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, though they have increased in recent years because of long African droughts and increased human activity in previously empty areas.

 

We are only starting to understand what happens between dust and hurricanes.  Scientists are much more able to track and measure the Saharan Air Layer (SAL), but its coverage and progress can’t be predicted with any useful accuracy more than a few days in advance.  It’s a three-dimensional puzzle with a lot of moving parts.

 

As the 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 Wave, 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.

 

In recent years, drought and human activity – war, agriculture, industry and tourism — in the Sahara and Sahel regions of Africa have kept a thick protective blanket of hot, dry air over the tropical Atlantic through most of the storm season.  This year may be different.  Both NOAA and the UK Met Service are predicting above-average rainfall for the Sahel through the July-September “wet season”.

 

For the last month a steady parade of tropical waves has rolled off the African coast and none has posed any serious threat of becoming a hurricane.  The tropical Atlantic is a lot (3oC) cooler than it was at this time last year, for two reasons.  High North Atlantic pressure has produced strong trade winds that cool the ocean by evaporation, wave mixing and upwelling of deep water.  The thick SAL shades the surface, keeping it too cool to be good hurricane fuel, while it keeps the Tropical Waves pinned down along its southern edge.

 

The cool sea surface has prompted three respected forecasters (Colorado State University, Tropical Storm Risk and WeatherBell) to reduce their 2018 storm outlooks slightly.  However, they point out that this could change quickly.  If the African wet season is a good one, there will be less hot dry air and a lot less dust in the tropical Atlantic through the busiest part of our hurricane season.  That means more African storms reaching the Caribbean in August – October, with more of their energy intact.  BE PREPARED.

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, though they have increased in recent years because of long African droughts and increased human activity in previously empty areas.

We are only starting to understand what happens between dust and hurricanes. Scientists are much more able to track and measure the Saharan Air Layer (SAL), but its coverage and progress can’t be predicted with any useful accuracy more than a few days in advance. It’s a three-dimensional puzzle with a lot of moving parts.

As the 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 Wave, 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.

In recent years, drought and human activity – war, agriculture, industry and tourism — in the Sahara and Sahel regions of Africa have kept a thick protective blanket of hot, dry air over the tropical Atlantic through most of the storm season. This year may be different. Both NOAA and the UK Met Service are predicting above-average rainfall for the Sahel through the July-September “wet season”.

For the last month a steady parade of tropical waves has rolled off the African coast and none has posed any serious threat of becoming a hurricane. The tropical Atlantic is a lot (3oC) cooler than it was at this time last year, for two reasons. High North Atlantic pressure has produced strong trade winds that cool the ocean by evaporation, wave mixing and upwelling of deep water. The thick SAL shades the surface, keeping it too cool to be good hurricane fuel, while it keeps the Tropical Waves pinned down along its southern edge.

The cool sea surface has prompted three respected forecasters (Colorado State University, Tropical Storm Risk and WeatherBell) to reduce their 2018 storm outlooks slightly. However, they point out that this could change quickly. If the African wet season is a good one, there will be less hot dry air and a lot less dust in the tropical Atlantic through the busiest part of our hurricane season. That means more African storms reaching the Caribbean in August – October, with more of their energy intact. BE PREPARED.

SAL with four Tropical Waves trapped along its southern edge
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