Unit Content
The Historical Context and Implications of Climate Change in the Modern Era
Our ancestors thrived in numerous climatic conditions from warm savannas to frigid Arctic environments. Throughout this time, we managed to successfully navigate interglacial and glacial events that expanded and contracted our populations around the globe. Today we are faced with another climatic event that our technocentric society must adapt to. However, there are key differences in the events of the past and what we face before us. Modern society lives in a globalized world with goods and services that span oceans and continents. We live with conveniences that are dependent on an aging electrical grid, monoculture crops, and communication services. Our ancestors were not sedentary as we are today and thus adapted to the onset of climatic shifts. In addition, the climate shift proposed by the Intergovernmental Panel on Climate Change (IPCC) suggests a much more rapid warming event than has previously been observed over the last 300,000 years. 4 For us to successfully navigate humanity from this disaster, it is imperative that generations fully understand the complexities that will materialize in the next 50 to 100 years. This unit seeks to tackle the potential impacts of extreme weather events, specifically with regards to tropical cyclone development. As oceans warm, coastal cities may increasingly experience flooding and storm damage in the wake of sweeping intense tropical cyclones. Areas within the DMV are prone to flooding and may experience challenges with flash floods and runoff especially as incidences of tropical cyclones begin to migrate northward (Figure 1). It is vital that students understand the physics that underpin the dynamics associated with tropic cyclones and successfully navigate data to better prepare for their future.
Figure 1. A line graph depicting tropical cyclone departures from average latitude by year
Energy Balance and Climate Change
To fundamentally understand the potential impacts that climate change poses on our way of life, it is imperative to grasp the importance of the energy balance that occurs between the sun, the Earth, and the Earth’s atmosphere. Life on Earth is intrinsically linked to the energy being emitted from the sun. The Earth is approximately 150 million km from the sun and receives 1034 Joules of energy every year. 5 This may appear to be an extraordinary amount of energy, and it is, but not all the energy is readily absorbed by earth. Interactions with the Earth’s atmosphere and magnetosphere reflect or divert incoming energy outwards. Approximately 30% of incoming solar radiation is reflected by clouds or other surfaces with high albedo. 6 The remaining energy that penetrates our atmosphere is absorbed by Earth’s surfaces (i.e., oceans, cities, forests) which radiate thermal energy (infrared radiation) back through the atmosphere. Some of the outbound thermal energy from Earth’s surface is absorbed by atmospheric gases, most notably water vapor, carbon dioxide, methane, nitrous oxide, and ozone. 7 Once absorbed, the long wavelengths of the thermal energy are redirected towards Earth, providing additional warming of Earth’s surface. This phenomenon is known as the greenhouse effect and provides the necessary mechanism that allows Earth’s surface temperature to be 13.9 degrees Celsius rather than the -18 degrees Celsius it would otherwise be without the greenhouse effect. 8 Earth’s atmospheric composition is directly linked to the amount of energy retained, which contributes to the distribution of energy around the world as well as the global climate regimes and dynamic weather patterns. Today the concentration of carbon dioxide exceeds 410 ppm which is the highest in more than 3 million years, when global surface temperatures were 2.5 to 4 degrees Celsius warmer than today. 9 As we continue to release CO2 into our atmosphere, we are fundamentally changing our planet’s energy balance, thus altering climate/weather stability, including the frequency and intensity of tropical cyclones around the tropics.
Tropical Cyclone’s Impact on Civilization
Only recently, have we been able to track tropical cyclones with remote sensing satellites and instrumentation. Historical written records have documented the destructive potential of tropical cyclones around the tropics. One of the most famous examples occurred in 1274 when Kublai Khan attempted to invade imperial Japan. As Khan’s naval forces sailed from Korea to Japan, they were met with a violent tempest which many believed to be a tropical cyclone (typhoon) that decimated Khans fleet. In central America, the Mayan’s worshipped the wind god, Huracan, who was one of the most powerful deities believed to be responsible for the creation of Earth.10 On a clifftop in Tulum, Mexico is a temple that served as a Mayan early warning system for inbound tropical cyclones. The temple contains an intricate web of holes that produces an extremely loud whistle which scientist believed warned residents from 1200 to 1450 AD. 11 Tropical cyclones have left a legacy on civilizations that have had the misfortune of settling in high-risk regions around the tropics. Even in the modern era, coastal communities still experience devastation as was documented in 1900 near Galveston, Texas. Between 1980 and 2021 tropical cyclones have caused over $1 trillion dollars of damage. 12The costliest tropical cyclone in the United States occurred in 2005 when Hurricane Katrina struck New Orleans which inundated the city with flood water leaving behind $186.3 billion dollars in damages. 13 Of the five costliest Atlantic tropical cyclones, three have occurred within the past five years. Washington, DC has not traditionally been in the maw of the tropical cyclones but have been impacted, nonetheless. In 2020 tropical storm Isaias hit parts of Maryland and DC with strong winds and heavy rains leaving behind 4.73 billion dollars in damages. 14
What, When and Where do Tropical Cyclones Occur?
