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Unusually warm Atlantic fueled busy hurricane season

By Skip Rigney

The 2020 Atlantic basin hurricane season, which includes the Gulf of Mexico and Carribean Sea, came to its official end this past Monday on November 30. This season will go into the history books as the most active in recorded history with a total of 30 tropical storms and hurricanes.

There’s no doubt some of this year’s named storms would never have been detected in the years before meteorological satellites. Still, by any standard this was an extremely active year. Tropical storm or hurricane warnings were issued for every coastal county on the Atlantic and Gulf of Mexico except for two according to data compiled by the National Weather Service. An unprecedented twelve named storms made landfall in the United States, eight of them in the Gulf of Mexico.

A major contributor to the favorable environment for tropical cyclone development was relatively weak upper level winds. When high-altitude winds are strong and change direction with height, it’s called wind shear, and it helps tear apart thunderstorms preventing them from becoming organized low pressure systems with closed circulations.

Wind shear tends to be weaker over the tropical cyclone formation regions in the Atlantic basin when sea surface temperatures in the eastern and central tropical Pacific are cooler-than-average, a condition known as La Nina, the opposite of El Nino. La Nina developed in August and has continued to strengthen since then.

Also, there was certainly abundant fuel for tropical cyclones in the Atlantic basin. For most of July through November sea surface temperatures (SSTs) in the western Atlantic, Carribean Sea, and Gulf of Mexico were warmer than the average of the last 30 years.

A number of researchers have shown that SSTs in the North Atlantic Ocean oscillate between warm and cool phases extending over multiple decades. The Atlantic basin was warm in the 1940s and 1950s, swung into a cool phase in the 1970s and 1980s, and then oscillated back into a warm mode that continues today.

Of course, there are also swings in ocean temperatures that have much longer periods. How does the current average SST in the North Atlantic basin compare to SSTs hundreds or thousands of years ago?

In an innovative bit of scientific sleuthing, a research team led by geoscientists from the University of Massachusetts in Amherst appear to have answered that question. Francois Lapointe and his colleagues examined layers of sediment collected from the bottom of a lake in the Canadian High Arctic. The concentration of the metal titanium deposited on the lake bed each year turns out to be highly correlated with the average SST across the North Atlantic that year.

The mechanism of how SST in the North Atlantic affects titanium deposits in a lake in northern Canada is obviously not direct. Instead, the connection exists because warmer or cooler SSTs in the North Atlantic result in different weather patterns affecting the Canadian Arctic, which in turn affects the lake’s sedimentation process.

According to the analysis done by Lapointe and his colleagues, over the last 2900 years, the North Atlantic was coldest between 1400 and 1800, which roughly corresponds to a period climatologists call the last “Little Ice Age.”

The warmest period of North Atlantic SSTs over the last three millenia? That would be the past ten years.