by Judith Curry
So, what can we expect for the 2012 Atlantic hurricane season? All of the seasonal forecasts are coming in for a near or below normal year. But already, we have seen two named storms, before the official start of the hurricane season on June 1.
What do I think the 2012 season holds?
A number of groups publicly issue seasonal hurricane forecasts. The main customer for such forecasts is the reinsurance sector. The timing for purchases is December, March, and end of May; hence seasonal forecasters put out their forecasts on this schedule. Bill Gray and Phil Klotzbach have recently abandoned their December forecasts since they have found they have no skill (something I have been saying for years). Their is little skill in March, but by the end of May and certainly in June, there is some predictability for the coming season.
So what do the latest (end of May) seasonal forecasts say?
NOAA‘s Climate Prediction Center says there’s a 70 percent chance of nine to 15 named storms (with top winds of 39 mph or higher), of which four to eight will strengthen to a hurricane (with top winds of 74 mph or higher) and of those one to three will become major hurricanes (with top winds of 111 mph or higher, ranking Category 3, 4 or 5). Based on the period 1981-2010, an average season produces 12 named storms with six hurricanes, including three major hurricanes.
Gray and Klotzbach anticipate that the 2012 Atlantic basin hurricane season will have reduced activity compared with the 1981-2010 climatology. The tropical Atlantic has anomalously cooled over the past several months, and it appears that the chances of an El Niño event this summer and fall are relatively high. We anticipate a below-average probability for major hurricanes making landfall along the United States coastline and in the Caribbean. Summary: 10 named storms, 4 hurricanes, 2 major hurricanes.
Weatherbug (Earth Networks): The Atlantic hurricane season will see a total of 11 to 13 named storms form in the Atlantic Hurricane Basin. The 30-year average is about 12 storms. Six or seven of these storms could become hurricanes, and two to four are predicted to become major hurricanes with possible winds in excess of 111 mph. The long-term average is about six hurricanes and three major hurricanes. The potential for a U.S. landfall appears to be near normal for the 2012 Atlantic Hurricane Season.
Weatherbell (Bastardi, Maue et al.) predict: The total number of storms and the ACE Index in the Atlantic are expected to be down for the 2012 Atlantic Hurricane Season. However, the potential for damage from land falling storms is high. This is because the pattern is such that development of over 50% of the storms may be within 300 miles of the US coast. Long tracked storms that turn out to sea, as we saw last year (with the exception of Irene), are not anticipated this year. Instead, storms that develop close to the coast are expected, which may reach maximum intensity as they reach shore. This would mean a tougher forecast year, in spite of fewer storms than the past couple of years. The earlier part of the season, June-August, may favor development near and just off the Southeast coast. Storms such as Belle (1976) or Bob (1991) come to mind in this case. During the mid and latter part of the season, the Gulf may be at a higher risk from the African waves that can make it across the Atlantic and develop late. One of the analog years is 2002, which is the year of Isidore and Lili.
TSR predicts a near normal year.
WSI /Weather Channel: This preseason forecast calls for 11 named storms, 6 hurricanes and 2 major hurricanes (Category 3 or higher on the Saffir-Simpson Hurricane Wind Scale). These forecast numbers are below the long-term average from 1950-2011 (12 named storms, 7 hurricanes, 3 major hurricanes) and well below the averages for the current active era from 1995-2011 (15 named storms, 8 hurricanes, 4 major hurricanes).
If there are other seasonal forecasts that you have spotted, let me know.
JC evaluation of these forecasts: the probabilistic approach of NOAA is far preferable to the deterministic approach of Gray and Klotzbach. The emphasis on named storms is rather pointless: the threshold for naming a storm is somewhat arbitrary; tropical depressions and tropical storms rarely cause much damage; and it is the number of landfalling hurricanes that really matter in terms of socioeconomic impacts. The more qualitative approach of Weatherbell is also a good approach, and I like that they separate out the dynamics of the early part of the season from the peak part of the season. WSI’s comparison with other years during the current active phase (e.g. since 1995) makes more sense than comparing to last 30 or 60 years.
Do I think any of these forecasts are likely to be correct? Some background on how I reason about this.
Seasonal predictability of Atlantic hurricanes
For background on this topic, see a report I wrote last year for a client in the reinsurance sector: Assessment of strategies used to project U.S. landfalling hurricanes. An excerpt (refers to figures in the document):
Interannual and multidecadal modes of climate variability have long been known to have an influence Atlantic hurricane activity. Several studies have explored the impact of different climate indices on Atlantic hurricane activity, including Atlantic and tropical sea surface temperatures, El Nino-Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), West African monsoon, Atlantic Multidecadal Oscillation (AMO), Atlantic Meridional Mode (AMM), Madden-Julian Oscillation (MJO), Quasi- Biennal Oscillation, and the solar cycle. In the analysis provided here, we focus on the ENSO, AMO and PDO because of their potential for longer-range predictability.
The El Nino Southern Oscillation (ENSO) dominates the interannual variability of North Atlantic hurricanes. Recent research by Kim, Webster, Curry (2009) highlighted the impact of the increasingly frequent Modoki El Nino on Atlantic hurricane activity. The Modoki is associated with central Pacific warming, rather than with eastern Pacific warming that characterizes the canonical El Nino.
Kim, Webster and Curry (2009) demonstrated that the two distinctly different forms of tropical Pacific Ocean warming have substantially different impacts on the frequency and tracks of North Atlantic tropical cyclones. There is a clear difference between the number of cyclones forming during EPW (El Nino) and EPC (La Nina) events, but there is almost as large a difference between the EPW (El Nino) and CPW (Modoki) events.
