This story will focus on the North Atlantic Gyre, the subtropical gyre that fills the middle of the ocean. This region is not only not influenced by the oceanographic oscillations in the North Pacific or the Atlantic, it is very stable. There are two main factors that determine the stability of a gyre at different basins. These factors are the size and relative surface currents. These two factors are discussed, along with the influence of other factors, in The Influence of the North Atlantic Gyre on the Atlantic Multi-decadal Variability, by Zuberi, et al. in the Journal of the Oceanophysical Society, vol. 71, no. 1, pages 1261-1271.
And the North Atlantic Gyre’s influence is likely not just limited to the North Atlantic.
While there is a positive relationship between the size of the North Atlantic gyre and the number of hurricanes in a tropical storm’s central pressure area, the influence of the North Atlantic Gyre is not as great as was originally believed. In the 1970s researchers theorized that, since wind strength and size are inversely related, the number of hurricanes in a tropical storm’s circulation region influences the size of the pressure region in which that storm forms and thus influences the overall strength of the storm. However, this was the only study that investigated just the North Atlantic, and also not with hurricanes that were at the center of such a belt. … The strongest (strongest) hurricane systems of the past have all formed in the Central Atlantic. And, indeed, we have some very strong hurricanes, including Rita and Lee, on the American East Coastbut they’re not located in the central part of the belt (which means that the number of hurricanes along the Atlantic coast must be very, very low…) . Now, if you want to have strong storms in the region, you’re going to have to have a storm that has a large central pressure. If you have a hurricane that’s at the center of a strong current where there aren’t any strong winds and no storm-fighting storm operations underway, it’s almost certainly not going to form over the most intense part of the belt (the middle of the Atlantic). A hurricane that’s near the Gulf of Mexico is unlikely to form, as the high pressure areas around the northern islands and low in the tropics allow hurricanes to form around the Gulf and Caribbean. However, if a hurricane is near the center of a large circulation area, where there’s already a lot of activity going on, there’s a good chance that we might see it move into a strong circumpolar jet. Indeed we do: there are a couple of super-hurricanes that happened in the Gulf of Mexico when the jet was present in their circulation path. For example, Hurricane Katrina tore off a half million square miles (over 700,000 km2) of Louisiana in less than 72 hours. The hurricane also created the equivalent of about 0.2% of Louisiana’s landmass (or about 600,000 sq mi or 1 m2).
These examples, of course, aren’t the only examples. In fact there are so many examples to illustrate that I don’t think it’s possible to list them. But my point was that these hurricanes were not all spawned by the North Atlantic. There are two reasons this is the case. First, you have to look at specific regions of the North Atlantic, and the second reason is that in most regions of the North Atlantic, small-scale internal variability is stronger than regional pressure variability. As a result, the strong internal variability that is present in most regions is also strong enough to help a hurricane form.
So back to the story. What explains the Atlantic Multi-decadal Variability? The long-term changes seen across the Atlantic Ocean indicate a weakening of the circulation that originates about 40S of the equator and crosses the continent across North Africa. This means that the northern circulation is weaker and more numerous in the Atlantic today. Given the long-term trend, it’s not at all clear why it’s weaker this season. But there’s only limited correlation between oceanographic changes and the change in the North Atlantic Multi-decadal Variability. And there’s one crucial factor that determines an influence on the Multi-decadal Variability: the influence of the North Atlantic. Now, the current direction of the ocean currents that lead to these cycles may be affected by climatic conditions. Since the beginning of the 20th century, Europe has become drier than usual due to its climate change caused by human-caused carbon dioxide increases. These changes in the eastern Atlantic have been associated with the presence of colder water in the North Atlantic, a result of the reduced evaporation from the subsurface. Thus, when the Eastern European Current, or EEC, passes by the North Atlantic, there’s potential for an event in the central region of the North Atlantic (the middle of the Atlantic) where cold, colder water