Posted on 4 November 2012 by Kevin C

This is a re-post from Ari Jokimäki's AGW Observer. Titles added by Kevin C

A new historical record of Atlantic hurricane threat

Homogeneous record of Atlantic hurricane surge threat since 1923 – Grinsted et al. (2012) “Detection and attribution of past changes in cyclone activity are hampered by biased cyclone records due to changes in observational capabilities. Here we construct an independent record of Atlantic tropical cyclone activity on the basis of storm surge statistics from tide gauges. We demonstrate that the major events in our surge index record can be attributed to landfalling tropical cyclones; these events also correspond with the most economically damaging Atlantic cyclones. We find that warm years in general were more active in all cyclone size ranges than cold years. The largest cyclones are most affected by warmer conditions and we detect a statistically significant trend in the frequency of large surge events (roughly corresponding to tropical storm size) since 1923. In particular, we estimate that Katrina-magnitude events have been twice as frequent in warm years compared with cold years (P < 0.02).” Aslak Grinsted, John C. Moore, and Svetlana Jevrejeva, PNAS October 15, 2012, doi: 10.1073/pnas.1209542109. [FULL TEXT]

Reconciling Pacific hurricane records

Consensus on climate trends in western North Pacific tropical cyclones – Kang & Elsner (2012) “Research on trends in western North Pacific tropical cyclone (TC) activity is limited by problems associated with different wind speed conversions used by the various meteorological agencies. This paper uses a quantile method to effectively overcome this conversion problem. Following the assumption that the intensity ranks of TCs are the same among agencies, quantiles at the same probability level in different data sources are regarded as having the same wind speed level. Tropical cyclone data from the Joint Typhoon Warning Center (JTWC) and Japan Meteorological Agency (JMA) are chosen for research and comparison. Trends are diagnosed for the upper 45% of the strongest TCs annually. The 27-year period beginning with 1984, when the JMA began using the Dvorak (1982) technique, is determined to be the most reliable for achieving consensus among the two agencies regarding these trends. The start year is a compromise between including as many years in the data as possible, but not too many that the period includes observations that result in inconsistent trend estimates. Consensus of TC trends between the two agencies over the period is interpreted as fewer, but stronger events since 1984, even with the lower Power Dissipation Index (PDI) in the western North Pacific in recent years.” Nam-Young Kang and James B. Elsner, Journal of Climate 2012, doi: http://dx.doi.org/10.1175/JCLI-D-11-00735.1.

Could climate change lead to storm surges flooding New York?

Physically based assessment of hurricane surge threat under climate change – Lin et al. (2012) “Storm surges are responsible for much of the damage and loss of life associated with landfalling hurricanes. Understanding how global warming will affect hurricane surges thus holds great interest. As general circulation models (GCMs) cannot simulate hurricane surges directly, we couple a GCM-driven hurricane model with hydrodynamic models to simulate large numbers of synthetic surge events under projected climates and assess surge threat, as an example, for New York City (NYC). Struck by many intense hurricanes in recorded history and prehistory, NYC is highly vulnerable to storm surges. We show that the change of storm climatology will probably increase the surge risk for NYC; results based on two GCMs show the distribution of surge levels shifting to higher values by a magnitude comparable to the projected sea-level rise (SLR). The combined effects of storm climatology change and a 1 m SLR may cause the present NYC 100-yr surge flooding to occur every 3–20 yr and the present 500-yr flooding to occur every 25–240 yr by the end of the century.” Ning Lin, Kerry Emanuel, Michael Oppenheimer & Erik Vanmarcke, Nature Climate Change 2, 462–467(2012), doi:10.1038/nclimate1389. [FULL TEXT]

Where will hurricanes occur under climate change?

