Time-Evolving, Spatially Explicit Forecasts of the Northern Gulf of Mexico Hypoxic Zone

The Mississippi–Atchafalaya River Basin delivers
large amounts
of freshwater and nutrients to the northern Gulf of Mexico promoting
the development of a large hypoxic zone every summer. Statistical
and semiempirical models have long been used to provide seasonal forecasts
of the mid-summer hypoxic extent using historic time series of spring
nutrient load and mid-summer hypoxic extent. These forecasts consist
of a scalar estimate of the hypoxic area with uncertainty but do not
include spatial distributions or temporal evolution of hypoxic conditions.
Three-dimensional (3D) circulation-biogeochemical models of the coastal
ocean simulate the temporal evolution of hypoxia in a spatially explicit
manner but have not yet been used for seasonal hypoxia forecasting.
Here, we present a hybrid method for seasonal, spatially explicit,
time-evolving forecasts of the hypoxic zone that combines statistical
forecasting with information from a 3D biogeochemical model. The hybrid
method uses spring nitrate load and a multiyear (1985–2018)
3D hindcast simulation to produce a seasonal forecast. Validation
shows that the method explains up to 76% of the observed year-to-year
variability in the hypoxic area. The forecasts suggest that the maximum
seasonal extent of hypoxia is reached, on average, on August 13, 2
weeks after the completion of the annual cruise. An analysis of month-to-month
variations in hypoxia forecasts due to variability in wind speed and
freshwater discharge allows estimates of weather-related uncertainties
in the forecast.