The amount of Atlantic hurricane activity in a season is most directly driven by two factors: the El Niño Southern Oscillation (ENSO) and the Atlantic Multidecadal Oscillation (AMO). These two oscillations are the primary indices used in forecasting activity for the Atlantic hurricane season. The AMO is a pattern of sea surface temperature anomalies (SSTAs) over the Atlantic Ocean, and is strongly correlated with the activity of Atlantic hurricane seasons. While the ENSO changes nearly every year, the AMO is a longer term cycle that usually shifts every 25-50 years. This post will explain the difference between a positive (warm) AMO phase and a negative (cold) AMO phase, as well as analyze the current AMO state.

Positive (Warm) AMO vs. Negative (Cold) AMO

In the positive (warm) phase of the AMO, sea surface temperatures are typically above normal in the Atlantic Main Development Region (MDR), the eastern subtropical Atlantic, and the far North Atlantic. Sea surface temperatures are typically below normal or near normal in the western subtropical Atlantic near the United States East Coast. The positive AMO phases since 1900 have been approximately from 1920-1970 and since 1995. The warm phase of the AMO is positively correlated with Atlantic hurricane activity, especially in the MDR. The five Atlantic hurricane seasons since 1950 with the most Accumulated Cyclone Energy (ACE) (2005, 1995, 2004, 1950 and 1961) have all occurred during a positive AMO phase.

In the negative (cold) phase of the AMO, sea surface temperatures are typically below normal in the Atlantic MDR, the eastern subtropical Atlantic, and the far North Atlantic. They are typically near or above normal in the western subtropical Atlantic. The negative AMO phases since 1900 have been approximately from 1900-1920 and 1970-1994. The negative phase of the AMO typically results in less active Atlantic hurricane seasons overall. The activity in the Atlantic MDR is the most strongly affected. However, there have been many intense or destructive hurricanes in negative AMO seasons. An example is Hurricane Gilbert in 1988, which was the most intense Atlantic hurricane on record at the time with maximum sustained winds of 185 mph and a minimum pressure of 888 mb. The amount of United States landfalls does not seem to be strongly correlated with the AMO state. The five Atlantic hurricane seasons since 1950 with the least Accumulated Cyclone Energy (ACE) (1983, 1977, 1972, 1982, 1994) have all occurred during a negative AMO phase.

The Current State of the AMO

The current state of the AMO is complicated to assess, and many experts disagree on whether we are in a positive or negative AMO phase. The current Atlantic SSTA pattern consists of above-normal SSTs in the Tropical Atlantic, subtropical East Atlantic, and subtropical West Atlantic. This accounts for an official AMO value for May 2017 of +0.320, calculated based on the overall SSTA of the Atlantic basin. Phil Klotzbach, meteorologist at Colorado State University (CSU) specializing in hurricane season forecasting, believes the AMO could have possibly switched to a negative phase. He believes the official AMO index value could be misleading because it is largely inflated by the above-normal sea surface temperatures in the western subtropical Atlantic. He created his own AMO index based on the SSTA in the far North Atlantic and sea level pressure anomaly in the Tropical Atlantic. His index has generally trended neutral to negative since 2014, largely as a result of higher than normal pressures in the Tropical Atlantic and a persistent “blob” of cooler than normal SSTAs in the far North Atlantic. To support his hypothesis, the Atlantic had three straight seasons of below normal activity from 2013 to 2015, although the Atlantic rebounded to an active-era level of activity in 2016.

In my opinion, the Atlantic is still in a positive AMO phase, and has been since 1995. For the most part, SSTAs in the Tropical Atlantic have been above average during the past few years, especially over the last year. During the 2016 Atlantic hurricane season, the Atlantic had more named storms (15) and major hurricanes (4) than any season of the 1970-1994 inactive phase. Last season’s Atlantic Accumulated Cyclone Energy (ACE) value of 141 was higher than all years over the 1970-1994 period except for 1980.  Although the 2016 Atlantic hurricane season was above average overall, there were some indications during the season to support the idea that the AMO could have flipped to the negative phase. For instance, the Atlantic Main Development Region (MDR) had only one hurricane form in the region all season – Hurricane Gaston, which peaked at minimal hurricane strength in the region. Gaston later strengthened into a major hurricane over the anomalously warm subtropical Atlantic. The overall Atlantic sea surface temperature pattern does not really resemble the classic positive or negative AMO signal, but based on the warmer than normal MDR and very strong African tropical waves, I highly doubt the active era of Atlantic hurricane activity has come to an end.

Do you think we are in a positive AMO or negative AMO? I have a Twitter poll open to determine the public opinion of the AMO debate.

The tropics are quiet globally at the moment. I will be back tomorrow for my weekly “A Look At The Atlantic” post.