Hurricanes and Global Warming: Results from Downscaling IPCC AR4 Simulations – Emanuel et al. (2011) “Changes in tropical cyclone activity are among the more potentially consequential results of global climate change, and it is therefore of considerable interest to understand how anthropogenic climate change may affect such storms. Global climate models are currently used to estimate future climate change, but the current generation of models lacks the horizontal resolution necessary to resolve the intense inner core of tropical cyclones. Here we review a new technique for inferring tropical cyclone climatology from the output of global models, extend it to predict genesis climatologies (rather than relying on historical climatology), and apply it to current and future climate states simulated by a suite of global models developed in support of the most recent Intergovernmental Panel on Climate Change report. This new technique attacks the horizontal resolution problem by using a specialized, coupled ocean–atmosphere hurricane model phrased in angular momentum coordinates, which provide a high resolution of the core at low cost. This model is run along each of 2,000 storm tracks generated using an advection-and-beta model, which is, in turn, driven by large-scale winds derived from the global models. In an extension to this method, tracks are initiated by randomly seeding large areas of the tropics with weak vortices and then allowing the intensity model to determine their survival, based on large-scale environmental conditions. We show that this method is largely successful in reproducing the observed seasonal cycle and interannual variability of tropical cyclones in the present climate, and that it is more modestly successful in simulating their spatial distribution. When applied to simulations of global climate with double the present concentration of carbon dioxide, this method predicts substantial changes and geographic shifts in tropical cyclone activity, but with much variation among the global climate models used. Basinwide power dissipation and storm intensity generally increase with global warming, but the results vary from model to model and from basin to basin. Storm frequency decreases in the Southern Hemisphere and north Indian Ocean, increases in the western North Pacific, and is indeterminate elsewhere. We demonstrate that in these simulations, the change in tropical cyclone activity is greatly influenced by the increasing difference between the moist entropy of the boundary layer and that of the middle troposphere as the climate warms.” Emanuel, Kerry, Ragoth Sundararajan, John Williams, 2008: Hurricanes and Global Warming: Results from Downscaling IPCC AR4 Simulations. Bull. Amer. Meteor. Soc., 89, 347–367. doi: http://dx.doi.org/10.1175/BAMS-89-3-347. [FULL TEXT]

Small increases in intense hurricanes in existing GCMs - but only by 2100

Tropical cyclones and climate change – Knutson et al. (2010) “Whether the characteristics of tropical cyclones have changed or will change in a warming climate — and if so, how — has been the subject of considerable investigation, often with conflicting results. Large amplitude fluctuations in the frequency and intensity of tropical cyclones greatly complicate both the detection of long-term trends and their attribution to rising levels of atmospheric greenhouse gases. Trend detection is further impeded by substantial limitations in the availability and quality of global historical records of tropical cyclones. Therefore, it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes. However, future projections based on theory and high-resolution dynamical models consistently indicate that greenhouse warming will cause the globally averaged intensity of tropical cyclones to shift towards stronger storms, with intensity increases of 2–11% by 2100. Existing modelling studies also consistently project decreases in the globally averaged frequency of tropical cyclones, by 6–34%. Balanced against this, higher resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20% in the precipitation rate within 100 km of the storm centre. For all cyclone parameters, projected changes for individual basins show large variations between different modelling studies.” Thomas R. Knutson, John L. McBride, Johnny Chan, Kerry Emanuel, Greg Holland, Chris Landsea, Isaac Held, James P. Kossin, A. K. Srivastava & Masato Sugi, Nature Geoscience 3, 157 – 163 (2010), doi:10.1038/ngeo779. [FULL TEXT]

Sea surface temperatures, multidecadal oscillations and hurricanes

Modeling the Dependence of Tropical Storm Counts in the North Atlantic Basin on Climate Indices – Villarini et al. (2010) “The authors analyze and model time series of annual counts of tropical storms lasting more than 2 days in the North Atlantic basin and U.S. landfalling tropical storms over the period 1878–2008 in relation to different climate indices. The climate indices considered are the tropical Atlantic sea surface temperature (SST), tropical mean SST, the North Atlantic Oscillation (NAO), and the Southern Oscillation index (SOI). Given the uncertainties associated with a possible tropical storm undercount in the presatellite era, two different time series of counts for the North Atlantic basin are employed: one is the original (uncorrected) tropical storm record maintained by the National Hurricane Center and the other one is with a correction for the estimated undercount associated with a changing observation network. Two different SST time series are considered: the Met Office’s HadISSTv1 and NOAA’s Extended Reconstructed SST. Given the nature of the data (counts), a Poisson regression model is adopted. The selection of statistically significant covariates is performed by penalizing models for adding extra parameters and two penalty functions are used. Depending on the penalty function, slightly different models, both in terms of covariates and dependence of the model’s parameter, are obtained, showing that there is not a “single best” model. Moreover, results are sensitive to the undercount correction and the SST time series. Suggestions concerning the model to use are provided, driven by both the outcomes of the statistical analyses and the current understanding of the underlying physical processes responsible for the genesis, development, and tracks of tropical storms in the North Atlantic basin. Although no single model is unequivocally superior to the others, the authors suggest a very parsimonious family of models using as covariates tropical Atlantic and tropical mean SSTs.” Villarini, Gabriele, Gabriel A. Vecchi, James A. Smith, 2010: Modeling the Dependence of Tropical Storm Counts in the North Atlantic Basin on Climate Indices. Mon. Wea. Rev., 138, 2681–2705. doi: http://dx.doi.org/10.1175/2010MWR3315.1. [FULL TEXT]

