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Two Hurricanes Colliding - Can Hurricanes Merge and Become One Big Hurricane?

Examining Reader Emails on the Formation of Hurricanes

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The Fujiwhara Effect Is Seen in the Interaction of Hurricane Ione and Hurricane Kirsten in 1974

NOAA-3 visible range VHRR image of Hurricanes Ione (left) and Kirsten (right.) The rare effect of two interacting hurricanes is termed the Fujiwhara effect. Photo Date: 1974 August 24 1749 GMT

NOAA Photolibrary, NOAA In Space Collection
Two hurricanes merging and combining to become one big hurricane is the topic of several reader emails in light of the close proximity of storms such as Gustav, Hanna, Ike, and Josephine 2008. One reader in particular sent this question
What would happen if two hurricanes come together? Would they make a big storm or would they tear each other apart? Such as if Ike caught up to Hanna what would happen?
To answer this question on combining hurricanes, let's go back to the beginning for a moment. Hurricanes need several key ingredients to form. These conditions are the optimal environmental ingredients needed to form a hurricane or tropical storm.
  1. Ocean waters must be warm. The optimal temperature is 80+ degrees Fahrenheit. The layer of water must also be relatively think for a powerful hurricane to occur.
  2. A tropical disturbance must also form. Atlantic hurricanes often begin off the coast of Africa when a low pressure zone forms. As the storm moves across the Atlantic gaining energy, the storm can develop into a tropical storm or hurricane.
  3. Low wind shear environments help hurricanes to form. The higher the wind shear, the more likely a storm is to be torn apart.

Hurricanes Fabian and Isabel 2003

Tropical cyclones can form so close together that often the storms will feed on each other. But hurricanes also suck up a lot of the warmth from the water as a source of fuel. In September of 2003, hurricane Fabian (formed August 27, 2003) crossed the Atlantic followed closely by hurricane Isabel. Category 5 hurricane Isabel would later make landfall in North Carolina on Septmber 18, 2003. As hurricane Fabian moved towards the United States, warm waters were essentially used up as a fuel source. Fabian would eventually become the strongest storm to hit Bermuda since August 1963 when hurricane Arlene struck the island.

Cold Water Trails

A cold water trail is a pathway left behind of cooler ocean waters after the passing of a hurricane. Satellite images and visualizations show what are called ‘cold water trails’ left behind from Fabian. Isabel then passed very near these cold water trails. The power of hurricane Fabian caused ocean water upwelling as cooler and deeper waters moved up from deeper in the ocean. The warm surface waters were then pushed outwards and away from the center of Fabian.

Fabian used up the stored solar heat in the ocean (the surface heat) and stored it inside the hurricane in the form of latent heat. A video animation from the NASA Science Visualization Studios shows cold water trails from Fabian and Isabel. So, essentially, two hurricanes develop independently from their own fuel source in the ocean. Had Isabel followed directly in the wake of Fabian, there may have been a reduction in her strength.

Another type of interaction that can occur between tropical cyclones is the Fujiwhara Effect. In the Fujiwhara Effect, two tropical cyclones can alter each other and/or become absorbed by the stronger of the two hurricanes. Multiple examples of the Fujiwhara Effect show how hurricane, typhoon, and tropical storm paths can be altered by their interactions with each other.

When two storms are very close together, instead of exhibiting a rotation around a common center as in the Fujiwhara Effect, the stronger of the two storms can help to cancel out the strength of the weaker second storm. One example is seen in Cyclones Nancy and Olaf. Cyclone Nancy, a Category 2 storm, was significantly weakened by the outflow from the very strong Cyclone Olaf, a Category 5 storm which would eventually pass over American Samoa in February 2005.

Think of two cars running on the same tank of gas. You could keep a straw pumping and exchanging the gas between the two cars, but eventually, one car will end up the victor. So, hurricanes and tropical system can interact with each other, but usually there is a sacrifice. Now, lets take for example two tropical storms merging in the central Atlantic. Those two storms can collide and become larger, but not more powerful. One storm is usually absorbed. The storm may later become more powerful, but that does not mean it was from the collision.

Sources:

Stormchasers: The Hurricane Hunters and Their Fateful Flight Into Hurricane Janet
NOAA National Data Center
Annual Summary of the 2004 Atlantic Hurricane Season
Annual Summary of the 1995 Atlantic Hurricane Season
Monthly Weather Review: An Example of the Fujiwhara Effect in the West Pacific Ocean
NASA Earth Observatory: Cyclone Gula
Cyclones Olaf and Nancy

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