Impacts of Tropical Cyclone Heat Potential Changes
between 1993 and 2012
Tropical Cyclone Heat Potential (TCHP) in theory determines the maximum potential intensity of a tropical cyclone given favorable atmospheric conditions. Between 1993 and 2012, sea surface temperature (SST) increases in the Atlantic and Pacific Oceans signaled widespread warming throughout the ocean column, implying the potential for increased tropical cyclogenesis. The extent to the increase in energy is calculated across the two basins and its effects on tropical cyclones are examined. The Atlantic basin sees great warming during this period, yet average annual SSTs do not reach the threshold required to be usable for tropical cyclogenesis on average. However, global climate dynamics create favorable circulation patterns for higher Atlantic Accumulated Cyclone Energy(ACE) values from 1992 to 2013. The Pacific basin experiences less warming during the period, yet because SSTs are more than high enough to hypothetically sustain tropical cyclone development, the region sees a ten times increase in TCHP compared to the Atlantic for the same period. However, the northwestern Pacific basin is dictated by El Nino-Southern Oscillation and PDO dynamics. TCHP is necessary for tropical cyclone development, but is not the most important factor in their formation.
Tropical Cyclone Heat Potential, or TCHP is defined as the total heat content of a column of ocean from the surface down to the depth of the 26° C isotherm (Wada, 2007). The equation for TCHP is given as:
where is the density of water, approximately 1000 kg/m³, Cp is the specific heat of water at constant pressure (4186 J/kg°C), T is the temperature of the ocean in celsius, and Z is the depth of the ocean layer in meters. TCHP depicts potential a column of water has in providing fuel to a tropical system. The units associated with this metric are KJ/cm².
Methods & Data
GLORYS2V1 is a global ocean model reanalysis with ¼ ° resolution and reliable temperature profiles of the world’s oceans for the 20 year period of 1993 to 2012 (Ferry, 2013). The total heat above 26°C is calculated for a vertical column of water as a quantitative estimate of the potential ‘fuel’ available to a passing low pressure disturbance. Ocean temperatures above 26°C are seen as necessary for tropical cyclogenesis, and so TCHP is an estimate of the the potential energy available to a cyclone (Wada, 2007). Anomalous values of TCHP over extended periods of time will capture particularly active tropical cyclone seasons. Atmospheric factors such as vertical wind shear and already existing mid-tropospheric moisture are not fully taken into account, but are necessary to the complete tropical picture (Briegel, 1997). This paper limits itself to an analysis of the oceanic component of tropical cyclogenesis with allusions to the atmospheric component..
Justifying TCHP’s Connection to Tropical Cyclones
TCHP can be used to predict the maximum possible tropical cyclone intensification. Tropical cyclones derive their energy from the latent heat released as the ocean evaporates. Typically, there is a negative feedback loop involved with a tropical cyclone’s fuel (Fu, 2007). When a storm intensifies over waters with high sea surface temperatures, the wind produced by the storm mixes the ocean and allows for upwelling of colder water, cutting the cyclone off from warm, favorable waters (Fu, 2007). When TCHP is relatively high, the water beneath the surface can sustain tropical cyclogenesis. On October 1st, 2015, Hurricane Joaquin strengthened to a Category 5 hurricane as it remained over the same area over ocean for three days (Hurricane Joaquin Advisory Archive, 2015). Storm intensity was not accurately captured in model predictions as winds increased to 155 mph due to low wind shear and exceptionally high TCHP north of the Bahamas (Figure 1). During this October 2015 storm, the area around Long Island of the Bahamas experienced TCHP values of around 110 KJ/cm^2 according to data by Remote Sensing Systems and NOAA/AOML. The 1993 to 2012 average October value of TCHP around the Bahamas is at most 75 KJ/cm^2, which occurs around 24.7°N and 80°W in Figure 2. As Hurricane Joaquin passed over this area, it underwent a near rapid intensification (Hurricane Joaquin Advisory Archive, 2015). Going beyond Joaquin, abnormally high values of TCHP have been identified as a reason for rapid intensification of individual tropical cyclones, yet long term trends have yet to be established.
