Storms

Storms

Large storms in coastal settings typically produce strong winds that can create larger than normal waves. Increased wave heights, and thus the energy associated with the waves, can very quickly change the overall morphology of coastal environments that are exposed to the storm. Even relatively smaller-scale storms, such as thunderstorms, can, with strong enough winds, create substantial changes to a beach or other environments within the coastal zone if they occur with a high frequency during the course of several days or weeks.

Wind can also modify coastal environments. For example, strong winds steadily blowing across a fine-grained beach and dune system for several days can lead to the transport of some of that sand from the beach and dunes and its deposition elsewhere. As a result, environments exposed to strong winds during the course of several days can be reduced in elevation through a process known as deflation.

There is a wide range of scale when it comes to discussing storms. Although capable of inducing change, it is not the random small-scale thunderstorm or even weeks of continuous thunderstorms that cause the most modification to coasts. The most powerful storms to potentially impact a coast are tropical cyclones. We will introduce these systems in the following, but also exploring them in more depth in Modules 5 and 6.

Tropical Cyclones

Tropical cyclones are low-pressure rotating weather systems that are created in the atmosphere by the transfer of heat from the ocean basins to the atmosphere. These low latitude systems (23.5 N and S of the equator) spin clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere and are referred to by different names depending on the ocean basin. In the Atlantic Ocean, they are known as tropical storms and hurricanes when the wind speed exceeds 119 km/hr (75 miles/hr), whereas, in the western Pacific Ocean, they are referred to as tropical cyclones and typhoons respectively. In both cases, tropical cyclones have the capability to generate very large, high, waves that have the potential to devastate the natural environments of a coastal zone and any human infrastructure that is present where the storms make landfall. Another critically important component to understand about tropical cyclones is the storm surges, or elevated levels of sea level, that occur as these low-pressure systems move through an ocean basin. Very large storm surges can be created depending upon the morphology of the coast, the gradient and width of the neighboring continental shelf, the strength of the storm, and the rate of forward motion of the storm. For example, a storm surge in excess of 7 m (25 feet) was created along parts of the northern Gulf of Mexico shore by Hurricane Katrina in 2005 and resulted in inland flooding several kilometers from the non-storm-surge shoreline. We will explore these systems in detail in Modules 5 and 6.

Extratropical Storms and Fronts

North of the Tropic of Cancer (23.5 N latitude) and south of the Tropic of Capricorn (23.5 S latitude), extratropical storms and weather fronts dominate as the major storm impact to coastal zones in these regions. Although not capable of packing the full magnitude of punch that a hurricane can produce, some storms outside of the tropics can produce wind speeds equivalent to those of weak hurricanes, resulting in very large waves and storm surges along coastal environments.

Northeasters

Along the east coast of North America, the classic example of an extratropical storm system is the Northeaster, which is a storm that tracks from the Midwest of the United States, northeast along the east coast of the United States and into Maritime Canada before being turned eastward into the northern Atlantic basin. The term Northeaster derives from the fact that the winds of these storms are from the northeast within this irregularly shaped but counterclockwise rotating storm system. They occur between late fall through spring when cold arctic air moving southeast comes in contact with warmer air moving north from more southerly latitudes to create a low-pressure system with a cold center (in contrast to a hurricane with a warm center). Although not as strong as hurricanes, northeasters are larger in diameter and can impact three to four times as much shoreline as a very large hurricane. Further, their frequency of occurrence is higher for the east coast of North America with 20 to 40 annually, although only several of these generate severe offshore wave heights greater than 5 meters (16 feet).

There are numerous other types of weather systems that can affect coastal zones. The most common along the northern Gulf of Mexico is what is known as a front. A front is a zone between two air masses with different temperatures, different levels of humidity, and densities. In the winter, low humidity cold air systems sweep southward across North America and encounter more humid, warmer air over the Gulf of Mexico. A strong southerly flow (winds blowing from south to north) persists along the northern Gulf in the days prior to the arrival of the cold front. These sustained winds blow as much as 30 to 50 km/hr and can generate large waves in the Gulf of Mexico that then travel northward and impact the shore, leading to erosion, deposition, and overall modification of the pre-frontal morphology. As the front passes, the wind direction shifts to blowing from the north, bringing much colder and dryer air. These north winds can similarly reach sustained speeds in excess of 30 km/hr persisting for a couple of days. Because these winds are from the north, they can generate large waves in the bays on the north side of the shorelines and have been shown to be very effective at eroding the marsh and other features on the north-facing shorelines of barrier islands and marsh platforms.