Teenager. Weather aficionado. Soccer fan. Realist. Posts subject to sarcasm. Goal: National Hurricane Center.
By: TropicalAnalystwx13, 1:59 AM GMT on April 21, 2014
First off, Happy Easter to all reading this.
It's a nice day out today. The sun is shining, the air is warm, the birds are chirping, and the bees are busy pollinating. Oh wait, just kidding, it's pouring down rain with gusts over 45 mph here; my yard is a swamp. Must be spring in southeastern North Carolina. With that said, we are quickly approaching the official June 1 start of the 2014 Atlantic hurricane season, and we're doing so on the heels of one of the biggest busts the meteorological community has ever seen after the events of last season. 2013 is a good example of how meteorology is an inexact science; it should be realized that any forecast from any individual or any organization should not be expected to turn out with 100% accuracy.
When dealing with seasonal forecasts, it's usually best to start with the largest contributor to activity...in this case, the Pacific Ocean. A first glance at the sea surface temperature anomaly map reveals a very different pattern than we've been accustomed to the past few years. The Pacific Decadal Oscillation (PDO) - an index used to describe the sea surface temperature configuration in the north Pacific north of 20°N - has been running positive since the start of 2014, conflicting the overall negative phase that began in earnest in 2007. A positive PDO features warmer-than-average waters stretching from the Aleutian Islands to the coastline of California to the Hawaiian islands, with cooler-than-average waters east of Japan. In a negative PDO, the anomalies are reversed. In addition, we notice a notably warmer equatorial Pacific compared to the stretch of cool Neutral or La Niña conditions that began in 2010.
Figure 1. Global sea surface temperature anomaly map (based on 1982-2010 climatology). Image credit to Levi Cowan of Tropical Tidbits.
There has been a lot of media attention the past few months towards the potential development of an El Niño event by summer 2014. An El Niño is a band of anomalously warm waters stretching from the northwestern coastline of South America into the central Pacific. This configuration leads to extreme changes in the global weather pattern, with warmth and drought conditions usually found across The Philippines, Indeonesia (and surrounding locations), northeastern South America, and Australia, and cooler and wetter conditions across the southern United States. El Niños typically lead to a spike in global temperatures, and this evidenced by the fact that the last two significant events...the 1997-98 and 2009-10 El Niños...both made the latter years among the warmest on record.
If we take a look at model projections for this likely El Niño, we see that there is some discrepancy as to how strong it becomes. The Climate Prediction Center/International Research Institute model suite is generally the most conservative (as is typical) with this upcoming event, showing a peak just above 1C. Meanwhile, the mid-April update of the ECMWF ensembles is more bullish, showing a peak near 1.5C. The CFSv2 forecasts a peak near 2C. Depending on the values, the potential El Niño can be broken down into categories: if the peak tri-monthly Niño 3.4 value falls on or between 0.5-0.9C, it can be classified as weak; if the value falls on or between 1-1.4C, it can be classified as moderate; if the value falls on or between 1.4-1.9C, it can be classified as strong. Finally, though this term is not official, it is widely used--if the value falls on or above 2C, the El Niño can be classified as super. Only the 1982-83 and 1997-98 El Niños have observed an Oceanic Niño Index (ONI; tri-monthly Niño 3.4 value mean) in this category. Given model projections and the current state of the ENSO, I feel it is almost certain that we see an El Niño event of any magnitude; a high chance that we see an El Niño event of moderate strength; a medium chance that we see an El Niño event of strong strength; and a medium chance that we see an El Niño event of super strength.
Figure 2. The IRI/CPC ENSO model suite from mid-April 2014. Image credit to the Climate Prediction Center and International Research Institute for Climate and Society.
Breaking down the current state of the ENSO, we can see that easterly winds have dominated the equatorial Pacific for the past few weeks. This is in contrast to the January-March period, which featured mainly westerly winds. Easterly winds are associated with stronger trade winds and more upwelling (cooler waters), while westerly winds are associated with reduced trade winds and less upwelling (warmer waters). This change has caused many people to second guess their predictions of an El Niño event, but overall, this is just "a drop in the pond". If we take a look at the CFSv2 forecast for the next month, we see that it shows a renewal in westerly winds across the equatorial Pacific for this upcoming week, lasting through the middle portions of May. This is in response to a strong pulse of the Madden-Julian Oscillation currently moving west to east across the western Pacific Ocean. The MJO is associated with westerly wind bursts along the equatorial Pacific, leading to overall warming. Low-latitude tropical cyclones...usually within 5N of the equator...can also produce westerly wind bursts.
Figure 3. CFSv2 forecast for 850mb winds for weeks 1-2 (left; 20 April-3 May) and weeks 3-4 (right; 4-17 May). Image credit to the Climate Prediction Center.
As a consequence of the first westerly wind burst in January, a downwelling oceanic kelvin wave was initiated and has since spread eastward across the equatorial Pacific Ocean. In response to this, an incredible amount of warm water has built near 150 meters, with anomalies of 6-7C above average visible on the latest Climate Prediction Center update. This is the strongest subsurface warm pool we've ever seen prior to the formation of an El Niño, and much stronger than the subsurface warm pool of 1997 at this time. This subsurface warm pool in itself won't lead to an El Niño of strong to historic proportions as we saw in 1997-98...continued tropical forcing is needed for that...but it does significantly increase our chances are seeing at least a moderate El Niño event.
