2011 Atlantic Hurricane Season Extended Outlook
By: MiamiHurricanes09 , 3:10 AM GMT on January 21, 2011
PDF Format: 2011 Atlantic Hurricane Season Extended Outlook
Currently I am forecasting for an above-average 2011 Atlantic Hurricane Season.
El Niño/La Niña-Southern Oscilliation
Before we get into the forecast of what ENSO conditions are anticipated to be like for this upcoming hurricane season, we first must understand what the ENSO is. ENSO is a quasi-periodic (a reoccurring phase that involves a certain component of unpredictability) climate pattern that occurs across the eastern equatorial Pacific. These pattern changes are characterized under two stages, a cool stage (La Niña) or warm stage (El Niño). A La Niña is characterized when the anomalous sea surface temperature across the eastern equatorial Pacific is below -0.5˚C. As opposed to an El Niño which is when the anomalous sea surface temperature across the eastern equatorial Pacific is above 0.5˚C. If the anomalous sea surface temperature is between the aforementioned temperatures, then it can be characterized as neutral conditions, however, these conditions do not last as long as an El Niño or La Niña phase.
But this is not all that is involved in the ENSO...there is also the Southern Oscillation Index (SOI) which is defined as the air pressure difference between Tahiti and Darwin. When the SOI is in its positive phase, then higher air pressure dominates over Tahiti and Darwin, producing for trade winds along the eastern equatorial Pacific to increase. When this occurs, upwelling of water takes place...bringing cooler water from below the surface upwards. This then causes for a La Niña phase to occur, or simply continues to increase the strength of an ongoing La Niña or weaken the strength of an ongoing El Niño. The opposite is when the SOI is in its negative phase. When this occurs, the air pressure over Darwin is greater as opposed to Tahiti, and so, trade winds over the equatorial Pacific are weaker, thus constituting less to none upwelling, and thus allowing for sea surface temperatures to increase or remain steady. When the SOI is in its negative phase, we are seeing an increase in trade winds over the Atlantic basin, which then constitutes for an increase in vertical wind shear (a factor of an El Niño season).
Based on the graphic below, it could be said that neutral seasons have more tropical cyclones than La Niña seasons. However, I don’t believe that this graphic is ‘complete’, per se. You see, not only are the numbers ‘inflated’ due to 2005 being incorporated into the average, but this is only using data from recent years and not years prior to 1995. The reason that I say that the neutral section of the graph is inflated is because one could argue that 2005 is only a once in a lifetime experience, or just simply an anomaly. Nevertheless, according to the graph below neutral years are more active than La Niña seasons.
Do not be fooled however, because NOAA does say that La Niña seasons are the most active. They’ve also factored a longer span of time...and so a definite deduction can not be made at this time saying that neutral seasons are the most active just solely based on this graph.
Figure 1. A comparison between El Niño, neutral, and La Niña conditions across the eastern equatorial Pacific after 1995.
Now on to the forecast. There are four sections to the Niño region, the Niño 1+2, the Niño 3, the Niño 3.4, and the Niño 4 region. The section that plays the largest role in the atmospheric conditions across the Atlantic basin is the Niño 3.4 region, and so, this is the region the will be discussed below.
Forecast models currently foresee that the current moderate La Niña weaken to a weak La Niña by the start of the 2011 Hurricane Season. They forecast that this weakening trend in the La Niña continue, and end up with neutral conditions just prior to the peak of the season. Unfortunately, the forecast ends there, but if extrapolation is used, we could hint that the final months of the season be under the effects of neutral conditions or a weak El Niño. Extrapolation however, is quite useless when dealing with something as unpredictable as the ENSO.
Figure 2. All statistical and dynamical plots for the sea surface temperatures across the Niño 3.4 region.
Should the forecast verify, we could anticipate below average to average levels of vertical wind shear be seen over the Atlantic basin this upcoming season.
