Written by Aaron Studwell, Senior Meteorologist and Tropical Specialist, Wilkens Weather Technologies LP
Thursday, 02 May 2013 13:12
The first section of this article will review the active, and quite unique, North Atlantic hurricane season of 2012. The remainder of the article will detail the North Atlantic basin tropical forecast for 2013. The forecast will examine the impact of factors ranging from sea-surface temperatures in the Atlantic and Pacific Oceans to wind patterns in the upper reaches of the troposphere. All of these factors point to above normal tropical activity for the 2013 Atlantic hurricane season.
Review of the 2012 Hurricane Season
Figure 1: Hurricane and tropical storm tracks for 2012. (Source: National Hurricane Center)
The 2012 North Atlantic hurricane season was more active than the long-term normal, with nineteen named storms, ten hurricanes, and two major hurricanes, Michael and Sandy. (A major hurricane is one that is classified as Category 3 or greater on the Saffir-Simpson scale, i.e., maximum sustained winds are 111 mph or greater.) This season tied with the 1887, 1995, 2010 and 2011 seasons as the third most-active season on record.
Reviewing the 2012 hurricane season shows some interesting facts:
Nearly US$78 billion in damages was caused across the United States and the Caribbean. The primary contributor to this total was Hurricane Sandy with damages estimated between $50 and $72 billion.
Second costliest season ever – Costliest season was 2005 (Hurricanes Katrina and Rita) – $108 billion.
Deadliest season since 2008 – 354 fatalities.
Before we begin our forecast for the 2013 North Atlantic tropical season, let’s take a look back over the past six decades. Across the North Atlantic basin, there have been various cycles of tropical activity during this period. The 60-year average for this basin is 10.9 named storms, 6.2 hurricanes and 2.6 major hurricanes (Table 1).
The inclusion of the 30-year median shows a rise in tropical activity across the North Atlantic basin. This can generally be attributed to the warm phase of the Atlantic Multidecadal Oscillation (AMO) that has been in place since 1995, which will be described in specific detail later. Over the past 16 years, the North Atlantic basin has been more active with five of the most active seasons (1995, 2005, 2010, 2011 and 2012) in the past 60 years occurring in this time frame.
Period of interest
1951 – 2010
1981 – 2010
1963 – 1994
(Cold AMO phase)
1995 – present
(Warm AMO phase)
Table 1: North Atlantic Ocean tropical season climatology showing the 60-year mean (1951-2010), 30-year median (1981-2010) and average values during the period in which the
Atlantic Multidecadal Oscillation (AMO) has been in its warm phase.
The sea-surface temperatures (SSTs) of both the Atlantic Ocean and the equatorial Pacific Ocean are factors contributing to the annual levels of tropical cyclone activity across the North Atlantic basin. For the Atlantic Ocean, warmer SSTs lead to higher evaporation rates, a more active Intertropical Convergence Zone (ITCZ), and the ITCZ moving further away from the Equator. These factors combine to yield a more active tropical season.
Across the equatorial Pacific Ocean, the variations in SSTs are known as the El Niño-Southern Oscillation (ENSO). There are two phases of ENSO, El Niño (warmer than normal SSTs) and La Niña (cooler than normal SSTs). During a La Niña event, the vertical wind shear (i.e., the difference between the upper-level and lower-level winds) is reduced across the North Atlantic basin (Fig. 2). In this reduced shear environment (Profile b on Fig. 2), the potential for tropical development across the North Atlantic basin is increased and vice versa for El Niño events.
Figure 2. Vertical wind profiles across the tropical North Atlantic basin. Profile (a) is associated with an inactive Atlantic tropical season and with El Niño, while Profile (b) is associated with an active Atlantic tropical season and with La Niña (Source: Colorado State University, Department of Atmospheric Science).
A recent image of the worldwide SST departures from normal is shown in Figure 3. While SSTs across the equatorial Pacific are generally below normal, this is classified as a neutral state. The criterion for an El Niño-Southern Oscillation (ENSO) designation as El Niño, La Niña or neutral is the Oceanic Niño Index (ONI). The ONI is defined as the three-month running-mean SST departures in the Niño 3.4 region (The Niño 3.4 region extends from 120° to 165°W, between 5°N and 5°S.). Cold water is upwelling off the Ecuadoran coast and has been spreading westward along the Equator. Across the tropical extent of the North Atlantic basin (highlighted in the red oval), as a whole, temperatures are above normal.
