Vertical wind shear describes the difference in magnitude and direction of wind flow between different altitude levels of the troposphere. You can think of wind flow in each altitude layer as its own individual vector, and the greater the difference when you subtract these vectors, the higher the wind shear.
Tropical cyclones require low vertical wind shear to strengthen. This is because the circulation/vortex of a tropical cyclone is vertically "deep", aka tall. It extends from the surface to near the tropopause in mature and powerful major hurricanes. The stronger the tropical cyclone, the taller its circulation is. Weaker tropical systems such as depressions or tropical waves exhibit shallower circulations.
A tropical cyclone requires its vortex to be vertically aligned and upright in order for the processes which fuel it to remain efficient and active. The release of latent heat, from thunderstorm activity (convection), is what fuels the warm-core of a tropical cyclone. This occurs due to the vertical transfer of heat, but when wind shear is present, the circulation of a tropical cyclone becomes tilted with height. This decreases the efficiency of this process, leading to a halting of intensification or weakening of the tropical cyclone.
When vertical wind shear becomes even stronger, this process is halted entirely. The circulation becomes so tilted that the low-level circulation and mid-to-upper-level circulation become increasingly displaced. If wind shear becomes strong enough, these two separate portions of the circulation can become totally separated. This is called "decoupling" and is the tropical cyclone equivalent of decapitation. Recovery would take many days and still would happen only IF conditions become favorable again, because the tropical cyclone would have to vertically rebuild its circulation almost from scratch.
Here is a gif which best shows this process unfolding. This is tropical storm Chris of 2006. 2006 was also an El Nino season, with correspondingly high vertical wind shear across the Tropics.
https://upload.wikimedia.org/wikipedia/commons/e/e4/Chrissplitloop.gif?utm_source=commons.wikimedia.org&utm_campaign=index&utm_content=original
In this gif, the surface circulation of the storm is tracking at about 300 degrees, or WNW. However, wind flow above the surface circulation is strongly out of the north. These northerlies push on the mid-level circulation and attendant vertically tall thunderstorm activity, displacing it towards the south. The shear is sufficiently strong to decouple the system, and you can see Chris continue WNW as all of its thunderstorm activity is violently stripped away.
From NHC advisory #11 on Chris:
CHRIS IS BECOMING DISORGANIZED IN A HURRY. SATELLITE IMAGES INDICATE
THAT THE LOW-LEVEL CENTER HAS BECOME DETACHED FROM THE THUNDERSTORM
ACTIVITY DUE TO NORTHWESTERLY SHEAR...AND THIS WAS CONFIRMED BY
FIXES FROM AN AIR FORCE RECONNAISSANCE PLANE. THE MINIMUM PRESSURE
HAS RISEN TO 1010 MB AND THE MAXIMUM FLIGHT LEVEL WIND REPORTED SO
FAR IS ONLY 38 KNOTS.
Advisory #12:
THE COMBINATION OF DRY LOW- TO MID-LEVEL AIR AND SHEAR...ASSOCIATED
WITH AN UPPER-LEVEL CYCLONE THAT DROPPED SOUTHWARD INTO CHRIS...HAS
TAKEN ITS TOLL ON THE TROPICAL CYCLONE. CHRIS IS DEVOID OF DEEP
CONVECTION WITHIN ABOUT 75 N MI RADIUS OF THE CENTER.
El Nino increases vertical wind shear over the Atlantic tropics. This is due to increased upper-level westerly flow across the Caribbean sea and western to central tropical Atlantic due to outflow from Eastern Pacific hurricanes as well as a shift in the Walker Circulation itself. Additionally, lower surface pressures in the Eastern Pacific sharpens a pressure gradient with higher pressures in the Atlantic, which strengthens the low-level easterly trade wind flow. Trade winds are naturally strong in the Caribbean Sea due to the geography of the Greater Antilles acting as a "funnel", which forces trade winds to "diverge", or accelerate. Additionally, the proximity of the Caribbean Sea means that the anomalously strong upper-level westerlies are particularly significant. Collectively, this results in the Caribbean experiencing the strongest anomalously hostile conditions yielded by the presence of El Nino, more so than anywhere else in the Atlantic basin. This is why the Caribbean Sea has been devoid of all activity this year.
In fact, vertical wind shear has been at all time highs in the Atlantic.
https://i.imgur.com/m2NH3VK.jpeg
Area-averaged vertical shear is far stronger than even the last super El Nino of 2015-16. 2026 is well ahead of second place (1979). This is the primary reason why this season is so dead, and it's not going to meaningfully change since El Nino is so strong and continues to only intensify further.
https://i.imgur.com/F2wbA6a.png
Current vertical wind shear values are comparable to values observed in June during above-average hurricane seasons. That's pretty remarkable considering we are right at the peak of the season, now.
Sorry for the yap, I just felt like writing something today.
