Current Hurricane Activity Raises Questions About The AMO - What Is It And Why Is it Relevant?

Have you taken a look at satellite view of the tropics right now? Hurricane Humberto, a major hurricane, threatens Bermuda. The remnants of Tropical Storm Imelda are drenching Southeast Texas, and several potential systems lurk in tropical regions that we look to at this time of the year. National Hurricane Center tropical meteorologist Eric Blake captures it best in this Tweet:

Anyone want a tropical storm? They are forming like roaches out there! 6 at once in both basins combined is thought to tie a modern NHC record , with two other disturbances adding the cherries on top of a crazy busy day!

The hurricane basins of the Eastern Pacific and Atlantic are very active as seen in the picture below that I took at The Weather Channel early Wednesday morning. While likely not at the forefront of your thought processes this week, this active week prompted me to wonder about the status of something called the Atlantic Multidecadal Oscillation (AMO). What is it and why am I bringing it up during hurricane season?

According to the University Corporation for Atmospheric Research (UCAR) website, the AMO is:

a coherent mode of natural variability occurring in the North Atlantic Ocean with an estimated period of 60-80 years. It is based upon the average anomalies of sea surface temperatures (SST) in the North Atlantic basin, typically over 0-80N.

The AMO has been at the center of many of the discussions about whether hurricane activity changes naturally or is being affected by climate change. I remember a particularly vigorous debate about these topics after the anomalously active 2005 hurricane season that gave us Hurricane Katrina and a series of storms taking on “Greek-letter names.” I haven’t heard as much about it recently, but it is still a “thing.” I often found the AMO-natural variability or anthropogenic climate change debate to be silly. I continue to be baffled by why these things are framed as “either/or” rather than “and.” The current scientific literature suggests the climate change signal on hurricanes will likely be apparent in intensity, forward motion, and surge inundation. The outstanding NOAA GFDL page on hurricanes and climate change points out that there is less conclusiveness on frequency. However, natural climate variability like the AMS is certainly in the mix. A 2017 study in Nature Scientific Reports argues that a negative AMO is emerging in spite of a warm subtropical region. A negative or cool phase is typically associated with fewer Atlantic hurricanes (graphic below).

I reached out to tropical expert Dr. Phil Klotzbach to get his latest thoughts on the AMO, and how this all aligns with what he is seeing in recent years. His group at Colorado State University issues seasonal hurricane forecasts. In their August update, they called for a “near normal” season in terms of activity.

I posed the question to Dr. Klotzbach, “So what’s going on with the AMO right now?” His answer:

That’s the million dollar question. The winters have looked like a very negative AMO with a cold SST tripole. But those cold anomalies have been much weaker in the summer when the far North Atlantic has a much shallower mixed layer.

Dr. Klotzbach also told me that when he examined sea surface temperature differences (SSTs) from 2014-2019 minus 1995-2012 averaged over the period August to October (excluding 2019), the far North Atlantic remains colder but the tropical Atlantic SSTs haven't shown much change. Klotzbach goes on to say:

There has been quite a bit of discussion about a weakening of the Atlantic Meriodional Ocean Circulation (AMOC) in the literature - including a couple of high profile papers published in Nature. The cold SST in the far North Atlantic bares that point out. However, the connection between the polar regions and the tropical regions doesn't seem to be there during the summer months. Normally a cold far North Atlantic drives a stronger subtropical which drives stronger trade winds that then anomalously cool the tropical Atlantic. This has certainly been the case in the winter months, but the relationship has broken down in the summer

I am providing links to 2017 and 2019 studies, respectively, in the Nature Climate Change.

Ultimately, September is a climatologically-active month so there is nothing unusual about seeing tropical waves, depressions, storms and hurricanes at this time of year. Eric Blake’s tweet just inspired me to revisit what people are thinking about the AMO since it was such a hot topic after the 2005 hurricane season.