Figure 4. Equatorial Pacific temperature anomaly map as of April 13, 2014. Image credit to the Climate Prediction Center.
At this point, some may be wondering why El Niño matters with regards to Atlantic hurricane activity. In an El Niño, warmer waters along the equatorial Pacific warm the air just above the surface. Because warm air is less dense than cool air, the air rises and expands, leading to showers and thunderstorms. The resultant outflow from these disturbances fans out, leading to increased wind shear across most of the Gulf of Mexico and Caribbean Sea. In addition, a stronger-than-average subtropical jet stream favors above-average wind shear across much of the aforementioned regions. Wind shear tilts the circulation of a developing tropical cyclone in addition to removing heat/moisture needed for the formation of a system. El Niños go hand-in-hand with above-average sea level pressures across the Atlantic, as indicated by the latest forecast from the ECMWF:
Figure 5. Mid-April ECMWF forecast of mean sea level pressure for the August-September-October 2014 period. Image credit to the European Centre for Medium-Range Weather Forecasts.
Now viewing a sea surface temperature anomaly map of solely the Atlantic Ocean, we notice that the basin also looks much different than the configuration we've become so accustomed to. While warm waters encompass much of the northwestern Atlantic, near-average to below-average waters are observed across the north-central, central, and eastern Atlantic: this is a negative Atlantic Multidecadal Oscillation (AMO) pattern. Because the eastern and central Atlantic are the birthplace for Cape Verde hurricanes - which make up a majority of the longest-lasting and strongest hurricanes in the Atlantic - this is not a favorable configuration for Atlantic hurricane activity. This pattern has been brought about by a strongly positive North Atlantic Oscillation (NAO) during the winter and early spring months. In a positive NAO, a stronger-than-average Bermuda High promotes stronger-than-average trade winds across most of the tropical Atlantic, leading to evaporational cooling of the water. If we take a look at the forecasts, we see that while the NAO is forecast to go negative for a brief period of time in late April, the majority of available models show a similar ocean temperature configuration lasting through the fall of 2014.
Figure 6. Atlantic sea surface temperature anomaly map. Image credit to Levi Cowan of Tropical Tidbits.
Other than the aforementioned factors, there are a few things to watch out for as we progress into this season. Vertical instability in the tropical Atlantic continues well below-average, as has been the case for the past few years. While this factor may not limit the number of named storms, it does limit the intensity of tropical cyclones that form; will its trend continue in the season? Meanwhile, we are beginning to transition from a period of below-average rainfall across the Sahel to one that features wetter-than-average conditions. A wet Sahel is correlated to stronger-than-average tropical waves that have a higher probability of developing once over the ocean. Despite cooler waters and increased shear, will the Cape Verde season be completely dead this year? In the Indian Ocean, sea surface temperatures are slightly above-normal, indicating a neutral to slightly positive Indian Ocean Dipole (IOD). A positive IOD is associated with a weaker western Africa monsoon circulation, and is unfavorable towards tropical activity in the eastern Atlantic as seen in 2007 and 2011. Finally, the Gulf of Guinea has been generally above-average this year, contrasting the generally cool waters that have existed there during the past few seasons. In years with a cool Gulf of Guinea, the Intertropical Convergence Zone (ITCZ) was shifted farther north and allowed for stronger-than-average tropical waves to form within the African Easterly Jet. Will we see a cooldown before the start of the season?
Figure 7. Timeseries of Sahel rainfall anomalies from 1900 to 2013.
Given the current state of the Pacific and Atlantic, as well as model projections, we can choose a set of analog years from the database. In doing so, I chose years that came on the heels of an extended period without an El Niño but at the same time entered an El Niño of at least moderate intensity, all the while maintaining cooler-than-average tropical Atlantic sea surface temperatures (mainly negative AMO years). The list is as follows: 1963, 1968, 1972, 1982, 1991, 1997, 2002, and 2009. If we take the mean of these years, we come up with 8.375 named storms, 3.75 hurricanes, 1.25 major hurricanes, and an Accumulated Cyclone Energy (ACE) index of 50.5 units. Given the possibility that the El Niño comes on sooner than many of the listed years and also peaks higher than the listed years, my forecast calls for 7-9 named storms, 1-3 hurricanes, 0-2 major hurricanes, and an ACE index of 35-55 units (roughly 40-60% of average). An interesting tidbit to note is that the focus of tracks during these analog years encompassed the extreme northern Gulf Coast and the Southeast United States. While every individual living along any stretch of coastline that borders the North Atlantic should be prepared during any season, this area may need to keep an extra eye to the tropics during the season. For more information about track congregation during moderate El Niño years, watch Levi Cowan's 2014 Atlantic hurricane season outlook video. As is emphasized every year, significant storms - such as Andrew during the 1992 Atlantic hurricane season - can still occur during an otherwise dead season.
Updated: 5:06 AM GMT on April 21, 2014