Deep Argo: Toward a Truly Global Ocean Observing System

The ocean is the flywheel of the climate system, capable of storing about 1000 times more heat than the atmosphere for the same change in temperature. Around 93% of Earth’s energy imbalance from 1971–2010 was stored in the oceans, with approximately 10% of ocean warming observed deeper than 2000 dbar over the last few decades. Furthermore, the deep ocean has a filling time of around a millennium based on current ventilation rates, so were atmospheric greenhouse gas concentrations to be stabilized, the deep ocean would continue to warm for a long time thereafter. Knowing both how much and where heat is being stored in the oceans is important for understanding how much, how fast, for how long, and where the seas will rise in addition to how much the atmosphere will warm. The international Argo Program’s array of profiling floats, with more than a decade of near-global coverage of the upper half of the ocean’s volume, has greatly reduced uncertainties in rates and patterns of ocean warming there. Deeper than the 2000-dbar pressure limit of conventional Argo floats, ship-based measurements have revealed a bottom intensified warming signature originating from around Antarctica. However, the full-depth, high-quality measurements from the international GO-SHIP network of oceanographic sections are spatially sparse, and only repeated at roughly decadal intervals. A global 5°x5°x15-day array of 1228 Deep Argo floats profiling the full depth of the ocean would reduce uncertainties of decadal deep (pressures > 2000 dbar) ocean heat storage from their current value near ±17 TW to around ±3 TW. This new deep array would boost annual coverage of the deep ocean’s heat content below 2000 dbar from about 20% to around 80%, close to the ~90% value achieved by Argo for 0–2000 dbar over the last several years.