Relative role of wind forcing and riverine nutrient input on the extent of hypoxia in the northern Gulf of Mexico

Seasonal hypoxia of the northern Gulf of Mexico has been observed for more than 25 years. It is generally accepted that the variation in the areal extent of hypoxia is determined by changes in nutrient addition from the Mississippi River. In this study, we investigate the statistical relation between the hypoxic area and a new variable, the duration of west wind, using the available measurements for the period 1985–2010. Special consideration was paid to the 1993–2010 period, a time when a large shift in the seasonal hypoxia pattern has been reported. When excluding the years in which hurricanes directly impacted the hypoxic area observation, we find that the duration of west wind is correlated with the hypoxic area at r2 = 0.32 for the 1985–2010 period, and r2= 0.52 for the 1993–2010 period. Multilinear regressions using both wind duration and May-June nitrate loading improve the statistical relationships for both periods tor2 = 0.69 and 0.74 for the long and short time periods, respectively. Mechanistically, the statistical relationships reflect the movement and changes in horizontal river plume position associated with the wind and the influence of stratification on the hypoxic area.