Paleoclimate reconstruction of hurricane activity over 1500 years

Atlantic hurricanes and climate over the past 1,500 years – Mann et al. (2009) “Atlantic tropical cyclone activity, as measured by annual storm counts, reached anomalous levels over the past decade1. The short nature of the historical record and potential issues with its reliability in earlier decades, however, has prompted an ongoing debate regarding the reality and significance of the recent rise. Here we place recent activity in a longer-term context by comparing two independent estimates of tropical cyclone activity over the past 1,500 years. The first estimate is based on a composite of regional sedimentary evidence of landfalling hurricanes, while the second estimate uses a previously published statistical model of Atlantic tropical cyclone activity driven by proxy reconstructions of past climate changes. Both approaches yield consistent evidence of a peak in Atlantic tropical cyclone activity during medieval times (around ad 1000) followed by a subsequent lull in activity. The statistical model indicates that the medieval peak, which rivals or even exceeds (within uncertainties) recent levels of activity, results from the reinforcing effects of La-Niña-like climate conditions and relative tropical Atlantic warmth.” Michael E. Mann, Jonathan D. Woodruff, Jeffrey P. Donnelly & Zhihua Zhang, Nature 460, 880-883 (13 August 2009) | doi:10.1038/nature08219.[FULL TEXT]

Oscillations and Cycles

Gulf Stream and ENSO Increase the Temperature Sensitivity of Atlantic Tropical Cyclones – Moore et al. (2008) “Controversy exists over the role of the recent rise in sea surface temperatures (SST) and the frequency of tropical cyclones or hurricanes. Here, 135 yr of observational records are used to demonstrate how sea surface temperature, sea level pressure, and cyclone numbers are linked. A novel wavelet-lag coherence method is used to study cause and effect relations over a large space of time scales, phase lags, and periods. It is found that SST and cyclones are not merely correlated, but are in a negative feedback loop, where rising SST causes increased numbers of cyclones, which reduce SST. This is statistically most significant at decadal and not at longer periods, which is contrary to expectations if long-period natural cycles are important in driving cyclone numbers. Spatial relationships are examined using phase-aware teleconnections, which at the dominant decadal period show the in-phase behavior of the Atlantic SST in the Gulf Stream region, reflecting the role of the transportion of heat northward from the tropical Atlantic. At 5-yr periods there is significant coherence when SST leads cyclones by 2 yr, and this is associated with tropical ENSO activity such that, as predicted, increasing numbers of El Niños cause fewer Atlantic cyclones. The pattern of coherence existing since 1970 strongly favors the decadal coherence band, and despite growing coherence at higher frequencies, there is none at the 5-yr band, perhaps explaining why the observed sensitivity between SST and cyclones is larger than that from general circulation model (GCM) predictions and becoming greater.” Moore, J. C., A. Grinsted, S. Jevrejeva, 2008: Gulf Stream and ENSO Increase the Temperature Sensitivity of Atlantic Tropical Cyclones. J. Climate, 21, 1523–1531. doi: http://dx.doi.org/10.1175/2007JCLI1752.1. [FULL TEXT]

Hurricane activity linked to sea surface temperatures, but the relationship is complex

Whither Hurricane Activity? – Vecchi et al. (2008) “Alternative interpretations of the relationship between sea surface temperature and hurricane activity imply vastly different future Atlantic hurricane activity.” Gabriel A. Vecchi, Kyle L. Swanson and Brian J. Soden, Science 31 October 2008: Vol. 322 no. 5902 pp. 687-689, DOI: 10.1126/science.1164396. [FULL TEXT]

A centennial hurricane history

Counting Atlantic tropical cyclones back to 1900 – Landsea (2007) “Climate variability and any resulting change in the characteristics of tropical cyclones (tropical storms, subtropical storms, and hurricanes) have become topics of great interest and research within the past 2 years [International Workshop on Tropical Cyclones, 2006].An emerging focus is how the frequency of tropical cyclones has changed over time and whether any changes could be linked to anthropogenic global warming.” Landsea, C. (2007), Counting Atlantic tropical cyclones back to 1900, Eos Trans. AGU, 88(18), 197, doi:10.1029/2007EO180001. [FULL TEXT]

Katrina events linked to temperatures

Estimated return periods for Hurricane Katrina – Elsner et al. (2006) “Hurricane Katrina is one of the most destructive natural disaster in U.S. history. The infrequency of severe coastal hurricanes implies that empirical probability estimates of the next big one will be unreliable. Here we use an extreme-value model together with interpolated best-track (HURDAT) records to show that a hurricane of Katrina’s intensity or stronger can be expected to occur, on average, once every 21 years somewhere along the Gulf coast from Texas through Alabama and once every 14 years somewhere along the entire coast from Texas through Maine. The model predicts a 100-year return level of 83 ms⁻¹ (186 mph) during globally warm years and 75 ms−1 (168 mph) during globally cool years. This difference is consistent with models predicting an increase in hurricane intensity with increasing greenhouse warming.” Elsner, J. B., T. H. Jagger, and A. A. Tsonis (2006), Estimated return periods for Hurricane Katrina, Geophys. Res. Lett., 33, L08704, doi:10.1029/2005GL025452. [FULL TEXT]

