Why so many U.S. hurricanes the past two years?

Before I answer the title question, let's talk about Epsilon. This hurricane refuses to die, despite cold waters and high wind shear. The latest model projections continue to keep it as a hurricane for at least two more days, which would make 2005 break the record for most number of hurricane days (50, set in 1995) and most number of days with a named tropical storm (120.5, set in 1995). Beyond three days from now, I really don't see how the storm can survive, since wind shear levels will increase to more than 50 knots, which will surely tear the storm apart. Nothing else is brewing in the tropics, although there is a marginal possibility of something developing north of Panama a week or so from now.

Why have so many hurricanes hit the U.S. in 2004 and 2005?
The 1995 - 2003 hurricane seasons were quite active, but only 3 of 32 (9%) major hurricanes that formed in the Atlantic made landfall in the United States, much below the climatological average of 30% for the entire 20th century. This lack of hurricane strikes occurred because a trough of low pressure was frequently located over the East Coast during these years, and the flow of winds through the trough tended to recurve hurricanes northeastward out to sea before they could strike land. However, in 2004 and again in 2005, this trough was mostly absent, and an unusually strong ridge of high pressure was in place which tended to steer hurricanes into the U.S. and not allow recurvature. Seven of 13 (54%) major hurricanes that formed in the Atlantic made United States landfall as major hurricanes during 2004 and 2005.

So, what caused this unusually strong ridge of high pressure to develop over the eastern U.S.? According to Dr. Bill Gray's hurricane forecast team at Colorado State University, this ridge formed in response to a strong warming of the ocean in the central North Pacific. Figure 3 shows the difference in Pacific sea surface temperatures during August-October 2004-2005 from August-October 1995-2003. Central North Pacific sea surface temperatures were up to 1.3C (2.3F) warmer in 2004-2005 compared to 1995-2003, leading to a deflection of the jet stream dowstream of the warm pool. The jet stream assumed a standing wave pattern, resulting in a ridge of high pressure over the central Pacific, trough of low pressure over the western U.S., and high of high pressure over the eastern U.S. It was this ridge of high pressure over the eastern U.S. that steered so many storms into the country during 2004-2005. Why did the ocean warm in the central North Pacific, and will that warm pool remain in place for the 2006 hurricane season? That is unknown.

Tune in tomorrow, when Dr. Bill Gray's forecast team releases their 2006 hurricane season forecast.



Figure 3. August-October 2004-2005 sea surface temperatures minus August-October 1995-2003 sea surface temperatures. Note the large warming of 1.3 degrees C in the central North Pacific. Image credit: Dr. Bill Gray's hurricane forecast team at Colorado State University.


Jeff Masters