Projected Atlantic Hurricane Surge Threat From Rising Temperatures

by Tim Storer

The ability to accurately predict and prepare for weather changes in the 21st century will be an invaluable asset to nations and policymakers across the globe, and because extreme hurricanes have traditionally been among the most destructive weather patterns, prediction of their patterns/intensities is useful. Because it is so difficult to directly predict storm activity, researchers have sought to take a roundabout approach: first investigate connections between local/global temperatures and past hurricanes, and then use future temperature predictions to predict future storms. Because the high winds associated with hurricanes are so closely accompanied by increased sea levels, called surges, the measured levels are a good measure of hurricanes. In addition, the researchers note that surge levels have shown to be better than wind speeds at indicating the damage potential of hurricanes. In the United States, the greatest hurricane threats are usually tropical storms along the eastern seaboard, and these storms are the primary focus. It has been widely predicted that global temperatures are expected to rise in the upcoming century, and researchers have now found that global temperatures are a surprisingly effective indicator of storm surge levels (Grinsted et al. 2013). Additionally, it was estimated that there would be many more Katrina-scale events in the upcoming decades, by at least a factor of two. Continue reading

Extreme Temperature and Precipitation in the Caribbean Intensifying

by Tim Storer

Climate change is an enormously complex subject, but thankfully copious temperature and precipitation data exist from around the globe that allow for detailed analyses of global and local patterns. In many parts of the globe, increasing trends in weather intensity have been observed, and the most recent data analysis of Caribbean weather reaffirms increased weather intensity throughout the last fifty years (Stephenson et al. 2013). There have been strong rising trends in surface temperatures at several land weather stations throughout the region, but much weaker trends in precipitation than those related to temperature. Still, there were trends of increased average yearly and daily rainfall. The Caribbean region is especially noteworthy because of its high potential for damage related to climate change and high intensity weather events. Continue reading

Dynamic Conditions of the Atmosphere Accounting for the Opposite Trends in Tropical Cyclone Count in the Pacific

Tropical cyclones<!–[if supportFields]> XE “cyclones” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> are considered to be one of the more devastating weather phenomena in terms of their effect on human life and global economy.  How global warming will affect these events is widely debated amongst scientists. It is understood that tropical cyclone genesis is dependent on sea surface temperature (SST)<!–[if supportFields]> XE “sea surface temperature (SST)” <![endif]–><!–[if supportFields]><![endif]–>, since a higher SST provides a cyclone with higher ocean thermal energy.  Tropical cyclones typically develop from a tropical disturbance, and only a small percentage of cyclones result from such disturbances. It is implied that with higher SST levels and a temperature increase there will be a similar increase in tropical storms, however, there are opposite trends of cyclone frequency in the western and central Pacific.  Li et al. (2010) created and examined a model that would explain the variations in the tropical cyclone genesis in different areas of the Pacific. Their results suggest that the major factor that accounts for the distinctly opposite tropical cyclone trends is the constantly changing condition of the atmosphere. They conclude that the projected shift in tropical cyclone activity might pose a great threat to Hawaii and the central Pacific islands. —Brian Nadler
 Li, T., Kwon, M., Zhao, M., Kug, J., Luo,J., and Yu, W. 2010.  Global warming shifts Pacific tropical cyclone<!–[if supportFields]>XE “cyclone”<![endif]–><!–[if supportFields]><![endif]–> location. Geophysical Research Letters 37, 1–5.

Li et al. examined data demonstrating opposite effects of global warming in different areas of the Pacific.  They created a high-resolution global model that can more accurately predict the results shown in previous studies. The model was then tested to ensure that it is applicable to a wider range of results than just Pacific tropical cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> increases and decreases.  The authors used data from the International Pacific Research Center in Hawaii, Korea Ocean Research and Development Institute, First Institute of Oceanography in China<!–[if supportFields]> XE “China” <![endif]–><!–[if supportFields]><![endif]–>, and the GFDL, NOAA in Princeton New Jersey to compile a suitable model to predict tropical cyclone genesis.  The results suggested an increase in variations in atmospheric conditions in the north central Pacific region (termed synoptic-scale disturbances), and a decrease in variations in the northwestern Pacific region. These changes in conditions create wind shears that have significant effects on the increases and decreases in tropical cyclone activity.
Li et al. discovered that global warming weakens the trade winds in the Pacific, and that the Walker circulation is weakened, causing a weakening of the North Pacific monsoon<!–[if supportFields]>XE “monsoon” <![endif]–><!–[if supportFields]><![endif]–> season. The weakening of heating caused by the monsoon season leads to a reduction of tropical cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> frequency. In contrast, in the central Pacific, the SST<!–[if supportFields]>XE “sea surface temperature (SST)”<![endif]–><!–[if supportFields]><![endif]–> gradient is reduced and there is a higher localized SST, which results in an increase in tropical cyclone frequency. The model tends to overestimate tropical cyclone genesis, so the authors suggest caution when interpreting the results—however, the shift of cyclone activity in the Pacific is significant to the millions of people living in Hawaii and central Pacific islands. 
Even with the overestimation, developing a model that can accurately predict tropical cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> activity and take into account so many variables is particularly important. There are still uncertainties with regards to physics in the models uses and SST<!–[if supportFields]> XE “sea surface temperature (SST)” <![endif]–><!–[if supportFields]><![endif]–> warming patterns, but being able to take these into account may have implications for determining the effects of tropical cyclone activity on a global scale, rather than limiting it to the Pacific.

