Offshore forcing on the “pressure point” of the West Florida Shelf: Anomalous upwelling and its influence on harmful algal blooms

Abstract

Gulf of Mexico Loop Current (LC) interactions with the West Florida Shelf (WFS) slope play an important role in shelf ecology through the upwelling of new inorganic nutrients across the shelf break. This is particularly the case when the LC impinges upon the shelf slope in the southwest portion of the WFS near the Dry Tortugas. By contacting shallow water isobaths at this “pressure point” the LC forcing sets the entire shelf into motion. Characteristic patterns of LC interactions with the WFS and their occurrences are identified using unsupervised neural network, self‐organizing map, from 23 years (1993–2015) of altimetry data. The duration of the occurrences of such LC patterns is used as an indicator of offshore forcing of anomalous upwelling. Consistency is found between the altimetry‐derived offshore forcing and the occurrence and severity of WFS coastal blooms of the toxic dinoflagellate, Karenia brevis: years without major blooms tend to have prolonged LC contact at the “pressure point,” whereas years with major blooms tend not to have prolonged offshore forcing. Resetting the nutrient state of the shelf by the coastal ocean circulation in response to deep‐ocean forcing demonstrates the importance of physical oceanography in shelf ecology. A satellite altimetry‐derived seasonal predictor for major K. brevis blooms is also proposed.

Winds on the West Florida Shelf: Regional comparisons between observations and model estimates

“…Being that coastal ocean ecosystem functionality begins with water property distributions that are largely determined by the coastal ocean circulation (e.g., see WFS examples in Weisberg and He [2003], Walsh et al [2003], Weisberg et al [2015Weisberg et al [ , 2016a, and Liu et al [2016]), our results are particularly germane to ecosystems-based management of the coastal ocean. Adding more in situ offshore wind observations will likely be a means for improving coastal ocean wind forcing fields and hence coastal ocean model fields of circulation and attendant water properties.…”

“…[], and Liu et al . []), our results are particularly germane to ecosystems‐based management of the coastal ocean. Adding more in situ offshore wind observations will likely be a means for improving coastal ocean wind forcing fields and hence coastal ocean model fields of circulation and attendant water properties.…”

Ocean Circulation in the Western Gulf of Mexico Using Self‐Organizing Maps

“…On the other hand, a larger map size will provide more details on different patterns but may be difficult to visualize, which is the case in Weisberg and Liu (), where they had to show 50 patterns of the LC evolution in two separate figures. For SOM applications in the eastern GoM, Liu et al (Liu et al, ; Liu, Weisberg, Vignudelli, & Mitchum, ) used 16 and 20 patterns. For our purpose, we employ a 7 × 7 map size (49 patterns) for the spatial analysis and a 2 × 3 map size (6 patterns) for the temporal variability.…”

“…Similar to Liu, Weisberg, Lenes, et al (), we perform a joint SOM‐wavelet analysis. The rectified wavelet power spectral analysis is applied to two time series to find additional relationships between the unmodified HYCOM salinity fields and the results of the SOM temporal analysis.…”

Causes of Interannual Variation of Summer Mean Alongshore Current Near the East Coast of Korea Derived From 16‐Year‐Long Observational Data

“…The CTWs are a hybrid of internal Kelvin waves and continental shelf waves (or topographic Rossby waves) that are trapped in the shelf/slope area and accompany pressure and alongshore current variations while propagating along the coast. On the other hand, the coastal current can be influenced by offshore pressure forcing, particularly in the vicinity of the WBCs (Archer et al, 2017;Csanady, 1978;Kelly & Chapman, 1988;Liu et al, 2016;Xu & Oey, 2011). It has been suggested that interannual variations in the alongshore current near the coast are related to remote wind forcing (Bachèlery et al, 2016), cross-shore wind stress curl gradient (Davis & Di Lorenzo, 2015), and alongshore (Li et al, 2014) and cross-shore (Feng et al, 2016) wind stress.…”