So far this year we’re looking at the prospect of a very active hurricane season, potentially one of the most active on record. Sea Surface Temperatures (SSTs) are so far this year at a record high with the sunspot cycle at a minimum lending itself to large hurricane favoring differences between temperatures in the upper and lower atmospheres. What will determine the scope of this year’s hurricane season are how strong wind shear remains to prevent development as well as what kind of dust storms come off of Africa and how they impact the formation of low pressure storm centers.
Sea surface temperature anomalies, May 2010. Image Credit: NOAA
Water temperatures in the Atlantic for the last 3 months have been the warmest on record being 1.46°C above average.
Past hurricane seasons that had high March SST anomalies include 1969 (0.90°C anomaly),2005 (1.19°C anomaly), and 1958 (0.97°C anomaly). These three years had 5, 7, and 5 intense hurricanes, respectively. Just two intense hurricanes occur in an average year. The total averaged activity for the three seasons was 15 named storms, 11 hurricanes, and 6 intense hurricanes (an average hurricane season has 10, 6, and 2.)
Compare the above water temperature differences from average for May 13th 2010 to those of last year on May 14th.
Sea surface temperature anomalies, May 2009. Image Credit: NOAA
We’re at present witnessing a weaker Azores-Bermuda high which reduces trade winds causing “less mixing of the surface waters with cooler waters down deep, plus less evaporational cooling of the surface water.”
The SSTs are already as warm as we normally see in July between Africa and the Caribbean, and we have a very July-like tropical wave approaching the Lesser Antilles Islands this weekend.
According to recent research there is a link between sunspot activity, changes in solar energy and the resulting atmospheric temperatures here on earth. During peak sunspot activity we can have slightly higher solar energy overall but significantly higher UV energy. During minimum sunspot activity we have lower energy overall a much lower UV energy. At the moment we’re at the minimum of the 11 year sunspot cycle meaning we’re facing an average of 10% lower UV energy. However, despite there being lower energy, this actually contributes to more hurricanes, not less.
Why we’ll see more hurricanes for less solar energy comes down to basics of how weather systems work. At their most basic notion weather systems work to redistribute solar energy. The sun heats up one area causing molecules to vibrate more rapidly and air to expand. This is what creates lower density, or “low pressure”. Areas with cool air remain less active and thus more dense creating “high pressure” zones. Air naturally tends to move from high pressure areas to low pressure areas which creates wind, develops clouds and can create storms.
Similarly we can also have differences between the temperatures of the upper and lower atmospheres. If temperatures closer to the surface to the surface of the earth are warmer and the upper atmosphere cooler, warm air rises. Often times it’ll follow the path of least resistance and the larger the difference between the upper and lower atmospheres the more chance there is for a significant low pressure center of rising air to form to allow the warm air to rise more rapidly. Such a low pressure centers are the foundations of tropical storms and hurricanes. Less sunspots mean less UV energy which directly contributes to heating the Ozone layer. A cooler Ozone layer contributes to a greater difference in temperature and pressure between the upper and lower layers of the atmosphere, increasing the likelihood of low pressure center development.
However, over the last few years research has discovered that various factors can throw a wrench into hurricane development forecasts and predictions. These primarily are wind shear and dust storms. Wind shear is caused by the difference in wind speeds and directions at different levels of the atmosphere. Pressure differences at each level of the atmosphere develop wind similarly as they do on the surface to move pressure from high to low. This difference in the winds at different altitudes can impact the development of the low pressure center of a tropical storm or hurricane as it ends up with more resistance for air to rise effectively.
Saharan Dust, Image Credit: NASA via myweatherlady.com
Dust storms form a part of the Saharan Air Layer (SAL), a mass of dry dusty air that can blow off the Saharan Desert in Africa. Periods of high dust storms can inject warm dry in the middle levels of the atmosphere. Warm dry air sitting in the middle of the atmosphere can disrupt the flow of warm air at the surface of the earth to the cool areas in the upper atmosphere. The reason being is that the SAL creates a form of temperature inversion where warmer air is injected above cooler air that lies in the atmospheric layer immediately below. This prevents the lower air from rising ceasing the development of low pressure storm centers. What we can note in the image below is that presently the SAL is a bit on the weaker side, though the pocket of strength to the west of the Cape Verde islands shows some potential.
Saharan Air Layer, Image Credit: NOAA
Thus at present it looks like the prospect for this year is that we’re facing a more active than normal hurricane season. Sea Surface Temperatures set to be extraordinarily high compounded with a low number of sunspots suggest we’ll have considerably active hurricane developments this year. Only time will tell however what impact wind shear and dust storms from the Sahara will have on hurricane development as they have been known to throw off predictions and expectations in the past.
Dr. Jeff Masters’ WunderBlog
Current Operational SST Anomaly Charts
The Saharan Air Layer and Hurricanes
The Saharan Air Layer suppresses hurricanes
Tropical Cyclones … A Satellite Perspective