Changes in Tropical Cyclone Frequency not Strongly Correlated with Sea Sur-face Temperature

Despite the devastating weather and climate events that have occurred globally throughout the past half-century, tropical cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> activity has decreased. This is true even while sea surface temperatures (SST<!–[if supportFields]> XE “sea surface temperature (SST)” <![endif]–><!–[if supportFields]><![endif]–>), which are positively correlated with potential energy in tropical storms, have increased.  A scientific dilemma is why, with the increase in SSTs and global warming that are necessary for typhoon genesis, would there be a decrease in numbers of tropical storms. Zhou et al. (2010) imply that the explanation for this phenomenon is based in the second law of thermodynamics. They arrived at this by examining the tropical storm genesis in concordance with the inter-tropical convergence zone (ITCZ<!–[if supportFields]> XE “ITCZ” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “intertropical convergence zone, ITCZ” <![endif]–><!–[if supportFields]><![endif]–>).  Their results suggest that sea surface temperatures are only one of the necessary conditions for tropical cyclone genesis, and that the low-level vorticity, or tendency for elements of the fluid to “spin”, associated with ICTZ variations should be a fundamental factor for tropical cyclone genesis. They conclude that the causality between SSTs and tropical storm frequency is suggested is not yet fully understood and should be examined further. —Brian Nadler
 Zhou, X., Liu, C., Liu, Y., Xu, H., and Wang, X., 2011. Changes in tropical cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> number in the Western North Pacific in a warming environment as implied by classical thermodynamics. International Journal of Geosciences 2, 29–35.

Zhou et al. examined the Western North Pacific (WNP) as a model that might explain the relationship between unusual SSTs and tropical storm number. They used NCEP/NCAR 2.5°x2.5° resolution reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric research to examine the correlation between SST<!–[if supportFields]> XE “sea surface temperature (SST)” <![endif]–><!–[if supportFields]><![endif]–> and surface wind divergence/convection<!–[if supportFields]> XE “convection” <![endif]–><!–[if supportFields]><![endif]–>. Zhou and company then used the second law of thermodynamics to try and explain the implications of SST variation and why sea surface warming around the western Pacific is not as dramatic as it is over the central and eastern Pacific.  Then, they developed equations, showing a predictable pattern in the changes in SST in various regions, which also helped to explain further temperature variation.
They discovered that there is a gradual increase in the 20-year mean of SSTs over the North Pacific, related to surface wind convection<!–[if supportFields]>XE “convection” <![endif]–><!–[if supportFields]><![endif]–> over the ITCZ<!–[if supportFields]> XE “ITCZ” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “intertropical convergence zone, ITCZ” <![endif]–><!–[if supportFields]><![endif]–>, but that originally warmer areas will experience a much weaker warming, because of heat lost through diffusion. This leads to the weakening of the ICTZ trough as seen in recent studies, thereby leading to the decreased number of tropical storms. Additionally, a wind component was discovered to have a much more important role over thermodynamic factors than previously considered.
The authors found that warmer SSTs in the western North Pacific can cause fewer tropical cyclones<!–[if supportFields]> XE “cyclones” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–>, indicating that sea surface temperatures are only one of the necessary conditions and do not definitively lead to an increase in tropical storm numbers. Examining the effects of other variables on tropical cyclone frequency will help better understand the results proposed by this study. 