Approximately 726 million people around the globe were impacted by tropical cyclones between 1998 and 2017. 15 Understanding these storm systems has been of paramount importance. A tropical cyclone is defined as a “warm-core, non-frontal, low pressure system that develop[s] over the ocean and has a definite organized surface circulation”. 16 The term cyclone was first used in 1848 to describe a rotating storm system; it originates from the Greek word kuklôn meaning cycle or wheel. 17 Tropical cyclones are more commonly known as hurricanes, typhoons, or simply cyclones. Figure 2 shows a typical Atlantic hurricane (tropical cyclone) season, beginning on June 1st and lasting until November 30th. The peak Atlantic hurricane months are between September 1st and October 1st. 18 A typical hurricane season has 15 named storms, 8 hurricanes, and 4 major hurricanes with winds up 165-224 kph. 19 Hurricanes develop in the tropics and occur all around the world from the Indian Ocean to the North and South Pacific.
Figure 2. Depicts the frequency of tropical storms and hurricane in the Atlantic Ocean from NOAA.
Tropical Cyclone Monitoring
As these phenomena regularly occur throughout the world, a global monitoring network was established to provide accurate data and projections for tracking named systems. Currently, there are six tropical cyclone Regional Specialized Meteorological Centers (RSMSc) and five Tropical Cyclone Warning Centers (TCWCs) around the world. 20 These agencies provide advisories and updates to government agencies about the projected track and intensity of the storm. Each region has various classification systems to designate potential damage of a tropical system. The Atlantic tropical systems utilize the Saffir-Simpson Scale which consists of five categories of expected damage (i.e., minimal (category 1), moderate (category 2), extensive (category 3), extreme (category 4), catastrophic (category 5). Tropical systems in this region are called hurricanes are named once maximum sustained wind speed reaches 119 kph. 21 The Australian region tropical cyclone utilizes a similar five category schema for determining potential damage of tropical systems. 22 Lastly, the Western North Pacific and Indian Ocean designate tropical systems into two or three categories based on the maximum sustained surface wind speed. These categories range regionally but use similar criteria for classification. To mitigate the loss of life, it is paramount to continue having a robust monitoring and research network to warn communities of tropical cyclones. This units seeks to investigate tropical cyclones that originate within the South Atlantic Basin.
Tropical Cyclogenesis
Solar Radiation and Earth’s Orientation
On the global scale, we know that the tilt of the Earth as well as the orientation of continents determine the likelihood of tropical cyclones developing in a particular region. Tropical cyclogenesis is initiated by the unequal heating of the Earth that gives rise to atmospheric convection. Incoming solar radiation, emitted from the sun, enters Earth’s atmosphere at varying degrees due to an axial tilt of 23.5 degrees of our planet. 23 This positional difference results in more oblique angles of sunlight at higher latitudes and more direct angles towards equatorial regions. This is precisely why polar ice caps are located at the North and South poles and the tropics host some of the warmest climates on earth. Once the incoming solar radiation enters Earth’s atmosphere, the energy is readily absorbed and reflected from varying surfaces. The vast majority of Earth’s surface is composed of water, ice, rock, sand, and vegetation each of which stores thermal energy at various amounts due to their material properties. The heat capacity, albedo, and state of matter of surfaces all interplay to determine how much thermal energy is stored in a particular area. 24 The differences in stored thermal energy and a material ability to facilitate heat exchange produce the necessary mechanism for convection to occur both within Earth’s oceans and atmosphere.