The above normal SSTs across the North Atlantic basin will provide a favorable environment for the development of tropical systems as well as their being sustained. The warmer SSTs contribute to increased evaporation. By having additional moisture in the lower atmosphere, there is more potential latent heat available, which is a critical factor in the development and maintenance of a tropical system. (Latent heat is the release of energy as the moist air rises, releasing energy to the atmosphere as the vapor condenses to droplets. By warming the air due to the release of latent heat, the atmosphere becomes increasing unstable, contributing to the development of additional convection in either a developing or maturing tropical system.)
Figure 3: Worldwide sea surface temperature anomalies (°C) as of April 27, 2013. (Source: NOAA – Earth System Research Laboratory)
The long-range climate models predict that the Pacific Ocean will remain at an ENSO-neutral state during the spring months and continue into the summer months. Figure 4 shows the probabilistic output from 24 forecast models for the Niño 3.4 region. For the peak three months of hurricane season, August through October, there is 78% probability that the Niño 3.4 region will be in either a neutral or La Niña state.
Figure 4: Niño 3.4 probabilistic forecast model outputs for 16 dynamical and 8 statistical models. This information is compiled and published by the International Research Institute for Climate Prediction at Columbia University.
The impact of the forecasted neutral ENSO state for the equatorial Pacific basin is that the vertical wind shear across the tropical Atlantic should be similar to the levels experienced last year throughout the tropical season. Last year was an atypical tropical season for the Atlantic basin with four named storms before July 1. (Typically, there is only one named storm prior to July 1.) While, another fast start cannot necessarily be expected again in 2013, the neutral ENSO pattern will be treated as a favorable factor for a more active 2013 Atlantic tropical season.
Atlantic Multidecadal Oscillation
The Atlantic Multidecadal Oscillation (AMO) is generally associated with long-term cycles of tropical activity in the North Atlantic basin. Table 1 shows the difference in the levels of tropical activity when AMO is in a cold phase versus a warm phase. The AMO is a naturally occurring pattern over the Atlantic basin which is typified by variations in both the sea surface temperature and the sea level pressure fields. It has been theorized that the AMO is linked to fluctuations in the strength of the large-scale oceanic circulation across the Atlantic Ocean, including the Gulf Stream. When the circulation is stronger than normal, the AMO tends to be in its warm phase. As indicated by the phase names, the “warm phase” of the AMO is generally associated with warmer SSTs and weaker trade winds.
Figure 5 shows the AMO pattern from 1880 through early 2013. The AMO has generally been in a warm phase since 1995 and this has coincided with more active tropical seasons across the North Atlantic basin.
Figure 5: Monthly values for the AMO index, 1880 through 2013.
The most recent AMO value of 0.188 (March 2013) is a moderately positive value. In aggregate, the 2012-13 Northern Hemisphere winter has had above normal readings. Generally, when the AMO has been in its positive phase since 1995 and the March value is positive (16 occurrences), there has only been one year (2002) where the AMO transitioned to a negative value. Given this trend, it is anticipated that the AMO will remain positive through the summer of 2013 and will be treated as a favorable factor for a more active 2013 Atlantic tropical season.
Coupled Forecast System
The Coupled Forecast System (CFS) v2 is a fully coupled model and represents the interaction between the Earth’s oceans, land and atmosphere. A majority of the runs of the CFS v2 show above normal precipitation during July and August for the Southeast U.S. The forecast trend of a wet July and August may be associated with an increased risk of landfalling tropical systems during these two months. (There was no clear pattern for the month of September but this will continue to be monitored.)
The Wilkens Weather team has examined all of the factors discussed in this report and our forecast for the 2013 North Atlantic hurricane season is presented in Table 2.
2013 WWT forecast
Major hurricanes *
* Major hurricane – A Category 3 or greater hurricane, as measured on the Saffir-Simpson scale. Maximum sustained winds are 111 mph or greater
Table 2: Wilkens Weather’s 2013 North Atlantic basin hurricane season forecast.
Wilkens Weather Technologies, LP has provided worldwide meteorological forecasting services to the offshore and energy industries for more than 30 years, with weather forecast centers in Houston, Texas and in Reading, UK. WWT will be exhibiting at the 2013 Offshore Technology Conference in Houston for the 29th consecutive year- Booth 5535 on the main aisle in Reliant Center at Reliant Park, 6-9 May 2013.www.wilkensweather.com
Written by: Aaron Studwell, Senior Meteorologist and Tropical Specialist, Wilkens Weather Technologies LP