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Good day everyone!
While there is a lull in the weather, and we are not to the point yet of that possible low, I figured this would be a good time for a discussion on the 2014 Atlantic Hurricane Season, and the negative factors contributing to a below average season.
The 2014 Atlantic Hurricane Season ended at midnight, Dec. 01, 2014 (the season runs from June 01, to November 30 of each year). The season closed out with the following totals:
NAMED STORMS: 8
MAJOR HURRICANES: 2
Even with the inhibiting factors, the season wasn’t that far below average. On average, a “normal” or “average” season produces 10 named storms, 6 hurricanes, and 2 major hurricanes…this average is prior to the upswing in 1995. Averages over the 100+ year time frame were increased to 11 named storms, 6 hurricanes and 2 major hurricanes.
There were some numerous factors involved this season, which led to the reduction in activity in the Atlantic Basin, in which some of them work hand in hand.
A news article citing information from NOAA provided an explanation of why activity was suppressed, and most of what I will cover is contained in the report. However, the following is sort of contradictory in the explanation of non development of El Nino, and wind shear over the Atlantic basin, in which I will explain my take on this, and may as well use it as a starting point. From the article:
* A small note on the article by Bell…the use of the term atmospheric instability should state, atmospheric stability. The sentence speaking of convergence, should read divergence, as divergence in the upper atmosphere causes sinking air.
The 2014 hurricane forecasts were more accurate than last year’s predictions. The 2013 hurricane forecast called for an above-average season, but had one of the slowest starts on record and fewer storms than predicted. This year, storm-crushing conditions in the Atlantic Ocean were already in place by spring, leading forecasters to predict fewer tropical storms. In May, NOAA projected for eight to 13 named storms, three to six hurricanes and two major hurricanes.
“What really suppressed the season was the strong wind shear and atmospheric instability across the Atlantic,” Bell told Live Science.
Most hurricanes begin as tropical disturbances offshore Africa or in the central Atlantic Ocean, a region called the Atlantic’s hurricane nursery. Strong vertical wind shear above the hurricane nursery can prevent these baby storms from growing stronger. That’s because wind shear, or winds blowing in different directions, tear apart a storm by pulling its bottom and top in opposite directions.
Other inhibiting factors over the central tropical Atlantic included dry air and an atmospheric phenomenon called convergence, which leads to sinking air. The sinking motion prevents storms from building tall thunderclouds, and dry air lacks the moisture that fuels storms. A weaker western Africa monsoon season is also one of the reasons for this year’s below-average hurricane season, Bell said. The monsoons kindle the tropical lows that eventually become hurricanes.
A quiet Atlantic hurricane season often occurs during an El Niño year, because the climate pattern triggers conditions that inhibit hurricanes. However, in the Pacific Ocean, the El Niño’s failure to launch meant the phenomenon had little impact on Atlantic hurricanes, Bell said. “El Niño never did form and it couldn’t affect the season,” he said.
I have read over the ENSO updates from the ENSO WRAP UP site over the past few months, and according to the findings, the teleconnection between the warmer sst’s in the Equatorial Pacific, and the atmosphere never occurred. This has me a little perplexed, in the fact that an increase in wind shear over the Atlantic, is a result from El Nino conditions, as the subtropical Jet, which is mainly a winter time feature, becomes energized from the extra heat energy added to the atmosphere on the PAC side, strengthening the flow, creating strong westerlies over the Atlantic ocean. Another phenomenon this season was, hurricane activity affecting Hawaii, which normally only occurs during an El Nino phase.
The Southern Oscillation Index (SOI) has been negative since the beginning of July. This tends to indicate a shift in the trade winds over the Equatorial Pacific to which they blow from west to east. To me, this would appear to signal at least some teleconnection between the atmosphere and the Nino regions. So in closing on this aspect, wind shear did play a factor, however observations I made during the season, indicated shear was not a constant.
