At the start of every hurricane season, the big question everyone asks is “How bad will this season be?” To prepare for the upcoming season, outlooks are released. Originally, the National Hurricane Center in Miami, Florida, predicted for an average to an above-average season.
As we are approaching the peak of the Atlantic Hurricane Season (mid/late September), forecasters and hurricane specialists are making an update to their forecast. Based on the first two months of the season, some key indicators have made them update their forecast and lean towards the possibility of an average to below-average season. In total, there are three reasons that have made forecasters and specialists update and, therefore, lower the outlook. Two of the three indicators are an increase in both wind shear and the Saharan Air Layer in the Main Development Region (MDR). The MDR is the part of the Atlantic Ocean where hurricanes are most likely to form and is positioned between Africa and the Caribbean Sea. The third reason is the most important: the presence of El Niño in the Central Pacific Ocean.
The peak of the Atlantic Hurricane Season occurs in mid/late September but begins to ramp up in August. The first indication of a below-average Atlantic Hurricane Season is the increase of wind shear over the MDR. According to the National Weather Service, wind shear is the rate at which wind velocity changes from point to point in a given vertical direction. When there is wind shear present in the MDR, the winds go against the winds associated with the tropical system. By the winds going in the opposite direction of the tropical system’s winds, the system gets torn apart and hinders any further tropical development. The wind shear forecast for August to October in the MDR illustrates winds greater than 20 knots: about 2-4 standard deviations above average.
The second indication in the MDR is the Saharan Air Layer. The Saharan Air Layer is simply hot, dry air that originates from the Sahara Desert in Africa. This hot, dry air leads to dust exiting the Sahara Desert, and entering the MDR. A “layer” is then created in the atmosphere, making it very difficult for tropical development. The Saharan Air Layer removes the moist air and convective processes that are crucial to tropical development. According to Michael Lowry, the Saharan dust was at a record high across the MDR for June and July. At one point at the end of June, the dust made its way all the way across the Atlantic and was noticed in Houston, Texas. If this continues to persist through August and into September, it will be extremely difficult for tropical development to unfold with any real degree of consistency.
The last and most notable indication for a quieter Atlantic Hurricane Season is El Niño in the Central Pacific Ocean. El Niño is the warming of the Central to Eastern Pacific Ocean to greater than or equal to at least 0.5 degrees Celsius above average. Historically, when El Niño is present in the Central Pacific Ocean, it affects the Atlantic Hurricane Season in a negative way. El Niño increases vertical wind shear and trade winds over the Atlantic Ocean and creates a more stable environment. In addition, El Niño causes sinking air in the Atlantic Ocean, which leads to cooler sea surface temperatures.
In the image above, there is a rectangle highlighting the MDR, which is illustrating cooler than normal sea surface temperatures. Due to El Niño’s effect on the cooling of the Atlantic Ocean, it results in less tropical development, as tropical systems depend on warmer waters to generate the deep convection needed for development. By the cooling of the MDR, it is more difficult for the systems to fully develop.
Due to the increased wind shear, the Saharan Air Layer, and El Niño all being present, forecasters and specialists have begun to update their outlook for the Atlantic Hurricane Season. If these conditions do persist into the peak of the season, we could see a quieter Atlantic Hurricane Season.
Credit: NOAA, Twitter.com @MichaelRLowry
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©2018 Weather Forecaster Andrew Lunavictoria