Atmospheric Convection and Regional Convection Cells
Convection is the process by which heat is transferred within fluids and gasses due to a temperature differential which govern movement. Warmer masses rise and expand which releases heat thus cooling the mass resulting in its descent. The greater the differential, the greater the rate of heat transfer and thus the masses motion. As molecules of air or water heat up (become more energetic) they begin to expand, become less dense, and rise. As the molecules move away from the source of energy, they release heat, subsequently becoming denser and sink back to the energy source. The expansion of rising warm air occurs near the sea surface and contracts in the upper troposphere with the condensation of water vapor. This circulation pattern converts a portion of the thermal energy into the system as mechanical work which typically manifests as fierce wind or rainbands. Convection can occur locally or at a global scale, both of which contribute to the patterns of tropical cyclogenesis that are currently observed. Atmospheric convection currents, which include the Hadley Cell, Polar Cell, and Ferrell Cell can be found at varying latitudes across the globe. 25 These large-scale currents circulate energy and facilitate heat exchange around the world as shown in Figure 2. The primary driver of tropical cyclones involves the Hadley Cell, which occurs 30 degrees north and south of the equator. 26 The heat exchange at the equator causes warm air to rise and descend near 30 degrees latitude.
Environmental Conditions that Promote Cyclogenesis
The conversion of moist static energy from the tropics to kinetic energy through convection provides the mechanism, termed cyclogenesis, for tropical cyclones to intensify. Initially, a disturbance must be present to promote a convective system. The stability and longevity of this convective system depends on vorticity, stability, and depth. 27 Many disturbances travel across the tropical ocean and yet only a small percent develop into tropical cyclones. Warm ocean waters of at least 26 degrees Celsius with a depth of 50 m are needed to provide the sustained energy for continued development. 28 Warmer waters promote the convective processes needed to form large volumes of moist air to rise. In addition, an atmosphere which cools fast enough to establish thunderstorm activity is required to release the heat from the rising moist air of the oceanic waters. A moist mid-troposphere (~5 km in altitude) further promotes thunderstorm development. The location of the initial disturbance with respect to its relative position above or below the equator will promote rotation force. Low vertical wind shear approximately 40 km/h or less from the surface offers favorable conditions for the organization of the eye wall to be established along with subsequent rainbands.29
Tropical Cyclone Formation and Anatomy
Once systems become organized, a mature tropical cyclone will consist of a boundary inflow layer, clear central eye, eyewall, cirrus cloud shield, rainbands, and upper tropospheric outflow. 30 In a tropical cyclone, warm air flows inward toward the central eye becoming more energetic as it approaches the eye wall. The rainbands and central eyewall act as convective zones that allow warm air to rise, cooling as it reaches higher altitudes creating an outflow of energy and moisture. This multi-directionality of air mass movement creates rapid atmospheric disturbance which allows tropical cyclones to develop and become large hazardous storms. The warmer the ocean water is, the greater the influx of energy and moisture which promotes further system organization and structure.
In summary, these dynamic systems require two conditions in order to form: (1) favorable warm oceanic waters that are positioned north or south of the equator and (2) limited shear winds so that tropical systems can properly organize and develop. A deeper understanding of cyclogenesis must start with the fundamental physics The development of tropical cyclones is limited by the available thermal energy supplied by the ocean water which is principally determined by the first law of thermodynamics. The mechanism by which tropical cyclones transfer heat utilizes the process of convection. In addition, the spiraling rotational motion of tropical cyclones which circulate mechanical energy manifests in the forms of rainbands and wind speeds.
Tropical Cyclone Physics
Thermodynamic Principles
The formation of tropical cyclones adheres to the law of conservation of energy which states that energy is neither created nor destroyed, energy can only be transferred from one form to another. Thus, the energy entering the system must equate to the energy exiting the system. The first law of thermodynamics is an adaptation of this principle and states that in a closed system (no transfer of matter into or out of a system) the energy within the system (U) is equal to the difference between the heat supplied to the system (Q) and the work (W) done by the system (Equation 1). Work is defined as the process of transferring energy from one system to another. 31The flow of heat or heat exchange in an example of energy transfer which is dynamically observed during tropical cyclogenesis.
ΔU = Q – W Eqn. 1
The direction of heat exchange observed in the convective currents within tropical cyclones can be explained by the second law of the thermodynamics. This principle introduces the concept of entropy or a measure of unusable energy or disorder within a closed or isolated system. The second law of thermodynamics as it pertains to heat transfer states that heat exchanges occur irreversibly from higher to lower temperature bodies. This provides the underlying mechanism of convective atmospheric cells and how heat exchange occurs within tropical systems. The greater the heat applied to the system (i.e., increase sea surface temperatures) the greater the amount of work (i.e., increased intensity of tropical cyclones) performed by the system. Conceptually the thermodynamic principles support scenarios of increased tropical cyclone activity.
