The Gulf Stream and the climate in the North Atlantic

A worrisome anomaly has been detected in the Gulf Stream in the last recent months. The world’s climate depends, to a great extent, on the ocean circulation and it is important to know the extent of the consequences that this anomaly could have in the future, especially in Europe and North America.

The Gulf Stream

This current circulates from the Gulf of Mexico to the western coast of Europe in the North Atlantic, passing through the east coast of the United States. Actually, it does not do it in a straight line, but the water moves in a complex way, forming a multitude of swirls.

The Gulf Stream influences Florida’s climate, softening temperatures throughout the year, where winters are less cold and summers are less hot. The same happens in Europe, where the warm water provided by this current has a regional impact on the climate of countries such as the United Kingdom, Norway or Iceland.

Unprecedented anomalies in 150 years

To begin with, we have to define an anomaly as a record that deviates from a standard or expected result at a certain place or time. When we talk about oceans, the most used record is the sea surface temperature (SST) anomaly, which in this case shows that the current is warmer than usual in some places in the North Atlantic, with 6 to 8 degrees above average.

Corriente del Golfo_anomalia — *1. North Atlantic surface temperature anomaly. Source: noaa.gov*

This anomaly is significant, and such values represent a greater amount of energy available for meteorological phenomena in the region. For example, the contact of the atmosphere with water at a temperature 8 °C above the usual, translates into a warmer air source that could meet an Arctic air mass, and produce big storms or blizzards such as the great snowfall that Spain suffered last January 2021.

Precisely that month the anomaly intensified, spreading over a larger area and with values higher than the average of recent years. And not only on the surface, the same change in water temperature has also been observed at depths between 100 and 500 meters.

Circulation in the North Atlantic

However, the Gulf Stream is just one part of something much larger: the South Atlantic Return Circulation, better known by the acronym AMOC.

The AMOC is a large system of ocean currents that move regularly based on several factors, but here we are going to pay special attention to salinity and temperature, since they are the parameters that control the density of water. Within the AMOC, the Gulf Stream is only a shallow part of the North Atlantic.

Salinity and ocean circulation

Salinity determines the water’s freezing point. The more salt a volume of water contains, the denser it becomes and this makes it more difficult for it to freeze, that is, it needs lower temperatures to form ice crystals. Density influences ocean circulation and affects the planet’s climate.

For example, the surface layer of the ocean is in contact with the atmosphere, so this layer ends up absorbing particles such as CO₂. When the density of water increases due to the combination of salt and low temperature, it sinks with the CO₂ that it has acquired on the surface.

ocean_circulation — *2. Oceanic circulation of the planet, where the vertical flow process in the Atlantic Ocean is shown. Source: ocean-climate.org*

Oceans are the largest CO₂ containers on the planet, and therefore are very important to remove it from the atmosphere, since once transported to the deeps of the seabed it remains there for thousands of years.

However, several processes can cause changes in the salinity of the water. Evaporation transforms water molecules into steam, leaving salt behind, so the ratio of salt to water increases. The same happens with the freezing of water. On the contrary, both the contribution of fresh water by rain or rivers to the sea, or the melting of ice water, cause a reduction in salinity through dilution.

All these processes combined make the oceans more or less saline in different regions of the Earth.

The salinity map that we show is from August 2020, and in it we can see that the ocean is more saline on both sides of the equator, where high pressures bring dry air to the sea surface and cause evaporation to be greater than precipitation.

Corriente del Golfo_Salinidad — *3. Ocean salinity map (August 2020). Source: noaa.gov*

Along the equator, precipitation is much higher, therefore salinity here is reduced. We also see how salinity tends to decrease as we approach the poles, where there is more precipitation than evaporation, and it also occurs at the mouths of large rivers such as the Amazon, the Congo or the Ganges.

The places with the highest salinity are the almost closed seas in arid zones (Mediterranean Sea, Red Sea and Persian Gulf).

Changes in salinity and temperature cause regular patterns in the world’s oceans, causing water to flow, sink and emerge. This is why the AMOC is so important: it is the only source of warm water for the North Atlantic.

The biggest concern for the United States and Europe is that this circulation would come to a halt, triggering a change in weather patterns in both regions. However, it is very difficult to calculate the extent of these changes, since, as we have seen, the climate and the ocean are inevitably connected.

AMOC and Gulf Stream are weakening

The reality is that the AMOC is weakening. The graph below shows that the Gulf Stream is warming while the waters of the North Atlantic are cooling. The difference between these two areas serves to quantify the intensity of the AMOC.

AMOC_weakening — *4. Differences in the anomalies of the Gulf Stream and the North Atlantic. Source: severe-weather.eu*

Although it sounds paradoxical that the North Atlantic is cooling in a context of global warming, where many areas of the ocean are increasing in temperature, the truth is that this makes a lot of sense.

Let us remember that both the Arctic ice sheet and Greenland are in contact with the waters of the North Atlantic, and that the increase in temperature in the atmosphere in recent decades has led to the melting of the Greenlandic glaciers and the Arctic ice pack. This means that the fresh and cold water in these frozen masses is reducing the salinity of the ocean, and therefore it is not dense enough to sink.

And what could happen if the water in these latitudes did not sink? Well, the usual ocean circulation would slow down, and the water would remain stagnant, causing a traffic jam. A simulation of this scenario warns us of the future consequences.

cooling_model — *5. Prediction of changes in surface air temperature if AMOC were to stop. Source: severe-weather.eu*

The northern hemisphere would decrease its average temperature by several degrees, also causing changes in current weather patterns. For example, we do not know what the new pressure patterns would be (low and high pressure cells), but what seems likely is that precipitation would be much scarcer, and would generate much colder and drier winters.

Naturally, this would be the result of decades or centuries after the arrest of the AMOC, but, although today it may seem distant, the trend of this process leads us to that scenario.

What does seem to be happening already is the increase in the frequency and violence of tropical storms and hurricanes. And indeed, the AMOC is very important in these phenomena.

Hurricanes and tropical storms

We all remember Hurricane Sandy. It was one of the most devastating of 2012, causing damages worth 70 billion dollars, and also killing 230 people throughout the 8 countries it affected from the Caribbean to Canada.

Indeed, Hurricane Sandy originated because the waters of the tropical Atlantic were warmer than normal, using this energy to enhance its magnitude.

After the passage of the hurricane, the satellites for climate observation recorded a sudden cooling of the surface layers of the ocean in those places traveled by the cyclone (between 4 and 6 degrees below the original temperature). Part of this cooling is due to the mixing of the different layers of the ocean, as the cooler water from the depths emerges and mixes with the warmer layers on the surface.

Extreme events and the Gulf Stream

Predictions show that, while the warm waters of the Gulf Stream continue to flow into the Arctic, the North Atlantic remains neutral and even colder than normal due to the melting of the icy waters we mentioned earlier. This is what the latest observations from late summer and early fall in 2020 suggest.

Through the prediction of the pluviometric anomaly, we can expect more rain than usual from West Africa, expanding across the Atlantic to the Caribbean, along with the east coast of the United States and the Gulf Stream.

This presence of warmer waters in the ocean, together with increased precipitation, is a very clear signal that points to increased activity in the Atlantic hurricane season.