Atlantic Tropical Season Begins

The 2010 Atlantic hurricane season officially began on Tuesday (June 1). This year promises to be very active.

Sea surface temperatures (SSTs) have been well above normal in the main development region (MDR) of the Atlantic for the 2010 spring. In fact the Tropical Northern Atlantic index (TNA) set an all-time record high in March ... and then again in April! The Atlantic Multidecadal Oscillation (AMO) also set a record for the month of April. These two indices are a strong indication of the hot waters across the tropical Atlantic and the presence of a distinct tripole (warm tropics, cool mid latitudes, and warm high latitudes).

This tripole and consequent above normal MDR SSTs is a result of the persistent strong negative North Atlantic Oscillation (NAO) this past winter. The NAO is the same force that supressed storms south of New England, while making for the snowiest winter on record for much of the Mid Atlantic region. In addition, this winter featured a strong El Nino in the central Pacific. In fact, this was the strongest west based El Nino on record. El Nino drives a zonal overturning cell, effectively an anomaly of the Walker Circulation, which creates subsidence over the MDR, allowing SSTs to warm more than normal.

In the last few months however, the strong El Nino has broken down significantly, and especially in the last four weeks, has made a complete reversal to slightly negative SST anomalies. Easterly wind anomalies have overtaken the equatorial Pacific, although westerlies remain strong in the subtropics and mid latitudes; a sign of El Nino remaining in the atmosphere. Long range forecast models are in agreement that El Nino will continue to die, with a strengthening La Nina into hurricane season. El Nino is associated with increased westerly shear across the Atlantic which is detrimental to tropical development, as it causes the low and mid level circulations to be decoupled. Neutral to weak La Nina conditions have proven to be the best for minimizing shear, which is precisely what is in store.

Also helping to reduce westerly shear will be anomalous easterlies in the stratosphere descending toward the tropopause. This is the product of the Quasi-Biennial Oscillation (QBO) which is a measure of the wind anomaly at 30mb that oscillates with a period of about 28 months. The QBO is currently in the easterly phase with easterly shear in the lower statosphere. By the thermal wind relationship, the easterly shear favors a colder stratosphere and high tropopause height over the equator and the opposite off the equator. The lower tropopause heights has the effect of reduced tropical activity, and this will have some effect through July. However, as the easterlies descend toward the tropospause, westerly shear develops in the stratosphere, raising tropopause heights over the MDR, encouraging tropical formation.

Reducing to intraseasonal time scale, the MJO has reflected a La Nina state for the last month, with a distinct low level divergence pattern over the central Pacific. In addition, the overturning cell favors convection over the Atlantic. This general bias will continue through hurricane season. Following the cycle of the MJO, expect enhanced tropical activity during mid to late July, and again in late August through September (also the climatological peak).

As for predominant track and landfall risk, all signs point toward an above normal Cape Verde season; that is, tropical storms originating in the eastern Atlantic. The seasonal oscillation of the NAO will switch phase coinciding with the peak in activity in late August and September, which is often a period of enhanced risk for an east coast landfall. The east coast has been relatively spared for the last few seasons, but 2010 will be different. The Carolinas, Virginia, as well as New England may be under the gun more than once. In addition, the Gulf of Mexico has warmed dramatically in the last few months and will fuel a number of intense hurricanes, as has been the theme of the last decade.

As for totals, my summer forecast, posted on March 16, specified 19 named storms, 10 hurricanes, and 5 major hurricanes. I continue to have the utmost confidence in these numbers. In addition, as support, the NHC outlook called for 14 to 23 named storms, 8 to 14 hurricanes, and 3 to 7 major hurricanes; and the CSU outlook in June called for 18,10,5. In summary, this season has the highest confidence ever reached for above normal to near record numbers.