US hurricane probabilities

Climatology Models for Extreme Hurricane Winds near the United States – Jagger & Elsner (2006) “The rarity of severe coastal hurricanes implies that empirical estimates of extreme wind speed return levels will be unreliable. Here climatology models derived from extreme value theory are estimated using data from the best-track [Hurricane Database (HURDAT)] record. The occurrence of a hurricane above a specified threshold intensity level is assumed to follow a Poisson distribution, and the distribution of the maximum wind is assumed to follow a generalized Pareto distribution. The likelihood function is the product of the generalized Pareto probabilities for each wind speed estimate. A geographic region encompassing the entire U.S. coast vulnerable to Atlantic hurricanes is of primary interest, but the Gulf Coast, Florida, and the East Coast regions are also considered. Model parameters are first estimated using a maximum likelihood (ML) procedure. Results estimate the 100-yr return level for the entire coast at 157 kt (±10 kt), but at 117 kt (±4 kt) for the East Coast region (1 kt = 0.514 m s⁻¹). Highest wind speed return levels are noted along the Gulf Coast from Texas to Alabama. The study also examines how the extreme wind return levels change depending on climate conditions including El Niño–Southern Oscillation, the Atlantic Multidecadal Oscillation, the North Atlantic Oscillation, and global temperature. The mean 5-yr return level during La Niña (El Niño) conditions is 125 (116) kt, but is 140 (164) kt for the 100-yr return level. This indicates that La Niña years are the most active for the occurrence of strong hurricanes, but that extreme hurricanes are more likely during El Niño years. Although El Niño inhibits hurricane formation in part through wind shear, the accompanying cooler lower stratosphere appears to increase the potential intensity of hurricanes that do form. To take advantage of older, less reliable data, the models are reformulated using Bayesian methods. Gibbs sampling is used to integrate the prior over the likelihood to obtain the posterior distributions for the model parameters conditional on global temperature. Higher temperatures are conditionally associated with more strong hurricanes and higher return levels for the strongest hurricane winds. Results compare favorably with an ML approach as well as with recent modeling and observational studies. The maximum possible near-coastal wind speed is estimated to be 208 kt (183 kt) using the Bayesian (ML) approach.” Jagger, Thomas H., James B. Elsner, 2006: Climatology Models for Extreme Hurricane Winds near the United States. J. Climate, 19, 3220–3236. doi: http://dx.doi.org/10.1175/JCLI3913.1. [FULL TEXT]

Increasing destructive potential

Increasing destructiveness of tropical cyclones over the past 30 years – Emanuel (2005) “Theory and modelling predict that hurricane intensity should increase with increasing global mean temperatures, but work on the detection of trends in hurricane activity has focused mostly on their frequency and shows no trend. Here I define an index of the potential destructiveness of hurricanes based on the total dissipation of power, integrated over the lifetime of the cyclone, and show that this index has increased markedly since the mid-1970s. This trend is due to both longer storm lifetimes and greater storm intensities. I find that the record of net hurricane power dissipation is highly correlated with tropical sea surface temperature, reflecting well-documented climate signals, including multi-decadal oscillations in the North Atlantic and North Pacific, and global warming. My results suggest that future warming may lead to an upward trend in tropical cyclone destructive potential, and—taking into account an increasing coastal population—a substantial increase in hurricane-related losses in the twenty-first century.” Kerry Emanuel, Nature 436, 686-688 (4 August 2005), doi:10.1038/nature03906.[FULL TEXT, Landsea comment, Emanuel reply]

How do hurricane frequencies vary between different climate models?

