Variability of the Deepwater Horizon Surface Oil Spill Extent and Its Relationship to Varying Ocean Currents and Extreme Weather Conditions

Abstract

Satellite observations and their derived products played a key role during the Deepwater Horizon oil spill monitoring efforts in the Gulf of Mexico in April–July 2010. These observations were sometimes the only source of synoptic information available to monitor and analyse several critical parameters on a daily basis. These products also complemented in situ observations and provided data to assimilate into or validate model. The ocean surface dynamics in the Gulf of Mexico are dominated by strong seasonal cycles in surface temperature and mixing due to convective and storm energy, and by major currents that include the Loop Current and its associated rings. Shelf processes are also strongly influenced by seasonal river discharge, winds, and storms. Satellite observations were used to determine that the Loop Current exhibited a very northern excursion (to approximately 28$^{\circ }$N) during the month of May, placing the core of this current and of the ring that it later shed at approximately 150 km south of the oil spill site. Knowledge gained about the Gulf of Mexico since the 1980s using a wide range of satellite observations helped understand the timing and process of separation of an anticyclonic ring from the Loop Current during this time. The surface extent of the oil spill varied largely based upon several factors, such as the rate of oil flowing from the well, clean up and recovery efforts, and biological, chemical, and physical processes. Satellite observations from active and passive radars, as well as from visible and infrared sensors were used to determine the surface extent of the oil spill. Results indicate that the maximum and total cumulative areal extent were approximately 45 $\times$ 10$^3$ km$^2$ and 130 $\times$ 10$^3$ km$^2$, respectively. The largest increase of surface oil occurred between April 22 and May 22, at an average rate of 1.3 $\times$ 10$^3$ km$^2$ per day. The largest decrease in the extent of surface oil started on June 26, at an average rate of 4.4 $\times$ 10$^3$ km$^2$ per day. Surface oil areas larger than approximately 40 $\times$ 10$^3$ km$^2$ occurred during several periods between late May and the end of June. The southernmost surface oil extent reached approximately 85$^{\circ }$W 27$^{\circ }$N during the beginning of June. Results obtained indicate that surface currents may have partly controlled the southern and eastern extent of the surface oil during May and June, while intense southeast winds associated with Hurricane Alex caused a reduction of the surface oil extent at the end of June and beginning of July, as oil was driven onshore and mixed underwater. Given the suite of factors determining the variability of the oil spill extent at ocean surface, work presented here shows the importance of data analyses to compare against assessments made to evaluate numerical models.