Hurricanes and global warming.
I've heard alot lately from people that global warming is the cause of the hurricanes. I thought this sounded fishy so I dug around a bit. Turns out to be wrong. But I digress. Let the facts speak for themselfs.
First lets define the term The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone". A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation (Holland 1993).
Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt, 39 mph) are called "tropical depressions" (This is not to be confused with the condition mid-latitude people get during a long, cold and grey winter wishing they could be closer to the equator ;-)). Once the tropical cyclone reaches winds of at least 17 m/s (34 kt, 39 mph) they are typically called a "tropical storm" and assigned a name. If winds reach 33 m/s (64 kt, 74 mph)), then they are called:
• "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E)
• "typhoon" (the Northwest Pacific Ocean west of the dateline)
• "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E)
• "severe cyclonic storm" (the North Indian Ocean)
• "tropical cyclone" (the Southwest Indian Ocean)
So how do we forecast hurricanes? A variety of hurricane track forecast models are run operationally for the Atlantic hurricane basin:
1. The basic model that is used as a "no-skill" forecast to compare other models against is CLIPER (CLImatology and PERsistence), which is a multiple regression statistical model that best utilizes the persistence of the current motion and also incorporates climatological track information (Aberson 1998). Surprisingly, CLIPER was difficult to beat with numerical model forecasts until the 1980s.
2. A statistical-dynamical model, NHC90 (McAdie 1991), uses geopotential height predictors from the Aviation model to produce a track forecast four times per day. The primary synoptic time NHC90 forecasts (00 and 12 UTC) are based upon 12 h old Aviation runs. A special version of NHC90, NHC90-LATE, is run at primary synoptic times with the current Aviation run, and is available a number of hours after NHC90. Both versions of NHC90 have been run operationally since 1990. An update to this model, NHC98, was implemented in 1998.
3. The Beta and Advection Model (BAM), follows a trajectory in the pressure-weighted vertically-averaged horizontal wind from the Aviation model beginning at the current storm location, with a correction that accounts for the beta effect (Marks 1992). Three versions of this model, one with a shallow-layer (BAMS), one with a medium-layer (BAMM), and one with a deep-layer (BAMD), are run. BAMS runs using the 850-700 mb layer,BAMM with the 850-400 mb layer, and BAMD with the 850-200 mb layer. The deep-layer version was run operationally for primary synoptic times in 1989; all three versions have been run four times per day since 1990.
4. A nested barotropic hurricane track forecast model (VICBAR) has been run four times daily since 1989. The primary synoptic time runs are run from current NCEP analyses, the off-time runs are run from six hour old data (Aberson and DeMaria 1994). Another barotropic model, LBAR, for Limited-Area Barotropic Model, is also being run operationally every 6 hours which performs slightly worse than VICBAR, but is available earlier for use by the NHC forecasters.
5. The NCEP Aviation and MRF models (Lord 1993) has been used for track forecasting since the 1992 hurricane season. These are global models.
6. A triply-nested movable mesh primitive equation model developed at the Geophysical Fluid Dynamics Laboratory (Bender et al 1993), known as the GFDL model, has provided forecasts since the 1992 hurricane season.
7. The United Kingdom Meterological Office's global model (UKMET) is utilized for forecasting the track of tropical cyclones around the world (Radford 1994). The National Hurricane Center starting receiving these operationally during 1996.
8. The United States Navy Operational Global Atmospheric Prediction Systems (NOGAPS) is also a global numerical model that shows skill in forecasting tropical cyclone track (Fiorino et al. 1993). This model was also first received operationally at the National Hurricane Center during 1996.
Despite the variety of hurricane track forecast models, there are only a few models that forecast intensity change for the Atlantic basin:
1. Similar to the CLIPER track model, the SHIFOR (Statistical Hurricane Intensity Forecast model) is used as a "no-skill" intensity change forecast. It is a multiple regression statistical model that best utilizes the persistence of the intensity trends and also incorporates climatological intensity change information (Jarvinen and Neumann 1979). SHIFOR has been difficult to exceed until recent years.
