On June 1, the Atlantic Hurricane Season will officially begin, however one named storm is already in the record books as Tropical Storm Arlene briefly formed over the northern Atlantic between April 19 through April 21. Even the NHC admits that without significant improvements in satellite technology over the past few years, the formation of Tropical Storm Arlene, an extremely compact warm core low, would not likely have been detected and bring to question whether similar storms were simply missed in the past. So with one named storm in the books, what will the rest of the Hurricane Season Bring? Let me give you my thought.s
The Atlantic Hurricane Season is going to be under the influence of three main factors. One, the changing state of the Atlantic Multidecadal Oscillation from positive to negative. Two, the influence of Saharan dust on the eastern tropical Atlantic. Three, the developing El Nino and the Sub Tropical jet stream.
AMO STATE AND CHANGE:
The AMO is still slightly positive but clearly trending towards a negative state after peaking in 2010 at +1.3C and has since fallen off to +0.23C as of last month. The changes in the Tropical Atlantic has been clear with SSTA near to below normal in the Gulf of Mexico, Caribbean Sea, and western Atlantic while the eastern Atlantic towards the African coast has been near to slightly above normal. I should note, compared to previous years, especially the past 10 years, the SSTA in the Tropical Atlantic are the coldest I’ve seen in a while heading into the hurricane season. What especially stands out is the well below normal SSTA to the north of the Greater Antilles and around the Bahamas, which if this anomaly remains a theme through the Summer would have a significant limiting influence on approaching tropical low pressure systems.
If trends continue, the shift from a positive to negative AMO state will have several impacts. For one, the heat content in the Tropical Atlantic will remain below normal through the Atlantic Hurricane Season (June through November), which means tropical low pressure systems will have less available warm waters and thus less latent heat energy to drive warm core low pressure systems. A neutral to negative AMO state also supports more below normal heights over eastern North America and the Gulf of Mexico, making for a hostile environment for warm core low pressure development.
A negative AMO state also typically supports a more positive NAO block in the northern Atlantic, however there is some debate on whether these high latitude blocks are driven more by stratospheric influence or SSTA. In my opinion, both factors have a role to place in the development of the 500 MB pattern and no two configurations are the same. While the NAO on average would eventually trend to a positive state in a negative AMO environment over the next 30 years, the influences in the stratosphere can override those factors, like what is happening now.
Another factor to watch is the development of the Saharan influence on the Tropical Atlantic. With the current high latitude blocking regime in place and not likely to collapse any time soon given stratospheric developments, the influence from the Saharan Desert will likely be limited to the eastern Atlantic. Of course, given most tropical disturbances come from the African coast, the transport of dust off the Sahara Desert can and likely will continue to have a lasting influence. With the Madden-Julian Oscillation (MJO) taking on a more El Nino influence over the next several months, the reduction in convective low pressure systems across Africa will be reduced and likely will help to enhance drought conditions in the Saharan Desert, which in turn will enhance dry dusty air transport off the African coast. This factor will help to reduce tropical low pressure development off the African coast this year.
As discussed yesterday, El Nino is on the way to developing and should be established by the end of the Summer, which is just in time for the heart of the hurricane season from August through October. This El Nino is expected to be weak and west based based on all available data, which would support a strong Sub Tropical jet stream influencing the Gulf of Mexico and North America. An El Nino influenced 500 MB and 250 MB pattern enhances the Sub Tropical jet stream in the Atlantic and leads to more troughs to dig from the mid latitudes into the tropics over the Atlantic Ocean. As a result, El Nino states tend to support above normal mid and upper level shear environments which inhibits tropical development.
Given the factors above, I’m expecting a below normal hurricane season given the various factors that will inhibit tropical development.