, rising sea levels and the intensification of extreme weather events are forcing operators to revise their design and maintenance standards. In addition, there are direct impacts on nautical activities: event postponements, increased risks for recreational boating, and temporary beach closures due to pollution or violent weather phenomena.
Heatwaves are no longer limited to land. By warming the oceans, they amplify the effects of climate change and blur established weather patterns. Currents, pressures, storms, cyclones: the entire atmospheric mechanism reacts to the temperature of the waters. For meteorologists, monitoring the sea is becoming as crucial as monitoring the atmosphere. And for residents and users of the coastline, this new reality requires a different way of anticipating risks – because the effects are very real.
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