2. A statistical-synoptic model, SHIPS (Statistical Hurricane Intensity Prediction Scheme ), has been available to the National Hurricane Center since the mid-1990s (DeMaria and Kaplan 1994). It takes current and forecasted information on the synoptic scale on the sea surface temperatures, vertical shear, moist stability, etc. with an optimal combination of the trends in the cyclone intensity.
3. The GFDL model, described above in the track forecasting models, also issues forecasts of intensity change for the National Hurricane Center.
4. A new statistical scheme for estimating the probability of rapid intensification has been developed (Kaplan and DeMaria 1999) and is now being used operationally. The RI scheme employs synoptic and persistence information from the SHIPS model to estimate the probability of rapid intensification (24 h increase in maximum wind of 35 mph or greater) every 6 hours.
So Dr. Nick now that we know what hurricanes are, and how we track them, lets get back the original question, does global warming have an effect on hurricanes? The short answer, The United Nation's Intergovernmental Panel on Climate Change (IPCC) has speculated that climate change due to increasing amounts of anthropogenic "greenhouse" gases may result in increased tropical sea surface temperatures (SSTs) and increased tropical rainfall associated with a slightly stronger intertropical convergence zone (ITCZ) (Houghton et al., 1990, 1992, 1996). Because tropical cyclones extract latent and sensible heat from the warm tropical oceans and release the heat in its upper tropospheric outflow to fuel the storm's spin up, early work of the IPCC expressed concern that warmer SSTs will lead to more frequent and intense hurricanes, typhoons and severe tropical cyclones. These concerns prompted the IPCC (Houghton et al. 1990) to suggest in 1990 that:
"There is some evidence from model simulations
and empirical considerations that the frequency
per year, intensity and area of occurrence of
tropical disturbances may increase [in a
doubled carbon dioxide world], though it is
not yet compelling."
However, any changes in tropical cyclone activity are intrinsically also tied to large-scale changes in the tropical atmosphere. As a result, SSTs by themselves cannot be considered without corresponding information regarding the moisture and stability in the tropical troposphere. What has been identified in the current climate as being necessary for genesis and maintenance for tropical cyclones (e.g. SSTs of at least 26.5°C [80°F] - Gray 1968) would change in an enhanced doubled CO2 world because of possible changes in the moisture or stability. It is quite reasonable that an increase in tropical and subtropical SSTs would be also accompanied by an increase in the SST threshold value needed for cyclogenesis because of compensating changes in the tropospheric moist static stability (Emanuel 1995). In addition to the thermodynamic variables, changes in the tropical dynamics also play a large role in determining changes in tropical cyclone activity. For example, if the vertical wind shear over the tropical North Atlantic moderately increased during the hurricane season in an increased CO2 world - as what is typically seen during El Nino-Southern Oscillation warm phases (El Nino events), then we would most likely see a significant decrease in tropical cyclone activity. This is due to the Atlantic basin having a marginal climatology for tropical cyclone activity because of its sensitivity to changes in vertical wind shear and lack of an oceanic monsoon trough (Gray et al. 1993). In other less marginal tropical cyclone basins, changes in the vertical shear profile typically result in alterations in the preferred location of development (e.g. Nicholls 1979, Chan 1985, Revell and Goulter 1986, and Lander 1994). These complications along with conflicting global circulation modeling (GCM) runs compelled the 1995 IPCC (Houghton et al. 1996) to express greater uncertainty about the nature of tropical cyclones in an enhanced CO2 environment:
"The formation of tropical cyclones depends not
only on sea surface temperature (SST), but also
on a number of atmospheric factors. Although
some models now represent tropical storms with
some realism for present day climate, the state
of the science does not allow assessment of
future changes."
