It’s that time of year again, folks.The 2011 Atlantic tropical season is upon us. We’ve heard predictions of a busier-than-average year, but coming off of La Niña this is to be expected. La Niña is a periodic cooling of waters over the Eastern Pacific Ocean off of the South American coast. (The opposite of El Niño, which is a periodic warming of the waters.)
This phenomenon shifts the global wind patterns. One result of this shift is that the Atlantic basin generally sees lighter upper-level wind shear. This is a huge ingredient in tropical cyclone formation; strong, upper-level wind shear rips apart tropical cyclones, effectively shutting them down. Contrarily, lighter wind shear offers disturbances and tropical waves a much better chance for development into a named system.
Looking back at similar La Niña events shows that 1989, 1999 and 2008 contained similar trends. However this is very limited data and thus no significant conclusion can be drawn from this data.
Given that we are coming off of La Niña and expected to be in an ENSO neutral cycle, amidst a multi-decadal cycle of above average tropical cyclone activity, and no real factors that would preclude tropical cyclone formation, we do believe that we will see an above average season storm-wise. But this does not necessarily mean that the Atlantic, Caribbean and Gulf regions will see an above-average swell season, it simply means that there is a chance for an above-average swell season. While the number of storms is an important fact to consider, more importantly we need to consider storm location, storm track, and storm intensity along that track. These factors will vary from storm to storm and will be driven by weather patterns as they evolve through the summer.
Analyzing the latest data available, sea surface temperatures across the Atlantic are basically what we’d expect for this time of year. Our sea surface temperature plot indicates that we do have warmer water than normal along the NE US coast — and this is great for the surfers in the region eager to shed the thick wetsuits needed during winter. But at this point, these warmer water temperatures are not a factor in the overall tropical outlook.
The mean pressure plots for the month of May over the Atlantic waters show a split ridge pattern. A strong Azores ridge has been in place with weaker ridging off of the SE US coast. There is a weakness in between these two ridges and we have seen a persisting trough in this region. This matches nicely with our May pressure anomaly chart — this chart shows the difference between what is expected on average and what has actually occurred. We see that right between the split in the ridge we are seeing lower pressures than normal extending into the Caribbean. If this pattern persists — which we think it will, for at least several more weeks — we will have a better-than-average chance of seeing an early-season system and if pressures remain lower than normal in this region, we will want to keep a close eye on this area.
Heading through the tropical season, June is usually a very inactive month and we average less than one named system per year. We generally do see a weak disturbance early in the month, such as what we have been monitoring in the Caribbean the past several days, but by and large June is quiet. June is a great time to get the honey-do list tightened up, knock out a summer project, and/or grab a quick trip to more favorable wave climates. But given those lower-than-normal pressures off of the East Coast, we think that there is a better than average chance of seeing a June storm.
July generally remains quiet with very little activity expected. But this year, things may be different. There is a good chance July will produce a named system or two. And if we can get a storm of some substance, these can tend to produce some really good swells. Why? The steering currents are generally weaker resulting in storms with a longer life cycle such as Chantal in 1995 and Bertha in 2008. Also, summer conditions along the coastlines are usually benign with a general land/seabreeze regime, which can set up favorable conditions for many regions. As seen on our most-likely formation/track images, we expect any named systems to form close to the US.
With August rolling around, the sun has been blazing over the tropical ocean for months. This imparts a great amount of heat into the Atlantic waters and not only do we have the highest sea surface temperatures occurring towards the end of this month and into September, we also see the largest amount of heating into the depths of the ocean. We call this “oceanic heat content” — and this plays a huge role in developing the most powerful hurricanes. These large reservoirs of heat supply a continuous source of energy for tropical cyclones. The largest reservoirs of heat occur over the NW Caribbean Sea with very warm pools of oceanic heat content extending well to the east over the Atlantic waters. Add to this a steady stream of easterly and Caribbean tropical waves and the stage is set for the business end of the season.
We can see the large difference in the heating of the ocean between June and September by examining a couple of hugely important maps. These are the Sea Surface Temperature (SST) map and the tropical cyclone heat potential (TCHP) map. Examining the maps for June, we see sufficiently warm water on the SST map extending well out into the Atlantic. But in comparison to our SST map from September 2010, we can see that the SST’s are much warmer across a broader expanse of the ocean. Digging further we can compare the June and September TCHP maps. The September TCHP map shows large reservoirs of warm water through the Caribbean, extending into the eastern Atlantic. In contrast, June shows little TCHP over the Atlantic and Gulf waters with the majority of TCHP located within the Caribbean Sea.
For this reason, by mid to late August, things really begin to stir and we expect several systems to begin taking shape into the end of August and beginning of September. By early September we are hitting the climatological peak of the season and it is not uncommon to see several tropical cyclones barreling through the Atlantic (2008, 2010). This is the money time of year and we can see some powerful swells around this timeframe. We almost always see a tropical swell around this stretch of the tropical season.
In summary, we are expecting an above-average year of tropical cyclone activity. Expect June to be slow, with potential for a system to take shape. Things are still slow but expect one or two systems to form in July. Heading into August we are expecting three to four named systems to take shape — and we may see some of these storms cross over into September. As August turns into September we expect several more systems — five to seven by the back half of September, giving a total of nine to 13 named storms by late September. Beyond September and heading into fall we will see a couple of more named systems, and we can see some behemoths develop in October — especially if they are over the NW Caribbean. By the end of the season we expect to be above average and finish out with around 15 named storms.
Be sure to stay tuned to Surfline’s Hurricanetrak for the latest updates. This tracking feature has every tool needed from up-to-the-minute storm tracking tools, historical records, forecasting tools, proprietary swell ruler and much more — basically anything needed for the novice all the way to the professional forecaster.