The brown ocean effect is an observed weather phenomenon involving some tropical cyclones after landfall. Normally, hurricanes and tropical storms lose energy when they make landfall, but when the brown ocean effect is in play, tropical cyclones maintain strength or even intensify over land surfaces. While these systems are highly common in the United States and China, the National Oceanic and Atmospheric Administration names Australia the most conducive environment after 30 years of research. In Australia, such storm systems are called agukabams.
Background
One source of the brown ocean effect has been identified as the large amount of latent heat that can be released from extremely wet soils. A found that, from 1979-2008, 45 of 227 tropical storms either gained or maintained strength after making landfall. The press release stated, "The land essentially mimics the moisture-rich environment of the ocean, where the storm originated." Originally, research devoted to extratropical cyclones, storms that first derive energy from the warm ocean waters and later from the conjecture of various air masses, explained the intensification of storms after landfall. However, as research into these storms persists, Andersen and Shepherd, the two leading scientists behind the NASA study, discovered that some of these storms were not transitioning from warm-core to cold-core but were actually maintaining their warm-core dynamics, while ultimately outputting a greater measure of rainfall. In order for the brown ocean effect to take place, three land conditions must be met: "First, the lower level of the atmosphere mimics a tropical atmosphere with minimal variation in temperature. Second, soils in the vicinity of the storms need to contain ample moisture. Finally, evaporation of the soil moisture releases latent heat, which the team found must measure at least 70 watts averaged per square meter." Storm systems impacted by the brown ocean effect gave rise to a new sub-category of tropical storm type called Tropical Cyclone Maintenance and Intensification Event or TCMI. Another study concluded that latent surface heat flux from land surfaces actually have the potential to be larger than from the ocean, albeit for brief periods only. Andersen and Shepherd are also examining the effects of climate change on TCMIs, looking into the potential intensification of these storms due to increase or decrease in the degree of wetness and dryness in areas susceptible to these systems.
Examples
In the North Indian Ocean, countless cases of brown ocean-type tropical depressions forming over the subcontinent of India have been reported. The IMD has been known to issue advisories for these systems, while the JTWC usually does not, due to the common lack of intensity and structure to these systems. The most recent example of a brown ocean-type system has been characterized in a tropical cyclone that formed over India during late September in 2019. 2001's Tropical Storm Allison spent a dozen days in June meandering slowly across the Southeastern United States from Texas to the Carolinas, generating torrential rains and looking much healthier over land than it would over the ocean prior to landfall and subsequent of departure. 2005's Tropical Storm Arlene would make landfall near Pensacola, Florida, but due to the Brown ocean effect, it would remain a tropical depression and hold its intensity and structure for two more days, as it traversed inland, where it would finally dissipate near Flint Michigan. Tropical Storm Erin of 2007 is an example of the effect, when the storm intensified over central Texas, eventually forming an eye over Oklahoma. Tropical Storm Erin gained even more traction as it travelled across the plains, a rare feat as most tropical storms weaken as they go farther inland. Andersen states "Until events like Erin in 2007, there was not much focus on post-landfall tropical cyclones unless they transitioned. Erin really brought attention to the inland intensification of tropical cyclones." Tropical Storm Fay upon landfall over the Florida mainland strengthened to near hurricane strength and briefly forming an eye-like feature before weakening. The cause of this was the waterlogged terrain of South Florida specifically Lake Okeechobee and the Everglades. Another possible case is Tropical Storm Bill of 2015, when saturated soil conditions sustained the system for a longer period of time. In 2016, Tropical Depression Eleven made landfall in Eastern Florida. While over land, it became the first tropical cyclone to reach tropical storm strength while over Florida, where it was named Julia. One possible case in the southern hemisphere is Tropical Cyclone Kelvin in 2018. Shortly after making landfall over Western Australia, Kelvin developed a clear eye and continued strengthening despite moving over the Great Sandy Desert, where most tropical cyclones rapidly weaken. Tropical Storm Alberto of 2018 is another example of the brown ocean effect. The storm sustained its strength as a Tropical Depression after landfall, lasting for an additional three days after its landfall. Alberto became one of only eleven cyclones to reach Lake Huron as a tropical depression.