Regions of Tropical Cyclogenesis
North Atlantic Basin
The tropical north Atlantic produces on average 6.1 hurricanes every year and 9.9 tropical storms per year (Landsea, 2015). Regions most likely to experience tropical cyclogenesis are reflected in Figure 3. The regions of tropical cyclogenesis for the peak of the tropical cyclone season in late September and early October are in the Gulf of Mexico, the northern Caribbean by the Bahamas extending across Haiti, Puerto Rico, and the west Indies, as well as up the east coast of the United States. High values of TCHP from 1993 to 2012 in October follows very closely with the locations most likely to see tropical cyclone development, as seen in Figure 4. Just east of the Yucatan Peninsula contains the maximum TCHP between 100 and 125 KJ/cm² on average for October 1st in this period. Wada (2008) linked high TCHP in the western North Pacific with increased typhoon activity, and the same principle holds true in the Atlantic. Atmospheric conditions must be favorable for the development of tropical cyclones, such as having little to no vertical wind shear (Briegel, 1997), but some of the most memorable tropical systems are linked to rapid development over thick, warm waters.
Anomalous TCHP in the Atlantic basin, specifically the Gulf of Mexico can be linked to the memorable hurricanes such as Hurricane Katrina and Hurricane Rita in 2005 (Jaimes, 2009). The Loop Current in the Gulf of Mexico, a warm core eddy associated with the Gulf Stream, has been identified as the culprit for the rapid strengthening of these systems due to the deep, warm water associated with it. The current rotates clockwise past the Yucatan Peninsula and into the Gulf of Mexico, before turning southeast around Florida, becoming the Florida Current and eventually the Gulf Stream (Jaimes, 2009). The Loop Current often breaks off warm core rings which propagate westward towards the continental shelf before dissipating. Anomalous TCHP in the Gulf of Mexico is often found within these eddies.
The TCHP of the Gulf of Mexico is shown in Figure 5 during Katrina’s pass in late August 2005. This hurricane quickly strengthened to a category five hurricane while over the warm waters of the loop current. According to Figure 3, the TCHP decreased in the wake of Katrina, a possible sign of upwelling colder water into regions where high TCHP was previously favorable. While TCHP defines the potential for tropical cyclone development, it can also reflect climate signals such as rising air temperatures, as the oceans are thought to be where much of the heat from rising air temperatures is being stored (AR5 IPCC, 2013).
Western North Pacific Basin
The northwest Pacific Ocean basin is the most active region on the planet in terms of number of tropical cyclones, or typhoons (Landsea, 2015). On average the northwestern Pacific experiences 26 tropical storm strength cyclones per year and 16.5 typhoons, exceeding the North Atlantic, according to data from 1981 to 2010 (Landsea, 2015). Because the region is warm enough, tropical activity is sustained all year round but peaks in August through September (JTWC, 2013). Figure 6 shows the total number of typhoons recorded between 1945 and 2013. Figure 6b shows the average TCHP in August, showing where tropical cyclones formation is favorable over the northwestern Pacific. The annual variability of typhoon development has been linked to monsoon dynamics of the Indian Ocean (Fu, 2007). The convergence of the easterly trade winds with the westerly monsoon leads to upward vertical motion as well as the development of strong convection in these regions (Fu, 2007). Large scale atmospheric features typically make the western north Pacific favorable for tropical cyclone development, and the western Pacific warm pool provides sufficient energy to cyclones. The existence of a low pressure disturbance north of 5°N will often result in the formation of a typhoon with favorable TCHP and atmospheric conditions (Baik, 1998). Due to the significance of the western warm pool, any further increase in TCHP may not be directly correlated to an increase in the conversion from oceanic energy to cyclonic energy. It is understood that there may be an upper limit to tropical cyclone energy (Wada, 2008)
Changes in Tropical Cyclone Heat Potential from 1993 to 2012
North Atlantic Ocean
With the interconnection between TCHP and tropical cyclone potential strength, changes in the ocean’s heat are important to track and understand. Theoretically, an increase in TCHP would increase the maximum potential strength of a tropical cyclone. From the period 1993 to 2012, the mean sea surface temperatures in the Atlantic basin, between the regions 20°W and 98°W as well as between 5°N and 40°N, rose from 25.14℃ to 25.66℃. Sea surface temperatures were averaged for June through November. The trend for this period of time was 0.0163℃ per year. Assuming the sea surface temperature is valid to a depth of ½ meters, and given the area of this region is approximately 70% water, the estimated heat gained is 8.27 x 10¹⁴ KJ per year from each five month averaged period over the ½ meters deep of the region. This assumption provides a generously large representation of the heat at the surface.