The Carnot Cycle
In 1824, Nicolas Leonard Sadi Carnot observed the fundamental thermodynamic principles associated with heat engines and eventually developed the Carnot Cycle. 32 His work was pivotal in understanding the limitations of energy efficiency in heat engines. The first law of thermodynamics assumes that energy entering a system is perfectly conserved however Carnot was unable to construct a “perfect” heat engine where all absorbed heat turned into work. The heat in natural systems likewise do not perfectly absorb heat and convert it into work, other processes and transformation restrict the efficiency of the system. 33 Tropical cyclones like heat engines operate based on temperature differentials that perform work and operate under the confines of the second law of thermodynamics. The isothermal and adiabatic processes that Carnot described cycle heat through a system to perform work. Tropical cyclones similarly function in the same way as a heat engine but occur on much larger scales with devasting consequence as the work produce in the low-pressure systems can come in the form of intense rainbands and wind gusts of 200 kph. 34 Figure 3 shows a cross-section of a thermodynamic cycle of a tropical cyclone which serves as an analog to the Carnot cycle. The evaporation of seawater drives the transfer of energy from sea to air which acts as the fuel source for the tropical system to further develop. The changes that take places within the tropical system are largely governed by alteration in atmospheric pressure and volume (i.e., expansion or compression) of air mass that ultimately drive wind speeds. High pressure atmospheric systems flow to low pressure atmospheric systems, this movement of air mass ultimately generates work in the form of wind speed. The greater the pressure differentials the greater the amount of work or wind speed.
Figure 3. Provides a cross section of a tropical cyclone with regards to the distance from the center of the storm, height of air mass and temperature differential at sea surface level and at the tropopause. The color schema is meant to reference hot and cold regions that are analogous to the Carnot Cycle.
Phase I – Isothermal Expansion (A to B)
First, air is directed into the eye wall of the tropical cyclone through a pressure gradient. The water vapor absorbs the heat from the tropical oceans which acts as a heat reservoir. Based on ideal gas laws, air will begin to expand once introduced to a low-pressure environment. The air mass thus undergoes isothermal (temperature remains constant) expansion along the sea surface moving towards the inner eyewall of the tropical cyclone. 35 Because of the internal energy of an ideal gas, in this case water vapor, nitrogen, oxygen, is a function of temperature change, the internal energy is zero during isothermal expansion (“The Carnot Cycle – University Physics Volume 2” n.d.). Work (W1) is being carried out through this process as the water vapor moves towards the center of the low-pressure system.
Phase II – Adiabatic Expansion (B to C)
The air ascends along large columns of cumulonimbus clouds through adiabatic expansion. The extremely low-pressure of the inner eye wall facilitates the rapid movement of the expanding air where work occurs (W2). The air mass is thermally isolated (adiabatic), resulting no change in heat which results in a decrease in temperature during the expansion (work) process. 36
Phase III – Isothermal Compression (C to D)
Next the air mass undergoes isothermal (temperature is constant) compression. The air mass is placed in contact with a cold reservoir and compressed. In the case of the air mass circulating within the tropical cyclone the air is propelled to the tropopause where temperatures can get as low as -73 degrees Celsius. Work (W3) is done on the gas whereby the air mass gives up heat to the surrounding cold reservoir. Again, no temperature or energy change occurs rather the air mass cools which causes the denser air mass to increase in pressure.
Phase IV – Adiabatic Compression (D to A)
The final step is adiabatic compression which results in no change in heat but an increase in temperature as well as an increase in energy. 37 Work (W4) is done on the gas where it returns to its initial state.
These four fundamental phases explain how tropical cyclones transfer heat through their dynamic systems to maintain or even further develop around the worlds oceans. The total work done within the system can be determine by Equation 2. Work is equal to the area of the enclosed loop (AàBàCàD). 38 Thus, the greater the temperature differential within the system the greater the area or work achieve by the system. As mean ocean temperature rise there is greater potential for tropical cyclones to achieve greater windspeeds, as the amount of wind generated is directly related to the work generated by the low-pressure system.