Impact of CO2-Induced Warming on Simulated Hurricane Intensity and Precipitation: Sensitivity to the Choice of Climate Model and Convective Parameterization – Knutson & Tuleya (2004) “Previous studies have found that idealized hurricanes, simulated under warmer, high-CO₂ conditions, are more intense and have higher precipitation rates than under present-day conditions. The present study explores the sensitivity of this result to the choice of climate model used to define the CO₂-warmed environment and to the choice of convective parameterization used in the nested regional model that simulates the hurricanes. Approximately 1300 five-day idealized simulations are performed using a higher-resolution version of the GFDL hurricane prediction system (grid spacing as fine as 9 km, with 42 levels). All storms were embedded in a uniform 5 m s⁻¹ easterly background flow. The large-scale thermodynamic boundary conditions for the experiments— atmospheric temperature and moisture profiles and SSTs—are derived from nine different Coupled Model Intercomparison Project (CMIP2+) climate models. The CO₂-induced SST changes from the global climate models, based on 80-yr linear trends from +1% yr⁻¹ CO₂ increase experiments, range from about +0.8° to +2.4°C in the three tropical storm basins studied. Four different moist convection parameterizations are tested in the hurricane model, including the use of no convective parameterization in the highest resolution inner grid. Nearly all combinations of climate model boundary conditions and hurricane model convection schemes show a CO₂-induced increase in both storm intensity and near-storm precipitation rates. The aggregate results, averaged across all experiments, indicate a 14% increase in central pressure fall, a 6% increase in maximum surface wind speed, and an 18% increase in average precipitation rate within 100 km of the storm center. The fractional change in precipitation is more sensitive to the choice of convective parameterization than is the fractional change of intensity. Current hurricane potential intensity theories, applied to the climate model environments, yield an average increase of intensity (pressure fall) of 8% (Emanuel) to 16% (Holland) for the high-CO₂ environments. Convective available potential energy (CAPE) is 21% higher on average in the high-CO₂ environments. One implication of the results is that if the frequency of tropical cyclones remains the same over the coming century, a greenhouse gas–induced warming may lead to a gradually increasing risk in the occurrence of highly destructive category-5 storms.” Knutson, Thomas R., Robert E. Tuleya, 2004: Impact of CO2-Induced Warming on Simulated Hurricane Intensity and Precipitation: Sensitivity to the Choice of Climate Model and Convective Parameterization. J. Climate, 17, 3477–3495. doi: http://dx.doi.org/10.1175/1520-0442(2004)0172.0.CO;2. [FULL TEXT]

Hurricanes and global warming in climate models - a perspective from 2001

Impact of CO₂-Induced Warming on Hurricane Intensities as Simulated in a Hurricane Model with Ocean Coupling – Knutson et al. (2001) “This study explores how a carbon dioxide (CO₂) warming–induced enhancement of hurricane intensity could be altered by the inclusion of hurricane–ocean coupling. Simulations are performed using a coupled version of the Geophysical Fluid Dynamics Laboratory hurricane prediction system in an idealized setting with highly simplified background flow fields. The large-scale atmospheric boundary conditions for these high-resolution experiments (atmospheric temperature and moisture profiles and SSTs) are derived from control and high-CO₂ climatologies obtained from a low-resolution (R30) global coupled ocean–atmosphere climate model. The high-CO₂ conditions are obtained from years 71–120 of a transient +1% yr⁻¹ CO₂-increase experiment with the global model. The CO₂-induced SST changes from the global climate model range from +2.2° to +2.7°C in the six tropical storm basins studied. In the storm simulations, ocean coupling significantly reduces the intensity of simulated tropical cyclones, in accord with previous studies. However, the net impact of ocean coupling on the simulated CO₂ warming–induced intensification of tropical cyclones is relatively minor. For both coupled and uncoupled simulations, the percentage increase in maximum surface wind speeds averages about 5%–6% over the six basins and varies from about 3% to 10% across the different basins. Both coupled and uncoupled simulations also show strong increases of near-storm precipitation under high-CO₂ climate conditions, relative to control (present day) conditions.” Knutson, Thomas R., Robert E. Tuleya, Weixing Shen, Isaac Ginis, 2001: Impact of CO2-Induced Warming on Hurricane Intensities as Simulated in a Hurricane Model with Ocean Coupling. J. Climate, 14, 2458–2468. doi: http://dx.doi.org/10.1175/1520-0442(2001)0142.0.CO;2. [FULL TEXT]

Atlantic hurricane activity 1996-2000

The Recent Increase in Atlantic Hurricane Activity: Causes and Implications – Goldenberg et al. (2001) “The years 1995 to 2000 experienced the highest level of North Atlantic hurricane activity in the reliable record. Compared with the generally low activity of the previous 24 years (1971 to 1994), the past 6 years have seen a doubling of overall activity for the whole basin, a 2.5-fold increase in major hurricanes (≥50 meters per second), and a fivefold increase in hurricanes affecting the Caribbean. The greater activity results from simultaneous increases in North Atlantic sea-surface temperatures and decreases in vertical wind shear. Because these changes exhibit a multidecadal time scale, the present high level of hurricane activity is likely to persist for an additional ∼10 to 40 years. The shift in climate calls for a reevaluation of preparedness and mitigation strategies.” Stanley B. Goldenberg, Christopher W. Landsea, Alberto M. Mestas-Nuñez, William M. Gray, Science 20 July 2001: Vol. 293 no. 5529 pp. 474-479, DOI: 10.1126/science.1060040. [FULL TEXT]

