Hurricanes and the Global Energy Balance
Beyond the dramatic images and the damage, hurricanes help balance energy inequalities across the planet
If the previous essay was something of an ode to the power and beauty of hurricanes, this one asks some more analytical questions: What role do these powerful heat engines play in redistributing energy within the climate system, and is that role changing as the planet warms?
Energy Imbalances Drive Weather and Climate
Weather and climate result from the unequal distribution of heat energy and moisture across the planet. Those imbalances can occur on a daily and local scale or a seasonal and regional-to-global one. They all derive ultimately from unequal absorption of energy received from the sun.
Here is a local, one-day example.
On a hot, humid summer day, sunlight passes through the atmosphere and preferentially warms the ground and the air immediately above it. Warm air rises. As it rises it cools. As it cools, the water vapor condenses creating clouds and maybe rain. If there is enough heat and moisture, thunderstorms can develop. If those storms are strong enough, and reach high enough, the jet stream can give them a twist that can be transmitted all the way down to the ground - and a tornado is born.
Those summer thunderstorms transfer heated air to the upper atmosphere, cooling the surface by downdrafts and falling rain. This reduces the temperature gradient, cuts off further uplift and shuts down the storm. That cycle might repeat tomorrow.
For a seasonal and regional example, imagine now that it is September and a set of thunderstorms forms over the tropical forests of West Arica and the prevailing winds blows them off the coast and over the tropical Atlantic Ocean. During the summer, the surface waters have been warmed, creating an energy imbalance between ocean and atmosphere that is ripe for harvesting by a hurricane.
As the storm sweeps the humid, saturated air off the ocean's surface and lifts it into the developing clouds, the same condensation that converts that humid air into clouds and rain releases tremendous amounts of heat energy that lifts the clouds higher, leading to more condensation, more clouds, more energy release.
While that summer thunderstorm over land cools the surface and shuts down the storm, this developing tropical storm has access to huge reservoirs of heat and moisture from the ocean. As long as it keeps moving and can continually find more warm humid air on which to draw, it can tap into a continuous source of energy.
For a developing hurricane, it's not the jet stream that starts the circular motion, but the Coriolis effect - the rotation of the Earth underneath the atmosphere. It is an interesting fact that hurricanes do not form right at the equator because the Coriolis effect there drops to zero.
As the storm starts to spin, a central core (the developing "eye") can form that begins to define the structure of a mature hurricane. Each rainband in this structure is like that summer thunderstorm, with uplift, condensation, rainfall, thunderstorms, maybe even tornadoes, but they are all drawn in a spiral towards the central eye. An upward and eventually outward spiraling of air around the eye transfers heat to the upper atmosphere.
One NASA site puts it this way:
A hurricane may travel thousands of miles and persist over several days or weeks. During its lifetime, a hurricane will transport a significant amount of heat up from the ocean surface and into the upper troposphere or even lower stratosphere. Even though hurricanes form only sporadically, they do affect the global atmosphere's circulation in measurable ways, although this is still an active area of research.
The churning sea surface under a mature hurricane brings cooler subsurface water up, and also distributes heat down into deeper layers, with implications for climate change discussed in a bit. Between the impact of cooler water delivered to the surface in rainfall and the turbulent upward transfer of cooler subsurface sea water, a passing hurricane can lower that sea surface temperature by several degrees.
If a developing hurricane stalls in place, it will lose energy as it cools the surface - like that summer thunderstorm. But if the steering winds drive it across new areas of accumulated heat and moisture, the continual uplift of that new air will continue to power the storm.
A hurricane can feed off this heat energy for weeks as long as it continues to travel across super-warm sea water. This is one reason that sea surface temperature over large ocean regions is so important to determining the intensity of a hurricane. Higher water temperatures lead to higher water vapor content near the surface, providing more moisture for condensation, energy release and storm intensification.
The eventual dissipation of a hurricane happens when it encounters cooler surface waters or if it makes landfall, where its energy is also lost to friction with the rough land surface.
Correcting the Global Energy imbalance
So hurricanes are concentrated heat engines that redistribute accumulated heat into the upper atmosphere and the deeper ocean along the storm's path. Do they also have a cumulative effect in correcting energy imbalances at the global level over the course of a year?
But first, how do those temperature inequalities come about?
Even without the seasonal variation due to the Earth's orbit, the increasing angle of the Earth's surface away from the sun as you move from the equator to the poles reduces the amount of solar energy received by a unit area of land or sea. So it becomes, on average, colder as you move from the tropics to the poles - no surprises there!
The difference in average temperature creates a gradient that should draw warm tropical air upwards and towards the poles. In textbooks, this happens not in a single cycling cell, but in three interlocked circulation systems (Hadley, Ferrel, and Arctic cells).
Those neat boundaries between cells in this figure get a lot messier as the jet streams they form roar around the planet in either straight lines or wildly "wavy" patterns called Rossby Waves.
The net effect of the activity of these cells and associated weather patterns is to move heat towards the poles. Here is one estimate of how much of that movement occurs through the atmosphere and how much through ocean currents.
Hurricanes can move heat away from the tropics through the atmosphere (especially through the outflow from the eye) and by driving ocean currents. How important are these processes?
It is interesting that despite the dramatic importance of hurricanes, there is still some uncertainty about their role in the global energy balance, but here is a simplified view.
A hurricane that travels across the Atlantic within the tropics would redistribute heat within the region, playing less of a role in redressing the equator-to-pole inequality shown above. But often the steering winds around the Atlantic Subtropical Ridge will turn the storm to the north following the most common track in the figure below. Slight deflections in that path can mean the difference between an interesting storm out to sea and a devastating landfall disaster.
