Predicting marine futures: The ability to predict how climate variations will affect marine ecosystems would allow better economic and ecosystem planning and management. Park et al. found that a global Earth system model skillfully predicted seasonal to multiannual ocean chlorophyll fluctuations in many regions. This could allow annual fish catches in some regions to be forecast 2 to 3 years in advance. Science , this issue p. 284

Reliable estimates of historical and current biogeochemistry are essential for understanding past ecosystem variability and predicting future changes. Efforts to translate improved physical ocean state estimates into improved biogeochemical estimates, however, are hindered by high biogeochemical sensitivity to transient momentum imbalances that arise during physical data assimilation. Most notably, the breakdown of geostrophic constraints on data assimilation in equatorial regions can lead to spurious upwelling, resulting in excessive equatorial productivity and biogeochemical fluxes. This hampers efforts to understand and predict the biogeochemical consequences of El Niño and La Niña. We develop a strategy to robustly integrate an ocean biogeochemical model with an ensemble coupled‐climate data assimilation system used for seasonal to decadal global climate prediction. Addressing spurious vertical velocities requires two steps. First, we find that tightening constraints on atmospheric data assimilation maintains a better equatorial wind stress and pressure gradient balance. This reduces spurious vertical velocities, but those remaining still produce substantial biogeochemical biases. The remainder is addressed by imposing stricter fidelity to model dynamics over data constraints near the equator. We determine an optimal choice of model‐data weights that removed spurious biogeochemical signals while benefitting from off‐equatorial constraints that still substantially improve equatorial physical ocean simulations. Compared to the unconstrained control run, the optimally constrained model reduces equatorial biogeochemical biases and markedly improves the equatorial subsurface nitrate concentrations and hypoxic area. The pragmatic approach described herein offers a means of advancing earth system prediction in parallel with continued data assimilation advances aimed at fully considering equatorial data constraints.

This study investigates the physical mechanism involved in an Earth system model (ESM)-based global marine biogeochemical prediction system providing successful forecasts of surface chlorophyll concentrations over the southern Pacific. The significant correlation skill of the surface chlorophyll concentration over the south-central Pacific (SP region, 160°–110° W, 10°–5° S) appears up to a 15-month lead. In contrast to the previously known role of the vertical nutrient supply on the predictive chlorophyll concentration forecasts, the NO₃ budget analysis indicates that this prediction skill over the SP region is mostly controlled by the meridional advection of nutrients. Further analysis indicates that the controlling mechanisms involved in chlorophyll variability over the SP region can be explained by atmospheric and oceanic dynamics during the ENSO events. During La Nina, equatorial NO₃ anomalies are increased due to enhanced equatorial upwelling, and the climatological southward current then advects nutrient-rich waters from the equator to the SP region $$\left( {{\text{i.e.,\,positive}}\, - \overline{v}\frac{{\partial {\text{NO}}_{{3}}^{\prime } }}{\partial y}} \right)$$i.e.,\,positive - v ¯ ∂ NO 3 ′ ∂ y . In addition, anomalous easterly surface winds blow over the SP region as a circulation response to atmospheric diabatic heating anomalies during La Nina, which leads to southward current anomalies over the surface-layer ocean. This advects high climatological NO₃ over the tropics to the subtropical south Pacific, which increases the NO₃ anomalies $$\left( {{\text{i.e.,\, positive}} - v^{\prime}\frac{{\partial \overline{{{\text{NO}}_{3} }} }}{\partial y}} \right)$$i.e.,\, positive - v ′ ∂ NO 3 ¯ ∂ y . This positive NO₃ advection over the SP region is realistically simulated only at lead times shorter than 9 month, and the multi-season persistency of the nutrients contributes to the surface chlorophyll bloom at lead times longer than 1 year.

Marine extreme temperature events (METs), including marine heatwaves (MHWs) and cold spells, have recently gained much attention owing to their vital influence on the marine ecosystem and social economy. Since METs can alter the upper ocean stratification and wintertime convective mixing in the northwestern North Pacific subtropical gyre (NPSG), their activities may modulate phytoplankton blooms by regulating entrainment of the subtropical mode water (STMW) with high NO 3 − . Furthermore, because STMW formed in the previous winter reemerges east of its formation site in the following winter, the METs activities imprinted in STMW affect phytoplankton blooms remote from its formation site. Here, we examined the relationship between the MET activities, STMW volume, and phytoplankton blooms using satellite observations and a data-assimilative coupled physical-biogeochemical model dataset. MET activities appearing in the STMW formation region during winter regulate the formation of STMW and the supply of NO 3 − from the subsurface, with the latter controlling the spring/autumn blooms in that region under NO 3 − -limited conditions. Subsequently, this water mass is transported eastward in the subsurface within the northern flank of the NPSG before reemerging east of the STMW formation site the following spring. This process results in a negative lag-correlation between MET activities and surface chlorophyll in the reemergence region; for example, MHWs in winter at the STMW formation site tend to lower the surface chlorophyll concentration one year later in the reemergence region. Our study suggests that the oceanic processes allow one year of predictability of the marine ecosystems by monitoring METs in the STMW formation site.