Most recently, Henderson-Sellers et al. (1998) addressed a few of the tropical cyclone-greenhouse warming problems. The first is that "there is no evidence to suggest any major changes in the area or global location of tropical cyclone genesis in greenhouse conditions." This conclusion is based upon Holland's (1997) thermodynamic tropical cyclone model which does show that in a greenhouse-warmed climate there is an upward alteration in the minimum SST from 26.5° to 28°C (80° to 83°F) needed for tropical cyclogenesis. The additional conclusion from Henderson-Sellers et al. (1998) suggests "an increase in [maximum potential intensity] MPI of 10%-20% [in central pressure or 5%-10% in maximum sustained winds] for a doubled CO2 climate but the known omissions (ocean spray, momentum restriction, and possibly also surface to 300 hPa lapse rate changes) all act to reduce these increases." This second finding is also based upon the thermodynamic models of Emanuel (1986) and Holland (1997), which also appear to corroborate similar findings for Northwest Pacific typhoons from a "downscaled" GCM to mesoscale model approach by Knutson et al. (1998). Henderson-Sellers et al. (1998) does not provide guidance for possible changes in tropical cyclone frequency, mean intensity, or area of occurrence. The most helpful paper that may predict changes in hurricane and typhoon frequency with some realism is the recent work by Royer et al. (1998). Based upon alterations to the large scale atmospheric and oceanic conditions (vertical shear, vorticity and thermodynamic stability), they suggest that only small changes to the tropical cyclone frequencies may result: up to 10% increase in numbers in the Northern Hemisphere (primarily in the Northwest Pacific) and up to a 5% decrease in numbers in the Southern Hemisphere. These values should be considered very preliminary. To summarize, our current assessment of how global warming may alter hurricanes, typhoons and tropical cyclones is as follows (from Henderson-Sellers et al. 1998, Knutson et al. 1998, and Royer et al. 1998):
• There is no evidence to suggest tropical cyclones will have any major changes in WHERE they form or occur.
• Preliminary analyses hint that only small to no change in the NUMBER of tropical cyclones may occur, and that regionally there may be areas that have small increases or small decreases in frequency.
• The PEAK INTENSITY of tropical cyclones may increase by 5-10% in wind speeds, but this may be an overestimate because of simplifications in the calculations.
• Little is known as to how the AVERAGE INTENSITY or SIZE of tropical cyclones may change due to global warming.
• Overall, these suggested changes are quite small compared to the observed large natural variability of hurricanes, typhoons and tropical cyclones. However, more study is needed to better understand the complex interaction between these storms and the tropical atmosphere/ocean. So tell everyone you know that bitches about increased activity of hurricanes that they are in fact wrong and should read more from better sources.
http://www.nhc.noaa.gov/
This is where I got all my info, the national hurricane center.
First lets define the term The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone". A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation (Holland 1993).
Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt, 39 mph) are called "tropical depressions" (This is not to be confused with the condition mid-latitude people get during a long, cold and grey winter wishing they could be closer to the equator ;-)). Once the tropical cyclone reaches winds of at least 17 m/s (34 kt, 39 mph) they are typically called a "tropical storm" and assigned a name. If winds reach 33 m/s (64 kt, 74 mph)), then they are called:
• "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E)
• "typhoon" (the Northwest Pacific Ocean west of the dateline)
• "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E)
• "severe cyclonic storm" (the North Indian Ocean)
• "tropical cyclone" (the Southwest Indian Ocean)
So how do we forecast hurricanes? A variety of hurricane track forecast models are run operationally for the Atlantic hurricane basin:
1. The basic model that is used as a "no-skill" forecast to compare other models against is CLIPER (CLImatology and PERsistence), which is a multiple regression statistical model that best utilizes the persistence of the current motion and also incorporates climatological track information (Aberson 1998). Surprisingly, CLIPER was difficult to beat with numerical model forecasts until the 1980s.
2. A statistical-dynamical model, NHC90 (McAdie 1991), uses geopotential height predictors from the Aviation model to produce a track forecast four times per day. The primary synoptic time NHC90 forecasts (00 and 12 UTC) are based upon 12 h old Aviation runs. A special version of NHC90, NHC90-LATE, is run at primary synoptic times with the current Aviation run, and is available a number of hours after NHC90. Both versions of NHC90 have been run operationally since 1990. An update to this model, NHC98, was implemented in 1998.
3. The Beta and Advection Model (BAM), follows a trajectory in the pressure-weighted vertically-averaged horizontal wind from the Aviation model beginning at the current storm location, with a correction that accounts for the beta effect (Marks 1992). Three versions of this model, one with a shallow-layer (BAMS), one with a medium-layer (BAMM), and one with a deep-layer (BAMD), are run. BAMS runs using the 850-700 mb layer,BAMM with the 850-400 mb layer, and BAMD with the 850-200 mb layer. The deep-layer version was run operationally for primary synoptic times in 1989; all three versions have been run four times per day since 1990.
