Atmospheric Cold Front–Induced Exchange Flows through a Microtidal Multi-Inlet Bay: Analysis Using Multiple Horizontal ADCPs and FVCOM Simulations

Abstract

Atmospheric cold fronts provide recurring forcing for circulations and long-term transport in estuaries with microtides. Multiple horizontal ADCPs were used to obtain time series data from three inlets in Barataria Bay. The data cover a period of 51 atmospheric cold fronts between 2013 and 2015. The weather and subtidal ocean response are highly correlated in the “weather band” (3–7 days). The cold front–associated winds produce alternating flows into, out of, and then back into the bay, forming an asymmetric “M” for low-pass filtered flows. Results show that cold front–induced flows are the most important component in this region, and the flows can be predicted based on wind vector time series. Numerical simulations using a validated Finite-Volume Coastal Ocean Model (FVCOM) demonstrate that the wind-driven oscillations within the bay are consistent with the quasi-steady state with little influence of the Coriolis effect for cold front–related wind-driven flows. The four major inlets (from the southwest to the northeast) consistently carry 10%, 57%, 21%, and 12% of the tidal exchange of the bay, respectively. The subtidal exchange rates through them however fluctuate greatly with averages of 18% ± 13%, 35% ± 18%, 31% ± 16%, and 16% ± 9%, respectively. Several modes of exchange flows through the multiple inlets are found, consisting of the all-in and all-out mode (45% occurrence) under strong winds perpendicular to the coastline; the shallow-downwind, deep-upwind mode (41%), particularly during wind-relaxation periods; and the upwind-in and downwind-out mode (13%) under northerly or southerly winds. These modes are discussed with the low-pass filtered model results and verified by a forcing–response joint EOF analysis.

Waves on Louisiana Continental Shelf Influenced by Atmospheric Fronts

“…The passage of cold front affects the main local weather pattern [39][40][41] . Many meteorological and oceanic phenomena are associated with cold front passages, such www.nature.com/scientificreports www.nature.com/scientificreports/ as wind and current variations, significant decrease in humidity and temperature, water level variation, flushing of bays 9 , and resuspension of sediment and transport of water and sediment [42][43][44][45] . Due to different configurations of the cold front, the distribution of cloud and precipitation near the cold front is also obviously different, some mainly occur behind the front, others mainly in front.…”

“…In winter and spring, however, the majority of the weather variability is caused by the wind associated with cold fronts or cold air outbreaks. According to the analysis of wind data, the front usually has the southwest-northeast orientation and plays an important role in controlling the coastal processes 9 , giving the east-west orientation of the northern Gulf of Mexico coast 46 .…”

“…The north wind is primarily after the passage of the front along the coastal Louisiana continental shelf. The southerly moving cold fronts produce strong winds (onshore wind) by which the higher amplitude waves are produced and water setup over the coast 9 . After the water setup over the coast, wind changes rapidly from the north (offshore wind) leading to water level set down, which drains the shallow bay area 9,28 .…”

“…The southerly moving cold fronts produce strong winds (onshore wind) by which the higher amplitude waves are produced and water setup over the coast 9 . After the water setup over the coast, wind changes rapidly from the north (offshore wind) leading to water level set down, which drains the shallow bay area 9,28 . The associated water setup and set down during frontal passage, when the wind quickly switched from south to north, have significant implications to Louisiana's bays and coastlines.…”

“…Although the northern Gulf of Mexico is located in a relatively low tidal and wave energy zone, the trend of the sedimentation and erosion in the low-lying plains of Louisiana Delta are strongly affected by coastal processes, of which subsidence and the effects of frequent winter-spring cold fronts and occasional tropical cyclones are significant 5 . A large number of ocean phenomena are associated with the passage of cold fronts, when temperature and humidity decrease significantly while wind switches to relatively strong northerly, resulting in flushing of bays, and sediment resuspension and transport [6][7][8][9] . Wind-driven coastal surge is an important contributor to the regional flooding, although high water levels are also influenced by tides, riverine currents, atmospheric pressure, wave and rainfall.…”