Frontal Dynamics of a Buoyancy-Driven Coastal Current: Quantifying Buoyancy, Wind, and Isopycnal Tilting Influence on the Nova Scotia Current
Document Type Article
The focus of this study is on the relative roles of winds and buoyancy in driving the Nova Scotia Current (NSC) utilizing detailed hydrographic glider transects along the Halifax Line. We define a Hydrographic Wind Index (HWI) using a simplistic two-layer model to represent the NSC and its frontal system. The HWI is based on local characteristics of the density front extracted from the glider data (e.g., frontal slope). The impact of wind-driven isopycnal tilting on the frontal slope is estimated and corrected for to accurately scale the buoyancy-driven component of the NSC. Observations from independent current profilers deployed across the NSC confirm that the HWI captures the low-frequency variability of the NSC. The monthly wind-driven flow is estimated to represent between 1.0% (±0.1%) and 48% (±1%) of the total alongshore currents, with a yearly mean of about 36% (±1%). We demonstrate that using local conditions is more appropriate to the study of buoyancy-driven currents ranging over distances on the order of O (100 km), compared to the traditional approach based on upstream conditions. Contrary to the traditional approach, the HWI is not affected by the advective time lag associated with the downshelf propagation of the buoyant water coming from the upstream source. However, the HWI approach requires high-resolution data sets, as errors on the estimates of the buoyancy- and wind-driven flows become large as the sampling resolution decreases. Despite being data intensive, we argue that the HWI is also applicable to multisource currents, where upstream conditions are difficult to define.