4. A nested barotropic hurricane track forecast model (VICBAR) has been run four times daily since 1989. The primary synoptic time runs are run from current NCEP analyses, the off-time runs are run from six hour old data (Aberson and DeMaria 1994). Another barotropic model, LBAR, for Limited-Area Barotropic Model, is also being run operationally every 6 hours which performs slightly worse than VICBAR, but is available earlier for use by the NHC forecasters.
5. The NCEP Aviation and MRF models (Lord 1993) has been used for track forecasting since the 1992 hurricane season. These are global models.
6. A triply-nested movable mesh primitive equation model developed at the Geophysical Fluid Dynamics Laboratory (Bender et al 1993), known as the GFDL model, has provided forecasts since the 1992 hurricane season.
7. The United Kingdom Meterological Office's global model (UKMET) is utilized for forecasting the track of tropical cyclones around the world (Radford 1994). The National Hurricane Center starting receiving these operationally during 1996.
8. The United States Navy Operational Global Atmospheric Prediction Systems (NOGAPS) is also a global numerical model that shows skill in forecasting tropical cyclone track (Fiorino et al. 1993). This model was also first received operationally at the National Hurricane Center during 1996.
Despite the variety of hurricane track forecast models, there are only a few models that forecast intensity change for the Atlantic basin:
1. Similar to the CLIPER track model, the SHIFOR (Statistical Hurricane Intensity Forecast model) is used as a "no-skill" intensity change forecast. It is a multiple regression statistical model that best utilizes the persistence of the intensity trends and also incorporates climatological intensity change information (Jarvinen and Neumann 1979). SHIFOR has been difficult to exceed until recent years.
2. A statistical-synoptic model, SHIPS (Statistical Hurricane Intensity Prediction Scheme ), has been available to the National Hurricane Center since the mid-1990s (DeMaria and Kaplan 1994). It takes current and forecasted information on the synoptic scale on the sea surface temperatures, vertical shear, moist stability, etc. with an optimal combination of the trends in the cyclone intensity.
3. The GFDL model, described above in the track forecasting models, also issues forecasts of intensity change for the National Hurricane Center.
4. A new statistical scheme for estimating the probability of rapid intensification has been developed (Kaplan and DeMaria 1999) and is now being used operationally. The RI scheme employs synoptic and persistence information from the SHIPS model to estimate the probability of rapid intensification (24 h increase in maximum wind of 35 mph or greater) every 6 hours.
So Dr. Nick now that we know what hurricanes are, and how we track them, lets get back the original question, does global warming have an effect on hurricanes? The short answer, The United Nation's Intergovernmental Panel on Climate Change (IPCC) has speculated that climate change due to increasing amounts of anthropogenic "greenhouse" gases may result in increased tropical sea surface temperatures (SSTs) and increased tropical rainfall associated with a slightly stronger intertropical convergence zone (ITCZ) (Houghton et al., 1990, 1992, 1996). Because tropical cyclones extract latent and sensible heat from the warm tropical oceans and release the heat in its upper tropospheric outflow to fuel the storm's spin up, early work of the IPCC expressed concern that warmer SSTs will lead to more frequent and intense hurricanes, typhoons and severe tropical cyclones. These concerns prompted the IPCC (Houghton et al. 1990) to suggest in 1990 that:
"There is some evidence from model simulations
and empirical considerations that the frequency
per year, intensity and area of occurrence of
tropical disturbances may increase [in a
doubled carbon dioxide world], though it is
not yet compelling."