Hurricanes and global warming in climate models - a perspective from 1999

Increased hurricane intensities with CO₂-induced warming as simulated using the GFDL hurricane prediction system – Knutson & Tuleya (1999) “The impact of CO₂-induced global warming on the intensities of strong hurricanes is investigated using the GFDL regional high-resolution hurricane prediction system. The large-scale initial conditions and boundary conditions for the regional model experiments, including SSTs, are derived from control and transient CO₂ increase experiments with the GFDL R30-resolution global coupled climate model. In a case study approach, 51 northwest Pacific storm cases derived from the global model under present-day climate conditions are simulated with the regional model, along with 51 storm cases for high CO₂ conditions. For each case, the regional model is integrated forward for five days without ocean coupling. The high CO₂ storms, with SSTs warmer by about 2.2 °C on average and higher environmental convective available potential energy (CAPE), are more intense than the control storms by about 3–7?m/s (5%–11%) for surface wind speed and 7 to 24 hPa for central surface pressure. The simulated intensity increases are statistically significant according to most of the statistical tests conducted and are robust to changes in storm initialization methods. Near-storm precipitation is 28% greater in the high CO₂ sample. In terms of storm tracks, the high CO₂ sample is quite similar to the control. The mean radius of hurricane force winds is 2 to 3% greater for the composite high CO₂ storm than for the control, and the high CO₂ storms penetrate slightly higher into the upper troposphere. More idealized experiments were also performed in which an initial storm disturbance was embedded in highly simplified flow fields using time mean temperature and moisture conditions from the global climate model. These idealized experiments support the case study results and suggest that, in terms of thermodynamic influences, the results for the NW Pacific basin are qualitatively applicable to other tropical storm basins.” T. R. Knutson, R. E. Tuleya, Climate Dynamics, July 1999, Volume 15, Issue 7, pp 503-519. [FULL TEXT]

How much was known in 1998?

Tropical Cyclones and Global Climate Change: A Post-IPCC Assessment – Henderson-Sellers et al. (1998) “The very limited instrumental record makes extensive analyses of the natural variability of global tropical cyclone activities difficult in most of the tropical cyclone basins. However, in the two regions where reasonably reliable records exist (the North Atlantic and the western North Pacific), substantial multidecadal variability (particularly for intense Atlantic hurricanes) is found, but there is no clear evidence of long-term trends. Efforts have been initiated to use geological and geomorphological records and analysis of oxygen isotope ratios in rainfall recorded in cave stalactites to establish a paleoclimate of tropical cyclones, but these have not yet produced definitive results. Recent thermodynamical estimation of the maximum potential intensities (MPI) of tropical cyclones shows good agreement with observations. Although there are some uncertainties in these MPI approaches, such as their sensitivity to variations in parameters and failure to include some potentially important interactions such as ocean spray feedbacks, the response of upper-oceanic thermal structure, and eye and eyewall dynamics, they do appear to be an objective tool with which to predict present and future maxima of tropical cyclone intensity. Recent studies indicate the MPI of cyclones will remain the same or undergo a modest increase of up to 10%–20%. These predicted changes are small compared with the observed natural variations and fall within the uncertainty range in current studies. Furthermore, the known omissions (ocean spray, momentum restriction, and possibly also surface to 300-hPa lapse rate changes) could all operate to mitigate the predicted intensification. A strong caveat must be placed on analysis of results from current GCM simulations of the “tropical-cyclone-like” vortices. Their realism, and hence prediction skill (and also that of “embedded” mesoscale models), is greatly limited by the coarse resolution of current GCMs and the failure to capture environmental factors that govern cyclone intensity. Little, therefore, can be said about the potential changes of the distribution of intensities as opposed to maximum achievable intensity. Current knowledge and available techniques are too rudimentary for quantitative indications of potential changes in tropical cyclone frequency. The broad geographic regions of cyclogenesis and therefore also the regions affected by tropical cyclones are not expected to change significantly. It is emphasized that the popular belief that the region of cyclogenesis will expand with the 26°C SST isotherm is a fallacy. The very modest available evidence points to an expectation of little or no change in global frequency. Regional and local frequencies could change substantially in either direction, because of the dependence of cyclone genesis and track on other phenomena (e.g., ENSO) that are not yet predictable. Greatly improved skills from coupled global ocean–atmosphere models are required before improved predictions are possible.” Henderson-Sellers, A., and Coauthors, 1998: Tropical Cyclones and Global Climate Change: A Post-IPCC Assessment. Bull. Amer. Meteor. Soc., 79, 19–38. doi: http://dx.doi.org/10.1175/1520-0477(1998)0792.0.CO;2. [FULL TEXT]

