About this time every year, my meteorological senses begin to turn toward the tropical skies in anticipation of the upcoming hurricane season. With the official start date less than four weeks away (June 1), I can’t help but wonder what the tropical atmosphere will look like during crunch time – those special few weeks during the season when tropical cyclone formation is imminent or when one is in actually in progress.
Although skillful forecasts of precisely when a hurricane will develop, even just a few days ahead of time, are beyond the current scope of our science, there are some interesting pieces of the hurricane-season puzzle that may soon come into view.
Long-range predictions for the 2011 season have been out for about a month. Some of these products are scientifically well-guided estimates of what kind of season we’ll get. This year, the consensus so far is that it’ll be busy once again, with more hurricanes than in recent multi-decadal averages.
But it remains true that hurricane season predictions made the spring prior to the hurricane season (or before) inherently leave plenty of wiggle room for things to turn out differently than advertised. They have ... so far ... not shown much skill in predicting the season’s activity in real-time. This is because intraseasonal (weekly or monthly) variations in the wind/temperature/humidity/dustiness of the tropical atmosphere will always play a big role in the productivity of our hurricane seasons –in particular, in the character of the steering currents - and it’s still too early to get a reliable handle on how these ambient conditions will vary locally when it really matters.
However, clues may soon be emerging that suggest this year’s dominant steering currents may very well be different than the ones that protected the U.S. Coast from landfalls just one year ago.
Last year’s storm tracks were essentially grouped into two distinct clusters. One set recurved northward over the western Atlantic and another traveled west or west-northwestward over the Caribbean.
The shape of this clustering may be largely attributed to certain high altitude wind patterns that frequently occupied this corridor of the globe during the heart of the 2010 season. Large anticyclonic gyres -rotating clockwise around a core of high pressure- were frequently observed at upper levels over the Central Atlantic and over the Central United States. These high pressure systems aloft (denoted by the blue Hs in the picture below) were associated with high-altitude southerly winds just east of North America and upper-level northwesterly or northerly flows over the Gulf of Mexico. The image below shows a time-averaged view (Aug-Oct) of the anomalous winds aloft associated with these systems, along with sketches of the two types of tracks we saw (in red and brown).
It’s pretty clear that for much of the late summer and early fall, the 2010 steering currents over this region of the world were infrequently aimed at North America. These currents either redirected approaching storms toward the north and back out to sea or blocked ones that formed in the Caribbean from turning northward in time to cross the U.S. Gulf Coast.
So what will it all look like during this upcoming season? It’s important to keep in mind that the atmosphere last season didn’t always look like those time-averaged pictures above, with a friendly steering flow protecting our shores. Intraseasonal variations in the governing dynamics did at times favor a pattern that, were a tropical system at the ‘right’ place, may have directed one onshore. One of the mechanisms that is quite capable of nudging these high pressure systems around –and thus the steering patterns- is the MJO (Madden-Julian Oscillation).
The MJO is a tropical weather system the size of an entire ocean basin. It propagates eastward (most of the time) around the tropics, and is intricately involved with varying the wind, cloudiness, and rainfall patterns in the hurricane development regions (not to mention the weather over North America as well). As the MJO moves, it pings the atmosphere in different places and alters the progression of weather systems around the world. As such, the MJO can greatly affect weather patterns everywhere, including the development and movement of tropical cyclones. It is one of the wild cards in the long-range hurricane forecasting scheme because we really don’t know how strong or where it’ll be several months from now.
Exactly how the MJO evolves during the late summer and early fall can have a lot to do with how our hurricane season turns out.
Even though these details are currently out of reach, some aspects of its future conduct may not be. I have observed there are some signs that the MJO cycles through preferred phases over the course of several months, visiting the same places repeatedly. During early summer 2011, we will be watching closely how it orbits the globe, and in which region(s) it preferentially passes through. If it continues to act as it has in the last couple of months, one of the atmospheric shapes we may see recur this hurricane season is one which favors upper-level high pressure near the Southeast U.S. and a steering pattern less likely than last year’s to divert hurricanes away from the coast.
If indeed the MJO follows this course and accentuates the risk for landfalls, it would be acting in accordance with some of the general predictions for this summer’s weather … particularly those that develop a pattern with anomalous heat in the Southeast part of the country. High altitude winds associated with temperature outlooks like these, with stronger westerly flow across the Western Atlantic and stronger southerlies over the Gulf of Mexico than what was observed last year, would tend to result in a westward shift (from 2010) of the longitudes along which tropical-cyclone recurvature takes place.
But because the atmosphere’s governing dynamics are so incredibly complex, there are plenty of other processes that will also shape the 2011 hurricane season. I’ve only mentioned one of them. And though the MJO cycles may only account for a fraction of the total variability in the steering patterns we see during the season, it still offers one of the great clues to solving hurricane-track mystery.