W = W1 + W2 – W3 – W4 Eqn. 2
Rotational Motion and the Coriolis Effect
The Coriolis effect describes a pattern in deflection that occurs when objects or systems travel along a rotating body such as the Earth. The Earth is a spheroid body with equatorial regions having the largest diameter and pole regions the smallest. Our planet rotates eastward around a tilted axis of approximately 23.5 degrees. 39 This rotating reference frame with respect to other moving bodies generates what is known as a Coriolis force. The magnitude of this force is proportional to the rotation rate of the reference frame. The Earth completes one rotation every 23 hours, 56 minutes, 4 seconds and since the circumference of the Earth is approximately 40,000 kilometers that means that the surface of the equator moves at roughly 460 meters per second. 40 This rotation rate is reduced the farther away from the equatorial regions of the earth since the circumference is smaller closer towards the poles. This differential in rotational rates causes objects that are traveling at fixed or relatively fixed rates to become deflected through the Coriolis force. For example, a plane taking off from the equator, traveling north will deflect eastward, while one traveling towards the equator from the northern hemisphere will deflect westward. The Coriolis effect is fundamentally important in generating sufficient rotational force to provide the feedback mechanism for heat exchange to efficiently transfer throughout the tropical cyclone.
Climate Change Projections and the Implications of Tropical Cyclones
Increase in Sea Surface Temperature
Since the 1800s, the IPCC asserts that the anthropogenic contribution to global mean temperature increased an average of 1 degree Celsius. 41 Due to a greater thermal capacity, a less pronounced warming trend has also been observed with respect to sea surface temperatures around the world. As shown in Figure 4, from 1901 through 2020, sea surface temperatures rose an average of 0.07 degrees Celsius per decade. 42 Sea surface temperatures have consistently been higher over the past 30 years than any other time in recorded history. Although on average sea-surface temperatures have increased worldwide, regionally some waters have cooled. These observations are mainly attributed to water’s high specific heat capacity and ocean currents facilitating the transport of thermal energy within water columns. The surface temperature of the world’s oceans is predominantly warmest towards the equator and progressively cools towards higher latitudes and similar to air temperatures, the degree of warming has been most pronounced towards the arctic region with annual sea ice vanishing. Since the oceans are intrinsically linked with atmospheric changes, increased sea surface temperatures promote greater evaporation and increase the amount of atmospheric water vapor.
Figure 4. A line graph illustrating the increase in sea surface temperature from 1880 to 2020 using the mean temperatures from 1971-2000 as a baseline.
Tropical Cyclone Frequency and Intensity
These temperature changes have major implications for tropical cyclone frequency and intensity, purely from a thermodynamic perspective. The projected climatic effects on global tropical cyclone activity include sea level rise, increased tropical cyclone rainfall rates, and increased tropical cyclone intensity. 43 However, due to the dynamic and complex nature of these low-pressure systems (tropical cyclones) the IPCC has not definitively correlated anthropogenic warming with increased Atlantic basin hurricane activity.
Vecchi et al. 2021 compared multiple datasets to determine any long-term trends in sea-surface temperatures with respect to increased frequency in landfall of hurricanes and number of higher category storms. Adjusting for limited observation data from the 1800s and current trends they that the frequency of tropical cyclones in the Atlantic has not increased significantly over the last 100 years (Figure 5). 44 In addition to increased sea surface temperatures, models project rapid upper tropospheric temperature warming coupled with increasing levels of vertical wind shears over the western portion of the Atlantic Ocean. 45 Tropical cyclone frequency will likely be suppressed because of increased upper tropospheric temperature and greater vertical wind shear, however storms will likely be stronger with greater average rainfall when they do develop. 46
Figure 5. Compares the recorded and adjusted occurrences of North Atlantic Hurricanes from 1860 to 2020
To understand the nuances of these projections in frequency we must reconcile the quality of the data prior to the development of satellite and digital instrumentation. However, thoughtful transparent normalization will help aid our understanding of tropical cyclones as we continue to develop our models to understand these complex and dynamic systems.
In summary, coastal city along the east coast, including the District of Colombia, should prepare for large intense tropical cyclones with heavy precipitation. As the Atlantic Basin continues to experience increased sea surface temperature there remains an increased likelihood of large low-pressure systems to develop into category 4 and 5 hurricanes that could wreak havoc to local infrastructure. We have demonstrated the thermodynamic principles that underpin the development of tropical cyclones and now understand how heat exchange occurs within these systems. With continued monitoring and data available perhaps soon we will be better suited to better predict the frequency and trajectory of these storms.
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