It should be another quiet Atlantic hurricane season in 2015, and the active hurricane pattern that began in 1995 may now be over, said NOAA in their May 27 seasonal hurricane forecast. They give a 70% chance of a below-normal season, a 20% chance of a near-normal season, and only a 10% chance of an above-normal season. They predict a 70% chance that there will be 6 - 11 named storms, 3 - 6 hurricanes, and 0 - 2 major hurricanes, with an Accumulated Cyclone Energy (ACE) 40% - 85% of the median. If we take the midpoint of these numbers, NOAA is calling for 8.5 named storms, 4.5 hurricanes, 1 major hurricane, and an ACE index 62.5% of normal. This is well below the 1981 - 2010 average of 12 named storms, 6 hurricanes, and 3 major hurricanes. Hurricane seasons during the active hurricane period 1995 - 2014 averaged 14.7 named storms, 7.6 hurricanes, and 3.5 major hurricanes, with an ACE index 142% of the median. Only three seasons since 1995 have been classified by NOAA as being below normal--including two El Niño years (1997 and 2009), and the neutral 2013 season.
The forecasters cited the following main factors that will influence the coming season:
1) The current borderline weak/moderate El Niño event is expected to persist or intensify during the 2015 hurricane season. El Niño events tend to suppress Atlantic hurricane activity in three ways:
- By creating high levels of wind shear over the tropical Atlantic, which tends to tear storms apart.
- By increasing sinking motion and high pressure over the tropical Atlantic.
- By making the air more stable over the tropical Atlantic.
2) Near-average sea surface temperatures (SSTs) are in place over the hurricane Main Development Region (MDR), from the Caribbean to the coast of Africa between between 10°N and 20°N. These SSTs are expected to be near or below average during the peak August - October portion of hurricane season, and are expected to be cooler than SSTs in the remainder of the global tropics (SSTs in the remainder of the global tropics were 0.31°C warmer than SSTs in the MDR in May.) This configuration of SSTs is often quite hostile to Atlantic tropical cyclone development.
3) The active period of hurricane activity that began in 1995 due to a natural decades-long cycle in hurricane activity called the Atlantic Multi-decadal Oscillation (AMO) may now be over. The SST pattern associated with that cycle is absent this year, and NOAA said: "There have been two seasons in a row, 2013 and 2014, with below-normal and near-normal activity respectively and neither had an El Niño event responsible for the reduced activity. The current configuration of SSTs in the Atlantic Ocean, both in the MDR and the entire North Atlantic, are suggestive that the AMO may no longer be in the warm phase."...MORE