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. 

Observing Climate Effect on Hurricanes Through Hurricane Clustering Methods

When measuring hurricane variability, there are different variables taken into account when measuring the data. Storm intensity, duration, frequency, genesis location, and track all contribute in part to the observed data. In addition to these direct effects that change the thermodynamic state of the storm, there are also indirect effects, most notably climate variations that affect circulation patterns such as the El Niño Southern Oscillation Event (ENSO) (Kossin et al., 2010). The changes in atmospheric currents and vertical wind sheer affect North Atlantic hurricane activity by altering the storm duration. There is an increasingly important need to examine hurricane track in order to determine if there is a correlative relationship between climate change and storm intensity. The results suggest that it is not beneficial to utilize Atlantic tracks when observing hurricane models when attempting to quantify the global effects of climate change and track.  It is necessary to include different variables for each track in order to determine how tropical storms and hurricanes have responded to changes in climate variability.  According to projections, systematic increases in landfall and statistics and distributions of storm intensity are likely to occur, which makes it difficult to predict climate activity.—Brian Nadler
Kossin, J.P., Camargo, S.J., and Sitkowski, M. Climate Modulation of North Atlantic Hurricane Tracks. Journal of Climate, Vol.23. 3057 – 3078.

         
          The possibility is raised that climate change might significantly affect storm frequency in areas such as the North Atlantic. In order to further explore such a possibility, Kossin et al. separated tracks from the North Atlantic hurricane database ranging from 1950 to 2007 and clustered them into four groups based on techniques used in other ocean basins. The composites of each group vary from each other and are remarkably similar, demonstrating that the different oscillation events have influential holds on North Atlantic tropical storms and hurricanes.
          J.P. Kossin and colleagues conducted this research at the University of Wisconsin. Data were obtained through the hurricane database (HURDAT) that is maintained by the National Oceanic and Atmospheric Administration (NOAA). A range of 58 years was covered, from 1950 to 2007, and composite analysis of the sea surface temperatures at those times were recorded using a reconstructed database. Any other regional composites were performed at the National Centers for Environmental Prediction—National Center for Atmospheric Research (NCEP—NCAR). The results showed that when considered by individual clusters, the largely documented increase in North Atlantic hurricanes is confined to deep tropical systems, correlated to regions that display positive SST trends.

          Throughout the study there were various differences in tropical storm longevity and intensity, and the proportion and destructiveness of landfalling storms were indentified. The results of the study suggest that it is not useful to consider Atlantic tracks in their entirety when quantifying the climatic control of tropical cyclogenesis and track, which storm activity is dependent on.  This adds to the challenge of predicting future hurricane or tropical storm activity because it requires that climate models do two things: capture systematic changes in circulation patterns throughout the atmospheric region, and observe mean thermodynamic state changes. More in-depth research will be necessary to improve these attempts at clustering data, for at best, the analyses in this study are only useful as a rough tool for separating tropical storm and hurricane tracks, and it is stated that caution must be used when relating differences within a tropical storm cluster to actual physical mechanisms. 

Increase in Frequency of Intense Hurricanes due to Climate Variability in Latter 21st Century

Many recent models of climate change and weather events suggest that the frequency of tropical cyclones will decrease, but the intensity will increase alongside the upward trend in rising climate. The models, however, are flawed and unable to project hurricanes with an intensity rating of category 3 or higher. Bender et al. (2010) examined the future of global warming on Atlantic hurricanes, using a method of downscaling that allowed for a more realistic distributional projection of hurricane intensity. The model depicts a significant increase in frequency of category 4 and 5 storms in the latter half of the 21st century, with the number of storms doubling, although the overall frequency of tropical cyclones globally is expected to decrease. The results were similar in two different operational models, indicating a high degree of certainty in the findings. Such data are also dependent on the global climate models used for determining environmental conditions, so future studies should reexamine the findings using updated climate models as well as improved hurricane simulation models, if they exist.—Brian Nadler
Bender, M.A., Knutson, T.R., Tuleya, R.E., Sirutis, J.J., Vecchi, G.A., Garner, S.T., and Held, I.M. Modeled Impact of Anthropogenic Warming on the Frequency of Intense Atlantic Hurricanes. Science 237, 454 – 460.

Due to rising sea surface temperatures and a possible increase in hurricane activity in the Atlantic, concerns have been raised that a positive correlation between the two events might exist. There is a great deal of variation among studies, however, a large portion suggest an increase to some degree of hurricane intensity. These models were unable to simulate major hurricanes of category 3 or higher, translating to winds exceeding 50 m/s. Bender et al. improved the simulations of the hurricane intensities by downscaling the models from a previous study, and applying a similar method to two hurricane models that yielded similar results.
M.A. Bender and colleagues conducted the studies while at the National Oceanic and Atmospheric Administration and Geophysical Fluid Dynamics Laboratory (GFDL). Comparisons were made between observed and control storm counts from the GFDL, downscaled, and categorized from 1980 to 2007. The results were used in the storm averages for the two hurricane models used in the study. The results do not take into effect the mix of aerosol effects. The rescale of the hurricane model shows a growing trend in hurricane activity that, although not devastating at the present time, could pose a significant threat in the latter half of the 21st century. The largest increase of intense hurricane activity is projected to occur in the western Atlantic, which was demonstrated in three of the four models run.
The authors found a significant relationship between climate change and hurricane frequency. In the downscaled models, in 80 years the number of category 4 and 5 hurricanes increased by a cumulative 81%. The hurricane season is predicted to shorten, as well as there being a decrease in the number of hurricanes in other areas around the globe, such as in the Caribbean. The authors suspected the data might be slightly skewed to increase in the latter half of the data due to more capable hurricane monitoring tools being more readily available.
An increase in hurricane intensity due to climate change will have potentially enormous economic and global consequences. Being able to determine intensity and locations of increase in hurricane frequency,  we can further determine how climate change affects tropical storms, as well as provide incentive to  limit climate change and to plan ahead in areas that are often in the path of hurricanes and major tropical storms.