Wither the Atlantic Hurricane Season?

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  Thirteen tropical storms, two hurricanes, no major hurricanes.
  
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    Climatology (1981-2010, median values): twelve named storms, six-seven hurricanes, and two major hurricanes.
    
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    See also: http://www.nhc.noaa.gov/climo/images/cum-average_Atl_1966-2009.gif
    
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  Only four named storms (Chantal, Dorian, Erin, and Humberto), with only one reaching hurricane intensity (Humberto) formed within the “main development region” (10-20 N, 20-60 W), which typically gives rise to the most intense storms in the basin (e.g., McTaggart-Cowan et al. 2008, MWR).
  
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  Accumulated cyclone activity (ACE) of 30 was only 32% of the 1981-2010 median value of 92.
  

Compare this to the seasonal forecasts issued both before and during the season...

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  NOAA (May 23, 2013): 13-20 named storms, 7-11 hurricanes, 3-6 major hurricanes. Why?
  
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    “A continuation of the atmospheric climate pattern, which includes a strong west African monsoon, that is responsible for the ongoing era of high activity for Atlantic hurricanes that began in 1995.”
    
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    “Warmer-than-average water temperatures in the tropical Atlantic Ocean and Caribbean Sea.”
    
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    “El Niño is not expected to develop and suppress hurricane formation,” with “weaker wind shear.”
    
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    http://www.noaanews.noaa.gov/stories2013/20130523_hurricaneoutlook_atlantic.html
    
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  NOAA (August 8, 2013): 13-19 named storms, 6-9 hurricanes, 3-5 major hurricanes.
  
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    “Our confidence for an above-normal season is still high because the predicted atmospheric and oceanic conditions that are favorable for storm development have materialized.”
    
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    “Reduced expectation for extreme levels of activity.”
    
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    “A decreased likelihood that La Niña will develop and bring its reduced wind shear that further strengthens the hurricane season.”
    
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    “Other factors are the lack of hurricanes through July, more variability in the wind patterns across the tropical Atlantic Ocean and slightly lower hurricane season model predictions.”
    
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    http://www.noaanews.noaa.gov/stories2013/20130808_atlantichurricaneupdate.html
    
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  CSU (April 10, 2013): 15-21 named storms, 6-11 hurricanes, 3-6 major hurricanes. Why?
  
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    “We anticipate an above-average Atlantic basin hurricane season due to the
    

El Niño.”

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  Their findings suggest that an anomalously warm tropical Atlantic in wintertime continues into the summertime and is accompanied by a weaker-than-normal subtropical high, below-normal vertical wind shear, above-normal vertical instability, and above-normal mid-tropospheric moisture.
  
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  Their findings also suggest that a La Nina base state in the tropical Pacific Ocean and its accompanying atmospheric conditions promote reduced vertical wind shear during the tropical Atlantic hurricane season.
  
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  http://hurricane.atmos.colostate.edu/Forecasts/2013/apr2013/apr2013.pdf
  
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  CSU (August 2, 2013): 12-16 named storms, 6-10 hurricanes, 2-4 major hurricanes. Why?
  
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    “We have decreased our seasonal forecast slightly from
    

tropical and subtropical eastern Atlantic.”

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  Continuation of cool-to-neutral ENSO conditions in the tropical Pacific and lower-than-normal sea level pressure across the tropical Atlantic; however, cooler-than-normal sea surface temperatures in the eastern tropical and subtropical Atlantic have developed. These are typically associated with less favorable thermodynamic conditions.
  
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  http://hurricane.atmos.colostate.edu/Forecasts/2013/aug2013/aug2013.pdf
  

El Nino-Southern Oscillation (ENSO) diagnostics...

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  May 2013 SST/SST anomaly: http://www.cpc.ncep.noaa.gov/products/CDB/CDB_Archive_html/bulletin_052013/Tropics/figt18.shtml
  
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  August 2013 SST/SST anomaly: http://www.cpc.ncep.noaa.gov/products/CDB/CDB_Archive_html/bulletin_082013/Tropics/figt18.shtml
  
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  Note continuation of La Nina equatorial east Pacific SST field, erosion of cooler-than-normal SSTs in the subtropical western north Atlantic, and erosion of warmer-than-normal SSTs in the subtropical eastern north Atlantic.
  

Other diagnostics...

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  Anomalous upper tropospheric convergence was found over much of the tropical Atlantic between August and October.
  
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    http://www.cpc.ncep.noaa.gov/products/CDB/CDB_Archive_html/bulletin_082013/Tropics/figt23.shtml
    
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    http://www.cpc.ncep.noaa.gov/products/CDB/CDB_Archive_html/bulletin_092013/Tropics/figt23.shtml
    
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    http://www.cpc.ncep.noaa.gov/products/CDB/CDB_Archive_html/bulletin_102013/Tropics/figt23.shtml
    
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    Note that is was typically, albeit not always, found with upper tropospheric divergence, just weaker than it normally would be. Does this imply a threshold value of upper tropospheric divergence – and thus deep-layer ascent – for genesis, at least on sub-seasonal scales?
    