SST appears to be directly linked with the increases in TCHP for the period 1993 to 2012 for the months June to November, as seen in Figure 7. TCHP increased at a rate 0.1372 KJ/cm² per year over the same 20 year period. Distributed across the area of the Atlantic basin, this is a change of 3.24 x 10¹⁶KJ per year. This calculation only accounts for energy ‘usable’ for tropical cyclone development and ignores all energy associated with changes below 26℃. The heat associated with the change in SST (using the ½ meter deep assumption) accounts for less than 5% of the total energy added to the ocean. However, it should be noted that the average temperature of the Atlantic basin (due to inclusion of northern areas) is below the 26℃ threshold for the five month hurricane season, so more than half of this energy is not ‘usable’ for tropical cyclone development.
TCHP by definition is only usable energy and is comparable to integrated kinetic energy (IKE), a method of measuring the energy of hurricanes. Hurricane Sandy’s IKE was 400 Terajoules (40 x 10¹⁰ KJ) (Kozar, 2015). The additional tropical cyclone heat potential added to the Atlantic basin each year from 1993 to 2012 is the equivalent of 80,000 Hurricane Sandys in terms of energy. During the same period, the number of named storms per year in the Atlantic basin saw a positive trend of .39 more named storms per year (Figure 8). It can be presumed that the average tropical cyclone developing in 1993 would be less energetic than the average tropical cyclone in the future given the same atmospheric conditions. This holds if current speculated mechanisms about the relationship between intensity of tropical cyclones and TCHP are true.
Accumulated Cyclone Energy (ACE) in the Atlantic
While named storms over time is indicative of how TCHP may be affecting the formation of tropical cyclones, a way to determine atmospheric-oceanic conversion of energy is necessary. Accumulated Cyclone Energy (ACE) is used for this comparison. ACE is a sum of the squares of maximum winds in a hurricane over the duration of the hurricane in 10⁴ knots² (Emmanuel, 2005). Records of ACE are kept by NOAA’s National Hurricane Center. Figure 9 shows an upward trend of ACE of 1.323 x 10⁴ knots² per year for storms in the north Atlantic basin. It is to be noted that the variability from year to year is large and that the 2005 season acts as an outlier, shifting the trend upward more than it otherwise would be.
Maue (2011) presents an analysis on the different ocean basin’s contribution to global ACE, which has been trending downward since the 2006 La Nina. The north Atlantic, on average, contributed only 14% to the worldwide ACE from 1979 to 2010, yet contributed almost one-third of ACE in years such as 2010 (Maue, 2011). Much of this can be attributed to La Nina creating favorable atmospheric conditions for hurricane development in the Atlantic. While the Atlantic basin’s trend in ACE has not been overwhelmingly positive, its contribution to global ACE has been. Kossin et al., (2010) attributes this to a shift in the type of Atlantic hurricanes to ‘Cape Verde’ hurricanes. Cape Verde hurricanes are named by their location and typical track. These hurricanes become more intense and exist for long periods of time, contributing to ACE (Kossin, 2010).
Northwest Pacific Ocean
In the 1993 to 2012 period, the average June to November SST rose from an average of 27.57℃ to 27.86℃ between the regions 105°E and 180°E, 5°N and 40°N. The trend for this period of time was 0.0113℃ per year, slightly less than that of the Atlantic Ocean (Figure 10). Contrary to the Atlantic over this period, the northwest Pacific Atlantic on average had already met the 26℃ requirement for tropical cyclone development. Assuming the sea surface temperature is valid to a depth of ½ meters, and given the area of this region is approximately 65% water, the estimated heat gained is 5.14 x 10¹⁴ KJ per year over the ½ meters deep of the region. This is slightly less heat gained than the Atlantic Ocean.