A northward path means that the storm also transfers tropical heat energy into the temperate zone, helping to correct the annual energy imbalance across latitudes.
One source has suggested that hurricanes might contribute half of the heat flux from the tropics toward the poles, and another suggests this could occur as the strong hurricane winds can strengthen the northward flow of the Gulf Stream that transfers tremendous amounts of heat energy from south to north along the east coast of North America.
Climate change and trends
And what role do these massive heat engines play in a changing climate system?
To date, oceans have absorbed about 90% of the increase in heat energy in the climate system due to global warming. Air temperatures would be significantly hotter if not for this massive heat sink in the oceans, and hurricanes play a role in transferring that heat energy into the oceans.
One recent study out of Brandeis University shows that mixing of warm water by surface turbulence is augmented by underwater wave action that pushes the warmer surface water 4 times deeper into layers where it may remain for some time. While reducing warming of the atmosphere, this could have important implications for ocean currents, and that stored heat will resurface at some point in the global ocean circulation system.
And as hurricanes redistribute tropical heat energy up into the atmosphere and down into deeper waters, global circulation systems can propagate that added heat throughout the oceans and atmosphere.
And a final climate change question is: are hurricanes increasing in frequency and/or intensity? Since hurricanes draw on sea surface heat, as the oceans warm we might expect an increase.
A simple listing of number of storms by intensity (on the 1-5 scale usually reported in the media) would give one view, but a more complete answer would require a standard and repeatable measure of total hurricane energy release over the course of a season and across many years.
The current standard index is Accumulated Cyclone Energy or ACE, calculated at six hour intervals as the square of the maximum sustained wind velocity within a storm, and accumulated for each storm over its lifetime. The seasonal total is the sum of all of the totals for those individual storms.
This has some serious shortcomings as an index to total storm energy as it does not consider the width or size of the storm, just the maximum velocity. Still, a good index is often a measurable and understandable simplification of a complex phenomenon - and hurricanes certainly fit that description.
An excellent recent paper has summarized several trends in hurricane number and intensity from 1990 to 2021, including ACE.
The possibly surprising result is that the global summary suggest that total hurricane energy has been declining over this period, but also that results differ by ocean basin.
The authors note that the overall global decline in ACE results mostly from reductions in the Pacific Ocean basins, in turn due to what they call a La Niña-like overall overall pattern of cooler ocean surface temperatures across this 30-year period.
They do note that there has been an increase in the occurrence of rapidly intensifying storms. These are ones that increase in maximum sustained velocity over a short period of time. Rapid intensification has played a major role in determining the total destruction wreaked by some storms, like Katina, that led to devastating landfall impacts. The authors also report small increases in the number of category 4 and 5 storms.
The pattern is more significant, however, if you look at ACE for the North Atlantic basin alone. This is the only one that shows a significant increase in the number of storms and an increase in ACE. Plotting the numbers for just this region, ACE, though variable year-to-year, has nearly doubled across the 30 year period.
So, beautiful or deadly, energy imbalance correctors or indicators of climate change, hurricanes bear watching in a changing climate system. I find myself looking at NOAA's National Hurricane Center site daily from August through October, as the annual hurricane season unfolds.
Sources
Photo of hurricane Isabel taken from the International Space Station September 2003.
https://www.nasa.gov/multimedia/imagegallery/image_feature_87.html
Image of stages in the life of a thunderstorm are from:
https://www.noaa.gov/jetstream/thunderstorms/life-cycle-of-thunderstorm
The map of average sea surface temperature for September and the graph of tropical storm and hurricane frequency by month are both from NOAA's National Hurricane Center:
https://www.nhc.noaa.gov/gifs/SST/AL_09_SEP_1971-2000_RSST.gif
https://www.nhc.noaa.gov/climo/
The diagram of airflow and energy transfer in a hurricane is from:
https://www.gfdl.noaa.gov/operational-hurricane-forecasting/
These articles describe how hurricanes draw on pre-existing warm, humid air over heated ocean surface waters.
https://www.sciencedirect.com/science/article/abs/pii/S0169809516306469?via%3Dihub
https://www.sciencedirect.com/science/article/pii/S0169809523001758
The NASA site quoted is:
https://gpm.nasa.gov/resources/faq/what-difference-between-tornado-and-hurricane
The image of sea surface temperatures showing the changes due to hurricanes Lee and Margot is from:
https://coralreefwatch.noaa.gov/product/5km/index_5km_sst.php
The images of transfer of heat energy from the equator towards the poles is from:
The paper describing the role of hurricanes in driving Atlantic Ocean currents is:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008GL036680
And the source for the estimate that the oceans account for half of the heat flux from the tropics toward the poles is:
https://agupubs.onlinelibrary.wiley.com/doi/pdfdirect/10.1029/2000JD900641
The graphic on mixing of ocean layers and heat content is from:
https://www.brandeis.edu/now/2023/june/hurricanes-conversation.html
Attribution- Sally Warner cc:NoDerivatives 4.0 International (CC BY-ND 4.0)
And is also here:
This Wikipedia site describes the calculation of Accumulated Cyclone Energy:
https://en.wikipedia.org/wiki/Accumulated_cyclone_energy
The paper on the 30 year trend in hurricane occurrence and intensity is here:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2021GL095774
The graph of ACE for Atlantic hurricanes was derived from data found here:
https://psl.noaa.gov/gcos_wgsp/Timeseries/Hurricane/hurr.atl.ace.data