August 23, 2022
… SAL vs. the hurricanes
Last week, we looked at some of the effects of Saharan dust on the Caribbean – soil enrichment, haze, breathing problems and fouled machinery on land, algae growth and plankton blooms in the sea. These effects have been known for decades, but we are only starting to understand what happens between dust and hurricanes.
As the Saharan Air Layer (SAL) carries its dusty load from Africa to the Americas, the grains of dust reflect some of the sun’s heat, keeping the sea surface under the clouds cooler than it would otherwise have been. This cooling effect can block or slow the development of hurricanes, which need the updraft from warm water to gain strength.
However, there is more to the relationship between dust clouds and tropical storms than just sea surface cooling. When the SAL meets a tropical disturbance, one of three things may happen.
First, the cooling effect may take enough energy out of the system that it dies away without becoming a named storm. We have seen this repeatedly in the last three years, as scores of disturbances have either failed to develop or blown themselves out in mid-ocean.
Second, if the meeting happens while the Saharan system is still very hot and dry, some of the water in the storm clouds may evaporate. This takes away energy, causing a cooler downdraft that may shear off part of the developing storm and keep it from getting organized. In fact, the wind carrying the dust may also shear through a storm, and this effect will be increased by evaporation.
Third, a storm that has already started to spin may just suck the dust into its circulation. If the dust stream has lost some of its heat, water vapour in the storm clouds may condense on the dust grains, forming raindrops or ice pellets. This releases energy and strengthens the storm. At the same time, the circular motion flings raindrops and ice pellets outward, forming rain bands or squall lines.
These effects were explored on a smaller scale in a 2005 study of thunderstorms in south Florida. In the presence of dust the “anvils” – flattened cloud tops where warm, moist thunderstorm meets cold upper air – were denser and better organized, producing smaller but more intense storms.
In recent years, drought in the Sahara and Sahel regions of Africa has kept a thick protective blanket of hot, dry air over the tropical Atlantic through most of the storm season. Dozens of possible storms have been snuffed out or greatly weakened before they could develop. This year may be different.
The Sahel drought began to break in late 2016. Regular rainfall continued through this spring, and some of the heavy downpours in early June extended north into the Sahara. Both NOAA and the UK Met Service are predicting above-average rainfall for the Sahel through September. By the end of the year, this formerly thriving agricultural region could be turning green again.
The early effects are already starting to show up. Last week, the University of Wisconsin’s excellent SAL tracking map (http://tropic.ssec.wisc.edu/real-time/splitEW.jpg) showed a big gap off the African coast with no dust at all, and the SAL “blanket” is still smaller, thinner and further north than usual.
If the forecasters are right, there will be less hot dry air and a lot less dust in the tropical Atlantic through the busy part of the hurricane season. That means more African storms reaching the Caribbean, with more of their energy intact. BE PREPARED.