Sea Surface Temperatures
A key ingredient in tropical cyclogenesis is sea surface temperatures. Tropical cyclones feed off of latent heat from the warm waters as water vapor rises into the cyclones circulation. This then allows for the cyclone to continue to strengthen and grow more powerful. So far, models foresee the sea surface temperatures once again rise to near-record level thresholds for this upcoming season...somewhat similar to what we saw this previous season. But sea surface temperatures also play a role in something called the Madden-Julian Oscillation (MJO) which plays a very large role in the development of tropical cyclones.
It should be noted however, that if sea surface temperatures warm excessively across the north Atlantic, we run the risk of losing the Atlantic tripole (anomalous cooling of the far north Atlantic), which acts to focus upward motion of the MJO into the Atlantic basin.
The Madden-Julian Oscillation (MJO) is an easterly propagating wave of enhanced or suppressed rainfall that travels over the warm(est) parts of tropical oceans across the Earth. The upward motion of the MJO, or the increased anomalous rainfall, tends to spend more time over warmer waters, as opposed to cooler waters. For example, 2010: this year had upward motion of the MJO focused mainly over the Atlantic ocean (hence one of the reasons we saw so much tropical activity). But at the same time, the downward motion was providing for suppressed rainfall over the Pacific, because that’s where the cool waters of the La Niña were dwelling.
This year is likely to have the upward motion of the MJO located over the Atlantic for increased periods of time because as previously mentioned, models forecast that sea surface temperatures in the Atlantic soar to near-record heights.
North Atlantic Oscillation
The North Atlantic Oscillation (NAO) is characterized as the pressure difference between the Icelandic Low and the Azores High. When the NAO is in it’s negative phase the subtropical ridge weakens, and thus allows for trade winds to slacken and cause less upwelling which in turn allows for sea surface temperatures to increase. When the NAO is positive, the subtropical ridge strengthens and causes for stronger trade winds and more upwelling of cooler waters from below the surface.
The NAO is also a main factor in the steering of tropical cyclones. When the NAO is in its negative phase, yes the subtropical ridge is weaker, but do to this troughiness along the eastern seaboard is also weaker, which then reduces the chance of a recurving cyclone, and increases the chances of a United States landfall.
Since mid October to just a few weeks ago, the NAO has been negative, which is for quite a remarkable time. Currently we are weakly positive, but models foresee that the NAO once again plummet into negative. Should this trend continue, we should see sea surface temperatures slowly increase in warmth as time progresses but also increase the chance of a United States landfall as the season approaches.
As mentioned before, when the NAO is positive, trade winds increase. In the process of this occurring, these winds blow off dry air (SAL -- Saharan Air Layer) into the mid to upper levels of the atmosphere from the Sahel. When this occurs, African easterly waves (AEW’s) have a tough time developing because they are being choked by the dry air, and are unable to develop consistent thunderstorm activity.
Mean Sea Level Pressure
Due to the fact that we are months away from the start of the season, it is almost useless to look at mean sea level pressure (MSLP) maps. But we’ll give a brief overview of them anyways. Low pressure favors condensation, upward motion, instability, etc...which is conducive for tropical cyclogenesis. High pressure favors sinking air and fair conditions, which is not conducive for the development of tropical cyclones. In the graph below, we see that below-average pressures are dominating over the eastern Atlantic, and slightly-above to average pressures are dominating over the western Atlantic for April, May, and June. Typically we would look for tropical cyclogenesis this early in the season to occur in the western Atlantic, Caribbean, and Gulf of Mexico.
Figure 3. Mean Sea Level Graph for the months of April, May, and June.
Vertical Wind Shear
Finally we look at vertical wind shear (VWS). Considering the fact that the NAO is anticipated to be negative for the start of the season, we should see a lessening of trade winds. As well as the fact that a weak La Niña should be present, which should also keep trade winds relatively low. This tells me that vertical wind shear will likely be average, or below average for the start of the season. Forecasting further into the season is just too far out to even hint at with any accuracy.
Currently I am forecasting for an above average 2011 Atlantic Hurricane Season with 17 named storms, 8 hurricanes, and 5 major hurricanes. Keep in mind that forecasts made this early before the season are subject to very large errors.
I will be releasing another outlook for the 2011 Hurricane Season on May 15, 2011 with some more accurate and in-depth information.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.