How strong was Monica?

Cyclone Monica has come and gone. Fortunately, the storm hit a very sparsely populated area. There no reports of deaths or injuries, and damage was light. Darwin, the capital of the Northern Territory, received just tropical storm-force winds. With Monica's departure, we are left puzzling over an important question--how strong was she? The Navy Research Lab, using a satellite intensity estimation technique called the "Dvorak Technique", rated Monica as the strongest cyclone ever recorded in the Southern Hemisphere, with a central pressure of 879 mb and 180 mph sustained winds. However, the Australian Bureau of Meteorology (BOM) rated Monica a much weaker storm, with a central pressure of 915 mb at that time. Curiously, the BOM give Monica a 905 mb pressure 12 hours earlier, at a time when the Navy Research Lab had her much weaker--892 mb.


Figure 1. Cyclone Monica at peak intensity at 0130 GMT April 24, 2006, the strongest storm in Southern Hemisphere history--180 mph sustatined winds, and a 879 mb pressure. Or was she 915 mb?? Image credit: Navy Research Lab.

So who's right? Well, today I was in the right place to find out! I am attending the American Meteorological Society's 27th annual conference on hurricanes in Monterey, California all week, and I had the opportunity to talk to an Australian hurricane expert. Bruce Harper of Systems Engineering Australia Pty Ltd in Brisbane, Australia, gave a talk titled, "On the importance of reviewing historical tropical cyclone intensities," and I had the opportunity after his talk to ask him about Monica. He told me that hurricane forecasters in eastern Australia, the North Pacific, and Atlantic all use a uniform technique for estimating pressure of tropical cyclones from satellite imagery, but the western Australian forecasters use a different set of equations for that ocean region. These region-specific equations were developed to better model the small and intense cyclones that typically affect the area, such as Tropical Cyclone Tracy of 1974. The equations were not developed with much data from large and intense Category 5 storms, and so the 915 mb pressure estimate for Monica is suspect.

In reality, we will never know just how strong Monica was. There are no hurricane hunter airplanes anywhere but the Atlantic. Satellite estimation techniques are getting better each year, but are still subject to large errors. Scientists who are researching the link between hurricanes and global warming are free to use either intensity estimate for Monica's lowest pressure. This underscores the difficulty of assigning much credence to the reported 80% increase in Category 4 and 5 hurricanes globally since 1970 reported by Peter Webster and Greg Holland last year in their controversial article in Science. While I do believe there has been some increase in these storms, the estimation of maximum cyclone intensities is so fraught with uncertainties that I do not believe a reliable estimate of how significant this increase can be done until a full re-analysis of all historical tropical cyclones is completed. Even then, I think we need at least another ten years of data, since our data set covers such a short period of time.

Jeff Masters