Seasonal forecast skill of upper-ocean heat content in coupled high-resolution systems

Abstract

Ocean heat content (OHC) anomalies typically persist for several months, making this variable a vital component of seasonal predictability in both the ocean and the atmosphere. However, the ability of seasonal forecasting systems to predict OHC remains largely untested. Here, we present a global assessment of OHC predictability in two state-of-the-art and fully-coupled seasonal forecasting systems. Overall, we find that dynamical systems make skilful seasonal predictions of OHC in the upper 300 m across a range of forecast start times, seasons and dynamical environments. Predictions of OHC are typically as skilful as predictions of sea surface temperature (SST), providing further proof that accurate representation of subsurface heat contributes to accurate surface predictions. We also compare dynamical systems to a simple anomaly persistence model to identify where dynamical systems provide added value over cheaper forecasts; this largely occurs in the equatorial regions and the tropics, and to a greater extent in the latter part of the forecast period. Regions where system performance is inadequate include the sub-polar regions and areas dominated by sharp fronts, which should be the focus of future improvements of climate forecasting systems.

Skills of the user-relevant ocean indicators

“…While both this deliverable and D4.6 cover the skill of forecast systems, D4.6 covers marine variables while this deliverable covers marine indicators. Moreover, much of the work presented here has been published in various scientific journals (McAdam et al 2022a;McAdam et al 2022b;von Schuckmann et al 2022). Combined, we believe they state the case for an increase in marine seasonal forecasting validation and application.…”

“…A key scientific result of EuroSea's activity on seasonal forecasting is that OHC and its derivative indicators are generally reliably forecast on seasonal timescales (McAdam et al 2022a). Skill is high due to reduced variability on the seasonal timescale.…”

Global ocean surface and subsurface temperature forecast skill over subseasonal to seasonal timescales

“…Ocean reanalyses have been used in recent subsurface temperature skill assessments, chosen for their benefits in geographic and temporal coverage along with compatibility of grid resolution with NEMO-based coupled models (e.g. McAdam et al 2022McAdam et al , 2023. Monthly HC300 and MLD values were extracted from the ECMWF Ocean Reanalysis System 5 (ORAS5; Zuo et al 2019) as an independent dataset for 1982-2018.…”

Copernicus Ocean State Report, issue 6

“…Even though SEAS5 cannot predict the exact observed weather features, it can provide indications on weather statistics on a monthly to seasonal basis. Seasonal forecast of ocean variables other than SST has so far received little attention, but recent work hints that SEAS5 forecast skill for the ocean heat content in the upper 300 m is comparable to the skill for SST in the Tropics, and even exceeds it in the Extratropics (McAdam et al 2022).…”