2:40 PM | *”Ring of cool” in the Atlantic Ocean and dry (Sahara Desert) air putting a damper on tropical activity so far in the Atlantic Basin*
Paul Dorian
Current sea surface temperature anomalies are shown for the North Atlantic Ocean with a widespread area of cooler-than-normal (blue) in the "breeding grounds" region of the tropical Atlantic (boxed-in area); courtesy NOAA, tropicaltidbits.com
Overview
The Atlantic Basin tropical season is still rather young having “officially” just begun a few weeks ago on June 1st, but there have been two factors so far that have suppressed activity and they may not let up anytime soon. First, sea surface temperatures have been running at below-normal levels in the all-important "breeding grounds" region of the tropical Atlantic Ocean in the region extending from the west coast of Africa to the Caribbean Sea. Second, there has been a persistent flow of dry air moving westward from the Sahara Desert region of Africa into this same "breeding grounds" region of the tropical Atlantic Ocean. Both of these factors tend to inhibit the formation of tropical storms or the intensification of storms that actually do manage to form.
Dry air originating over the Sahara Desert region of Africa extends westward in much of the same "breeding grounds" region of the tropical Atlantic that is currently experiencing cooler-than-normal water temperatures; courtesy University of Wisconsin/CIMSS, NOAA
Details
In terms of sea surface temperatures, there has been an impressive “ring of cool” in the North Atlantic with persistent below-normal readings for this time of year and this cooler-than-normal water extends from Africa’s west coast into the Caribbean Sea. In fact, some of this tropical Atlantic region is experiencing some of the coldest water for mid-June going all the way back to the early 1980's when consistent monitoring began. Observations have generally shown that sea surface temperatures of at least 26°C are required for the formation of tropical storms and a good part of the tropical Atlantic is currently below that empirical threshold. The sea surface temperature requirement is related to how tropical systems derive their energy. A tropical storm can be thought of as an engine that requires warm, moist air as fuel. This warm, moist air cools as it rises in convective clouds and condenses into water droplets releasing heat (known as the latent heat of condensation). This heat provides the energy for the tropical storm. The 26°C value is tied to the thermal properties of the atmosphere in the tropics. Above this particular temperature deep convection can occur, but below this value, little or no convection is usually found.
Actual sea surface temperatures with much of the "breeding grounds" region below the threshold of 26 degrees C which is usually a requirement for tropical storm formation; courtesy NOAA, tropicaltidbits.com
In addition to cooler-than-normal sea surface temperatures, an inhibiting factor related to tropical storm development in the Atlantic Basin can be the emergence of dry air at multiple layers of the atmosphere. If in sufficient quantity, dry air can reduce the chances for convective activity and the release of latent heat of condensation which, as described above, is a requirement for tropical storm formation. One way to get dry air over the tropical Atlantic Ocean this time of year is for seasonal trade winds to carry Sahara Desert air masses from east-to-west. Indeed, there has been quite an expansive area of Sahara Desert dry air in recent weeks extending all the way from the west coast of Africa to the Caribbean Sea and this has no doubt had a suppressing effect on tropical storm formation.
Sea surface temperatures in the North Atlantic Ocean have been running at below-normal levels for several weeks; courtesy NOAA, tropicaltidbits.com
The only named tropical storm so far this season in the Atlantic Basin has been “Alberto” which was actually designated as a “pseudo” or “sub-tropical” system. This system never strengthened into hurricane (category 1, weak) status. In recent days, an area of convection over the northwestern part of the Caribbean Sea headed northwest to the Texas/Louisiana border region of the Gulf of Mexico and, while it resulted in heavy rainfall for parts of both states, it never strengthened into hurricane or even tropical storm status.
One other factor that soon may play an important role in the Atlantic Basin tropical season is the potential formation of an El Nino in the tropical Pacific Ocean. Many computer forecast models suggest El Nino conditions (i.e., warmer-than-normal water) may form in the equatorial part of the Pacific Ocean by the latter stages of the tropical season and this is usually an inhibiting factor in the tropical Atlantic Basin as it tends to be correlated with high levels of wind shear.
Stay tuned as we’ll continue to closely monitor the sea surface temperature anomaly patterns in both the Atlantic and Pacific Oceans over coming weeks as well as the westward extent of dry (Sahara Desert) air from western Africa.
Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com