New Study Reveals Unexpected Decline in Ocean Evaporation Amid Rising Sea Temperatures – Watts Up With That?

A recent study published in Geophysical Research Letters, has turned up a surprising result: global ocean evaporation, a cornerstone of the hydrological cycle, has been declining since the late 2000s despite steadily warming sea surfaces. This runs counter to the widely held view that a warmer climate should boost evaporation rates. For those tracking discrepancies in climate science, this is worth a closer look.

The researchers analyzed satellite data spanning 1988 to 2017, drawing from four independent products: J-OFURO3, SeaFlux, HOAPS, and IFREMER. Their findings show that global ocean evaporation—which supplies about 85% of the atmosphere’s water vapor—rose sharply over the first two decades of the period, peaking around 2008. Then, the trend flipped. From 2008 to 2017, the global average dipped slightly, with two-thirds of the ocean experiencing reduced evaporation. This was validated, where possible, against buoy observations from the Global Tropical Moored Buoy Array, though coverage is sparse beyond the tropics.

Abstract

Ocean evaporation (E_o) is the major source of atmospheric water vapor and precipitation. While it is widely recognized that E_o may increase in a warming climate, recent studies have reported a diminished increase in the global water vapor since ∼2000s, raising doubts about recent changes in E_o. Using satellite observations, here we show that while global E_o strongly increased from 1988 to 2017, the upward trend reversed in the late 2000s. Since then, two-thirds of the ocean have experienced weakened evaporation, leading to a slight decreasing trend in global-averaged E_o during 2008–2017. This suggests that even with saturated surface, a warmer climate does not always result in increased evaporation. The reversal in E_o trend is primarily attributed to wind stilling, which is likely tied to the Northern Oscillation Index shifting from positive to negative phases. These findings offer crucial insights into diverse responses of global hydrological cycle to climate change.

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL114256

What’s driving this? The study points to “wind stilling”—a measurable drop in near-surface wind speeds, particularly pronounced in the Southern Hemisphere. They link this to the Northern Oscillation Index (NOI), a measure of pressure differences between the North Pacific High and near Darwin, Australia, shifting from positive to negative phases after 2008. This phase change aligns with weakened trade winds and a broader slowdown in atmospheric circulation, outweighing the evaporation boost expected from rising sea surface temperatures (SSTs).

This challenges a core assumption in climate science: that evaporation scales predictably with temperature via the Clausius-Clapeyron relationship, where warmer air holds more vapor and thus pulls more water from the surface. The study’s data shows SSTs climbing as expected, increasing the air-sea humidity deficit—a key driver of evaporation. Yet wind speed’s decline overpowered that effect, cutting evaporation anyway. The authors quantify this using a multiple regression model, finding wind speed accounted for 62% of the post-2008 drop, compared to 38% from humidity changes.

The ramifications are significant. First, a weaker evaporation signal could reduce the moisture feeding precipitation, especially for land regions relying on ocean-derived vapor. The study notes spatial variation—declines were strongest in the southern tropical Pacific, Indian Ocean, and extratropical Atlantic—suggesting uneven impacts on rainfall patterns. Second, it could alter ocean salinity trends. Evaporation typically concentrates salt at the surface, but a slowdown might ease that process, particularly in subtropical zones where it dominates over precipitation. This could subtly influence density-driven ocean currents, though the study doesn’t claim dramatic shifts like an Atlantic Meridional Overturning Circulation disruption.

For climate modeling, this is a wrinkle. Most global circulation models (GCMs) project an intensified hydrological cycle under warming—more evaporation, more rain, more extremes. But if wind speed can override temperature effects, those projections might overshoot reality. The authors frame this as a possible natural cycle tied to decadal oscillations like the NOI or Pacific Decadal Oscillation, not a permanent climate change signature. Still, they leave the door open: if human-driven warming (e.g., polar amplification) is quietly slowing winds, this could signal a longer-term shift. With data ending in 2017, we’re stuck waiting for updates to settle that question.

Uncertainties linger. The satellite datasets aren’t flawless—HOAPS, for instance, shows a steeper drop, possibly due to quirks in its microwave sensor platforms (SSMI/SSMIS), while the others temper that signal by blending multiple sources. Buoy validation helps, but it’s spotty outside tropical zones, leaving high-latitude trends less certain. The study also sidesteps intra-annual shifts and secondary drivers like radiation or upwelling, focusing narrowly on wind and humidity. Future work, they suggest, could refine this with better measurements—think next-gen satellites or ocean drones.

It’s another case where observations don’t match the script. The “warming equals wetter” narrative takes a hit when a natural factor like wind can call the shots. It’s not a knockout blow to climate orthodoxy—evaporation rose for decades before 2008, tracking warming—but it exposes gaps in the models and the risk of over-relying on simplified physics. Is this a strong indicator that atmospheric dynamics are more in charge than CO2-driven forecasts admit? The study doesn’t preach; it just lays out the numbers. That’s plenty of fuel for discussion.

[Link to study: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2024GL114256]

H/T Dr. Judith Curry