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    This also shows up in Figure 2 of the NHC 2013 season report: http://www.nhc.noaa.gov/data/tcr/summary_atlc_2013.pdf
    
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  The NHC 2013 season report indicates below-normal moisture conditions between 0-30 N, 40-60 W, at least above 850 hPa – http://www.nhc.noaa.gov/data/tcr/summary_atlc_2013.pdf.
  
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    The origin for this relatively dry air does not appear to be tied to Saharan Africa, and thus to Saharan air layer outbreaks, as per http://hurricane.atmos.colostate.edu/Forecasts/2013/nov2013/nov2013.pdf, which suggests that below-normal moisture conditions are primarily confined to oceanic regions.
    
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    Possibly tied to enhanced deep-layer subsidence, a drying process...?
    

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  Anomalous upper-level trough in the western portion of the basin (10-30 N, 60-90 W) with anomalously high westerly vertical wind shear in this region (Figure 4 of http://www.nhc.noaa.gov/data/tcr/summary_atlc_2013.pdf).
  
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    Does this explain the dearth of activity in the western basin in 2013, whether in terms of genesis or intensification (noting that this is where the warmest, both at the surface and over a deep layer, waters are typically found in the entire basin)?
    
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    What is the cause of this? How well can such a semi-permanent feature be predicted?
    
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  Other ideas: subseasonal variability in the Atlantic Multidecadal Oscillation (AMO), tied to variability in the Atlantic's thermohaline circulation (e.g., sub-surface transports)?
  
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    Typically varies on scales of 25-35 years, but seasonal to sub-seasonal variability is possible.
    
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    Figure 33 of http://hurricane.atmos.colostate.edu/Forecasts/2013/nov2013/nov2013.pdf – positive AMO: warmer north Atlantic SST, increased rain in the Sahel of Africa, warmer tropical north Atlantic SST, reduced sea level pressure in the tropical north Atlantic, weakened easterly trades, mitigated El Nino activity, and enhanced tropical cyclone activity.
    
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    Substantially weakened in spring before restrengthening in summer; possible atmospheric lag to the oceanic state responsible for reduced Atlantic hurricane season activity?
    
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      Argument: all of what we've seen above was caused by the substantial weakening in the AMO.
      
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      Question: what causes subseasonal changes in the AMO? What causes changes in the AMO on longer time scales, too?
      

What are some of the scientific questions that arise from all of this?

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  Predictability: some of the factors influencing tropical cyclone activity evolve on long time scales, while some evolve on short time scales. For some of the longer-scale factors, we can recognize changes as they occur but our understanding of how and why they occur is incomplete at best. What is needed to improve our understanding, and thus our predictive capabilities, for these factors – e.g., ENSO, the Atlantic Meridional Mode (which is strongly related to the subtropical Atlantic base state), Sahel rainfall (modulating and modulated by African easterly waves), and so on?
  
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    In other words, should we have known better? Should our models – statistical, dynamical, or some combination thereof – have known better?
    
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  Disturbance genesis efficiency: McTaggart-Cowan et al. (2013, MWR) examined global tropical cyclone development efficiency as a function of genesis pathway. They defined development efficiency as a ratio of development events by pathway to the climatological frequency of synoptic-scale conditions associated with each pathway. They found that most pathways have a peak efficiency of 5-7%. What is responsible for intra- and inter-seasonal variability in disturbance genesis efficiency? What is responsible for intra- and inter-seasonal variability in the presence, intensity, structure, etc. of disturbances that could act as tropical cyclone seedlings?
  
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    http://journals.ametsoc.org/na101/home/literatum/publisher/ams/journals/content/mwre/2013/15200493-141.6/mwr-d-12-00186.1/20130526/images/large/mwr-d-12-00186.1-f15.jpeg
    
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  Synoptic time/space scales: the NHC 2013 season report http://www.nhc.noaa.gov/data/tcr/summary_atlc_2013.pdf, page 4, notes that “All of the
  

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  Such biases are not likely linked to seasonal/climate-scale processes...
  
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  But, what are the causes of this bias?
  
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  Note that we don't typically have great skill at forecasting tropical cyclone intensity, nor has it gotten appreciably better over time, at least at short lead times (http://www.nhc.noaa.gov/verification/figs/ALinerrtrd.jpg).
  
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  Aggregated over an entire season, do all forecasts – human or model – share this bias? (Similar information was not readily available for other seasons.)
  
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  What are common characteristics of well-predicted vs. poorly-predicted tropical cyclones?