Hurricanes and global warming in climate models - a perspective from 1998

Simulated Increase of Hurricane Intensities in a CO₂-Warmed Climate – Knutson et al. (1998) “Hurricanes can inflict catastrophic property damage and loss of human life. Thus, it is important to determine how the character of these powerful storms could change in response to greenhouse gas–induced global warming. The impact of climate warming on hurricane intensities was investigated with a regional, high-resolution, hurricane prediction model. In a case study, 51 western Pacific storm cases under present-day climate conditions were compared with 51 storm cases under high-CO₂ conditions. More idealized experiments were also performed. The large-scale initial conditions were derived from a global climate model. For a sea surface temperature warming of about 2.2°C, the simulations yielded hurricanes that were more intense by 3 to 7 meters per second (5 to 12 percent) for wind speed and 7 to 20 millibars for central surface pressure.” Thomas R. Knutson, Robert E. Tuleya, Yoshio Kurihara, Science 13 February 1998: Vol. 279 no. 5353 pp. 1018-1021, DOI: 10.1126/science.279.5353.1018. [FULL TEXT]

How strong can a hurricane get?

The Maximum Potential Intensity of Tropical Cyclones – Holland (1997) “A thermodynamic approach to estimating maximum potential intensity (MPI) of tropical cyclones is described and compared with observations and previous studies. The approach requires an atmospheric temperature sounding, SST, and surface pressure; includes the oceanic feedback of increasing moist entropy associated with falling surface pressure over a steady SST; and explicitly incorporates a cloudy eyewall and a clear eye. Energetically consistent, analytic solutions exist for all known atmospheric conditions. The method is straightforward to apply and is applicable to operational analyses and numerical model forecasts, including climate model simulations. The derived MPI is highly sensitive to the surface relative humidity under the eyewall, to the height of the warm core, and to transient changes of ocean surface temperature. The role of the ocean is to initially contribute to the establishment of the ambient environment suitable for cyclone development, then to provide the additional energy required for development of an intense cyclone. The major limiting factor on cyclone intensity is the height and amplitude of the warm core that can develop; this is closely linked to the height to which eyewall clouds can reach, which is related to the level of moist entropy that can be achieved from ocean interactions under the eyewall. Moist ascent provides almost all the warming above 200 hPa throughout the cyclone core, including the eye, where warm temperatures are derived by inward advection and detrainment mixing from the eyewall. The clear eye contributes roughly half the total warming below 300 hPa and produces a less intense cyclone than could be achieved by purely saturated moist processes. There are necessarily several simplifications incorporated to arrive at a tractable solution, the consequences of which are discussed in detail. Nevertheless, application of the method indicates very close agreement with observations. For SST < 26°C there is generally insufficient energy for development. From 26° to 28°C SST the ambient atmosphere warms sharply in the lower troposphere and cools near the tropopause, but with little change in midlevels. The result is a rapid increase of MPI of about 30 hPa °C⁻¹. At higher SST, the atmospheric destabilization ceases and the rate of increase of MPI is reduced.” Holland, Greg J., 1997: The Maximum Potential Intensity of Tropical Cyclones. J. Atmos. Sci., 54, 2519–2541. doi: http://dx.doi.org/10.1175/1520-0469(1997)0542.0.CO;2. [FULL TEXT]

Forecasting seasonal hurricane activity

Predicting Atlantic Basin Seasonal Tropical Cyclone Activity by 1 June – Gray et al. (1994) “This is the third in a series of papers describing the potential for the seasonal forecasting of Atlantic basin tropical cyclone activity. Earlier papers by the authors describe seasonal prediction from 1 December of the previous year and from 1 August of the current year; this work demonstrates the degree of predictability by 1 June, the “official” beginning of the hurricane season. Through three groupings consisting of 13 separate predictors, hindcasts are made that explain 51%–72% of the variability as measured by cross-validated agreement coefficients for eight measures of seasonal tropical cyclone activity. The three groupings of predictors include 1) an extrapolation of quasi-biennial oscillation of 50- and 30-mb zonal winds and the vertical shear between the 50- and 30-mb zonal winds (three predictors); 2) West African rainfall, sea level pressure, and temperature data (four predictors); and 3) Caribbean basin and El Niño–Southern Oscillation information including Caribbean 200-mb zonal winds and sea level pressures, equatorial eastern Pacific sea surface temperatures and Southern Oscillation index values, and their changes in time (six predictors). The cross validation is carried out using least sum of absolute deviations regression that provides an efficient procedure for the maximum agreement measure criterion. Corrected intense hurricane data for the 1950s and 1960s have been incorporated into the forecasts. Comparisons of these 1 June forecast results with forecast results from 1 December of the year previous and 1 August of the current year are also given.” Gray, William M., Christopher W. Landsea, Paul W. Mielke, Kenneth J. Berry, 1994: Predicting Atlantic Basin Seasonal Tropical Cyclone Activity by 1 June. Wea. Forecasting, 9, 103–115. doi: http://dx.doi.org/10.1175/1520-0434(1994)0092.0.CO;2 . [FULL TEXT]

