Cyclonic activity in the eastern Gulf of Mexico: Characterization from along-track altimetry and in situ drifter trajectories

Abstract

The shedding sequence of the Loop Current (LC) inside the Gulf of Mexico (GoM) is strongly influenced by cyclonic frontal eddies around its edge. Along-track altimetry data, analyzed based on a wavelet decomposition to provide estimates of individual cyclones’ diameter, amplitude and relative vorticity, and in situ surface drifter data from the Global Drifter Program, are used to investigate the cyclonic activity in the eastern Gulf of Mexico, where the LC extends and retracts. By analyzing this ∼20year long (1992–2011) combined set of observation data records, we were able to complement previous findings, to confirm results from modeling studies and to provide new insights on the LC frontal dynamics.Drifter data indicate, for the first time, that Loop Current Frontal Eddies (LCFEs) are in solid-body rotation close to their core. This property makes relative vorticity the most robust diagnostic from along-track altimetry for characterizing LCFEs in the eastern GoM, based on consistent comparisons with drifter data. Both data sets are complementary for describing the LCFEs’ regional variability.LCFEs observed in the deep southeastern GoM show intense relative vorticity, but they are not frequently observed. The study of an unprecedented, long drifter trajectory suggests that they are not intensified locally. This implies that, among LCFEs coming from the northern GoM, only intense ones reach the deep southeastern GoM.The observation datasets provide, for the first time, quantitative evidences of processes so far only identified with models: LCFEs are intensified when they are advected over the Mississippi Fan in the northern GoM; a small area north of Campeche Bank shows intense LCFE activity.The altimetry and drifter data confirm and complement results from more limited datasets: LCFEs forming the Tortugas Eddies, at the entrance of the Straits of Florida, are the most intensely observed LCFEs in altimetry. Coming from the GoM interior, they can be modified, in size and intensity, just before or during their stay in the Dry Tortugas area, whereas they were long considered to be modified only after they were advected in the Straits of Florida. A single drifter trajectory illustrates in detail the erosion of a LCFE in the southeastern GoM, presumably under the influence of the LC, which leads to the re-arrangement of vorticity and the formation of a distinct LCFE, just upstream the Dry Tortugas area.The study also confirms that LCFEs have diameters of 80–120km, generally larger east and north of the LC. They tend to stay for long periods in the northeastern GoM, and thus may undergo vortex merging with incoming eddies, as previously noted on individual episodes. Altimetry reveals that the largest LCFEs reside in the central eastern GoM, in a location where they are known to block the LC extension; however, our long altimetry dataset shows that these large eddies are not always involved in such LC blocking.Finally, this study led to improvements in data treatment, for both along-track altimetry and drifter trajectories. The wavelet approach used to derive eddy characteristics from along-track altimetry allows robust estimates of the eddy relative vorticity. The drifter dataset has been extended to include data from drifters even after they lost their drogue; the latter data treatment can be applied in areas of moderate winds.