Using Analysis of Climate Change Effects on Cyclone Frequency and Intensity to Mitigate Damage Risk

In terms of damages, hurricanes<!–[if supportFields]> XE “hurricane” <![endif]–><!–[if supportFields]><![endif]–> and tropical storms are a significant source of social disruption and economic hardship. According to many scientific studies, as well as the Intergovernmental Panel on Climate Change<!–[if supportFields]> XE “Intergovernmental Panel on Climate Change (IPCC)” <![endif]–><!–[if supportFields]><![endif]–>, the enhanced greenhouse conditions will lead to stronger tropical storms, and therefore higher levels of damage cost. A study conducted by Li and Stewart(2011) developed an analysis to assess growing cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> damage risk and the economic viability of hazard mitigation strategies in order to reduce the potential impact of these events.  The analysis utilized an approximation for cyclone wind intensity and frequency in combination with reconstruction patterns in Queensland, Australia<!–[if supportFields]> XE “Australia” <![endif]–><!–[if supportFields]><![endif]–>.  The mean annual wind changes over 50 years were examined in order to rule out annual variability in wind speed change. It was determined that average wind speeds are increasing rapidly, and that building new houses to withstand higher wind velocities and retrofitting older houses to withstand the wind velocities would be economically beneficial in the long run.  Further studies will need to be conducted to further examine increases in flooding and storm surge activities due to climate change and their impact on an economic and social level. —Brian Nadler
Li,Y., and Stewart, M.G. 2011. Cyclone damage risks caused by enhanced greenhouse conditions and economic viability of strengthened residential construction. Natural Hazards Review, 9–18.

          According to evidence in recent scientific reports, enhanced greenhouse conditions will lead to an increase of intensity in tropical storms and cyclones<!–[if supportFields]> XE “cyclones” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–>.  While there is regional variability, the overarching idea is that frequency and intensity will increase. Li and Stewart examined the impact of tropical storms and cyclones on Queensland Australia<!–[if supportFields]> XE “Australia” <![endif]–><!–[if supportFields]><![endif]–> in various predictor models that cover a wide range of wind speed increases.  Other parameters that are not yet fully understood, such as sea surface temperature (SST)<!–[if supportFields]> XE “sea surface temperature (SST)”<![endif]–><!–[if supportFields]><![endif]–>, storm trajectory, rainfall rates, etc, are not taken into account and are left for future study. Three different mitigation strategies were created in order to examine their cost-effectiveness over time at reducing tropical storm damages, all of which assumed that wind speeds would gradually increase to 110% of current levels.
          The first mitigation strategy involved retrofitting foreshore construction, or housing construction closer to the ocean, which is projected to reduce damages by 15.8 million dollars if a severe cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> hits, 15% less damage than if nothing were done.  The second strategy involves retrofitting all houses built pre-1980s, but there is only minimal benefit from the upgrades. The third strategy is an attempt to improve enhancements in new construction farther away from the shoreline, which has a varying effect due to economy viability of the times.  It was discovered that the retrofitting of foreshore construction would be the most useful method of protection due to the high likelihood of stronger tropical storms along the coastal areas.
          For future studies, it is recommended that other variables be considered rather than merely examining risk in comparison with damage costs, since some of the strategies developed are not the most economically viable when correlated with the increasing damages by gradually higher tropical storm winds. Also, being able to look at effects of SST<!–[if supportFields]> XE “sea surface temperature (SST)” <![endif]–><!–[if supportFields]><![endif]–> and other causes of heightened tropical storm activity in correlation with damage increases would also be beneficial in examining how much potential damage might be caused in the future due to the combination of all the effects warming has on tropical storm activity, and running an predictor analysis on the data. 

Predicting Tropical Storm Frequency and Intensity By Expanding Studies and Predictor Models

Being able to explain, predict, and understand changes in tropical storm activity is of great societal importance in terms of economic and social impact, and has been studied intensely by scientists around the globe.  There is a great variation in tropical storms on different time scales, varying from intra-seasonal to multi-decadal, and a great deal of argument about whether tropical storm frequency and intensity are sensitive to climate change. Villarini et al. (2010) examined the empirical understanding between tropical storm frequency and large-scale climate conditions by examining the climate indices that tropical storms are often associated with. The group of scientists modeled not only the North Atlantic climate basin, which is the typical target of studies, but also expanded their analysis to include all tropical storm activity that lasted longer than two days and was recorded as U.S. landfall events. Only tropical storms that last longer than two days were recorded since shorter storms are likely to produce negative results.  The authors found that it would be best to use a family of models with Atlantic and tropical storms as covariates. —Brian Nadler
Villarini, G., Vecchi, G.A., and Smith, J.A., 2010. Modeling the dependence of tropical storm counts in the North Atlantic basin on climate indices. Monthly Weather Review 138, 2681–2705.

