Ocean salinity may help forecast cyclone intensity

[MONTEVIDEO] Forecasting tropical cyclone intensity — a key factor in the damage they cause — could be made more accurate by monitoring ocean salinity, according to a study.

The differences in salinity caused by an inflow of big rivers in tropical regions cause a layering of ocean water temperature that may reduce the intensity of cyclones passing over its surface, scientists report in the Proceedings of the National Academy of Sciences (PNAS) last week (13 August).

Although previous studies have showed that ocean surface cooling has a significant impact on the intensity of tropical cyclones, researchers say their study is the first time it has been shown on a global scale that upper ocean salinity may affect intensity of the storms.

Tropical cyclones are one of the world’s most highly occurring and damaging types of natural disasters, and while scientists can track their movement fairly accurately, forecasting their intensity has been more challenging.

Cyclones usually absorb heat from oceans, but as they pass over an ocean, winds agitate the water, bringing up colder water from further down. This, in turn, reduces both surface temperatures and the amount of energy cyclones absorb from the ocean’s surface.

But in certain tropical regions where there is a high fresh water input — like the Amazon basin or Ganges delta — the difference in salinity between the ocean’s surface and subsurface causes a ‘barrier layer’ to form below the surface, effectively preventing the storm’s access to cooler ocean layers.

Researchers used observations and high resolution climate model simulations to analyse almost 600 tropical cyclones in the major tropical barrier layer regions — the Western Tropical Pacific, Tropical Northwestern Atlantic, and Northern Indian Ocean tropical basins — from 1998 to 2007.

Based on their analyses, the researchers found that when hurricanes pass over barrier layer regions, the amount of ocean surface cooling could be 36 per cent less than in regions without barrier layers, and hurricane intensity rates drop by up to 50 per cent.

"When the ocean surface stays warm, it can pump heat into the hurricane," raising intensity rates, Karthik Balaguru, a researcher at the Pacific Northwest National Laboratory, United States, and lead author of the study, told SciDev.Net.

The researchers concluded that the role of barrier layers should not be overlooked, as even modest improvements in forecasting intensity "can aid societal response and help mitigate these storms’ destructive power".

"Scientists could potentially integrate measurements of upper salinity into the hurricane intensity forecast system to improve predictions," Balaguru added.

Greg Holland, director of the Earth System Laboratory at the National Center for Atmospheric Research (NCAR), told SciDev.Net: "There is good evidence here for salinity impact, but this would need to be balanced against other observational priorities".

He added: "I see these findings more as providing valuable information on the type of effects and observations that we should look to within current prediction tools".

Omar García, a researcher at the Astronomy and Meteorology Institute at the University of Guadalajara, Mexico, said: "In the past three decades there has been no significant progress in the forecast of cyclones intensity, therefore, any research related to this issue is of great interest",

But Garcia told SciDev.Net that "more research is needed to understand the physics that could explain if [salinity] is a favorable condition for intensification [of cyclones]". 

Link to full paper in PNAS