Brown ocean effect

The remnants of Erin over Oklahoma, with what appears to be an eye-like structure

The brown ocean effect is an observed weather phenomenon where tropical cyclones, which are commonly expected to lose energy when they make landfall, instead maintain strength or intensify over land surfaces.^[1] While these systems are highly common in the United States and China, the National Oceanic and Atmospheric Administration (NOAA) names Australia the most conducive environment after 30 years of research. In Australia such storm systems are called agukabams.^[2]

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.^[1][3] A 2013 NASA study found that, from 1979-2008, 45 of 227 tropical storms either gained or maintained strength after making landfall.^[4] The press release stated, "The land essentially mimics the moisture-rich environment of the ocean, where the storm originated." Originally, countless 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.^[5] However, as research into these storms persists, Anderson 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.^[5] 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."^[5] 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.^[5] 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.^[6] Anderson 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.^[5]

Examples

Allison with an eye-like feature over Mississippi

2001's Tropical Storm Allison spent a dozen days in June meandering slowly across the southeastern US 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.

Tropical Storm Erin of 2007 is an example of the effect, when the storm intensified over central Texas, eventually forming an eye over Oklahoma.^[1][3][7] 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.^[7] Anderson 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."^[5]

Another possible case is Tropical Storm Bill, when saturated soil conditions sustained the system for a longer period of time.^[8]

References