          Villarini and colleagues used a statistical approach to examine the relationship between tropical storm count and climate indices, expanding on further studies by also including tropical storms recorded as U.S. landfall events, rather than only covering the North Atlantic basin. A Poisson distribution model was also utilized to examine the dependence of counts on climate indices, accounting for over-dispersion or under-dispersion of tropical storm counts. The model is able to predict inter-annual variability, however, another model will be necessary to examine decadal variability as well. For all the models, Atlantic and tropical sea surface temperatures (SST<!–[if supportFields]>XE “sea surface temperature (SST)”<![endif]–><!–[if supportFields]><![endif]–>) are retained as significant covariates, supporting an idea proposed by Vecchi that the increases or decreases in Atlantic SST are preferable to the values of tropical SST in predicting tropical storm count in the U.S. land areas and the North Atlantic basin. The Poisson model of distribution was determined to be the best method for evaluating the data. The scientists suggest running a further experiment modeling U.S. landfall count with the overall storm count for the North Atlantic in order to get a wider swath of data that would be much more accurate in predicting tropical storm changes and reduce anomalies in data.
Expanding such studies will allow for a better understanding of the ways that we can further predict tropical storm variability, along with patterns and variation. 

Projections of Changing Cyclone Fre-quency in Relation to Climate Change Demonstrating Uncertainty

While a steadily increasing global temperature is not much in dispute, the effect of such a warming on climate is subject to much debate.  Knutson et al. (2010) compared older modeling studies, which tended to project a decrease in overall cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> frequency with newer, higher resolution studies, which are more likely to predict an increase in the most intense cyclones<!–[if supportFields]> XE “cyclones” <![endif]–><!–[if supportFields]><![endif]–>.  The newest methods of projection, satellite analysis, and downscaling techniques are examined, as well as the newer, high-resolution projection models of tropical storm activity. The results suggest that while climate model<!–[if supportFields]> XE “climate model” <![endif]–><!–[if supportFields]><![endif]–>s are progressively more reliable, we cannot identify anthropogenic signals in past cyclone data, and therefore are severely limited in our ability to make projections with current data. Further research is highly recommended by Knutson et al. in order to enhance the reliability of climate-relevant observations in the future, since there is a high level of societal impact of tropical storms.—Brian Nadler
Knutson, T.R., McBride, J.L., Chan, J., Emanuel, K., Holland, G., Landsea, C., Held, I., Kossin, J.P., Srivastava, A.K., and Sugi, M. 2010. Tropical Cyclones and Climate Change. Nature Geoscience, 3, 157–163.

          The primary challenge for tropical cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> detection and attribution research is determining whether or not an observed change in tropical cyclone activity exceeds the natural variability of the event, and if so, attributing the change to a specific climate forcing<!–[if supportFields]> XE “climate forcing” <![endif]–><!–[if supportFields]><![endif]–>.  For projections in the future, the ultimate goal is to develop a reliable projection of these changes in factors that influence cyclone activity, as the resulting effect on storm frequency, track, and distribution.
          T.R. Knutson and colleagues conducted their research with the World Meteorological Organization, along with support by the West Australian Government Indian Ocean Climate Initiative.  In all models, there was a strong tendency to project an increase in stronger tropical cyclones<!–[if supportFields]> XE “cyclones” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> over the 21st century.  Detection and attribution was observed for characteristics such as tropical cyclone rainfall, genesis, tracks, duration and surge flooding, as well as activity versus sea surface temperature.  It was also observed that tropical cyclone frequency would likely remain the same overall, but that there would be a shift to more radical tropical storm activity, of a range of category 4 or higher.
          Throughout the study there were numerous variables that were identified as having a potential effect on tropical storm activity.  Knutson and colleagues were able to improve several aspects of cyclone<!–[if supportFields]> XE “cyclone” <![endif]–><!–[if supportFields]><![endif]–> activity projections, resulting in their predictions that tropical cyclone frequency will remain essentially the same, along with a global increase in the average frequency of strongest tropical cyclones<!–[if supportFields]> XE “cyclones” <![endif]–><!–[if supportFields]><![endif]–>.  It was recommended that newer models be created that have an increasingly more detailed spatial resolution and new approaches for observing past tropical cyclone records that would reduce uncertainty of causes of past changes, and be able to better predict future tropical cyclone activity.  Future projections of variables such as sea-level rise<!–[if supportFields]> XE “sea-level rise (SLR)” <![endif]–><!–[if supportFields]><![endif]–>, regional storm structure, and storm characteristics, need to be taken into account, as well as examining the assumption that there will be no future changes that have a markedly different effect on tropical cyclone behavior than is seen today.