The main development region for Atlantic hurricanes heated up rapidly in April while the Gulf Stream slowed down for 2 weeks . Tides on the southeast coast have been a half a foot to a foot higher than normal since mid-April in response to the Gulf stream slowdown and wind changes across the Atlantic. An abnormal wind pattern caused by an atmospheric bridge from the intense El Niño in the tropical Pacific ocean is heating the tropical north Atlantic. The same processes that caused very intense hurricane seasons in 1998, 2005 and 2010 are taking place now. An extreme Atlantic hurricane season is likely in 2016. Higher than normal east coast tides and rapidly rising sea levels will make the east coast extremely vulnerable to storm and surf damage this summer and fall.
Strong high level winds caused by intense thunderstorms in the tropical east Pacific have crossed over Mexico, the Gulf of Mexico and over the subtropical Atlantic carrying heat and humidity. Weak low pressure areas have formed under waves in this “subtropical jet stream”, weakening the trade wind flow across the Atlantic. Southerly flow across the equator has increased. These weather shifts brought on by the atmospheric bridge have intensified the normal spring time warming of the tropical Atlantic ocean.
An atmospheric bridge of strong winds is carrying heat and water vapor from the tropical Pacific to the tropical Atlantic. This image shows winds at the jet stream level (250mb) and total water vapor levels over the tropical western hemisphere for morning 3 May 2016. Note the big dome of warm air south of Mexico where exceptional storminess has injected heat and water vapor into the atmosphere.The warm waters of the fading El Niño have intensified the rising motion of air over the north eastern Pacific and increased subsidence over the tropical south Atlantic. Increased subsidence over the tropical south Atlantic, combined with lower pressure over the tropical north Atlantic has increased the southerly flow of air across the equator.
More uplift than normal (green) has been caused by warm El Niño waters in the north eastern tropical Pacific. The tropical Atlantic south of the equator has had more subsidence than normal (brown) while more uplift than normal has been induced by El Niño enhanced winds north of the equator. The shifts in surface pressure patterns have caused a southwesterly wind anomaly in the tropical north Atlantic.This southerly flow causes cooling on the equator in the mid-Atlantic and warming north of the equator. Moreover, The Gulf Stream is mostly wind driven. When the trade winds weaken in the tropical north Atlantic there is less force acting on the currents in the Caribbean that drive water into the loop current in Gulf of Mexico and the Gulf Stream. Thus the Gulf Stream is prone to suddenly weaken in this situation. When the Gulf Stream weakens tides jump up on the east coast.
Preliminary water levels from Fort Worth pier, Florida, near where NOAA observes Gulf Stream flow rates across a cable that runs from Florida to the Bahamas. Note the jump in water levels to up to 1 foot above normal occurred at the same time as the Gulf Stream slowed.The recent years of 2010, 2005 and 1998 with extreme Atlantic hurricane activity followed El Niño events in the tropical Pacific. However, the super El Niño of 1982 was followed by a low activity year in the Atlantic because the El Niño persisted into summer 1983. Wind shear from westerly winds off of the Pacific in El Niño years rips apart developing tropical storms in the Atlantic. A recent 2008 study by NOAA’s AOML in Geophysical Research Letters found that El Niños that don’t continue through the winter into spring fail to produce Atlantic warming. Thus, to have a strong bridge from the Pacific to the Atlantic that induces an intense hurricane season after an El Niño the equatorial eastern Pacific must stay warm into the spring but cool by mid summer. That’s just what’s happening now.
This year the intense warming wasn’t apparent until April because intense storminess continued well into March in the seas around Greenland. Suddenly the pressure patterns reversed over Greenland on about April 10 and wind patterns shifted across the whole north Atlantic. That’s when the Gulf Stream slowed about 25% for a week and tides rose.
I predicted last September that the 2016 hurricane season in the north Atlantic would be intense based on the atmospheric bridge from the warm tropical Pacific and based on NOAA’s CFS2 climate model. However, the CFS2 model broke down in late fall. It couldn’t handle the strong layered currents in the equatorial Atlantic. It mixed in far too much cold water from depth, water that is held down by the warm fresh light Amazon river water layer. This year literally broke the model.
The Colorado State team did not predict an intense hurricane season in their early forecast but they made their forecast before the rapid warming of the tropical Atlantic took place. Tropical storm forecasts made before April have very little predictive power because of the problems I discussed above.
The cold pool south of Greenland began rapidly dissipating as extraordinarily high pressure warmed the region in April. With the dissipation of the cold pool and the weakening up trade winds that cause cool water to well up in the Caribbean, the Yucatan and off of north Africa, the tropical Atlantic has rapidly, in the month of April, become much more favorable for tropical storm formation. At the same time, cool water has started to appear along the equator in the eastern Pacific indicating El Niño is ending and La Niña is coming. Conditions in the tropics now appear to be maximal for much above normal Atlantic basin tropical storm activity in summer and fall 2016. Heat is building up in the main development region in a very similar manner to how it built up in 2005. However, this year the heat content started out much higher on the east coast of the U.S. And tide levels are higher.
Gulf Stream flow has returned to normal between Florida and the Bahamas but tides remain about a half foot above normal. More episodes of Gulf Stream slowing and high water on the east coast can be expected as the atmospheric bridge and the developing “Atlantic Niño” continue to weaken the trade winds that push the Gulf Stream. The combination of high water with possible intense hurricanes puts the east coast and the Gulf Coast at high risk of storm damage this year.
The Gulf Stream typically flows in the low to mid 30 Sverdrups. It slowed to 19.8 on April 18, 2016. The last time a day had so little flow followed hurricane Sandy's disruption of the Florida current in late 2012.The Florida Current cable and section data are made freely available on the Atlantic Oceanographic and Meteorological Laboratory AOML) web page (www.aoml.noaa.gov/phod/floridacurrent/) and are funded by the DOC-NOAA Climate Program Office-Climate Observation Division