Natural Variability in a Changing Ocean: Emergence and Impacts

Abstract

Anthropogenically-forced changes in the ocean are underway and critical for the ocean’s role as a carbon sink and marine habitat. Detecting such changes will require quantification of not only the magnitude of the change (anthropogenic signal) but also the natural variability inherent to the climate system (noise). This work uses Earth System Models (ESMs) to (1) evaluate timescales over which anthropogenic signals in the contemporary ocean emerge from natural climate variability and (2) interpret observed variability in the Pacific Basin.

We apply time-of-emergence (ToE) diagnostics to a Large Ensemble experiment of an ESM, providing both a conceptual framework for interpreting the detectability of anthropogenic impacts on the ocean carbon cycle and observational sampling strategies required to achieve detection. We find ToEs for different components of the ocean carbon cycle range from under a decade to over a century, a consequence of the time-lag between chemical and radiative impacts of rising atmospheric CO2 on the ocean. Processes sensitive to chemical changes emerge rapidly, such as impacts of acidification on the calcium-carbonate pump (10-20 years), and the invasion of anthropogenic CO2 into the ocean (20-30 years). Processes sensitive to the ocean’s physical state, such as the soft-tissue pump, which depends on nutrients supplied through circulation, emerge decades later (20-80+ years).

Next, we evaluate the model- and scenario-sensitivity of ToEs through comparing Large Ensembles from four ESMs. We find ToEs are robust across models for variables that are tied directly with the rise in atmospheric CO2 namely rising sea surface temperature and the invasion of anthropogenic CO2 into the ocean (ToE 20-30 years). For the soft-tissue pump, ocean color, and sea surface salinity, ToEs are longer (50+ years), less robust across the ESMs, and more sensitive to the forcing scenario considered.

Finally, we investigate potential mechanisms responsible for recent variability in the Pacific Ocean. We conduct wind-substitution simulations with GFDL-ESM2M in which decadal trends in the trade-winds are nudged toward observed values. These simulations provide better agreement between simulated and observed variability in ocean temperature, circulation, sea-level and air-sea CO2 exchange, indicating this variability could be attributed to strengthening trade winds.