Abstract
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). 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). 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.
Highlights
Earth system models (ESMs) are the most complex among all interrelated system models, and they can be used to represent associated physical, chemical, and biological processes (Collins et al 2011; Watanabe et al 2011; Dufresne et al 2013; Dunne et al 2013; Lindsay et al 2014)
This study examines the physical mechanism involved in using an ESM to make successful surface chlorophyll concentration forecasts over the south-central Pacific with a lead time of up to more than 1-year
The NO3 budget analysis indicates that nutrients required to accumulate the surface chlorophyll concentration over the SP region are mostly provided by the meridional advections
Summary
Earth system models (ESMs) are the most complex among all interrelated system models, and they can be used to represent associated physical, chemical, and biological processes (Collins et al 2011; Watanabe et al 2011; Dufresne et al 2013; Dunne et al 2013; Lindsay et al 2014). While there is a clear need to predict marine biogeochemistry, developing biogeochemistry forecasting systems is challenging due to the model uncertainty and problems relating to biogeochemical initialization In this respect, model uncertainty includes imperfections in the model structure and relates to parameters in both physical and biogeochemical models, whereas the biogeochemical initialization problem relates to the relatively sparse number of global biogeochemistry observations, the large number of biogeochemical tracers in ESMs, and the extremely high sensitivity of ocean biogeochemistry to dynamical imbalances induced during data assimilation, all of which hamper the development of fully coupled physical-biogeochemical initialization systems (Ford et al 2012; Raghukumar et al 2015; Song et al 2016). The study used the extensive retrospective forecast experiments with an ESM and showed skillful marine ecosystem predictions in many ocean basins It showed the possibility of making skillful predictions up to 2 years in advance in some areas, as the ESM is capable of successfully simulating the evolution of marine biogeochemical responses to physical climate perturbations. The high biogeochemical predictability and its underlying mechanism will be of great interest for the management of marine living resources, given the high number of tuna fisheries and aquaculture activities in the tropical Pacific (Lehodey et al 1997)
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