TCHP gained on average 1.02 KJ/cm² per year in the northwest Pacific basin (Figure 10). Distributing this across the area of the Pacific basin studied equates to a total added usable energy for tropical cyclogenesis of 2.44 x 10¹⁷ KJ each year. The total tropical energy added each year to the Pacific basin is ten times the amount added to the Atlantic basin, despite the fact that the Atlantic’s average SST is increasing at a higher rate. This is due to the Pacific’s larger area of ocean with average SST above 26⁰C. Much of the Pacific contains energy usable for tropical cyclogenesis, so increases in the energy of the Pacific due to SST increases are directly transferred to TCHP. Describing TCHP as ‘usable’ energy may be misleading however, as water many meters under the surface is obviously not accessible to a passing tropical cyclone. It is only accessible after hours of churning over the ocean and significant upwelling occurs. Also, one basin may be more affected by changing atmospheric circulations.
Accumulated Cyclone Energy (ACE) in the Pacific
As was done for the Atlantic basin, trends in the number of storms per season as well as the accumulated cyclone energy (ACE) will be examined along with TCHP. Figure 11a demonstrates the negative trend over the period 1993 to 2012 of the number of named storms in the northwestern Pacific Ocean. The downward trend is -0.435 named storms per year for 20 years. This acts in the opposite direction of the increasing TCHP, suggesting a disconnect between tropical oceanic energy to atmospheric cyclonic energy after the 2005 typhoon season. The ACE displays a similar picture.
Figure 11a shows a dramatic -7.72 10⁴ knots² trend through the period 1993 to 2012. A significant decrease in the energy accumulated by typhoons is prevalent in this, mostly affected by the drop after the 2005 season. This trend is contrary to the upward trend in TCHP over the 20 year period, but is most likely explained by a change in the large scale climate through El Nino-Southern Oscillation (La Nina) or the Pacific Decadal Oscillation (PDO), which Maue (2011) discusses. An analysis of the western North Pacific’s TCHP, which is done in this study, does not fully capture El-Nino Southern Oscillation dynamics such as the 2006 La Nina. Sea surface temperatures drop after 2005, but the TCHP rises in the northwest Pacific, albeit at a slower pace.
Tropical Cyclone Heat Potential provides access to understanding the energetic limitations of tropical cyclones in a particular region, but can not provide a complete picture. Over the period 1993 to 2012, the north Atlantic exhibited moderate increases in TCHP that are correlated with small rises in ACE and number of named storms. While extra heat in the ocean may be a contributor to the atmospheric response, the main reason behind the greater energy associated with tropical cyclones in this region was a shift in the spatial distribution of tropical cyclogenesis, which is favored by atmospheric events. The switch to tropical cyclones that had a longer duration and was on average more intense affected the overall energy of the north Atlantic. TCHP and SSTs having an effect on the type of hurricane that is most likely to form can not be excluded to be associated with the La Nina events, but longer periods of data would be necessary to perform a thorough analysis on the likelihood of this.
The Pacific Ocean is also modulated by large scale climate dynamics that are not always expressed in a 20 year dataset of only oceanic variables. A shift to a negative PDO as well as a higher frequency of La Nina events after 2006 created a negative correlation between TCHP and ACE/number of named typhoons for the period 1993 to 2012 (Maue, 2011). The PDO acts on timescales greater than the dataset being used, and so long term climate dynamics were not captured. Had the global ocean average TCHP and SST been calculated over the time period, long term global climate dynamics may have been assessed and quantified. Computer processing power and time was a limiting factor in this analysis, as well as the availability of a robust longer-term global ocean model reanalysis. Ultimately, higher TCHP will help tropical cyclones to develop given favorable atmospheric conditions on small timescales. On longer timescales, an investigation of global climate dynamics is necessary to prove whether or not higher TCHP will cause an upward trend in year to year variability.
Figure 1 - Tropical Cyclone Heat Potential of the north Atlantic basin on October 1st, 2015 as per Remote Sensing Systems and NOAA/AOML
Figure 2 - Average Tropical Cyclone Heat Potential (KJ/cm²) for the month of October in the tropical Atlantic near the Bahamas. Dark blue values indicate land. Using GLORYS.
Figure 3 - NOAA’s National Hurricane Center graphic of regions most likely to see tropical cyclone development during the month of September. Source: NOAA National Hurricane Center.
