The 2013 Atlantic hurricane season is predicted to be very active by every group making such forecasts. It’s been a quiet start, but it’s far from over.
Through today, we’ve seen five named storms in the Atlantic (Andrea, Barry, Chantal, Dorian, and Erin), but no hurricanes. While this is a bit on the slow side, it’s certainly not unprecedented, nor does it allow for too many conclusions to be drawn about the remainder of the season.
In the past 30 years, only three years had no hurricanes through the end of August: 2002, 2001, and 1984.
As the chart below shows, the average date of the first hurricane formation (defined as first advisory written on a storm at hurricane intensity) is August 4, but there’s a rather large standard deviation of 26 days. Based on that, we’d say that the date of first hurricane formation typically ranges between July 9 and August 30. In this time series, the three latest dates are Sep 9, Sep 10, and Sep 11- so we have a climatologically-active stretch to go until we pass those dates.
According to my long-time friend and colleague, Dr. Phil Klotzbach at Colorado State University, if we extend the search back to 1960, and change the requirement to just no hurricanes during August, then the list broadens to include four additional years: 1961, 1982, 1988, and 1997, for a total of seven analogs. Among those, 1961, 1988, and 2001 all ended up being very active and/or destructive hurricane seasons.
1961 is an especially interesting year, in that August had no tropical cyclones and followed that up with the most active September on record with four major (Category 3-5) hurricanes forming! On the other hand, 1982 and 1997 remained quiet throughout the season.
In terms of ACE (Accumulated Cyclone Energy), the season stands at 7.6 units, compared to the average of 20 for this date. But a single strong hurricane can quickly add to the tally. For example, Hurricane Ivan in 2004 generated over 70 ACE units by itself! An average season ends up at 104, so the bulk of the action has historically not occurred yet.
Looking at the graph below that I like to show now and then, you can see where we stand today in terms of climatological activity.
As I mentioned in my outlook at the beginning of August, there are a few reasons why August was expected to be fairly quiet, and why the seasonal outlook was still for above average activity. Those reasons still stand, and have actually been reinforced in the meantime. The signals pointing to a quieter August became stronger, and the signals pointing to an active September have also gotten stronger.
Throughout the season, the core of the deep tropics has been characterized by abnormally dry and stable air masses. This plot shows a time series of an area-averaged measure of instability.
What stands out is that this year (blue line) is that tropical atmosphere has been much more stable than average (black line). Hurricanes need a moist unstable atmosphere to form and intensify… otherwise they choke.
Heading into the end of August, surface pressures across the basin are abnormally low, the African easterly jet has been active, Saharan dust outbreaks have diminished, and water temperatures across the basin are slightly warmer than average for this date. All of these factors are indicative that the tropical Atlantic is poised to come alive in the coming weeks.
Remember that MJO thing I’ve mentioned a few times in the past… the Madden-Julian Oscillation? It’s a large-scale forcing that propagates around the world on the timescale of 1-2 months and acts to enhance or suppress thunderstorm activity. Hurricanes are more likely to form and/or intensify when the enhancement phase is over them. As Phil pointed out to me Thursday, the latest model forecasts all indicate that the MJO should not only become stronger in amplitude (it’s presently fairly weak) but also enter a phase that favors Atlantic thunderstorm development.
So, just because the season is off to a slow start does not mean the seasonal forecasts were wrong or that we’re out of the woods. September and October can easily make up for lost time.
* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.