US hurricanes correlated with West African rainfall

Strong Association Between West African Rainfall and U.S. Landfall of Intense Hurricanes – Gray (1990) “Intense hurricanes occurred much more frequently during the period spanning the late 1940s through the late 1960s than during the 1970s and 1980s, except for 1988 and 1989. Seasonal and multidecadal variations of intense hurricane activity are closely linked to seasonal and multidecadal variations of summer rainfall amounts in the Western Sahel region of West Africa. The multidecadal nature of West African precipitation variations and their association with variations of intense Atlantic hurricane activity can be observed in data going back nearly a century. The apparent recent breaking of the 18-year Sahel drought during 1988 and 1989 suggests that the incidence of intense hurricanes making landfall on the U.S. coast and in the Caribbean basin will likely increase during the 1990s and early years of the 21st century to levels of activity notably greater than were observed during the 1970s and 1980s.” William M. Gray, Science, New Series, Vol. 249, No. 4974 (Sep. 14, 1990), pp. 1251-1256, DOI: 10.2307/2877855.

Hurricanes and global warming in (simple) climate models - a perspective from 1987

The dependence of hurricane intensity on climate – Emanuel (1987) “Tropical cyclones rank with earthquakes as the major geophysical causes of loss of life and property. It is therefore of practical as well as scientific interest to estimate the changes in tropical cyclone frequency and intensity that might result from short-term man-induced alterations of the climate. In this spirit we use a simple Carnot cycle model to estimate the maximum intensity of tropical cyclones under the somewhat warmer conditions expected to result from increased atmospheric CO₂ content. Estimates based on August mean conditions over the tropical oceans predicted by a general circulation model with twice the present CO₂ content yield a 40–50% increase in the destructive potential of hurricanes.” Kerry A. Emanuel, Nature 326, 483 – 485 (08 April 1987); doi:10.1038/326483a0. [FULL TEXT]

El Niño and the Quasi-Biennial Oscillation

Atlantic Seasonal Hurricane Frequency. Part I: El Niño and 30 mb Quasi-Biennial Oscillation Influences – Gray (1984) “This is the first of two papers on Atlantic seasonal hurricane frequency. In this paper, seasonal hurricane frequency as related to E1 Niño events during 1900–82 and to the equatorial Quasi-Biennial Oscillation (QBO) of stratospheric zonal wind from 1950 to 1982 is discussed. It is shown that a substantial negative correlation is typically present between the seasonal number of hurricanes, hurricane days and tropical storms, and moderate or strong (15 cases) El Niñ off the South American west coast. A similar negative anomaly in hurricane activity occurs when equatorial winds at 30 mb are from an easterly direction and/or are becoming more easterly with time during the hurricane season. This association of Atlantic hurricane activity with El Niño can also be made with the Southern Oscillation Index. By contrast, seasonal hurricane frequency is slightly above normal in non-El Niño years and substantially above normal when equatorial stratospheric winds blow from a westerly direction and/or are becoming more westerly with time during the storm season. El Niño events are shown to be related to an anomalous increase in upper tropospheric westerly winds over the Caribbean basin and the equatorial Atlantic. Such anomalous westerly winds inhibit tropical cyclone activity by increasing tropospheric vertical wind shear and giving rise to a regional upper-level environment which is less anticyclonic and consequently less conductive to cyclone development and maintenance. The seasonal frequency of hurricane activity in storm basis elsewhere is much less affected by El Niño events and the QBO. Seasonal hurricane frequency in the Atlantic and the stratospheric QBO is hypothesized to be associated with the trade-wind nature of Atlantic cyclone formation. Tropical cyclone formation in the other storm basins is primarily associated with monsoon trough conditions which are absent in the Atlantic. Quasi-Biennial Oscillation-induced influences do not positively enhance monsoon trough region vorticity fields as they apparently do with cyclone formations within the trade winds. Part II discusses the utilization of the information in this paper for the development of a forecast scheme for seasonal hurricane activity variations.” Gray, William M., 1984: Atlantic Seasonal Hurricane Frequency. Part I: El Niño and 30 mb Quasi-Biennial Oscillation Influences. Mon. Wea. Rev., 112, 1649–1668. doi: http://dx.doi.org/10.1175/1520-0493(1984)1122.0.CO;2. [FULL TEXT]