Satellite analysis showing the Saharan Air Layer over the tropical Atlantic
The Atlantic Basin is fairly quiet as we start the new week. There is one area, invest 94L, that bears watching but I think that the odds of development anytime soon are slim. The reason? Dry, sinking air.
Tropical cyclones are driven by convection (thunderstorms). Without this convection, heat cannot be released and the process fails. One sure fire way to keep a lid on tropical convection, almost literally, is to have too much dry or sinking air around.
The dry air part is probably easy to understand. Moist air is lighter than dry air, believe it or not, and it lifts easier. This instability is essential in the development of convection, especially over the tropics. A stable layer of the atmosphere due to drier air keeps the convective process limited at best. In most cases, the dry air we see originates from Africa in the form of huge outbreaks of dust and warm, stable dry air from the Sahara. Thus, it is termed the Saharan Air Layer. As you can see in the image, the oranges and reds represent a dry and/or dusty air mass. Right now, the SAL is the dominant feature across the tropical Atlantic. Until and unless it abates, it will be extremely difficult to have anything develop in the area between Africa and the Lesser Antilles.
Velocity chart showing areas of upward motion (green) vs areas of sinking air or subsidence (brown)
The other obstacle for tropical cyclone formation is subsidence or sinking air. Thunderstorms thrive in an upward motion environment where ascending air is favored over sinking air. Again, we’re talking about lift in the atmosphere and lift leads to cloud formation and ultimately, convection. Right now, the Atlantic is generally unfavorable in terms of lift in the atmosphere. The image shows this nicely with browns indicating sinking air and greens indicating rising air. This is an over-simplification of the process but that’s the general idea. As with the SAL issue, until we see a widespread change in the upward motion pattern, I doubt we will have much activity brewing in the deep tropics.
Vertical wind shear over the tropical Atlantic is actually quite favorable right now
On the other hand, vertical wind shear, the difference in wind speed with height in the atmosphere, is running just a little bit below the long-term average for this time of year. This is a big plus for development but without an unstable, moist environment, tropical waves are going to have a very difficult time developing.
These three aspects of tropical cyclone formation are just part of the overall picture that includes other elements such as vorticity, sea surface temperatures and heat content, etc. However, as we can clearly see, the dry air is an overwhelming factor and one that apparently trumps the other positive parameters that are in place. So the question then becomes, will the dry air ever let up? It should in due time. We are still a couple of weeks away from the traditional start to the busiest part of the hurricane season. There is a reason for that and it’s not just about warm sea surface temperatures. After the 15th of the month, we typically see a seasonal shift in pressure patterns that helps to limit the amount of dry air outbreaks from Africa. We also note that over time, we usually have a period of favorable upward motion in the atmosphere that could allow for development. None of these things are guaranteed to happen and we could keep on going right in to September with no changes in sight. For now, the tropics are not an issue. Enjoy the quiet while it lasts.
I’ll have more tomorrow.
M. Sudduth 9:17 AM ET Aug 11