When we talk about trends in Atlantic Basin tropical cyclones there are many interesting sides to the problem. Global warming's influence on tropical cyclones has been in the news lately and has been hotly debated among scientists. A few details on that problem near the top of the list are; 1) trends in how well we can see and measure tropical storms and hurricanes and 2) the overall short record we have on global tropical cyclone behavior. I will discuss the first one here.
We have seen an increase in the total number of named storms in the Atlantic Basin since about the early to mid 1980s if you average through time (i.e. smooth out high and low years) and a very active period since 1995. Below is a nice example taken from the National Climate Data Center for the period 1967-2006.
Your immediate response might be, "it must be related to global warming." You might be right, but let me show you how things can get messy. The graph below is a tally of the number of unnamed "tropical depressions" identified by the NHC for each of those same years (courtesy of Dr. Mark DeMaria at CIRA Colorado State University). That is the number of tropical depressions that never got strong enough to be named a tropical storm.
Obvious is the very large downward trend; in the 60s and 70s the number of unnamed tropical depressions average about 10-12. Since the 90s the average is closer to 2 unnamed tropical depressions (1998, 1996, 1998 and 2006 had no unnamed tropical depressions). Is this a real trend or partly a man-made trend? Either way today the NHC is not identifying near as many tropical depressions that never get to storm as it did in the "old days." Could this trend be related to internal policy changes at the NHC (e.g. go straight to storm for some developing circulations in the Atlantic)? Or could it be related to real atmospheric conditions, namely anything that develops goes quickly through tropical depression stage and becomes a named storm. Then again, I wonder how many of the tropical depressions of the 60s and 70s might have been "named storms" today if we had back then the excellent satellite observing network we have today; we are now able to see and monitor tropical cyclones at high resolution and at very short time intervals. Also microwave satellite imagery now allows us to estimate with high accuracy surface winds in and around a tropical cyclone (away from heavy rain) which could increase the likelihood that NHC finds an area of tropical storm winds somewhere.
The takeaway you ask? What appears to be an increasing trend in Atlantic named storms since about 1980 is at least partly blurred by improvements through time in how we see them and possibly by policy changes in how the NHC gives a circulation a name (storm) rather than a number (tropical depression).