The SAL (Saharan Air Layer) or African dust seemed to have some increase in episodes this season, which added to the drier air we noted almost all season. I don’t really have the time to research why, but a good portion of this dust seemed to originate over the same area as last season, NE of the Sahara, and flowing down just north and into portions of the MDR (Main Development Region).
To make it clear as to where the Main Development Region (MDR) is located, as it will be mentioned here, I have an old sst map which has the outline of the region
You will note, in the following sst (Sea Surface Temperature) anomaly images, anomalies were cooler than average over the MDR for the majority of the season. You will also note, anomalies were much warmer north of 20N Latitude. SST Anomalies were at average or below for most of the season, especially during the peak months. One main factor for average to cooler anomalies, is the fact, that when averaged out, the NAO (North Atlantic Oscillation) pretty much stayed on the positive side. As noted during my daily analyses during the season, we began with a strong Azores Bermuda high, or sub-tropical ridge, with this reoccurring during various times within the season. This allows for a few things to happen…stronger trade winds allow for too much of an evaporation rate at the oceans surface, not allowing for heat to build vertically, and cooling the sea surface in the process, thus the heat is spread out over a larger area vice being concentrated for the energy needed to build thunderstorm activity. Stronger trade winds also negate any surface convergence.
Stronger trade winds also allow for upwelling of colder water off of the African coast. Along with this, heavier outbreaks of SAL block out the sun, which cuts down on insolation (Incoming Solar Radiation).
ALL of this together creates the following effect: With warmer sst anomalies to the north, this is where most of the net upward motion, of lift occurred in the Atlantic, as although the Atlantic sst’s were warm, nature “sees’ these anomalies, and responds accordingly. So by the convective process, if we have rising air north of the MDR, we have air spreading out in the upper atmosphere, cooling, and then sinking to the south. As the air sinks, it warms due to compression. As the air warms, it dries out the surrounding atmosphere, hence all of the dry air we noted for a majority of the season. Another factor was a slightly stronger than average subtropical ridge, which enhanced the sinking motion. This is why we experienced a lack of atmospheric instability over the Atlantic for the season.
The Atlantic Ocean Tripole was lacking during the season as well. The Tripole is where we notice warmer sst anomalies above 40N Latitude, as a general rule, colder sst anomalies extending from approximately 40N, south to 30N, then much warmer sst anomalies in the MDR. The difference in the colder anomalies, and warmer anomalies in the MDR allow for maximum net upward motion (warm rising air) in the MDR, thus creating a reversal in what I just mentioned previously. This enhances moisture in the tropics, and vertical motion for thunderstorm development. These warmer anomalies in the Atlantic occur when the subtropical ridge is weaker than normal, or during a sustained negative NAO phase. The first graphic shows a fairly good Atlantic Ocean Tripole setup, which favors enhanced tropical activity.
The next SST map shows a classic tripole setup on May 17, 2010
The following sst anomaly maps are from this season during mid month from June through September. Notice the difference in the anomalies from 2010
JUNE 16, 2014 SST ANOMALIES
The one last item I’ll mention is, I’m sure everyone noticed we did not really have a Cape Verde season to speak of. This was attributed to a weaker African Monsoon season. Albeit we saw the West African Monsoon trof modulate almost every two weeks in the far Eastern Atlantic, energy and moisture was just not available to energize it. The main contributing factor in this is the mainly negative phase of the Indian Ocean Dipole. There is a positive and negative phase of this phenomenon, not too much unlike the Atlantic Ocean Tripole. When sst anomalies are cooler over the western portion of the Indian Ocean, the Dipole is said to be in a negative phase, and vice versa.
As you can notice, a NEGATIVE IOD phase doesn’t allow for energy or moisture to cross into the African continent, pretty much leaving dry conditions. Not very favorable for formation of African waves over the continent.
This pretty much summarizes the combination of factors which led to below average Tropical Cyclone Activity for the 2014 Atlantic Hurricane Season.
Have a blessed day!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST / SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS
OFFICIAL SKYWARN SPOTTER (ADVANCED)