Figure 4 - Tropical Cyclone Heat Potential average for October 1st for the period 1993 to 2012 for the north Atlantic, using GLORYS.
Figure 5 - Tropical Cyclone Heat Potential (KJ/cm²) for August 26th, August 28th, August 29th, and August 30th, as Katrina was closing in on New Orleans. Location of warm core eddy is around 27N and 90W. Using GLORYS.
Figure 6 - In the Joint Typhoon Warning Center’s 2014 Annual Tropical Cyclone Report on the Western North Pacific. Statistical data on typhoon frequency per month for the periods 1945 to 1958, 1959 to 2013. .
Figure 6b - Tropical Cyclone Heat Potential in the western Pacific for the month of August from the period 1993 to 2012. Using GLORYs.
Figure 7 - June through November Atlantic Tropical Cyclone Heat Potential (TCHP) and Sea Surface Temperatures (SST) for the region between regions 20°W and 98°W as well as between 5°N and 40°N. Upward trend of 0.0163℃ per year for SST observed. Upward trend of .1372 KJ/cm² per year.
Figure 8 - Atlantic Tropical Cyclone Heat Potential and number of named storms from the period 1993 to 2012. Upward trend of 0.39 named storms per year.
Figure 9 - Atlantic Tropical Cyclone Heat Potential (June to November) with Accumulated Cyclone Energy (ACE) for each year. Upward trend of ACE of 1.323 x 10⁴ knots² per year.
Figure 10 -
June through November Pacific Tropical Cyclone Heat Potential (TCHP) and SST (Sea Surface Temperatures) for the region between regions 105°E and 180°E as well as between 5°N and 40°N. Upward trend of 0.0113℃ per year for SST observed. Upward trend of 1.02 KJ/cm² per year.
Figure 11a - Tropical Cyclone Heat Potential, number of named storms in the northwestern Pacific, and the negative trendline of -.435 typhoons per year over the period 1993 to 2012.
Figure 11b - Tropical Cyclone Heat Potential, Accumulated Cyclone Energy in the northwestern Pacific over the period 1993 to 2012. ACE trend of -7.72 x10⁴ knots² over the period.
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Term Paper Topic: Tropical Cyclone Heat Potential Through Time
Using the GLORYS2V1 ocean model reanalysis and its ¼ degree, global temperature profile of the oceans for the 20 year period of 1992 to 2012, I will calculate the global Tropical Cyclone Heat Potential (TCHP), a measure useful in predicting the potential tropical cyclone development. TCHP is defined as the total heat content of a column of ocean from the surface down to the depth of the 26 degree C isotherm. The paper Importance of Tropical Cyclone Heat Potential for Tropical Cyclone Intensity and Intensification in the Western North Pacific (2006) by A. Wada and N. Usui in the Journal of Oceanography details the specifics of calculating the TCHP of the ocean and describes it usefulness. The equation for TCHP is given as:
where is the density, Cp is the specific heat at constant pressure, T is the temperature of the ocean, and Z is the depth of the layer. TCHP can accurately depict the potential a column of water has in providing fuel to a tropical system. The units associated with this metric is KJ per unit area.
Typically, there is a negative feedback loop involved with a tropical cyclone’s fuel. When the storm intensifies over waters with high sea surface temperatures, the wind produced by the storm mixes the ocean and allows for upwelling of colder water, cutting the cyclone off from its fuel. When TCHP is high, the water beneath the surface can be as warm as the sea surface, allowing for potentially stronger tropical cyclones. A recent example is Hurricane Joaquin, which continued to strengthen despite remaining over the same patch of ocean for 3 days. While many models suggested Joaquin would weaken, the storm reached winds of 155 mph due to what was most likely high TCHP around the Bahamas. Along with the calculation of the TCHP, I will analyze how the average TCHP varies during peak cyclone seasons from the years 1992 to 2012 from region to region and whether there is a strong trend globally. Then, I will determine if there is a correlation between average TCHP for a region during a season and tropical cyclone development using easily accessible archived data from the national hurricane center. TCHP only defines the potential for tropical cyclone development, but it can reflect climate signals such as rising air temperatures, as the oceans are thought to be where much of the heat from global warming is being stored. This investigation will be one further step in showing this.