Ocean heat transport, not air-sea flux, drives regional warming in the top 200 m

Why it matters

Tracking ocean heat content alone cannot tell you what is causing regional warming. The paper argues that full heat budgets are needed to explain warming patterns, including marine heatwave hotspots. It also reports that the sunlit layer is taking up heat at a high rate that is expected to continue.

What they did

The author defines a “sunlit ocean layer” energy imbalance for 0-200 m. It is a 10-year running mean of a closed heat budget that splits air-sea heat uptake from heat transport convergence. He estimates it over 1980-2022 with an ocean state estimate (OCCA2) that conserves heat. Argo float profiles and the IAP V4 gridded product are used to measure the leftover, non-conserving term. Anomalies are taken relative to a climate normal for 1985-2009, with an interdecadal oscillation pattern removed.

Key findings

• Global accumulation in the sunlit layer swung between near zero (around 1993 and 2008) and 30-60 ZJ/decade (around 1987, 2001 and 2015).
• Over 2013:17 the accumulation was 49.6 ZJ/decade in the Argo-adjusted estimate, 55.4 in the closed-budget estimate and 44.1 in IAP V4. Over 85:09 it was 15.1 and 24.0 in the last two.
• Regional warming changes in recent years are mainly set by heat transport convergence. Local air-sea uptake mostly acts as a feedback.
• The North Atlantic subpolar gyre cooled in the sunlit layer despite more heat entering through the surface, because heat transport divergence increased.
• Subtropical warming seen in 2007 appears near the equator below the surface by 2022, which the author takes as about 15 years of transit.

Limitations

• Uncertainty is larger before Argo (before 2004), including the 1993 minimum, and a 40-year record is too short to cleanly separate interdecadal variability from the long-term trend.
• Heat transport is the least well constrained term, and marginal seas and open-ocean latent and sensible heat fluxes are poorly observed, so the budget terms remain uncertain.
• The forecasts, such as 0.5 K or more of tropical subsurface warming within 15 years and the Atlantic gyre as a lead indicator for the Pacific, are conjectures that need further study.

Glossary

• Sunlit ocean layer (SOL): The ocean from the surface to 200 m depth, where solar radiation directly heats the water.
• SOL-EI: The 10-year mean heat budget of the sunlit layer, which splits air-sea heat uptake from heat transport convergence.
• Ocean heat transport (OHT) convergence: Net heat brought into a region by currents. If it is negative, heat is exported from the region.
• Interdecadal oscillation pattern (IOP): A leading decade-scale pattern found directly in the sunlit-layer maps. It resembles the Interdecadal Pacific Oscillation (IPO).