Actually, FlaMommy...that is a really good question!
I'm sure that you are already aware that recent ....years (really) increased in Atlantic Basin tropical cyclone frequency is fitting into a periodicity known as the multi-decadal oscillation quite nicely. Basically, in the 1940's, '50's and middle 1960's there was tons of action out there, then there was a relative lull, which lasted through the early 1990's. This cyclic behavior of hurricane frequency is thought to be associated with the SOI (Southern Oscillation Index) in the Pacific Ocean. Approximately every 30 years, the SOI tends to favor La Nina’s over El Nino’s as the preferred state of the Pacific equatorial waters. (Recently there has been study that the NAO (North Atlantic Oscillation) may play some role by exerting strong high pressure at middle latitudes, which intensifies the easterly components near and beneath 30N). Between 1940 and the middle 1960’s the Pacific SSTs tended to be neutral to negative, then, who can forget the incredible press coverage over El Nino that raged throughout the 1980’s.
How does the SOI effect hurricanes: La Nina states pile warmer, deeper thermocline rich waters around the western Pacific. The winds in the upper levels of the atmosphere respond accordingly by blowing lighter over central America and the SW Atlantic Basin. This is a spatial relationship of having less convection/upward moving air situated around the east Pacific equatorial regions. The resulting favorability is that it supplies less upper-level westerlies (i.e. shearing components) down wind; a favorable environment for tropical cyclone genesis occurs closer to home. Where as, if you pile warmer water in the east Pacific, greater convection/upward moving air is imparts stronger shear over the Caribbean and SW Atlantic.
Complicating this further, we are in a multi-year warm SST departure in the Atlantic Basin; which is to say, more energy available annually in the upper oceanic heat content across recent times. It becomes entirely intuitive thus, that more tropical cyclone production (notwithstanding intensities) would launch for having more energy(fuel). Compounding this factor with cool to neutral SST in the Pacific Basin creates high-confidence connection to our recent amazements. As a result, a 2.5fold increase in major hurricane production has been noted since about 1994, with is as much as a 5fold increase in the Caribbean alone! This makes the theoretical understanding of what teleconnects everything fairly well behaved at this time. This makes for a dire prediction for all these coastal interests here at home because these active times are not signaled to have an early lease.
What does this have to do with our ensuing winter?
Here’s the catch: the truth is, La Nina states or even just weak signal La Nina's may be very good for hurricane production, but that doesn’t not necessarily mean so for winter storms. Conversely, it has actually been demonstrated statistically that is weak La Nino states are concurrent with cold storminess in the E 1/3 of NA. This is then augmented much further by the NAO (Arctic Oscillation Index) (…also, a nested component of the AO (Arctic Oscillation)), and the PNA (Pacific-North-American).
There are a lot of different climate indices that have been defined, usually associate with specific weather behaviors around the northern hemisphere. Of the five primary indices, PDO, SOI, PNA, NAO, and the Northern Hemisphere temperatures, it is the NAO and PNA indices which show remarkably dependable correlation to intensities/severity of winters over North America (particularly the eastern 1/3). Complicating that even further…the PNA aspect has been weakly correlated to the PDO and SOI. I haven’t found any study that formally connects or attempts to connect the NAO to these, but who knows…
It is incredible how well these known indexes connect probabilistically to model corrections. If you’re looking at an 8 day lead time storm that could be a big one, check for your predominant PNA and NAO phases! If the former is positive, and the latter is negative (not to mention, you are in a weak ENSO east year) duck! I’ve seen storms in the modeled for 8 straight days that suddenly disappear just 2 days prior to the event, and then noted that my phases were not really that good all along. Also, the PNA has a shorter duration, extreme variability and the NAO is just slightly longer and more dependable. Sometimes, some years, they are remarkably fixed; other years they flip phases negative to positive so fast the very few forecast models (including people) demonstrate much skill.
There are of course exceptions to rule but for majority of the so-called 'block-buster' storms, the ones that can impart enough cold air to deeper latitudes to bring snow where it does not normally snow, can and are statistically correlated with positive PNA and negative NAO; while weak signal El Nino events make better snow producers (more moisture)
It is important to note this for this season because we are currently losing the weak La Nina signal that persisted for much of 2004 through the middle summer months. We are very darn close to neutral across the majority of the Pacific Basin. The graphical analysis of seasonal snow depths, as well as temperature behaviors for the U.S. during neutral scale is erratic and unpredictable... Meaning, nominal skill usually within normal will tend to result. But, that of course does not mean we are not going to get a deep latitude event, nor does it presuppose an active Nor'easter season. Last year we had more storms with CCB (cold conveyor belt) set ups then I can remember in a particular season. We had record breaking cold and tons of snow in all winters between 2002 and 2004. And guess what, that took place in a weak La Nina period... Basically, although the weak La Nina was probably a negative factor, the powerful –NAO and +PNA just overwhelmed.
There is also another very interesting (from the recent studies I've read on the matter) statistical connection. It has to do with the amount of late summer and autumn snow deposition that takes place over northeast Siberia.. Increased or positive anomaly snow pack in the sun-dimming months of the autumn up there for lack of better words, gets the momentum of cold air mass cryogenics working early. If the NAO turns negative in later October and November, then these arctic air masses get yanked over the NW Passage and Pole at least excuse to do so. Usually this excuse consists of a fairly high amplitude +PNA, where a ridge surges in latitude to encompass the Gulf of Alaska and W. Coast. The next thing you know, some reporter is demonstrating how to freeze a boiling cup of water by merely throwing it in the air some –40F morning in Tower Minnesota. I once saw Tower Minnesota go from the upper 40s to minus 20 in an afternoon in early December in a year such as this being described; these early cold snaps that coincide with the lowest solar irradiance of the annular cycle can be truly deep blue vicious. Even by early February you are getting nick by increased daylight – though admittedly you really need to get into early March to worry about sun messing up your cold.
So, if you have had enough of hurricane lore for one season and need the break… you got about 8 month reprieve. Next season is climate signaled to be big again. During the next few seasons for that matter, the lamest seasons will be average, but the big seasons will demolish cities – potentially.
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