Response to Reviewer #1

Abstract

The circulation in the northwestern North Atlantic Ocean is highly complex, characterized by the confluence of two major western boundary current systems and several shelf currents. Here we present the first comprehensive analysis of transport paths and timescales for the northwestern North Atlantic shelf, which is useful for estimating ventilation rates, describing circulation and mixing, characterizing the composition of water masses with respect to different source regions, and elucidating rates and patterns of biogeochemical processing, species dispersal, and genetic connectivity. Our analysis uses dye and age tracers within a highresolution circulation model of the region, divided into nine subregions, to diagnose retention times, transport pathways, and transit times. Retention times are shortest on the Scotian Shelf (∼ 3 months), where the inshore and shelf-break branches of the coastal current system result in high alongshelf transport to the southwest, and on the Grand Banks (∼ 3 months). Larger retention times are simulated in the Gulf of St. Lawrence (∼ 12 months) and the Gulf of Maine (∼ 6 months). Source water analysis shows that Scotian Shelf water is primarily comprised of waters from the Grand Banks and Gulf of St. Lawrence, with varying composition across the shelf. Contributions from the Gulf of St. Lawrence are larger at near-shore locations, whereas locations near the shelf break have larger contributions from the Grand Banks and slope waters. Waters from the deep slope have little connectivity with the shelf, because the shelf-break current inhibits transport across the shelf break. Grand Banks and Gulf of St. Lawrence waters are therefore dominant controls on biogeochemical properties, and on setting and sustaining planktonic communities on the Scotian Shelf.

A Numerical Model Analysis of the Mean and Seasonal Nitrogen Budget on the Northeast U.S. Shelf

“…The total yearly influx of~211 Gmols N year −1 is comparable in magnitude to the GoM/GB TN inventory (205 Gmols N), suggesting a turnover time of the nitrogen pools of~1 year. This falls into the range of studies by Rutherford and Fennel (2018), which reported the mean residence time of 251 days in the GoM, and the observed 1-to 2-year transit time of slope water anomalies through the deep basins of the GoM by Hopkins and Garfield (1979). Inflows are balanced primarily by net outflows near the Northeast Channel, the southern flank of GB, and across the Nantucket transect.…”