Hurricanes and the Climate, Part 2
Recent storms, such as Hurricane Sandy, have raised questions about the nature of hurricanes and their impact on humanity. In part 1 of this series, we addressed the issue of global warming’s influence on tropical cyclones, as well as the role of human behavior in the increase in economic damage caused by hurricanes. Now we consider if it is possible for good to come out of such natural disasters.
Focusing on the negative impacts of hurricanes, or tropical cyclones, seems easy enough. Yet these systems do benefit the climate system’s balance. A primary benefit of tropical cyclones involves the transfer of heat out of tropical regions. Additionally, tropical cyclones provide meaningful precipitation to some regions and also play a role in nutrient recycling and flushing of coastal ecosystems.
Sea surface temperatures (SST)1 may decrease up to 2˚C in the wake of tropical cyclones. The associated seawater mixing caused by tropical cyclone winds results in heat redistribution within ocean waters as well. An SST anomaly induced by a tropical cyclone may largely disappear within a month; however, remaining semi-permanent anomalies amount to about -0.2˚C and continue to affect heat transfer between the ocean and atmosphere.2 Subsurface temperature anomalies may persist for much longer periods. In the aftermath of a tropical cyclone, ocean heat tends to redistribute the ocean’s vertical temperature profile, with net cooling in the surface waters (top 30 meters) and at depth, but with a net warming of layers in between.
This overall heat redistribution in the upper ocean results in a net heat gain due in part to greater absorption from air-sea heat fluxes, which remove heat from the atmosphere and allow for heat transfer out of tropical regions via ocean currents.3 Additionally, tropical cyclone-induced ocean mixing enhances heat movement back toward the equator in some cases, resulting in enhanced precipitation for some equatorial regions.4 Tropical cyclones accomplish much of this heat transfer by strengthening the trade winds in the outer tropics (18–30˚ from the equator) and weakening those in the inner tropics (5–18˚ from the equator). Most of these effects occur at timescales much shorter than the annual cycle. One research team, led by E. Scoccimarro,5 suggests that tropical cyclones could be responsible for up to 10 percent of ocean heat transport out of the tropics. However, some studies have suggested that the associated heat transport is somewhat weaker and that these effects are only apparent for strong tropical cyclones (i.e., those that cause vertical heat redistribution at greater depth).6
While tropical cyclones strengthen evaporation rates (net warming), they also reduce SSTs (net cooling) in the regions they affect. These opposite factors enhance horizontal temperature gradients in some tropical areas, increasing beneficial precipitation in some cases. Many arid tropical regions receive a significant portion of their annual rainfall from tropical cyclones. In subtropical areas, seasonal drought may be reduced by the influence of tropical cyclones. For example, coastal regions of the southeastern United States receive as much as 10–15 percent of warm-season rainfall from tropical cyclones.7 When tropical cyclones reach into the midlatitudes, they frequently transform into extratropical systems that also may provide beneficial precipitation to those regions.
In light of these factors, the persistence of tropical cyclone frequency at nearly steady long-term rates historically demonstrates the tropical climate system’s stability. Climate projections do not appear to suggest significant changes to these frequencies. Without tropical cyclones, the transfer of excess heat out of the tropics would be less efficient and many tropical, subtropical, and midlatitude regions might receive less precipitation.
Tropical cyclones (warm-core low pressure areas) are not specifically referenced in Scripture. However, a nor’easter or extratropical cyclone (Eurakylōn) is mentioned in the recounting of the apostle Paul’s shipwreck in the Mediterranean Sea (Acts 27:14). This storm is similar to the type of system that tropical cyclones, in general, transform into. (Hurricane Sandy was in the process of transforming into this type of system just before landfall.) Although these storms sometimes bring great damage, the evidence presented here shows that tropical cyclones are an important and valuable part of Earth’s climate system. In Paul’s case, though he suffered the distress of being shipwrecked, the experience provided an opportunity to bring the Gospel to the Island of Malta.
- Wei Mei and Claudia Pasquero, “Restratification of the Upper Ocean after the Passage of a Tropical Cyclone: A Numerical Study,” Journal of Physical Oceanography 42, no. 9 (September 2012): 1377–401.
- S. Jullien et al., “Impact of Tropical Cyclones on the Heat Budget of the South Pacific Ocean,” Journal of Physical Oceanography 42, no. 11 (2012): 1882–906.
- R. L. Sriver and M. Huber, “Climatic Feedbacks between Tropical Cyclones, Temperature, and Vertical Ocean Mixing,” paper presented at the American Geophysical Union Fall Meeting (San Francisco, CA, 2007); Claudia Pasquero and Kerry Emanuel, “Tropical Cyclones and Transient Upper-Ocean Warming,” Journal of Climate 21, no. 1 (January 2008): 129–41.
- Enrico Scoccimarro et al., “Effects of Tropical Cyclones on Ocean Heat Transport in a High-Resolution Coupled General Circulation Model,” Journal of Climate 24, no. 16 (August 2011): 4368–84.
- Malte F. Jansen, Raffaele Ferrari, and Todd A. Mooring, “Season Versus Permanent Thermocline Warming by Tropical Cyclones,” Geophysical Research Letters 37, no. 3 (February 2010): L03602.
- “Agriculture: Tropical Cyclones are Welcome Visitors,” World Climate Report (blog), September 21, 2012, http://www.worldclimatereport.com/index.php/2012/09/21/agriculture-tropical-cyclones-are-welcome-visitors/.
By Dr. Kevin Birdwell
Kevin R. Birdwell received his PhD from the University of Tennessee in 2011 and currently serves as a meteorologist and atmospheric researcher in Oak Ridge, Tennessee.