However, any changes in tropical cyclone activity are intrinsically also tied to large-scale changes in the tropical atmosphere. As a result, SSTs by themselves cannot be considered without corresponding information regarding the moisture and stability in the tropical troposphere. What has been identified in the current climate as being necessary for genesis and maintenance for tropical cyclones (e.g. SSTs of at least 26.5°C [80°F] - Gray 1968) would change in an enhanced doubled CO2 world because of possible changes in the moisture or stability. It is quite reasonable that an increase in tropical and subtropical SSTs would be also accompanied by an increase in the SST threshold value needed for cyclogenesis because of compensating changes in the tropospheric moist static stability (Emanuel 1995). In addition to the thermodynamic variables, changes in the tropical dynamics also play a large role in determining changes in tropical cyclone activity. For example, if the vertical wind shear over the tropical North Atlantic moderately increased during the hurricane season in an increased CO2 world - as what is typically seen during El Nino-Southern Oscillation warm phases (El Nino events), then we would most likely see a significant decrease in tropical cyclone activity. This is due to the Atlantic basin having a marginal climatology for tropical cyclone activity because of its sensitivity to changes in vertical wind shear and lack of an oceanic monsoon trough (Gray et al. 1993). In other less marginal tropical cyclone basins, changes in the vertical shear profile typically result in alterations in the preferred location of development (e.g. Nicholls 1979, Chan 1985, Revell and Goulter 1986, and Lander 1994). These complications along with conflicting global circulation modeling (GCM) runs compelled the 1995 IPCC (Houghton et al. 1996) to express greater uncertainty about the nature of tropical cyclones in an enhanced CO2 environment:
"The formation of tropical cyclones depends not
only on sea surface temperature (SST), but also
on a number of atmospheric factors. Although
some models now represent tropical storms with
some realism for present day climate, the state
of the science does not allow assessment of
future changes."
Most recently, Henderson-Sellers et al. (1998) addressed a few of the tropical cyclone-greenhouse warming problems. The first is that "there is no evidence to suggest any major changes in the area or global location of tropical cyclone genesis in greenhouse conditions." This conclusion is based upon Holland's (1997) thermodynamic tropical cyclone model which does show that in a greenhouse-warmed climate there is an upward alteration in the minimum SST from 26.5° to 28°C (80° to 83°F) needed for tropical cyclogenesis. The additional conclusion from Henderson-Sellers et al. (1998) suggests "an increase in [maximum potential intensity] MPI of 10%-20% [in central pressure or 5%-10% in maximum sustained winds] for a doubled CO2 climate but the known omissions (ocean spray, momentum restriction, and possibly also surface to 300 hPa lapse rate changes) all act to reduce these increases." This second finding is also based upon the thermodynamic models of Emanuel (1986) and Holland (1997), which also appear to corroborate similar findings for Northwest Pacific typhoons from a "downscaled" GCM to mesoscale model approach by Knutson et al. (1998). Henderson-Sellers et al. (1998) does not provide guidance for possible changes in tropical cyclone frequency, mean intensity, or area of occurrence. The most helpful paper that may predict changes in hurricane and typhoon frequency with some realism is the recent work by Royer et al. (1998). Based upon alterations to the large scale atmospheric and oceanic conditions (vertical shear, vorticity and thermodynamic stability), they suggest that only small changes to the tropical cyclone frequencies may result: up to 10% increase in numbers in the Northern Hemisphere (primarily in the Northwest Pacific) and up to a 5% decrease in numbers in the Southern Hemisphere. These values should be considered very preliminary. To summarize, our current assessment of how global warming may alter hurricanes, typhoons and tropical cyclones is as follows (from Henderson-Sellers et al. 1998, Knutson et al. 1998, and Royer et al. 1998):
• There is no evidence to suggest tropical cyclones will have any major changes in WHERE they form or occur.
• Preliminary analyses hint that only small to no change in the NUMBER of tropical cyclones may occur, and that regionally there may be areas that have small increases or small decreases in frequency.
• The PEAK INTENSITY of tropical cyclones may increase by 5-10% in wind speeds, but this may be an overestimate because of simplifications in the calculations.
• Little is known as to how the AVERAGE INTENSITY or SIZE of tropical cyclones may change due to global warming.
• Overall, these suggested changes are quite small compared to the observed large natural variability of hurricanes, typhoons and tropical cyclones. However, more study is needed to better understand the complex interaction between these storms and the tropical atmosphere/ocean. So tell everyone you know that bitches about increased activity of hurricanes that they are in fact wrong and should read more from better sources.
http://www.nhc.noaa.gov/
This is where I got all my info, the national hurricane center.