The location of the tropical Pacific warming (central or eastern) also affects the location of cyclogenesis and the tracks of tropical cyclones. During an EPW (El Nino), track density is reduced over most of the North Atlantic, with a concentration in the western and Caribbean regions. The tracks during a CPW (Modoki) event differ markedly from those occurring during an EPW event: track density for CPW increases across the Caribbean, the Gulf of Mexico, and the U.S. east coast, but it decreases in the central and western North Atlantic. During an EPC (La Nina) event, large increases in track density occur across the entire North Atlantic.
We are currently in the warm phase of the AMO and the cool phase of the PDO. The total number of Atlantic hurricanes has strong interannual and interdecadal variability, but the highest numbers are characterized by warm AMO and cool PDO. The AMO and PDO also provide a signal regarding the location of the landfalls:
- Atlantic coast: more frequent landfalls during warm AMO, and cool PDO
- Florida coast: more frequent landfalls during warm AMO
- Gulf coast: no strong multidecadal signal
2012 regimes and teleconnection modes
The lower than normal prediction for 2012 is primarily associated with the anticipation of an El Nino. A summary of the latest ENSO forecasts is provided here. A discussion of the various forecasts from the IRI page is:
Although most of the set of dynamical and statistical model predictions issued during late April and early May 2012 predict continuation of neutral ENSO conditions through the middle of northern summer (i.e., June-August), slightly more than half of the models predict development of El Nino conditions around the July-September season, continuing through the remainder of 2012. Still, a sizable 40-45% of the models predict a continuation of ENSO-neutral conditions throughout 2012. Most of the models predicting El Nino development are dynamical, while most of those predicting persistence of neutral conditions are statistical. It is clear that uncertainty remains regarding the ENSO state during the second half of 2012.
Individual model performance can be found here. For CFAN’s ENSO forecasts, we mainly look at ECMWF (secondarily at NCEP). While sometimes ENSO is predictable 6+ months in advance, a key issue in ENSO forecasts is the springtime predictability barrier, whereby forecasts across Mar, Apr, May have low skill. Predictability picks up for forecasts initialized later in May or Jun.
A good article assessing ENSO predictability was just published in BAMS, Skill of Real-Time Predictions of ENSO During 2002-2011: Is our skill increasing?
Bottom line ENSO forecast: many forecasters think we are headed for El Nino by late summer/autumn. I agree that the most likely scenario is to have the ENSO index positive, it is not clear whether it will stay in neutral territory or make it to El Nino. Note: there is no sign of a Modoki (central Pacific warming), which is more predictable than ENSO.
With regards to AMO and PDO, the AMO index is currently moderately positive, the PDO index is currently moderately negative.
In terms of analogue years for 2012:
- weakly positive AMO: 2011, 2003, 1999, 1967, 1961, 1955, 1954, 1951, 1949
- of these, negative PDO: 2011, 1999, 1967, 1955, 1954, 1954, 1951, 1949
- El Nino years (late summer) for warm phase of the AMO: 2009, 2002, 1997, 1957, 1951
By these criteria, 1951 is the best analogue year for North Atlantic hurricanes. According to the Wikipedia, in 1951 there were 10 total storms, 8 hurricanes, and 5 major hurricanes. The first hurricane of the season, Able, was the earliest major hurricane in Atlantic hurricane history. There were no U.S. landfalls this year, although major Hurricane Charlie struck Jamaica and caused considerable damage and loss of life. Apart from Able, the other hurricanes formed in the prime part of the hurricane season (Aug, Sept) and apparently originated from African easterly waves.
The positive AMO index is especially relevant for the Cape Verde type storms that form from African Easterly Waves during the peak of the hurricane season (Aug/ Sept). The frequency of AEWs is moderately tied to the AMO index, and also the AMM, which is the tropical expression of the AMO. Whereas the AMO index is moderately positive right now and is likely to stay that way through the summer, the AMM SST index is significantly negative. Generally, the AMM/AMO are very strongly correlated. However, there’s still plenty of time for this to change especially given how sensitive Gulf of Guinea SSTs are this time of year. Therefore, the AMO may not be as important this season in terms of its impact on Cape Verde type hurricanes from African easterly waves. The conflicting signs of the AMO and AMM suggest that we are likely to see either average to slightly lower than normal AEW activity during the peak part of the hurricane season.
Early 2012 activity
See the Wikipedia for a summary of the season so far (two named storms in late May). Several factors have contributed to the early activity in late May (as per James Belanger, CFAN’s lead hurricane forecaster):
- Easterly wave genesis and trajectories in the southern Caribbean have been shifted northward relative to climatology this year. In fact, the first two East Pacific TCs appear to be entirely due to easterly waves generated in-situ in that basin.
- High-latitude blocking in the eastern U.S. has been unusually persistent which has allowed a series of weak upper-level cut-off lows to penetrate beneath the ridges and into the Gulf of Mexico/SE U.S.
- The interaction between the cut-off lows along with easterly waves has favored the transient-trough interaction TC pathway to occur. This process is facilitated by high-latitude ridging as it provides a region where relatively weak deep-layer shear is present.
- in the early part of the season, we may see additional activity with the formation of tropical and subtropical storms.
- even if the number of named storms turns out to be average or below average, we may still see an average to above average number of hurricanes and major hurricanes, particularly in the main part of the hurricane season (Aug/Sept).
- the Gulf of Mexico is relatively more vulnerable than say for the last few years, owing to El Nino tracks (which tend to stay away from the Atlantic coast) and the warm temperatures in the Gulf.
- assuming that we are headed for an El Nino, a key issue is when the transition occurs. If in July, then ENSO’s impact on the hurricanes will be very significant. But if in late August, then the impact will be much smaller. Note, changes in the Walker circulation and downstream wind shear impacts on the North Atlantic (that impact hurricanes) typically takes order 1 month to be manifested once moderate ENSO state is established.