Once a hurricane reaches land, it typically signals the beginning of the storm’s demise. However, researchers revealed in January at the American Meteorological Society’s conference in Baltimore that a hurricane can experience a surge of energy, rejuvenating its intensity through a phenomenon known as the brown ocean effect. This effect mirrors how tropical cyclones draw strength from the ocean by providing them with a continuous source of water and warmth. Understanding this phenomenon could enhance forecasters’ ability to alert residents that an inland hurricane might paradoxically intensify.
Geologist Dev Niyogi and his team analyzed satellite data on wind speeds, precipitation, and soil moisture to investigate the brown ocean effect during the events in the Carolinas in September 2018. The findings unveiled a significant feedback loop: as Hurricane Florence traversed over already saturated soil, the rainfall intensified, resulting in unprecedented levels of precipitation and subsequent flooding.
This study marks one of the initial observational validations of the long-theorized brown ocean effect, according to Niyogi from the University of Texas at Austin. He further suggests that this effect can assist hurricanes in maintaining their strength as they move across land, enabling them to penetrate further inland before weakening.
The concept of the brown ocean effect was first introduced in 2013 by atmospheric scientist Marshall Shepherd and geographer Theresa Andersen, inspired partly by the peculiar behavior of Tropical Storm Erin in 2007. Despite being weak and disorganized upon landfall, the storm unexpectedly intensified as it advanced northwest towards Oklahoma, encountering waterlogged terrain due to heavy rains that year. This sudden strengthening, including the formation of an eye, puzzled scientists.
Computer simulations have long indicated that higher soil moisture levels can amplify monsoon rains and cyclones. Shepherd, from the University of Georgia in Athens, explains that water evaporates from the ocean as vapor, but within hurricanes, it condenses back into liquid form, releasing heat and bolstering the storm’s energy.
Anderson’s research at the University of Georgia identified over a dozen storms between 1997 and 2008, including Tropical Storm Erin, that likely experienced post-landfall strengthening due to this effect. Shepherd emphasizes the non-discriminatory nature of tropical storms in sourcing energy, whether from wet soils, oceans, or wetlands.
The recent study by Niyogi and his team utilized detailed satellite data to track soil moisture, temperature, and rainfall changes before, during, and after Hurricane Florence’s landfall. The analysis showcased a robust positive correlation between intense rainfall bursts and soil moisture. The duration of soil saturation before the storm’s passage also played a crucial role, with the most intense rains occurring over soil that had been saturated for approximately three days. This period allowed for optimal humidity levels near the ground, facilitating energy transfer to the storm.
Apart from saturated soil, the team identified soil temperature as a critical factor. Once the soil reaches a certain temperature threshold, which is yet to be precisely determined, the conditions become favorable for storm intensification. Shepherd, in a separate study presented at the same conference, delved into how this effect influenced storm dynamics.
A notable instance of the brown ocean effect occurred in 2021 when Hurricane Ida made landfall in Louisiana, nearly replicating the path of Hurricane Katrina in 2005. Before dissipating, likely fueled by prolonged rainfall saturating the ground and elevating humidity levels prior to landfall, Hurricane Ida demonstrated the impact of the brown ocean effect.
Niyogi emphasizes that the existence of the brown ocean effect is evident in the data, underscoring the need for its consideration in storm forecasting and modeling. Shepherd concurs, highlighting the necessity for weather models to integrate the influence of land, ocean, and atmosphere on tropical storms to enhance predictions of the risks posed to inland populations. The incorporation of wet soil conditions into these models is crucial for accurately assessing the potential threats posed by storms moving inland.