A regional comparison between the seasonal cycles of the Western Irish Shelf and the Georges Bank, Gulf of Maine ecosystems: The impact of physical variability

The seasonal response of two shelf ecosystems in the North Atlantic to fluctuations in climate variability is investigated. The Western Irish Shelf and the Georges Bank/Gulf of Maine are highly productive shelf systems (>300 gC m-2 y-1) located at mid-latitudes between the subpolar and subtropical gyres in the North Atlantic, making them interesting oceanographic transition zones where different water masses, flora and fauna mix. Their boundaries expand, contract and shift position in response to fluctuations in climate variability. In this study, positive and negative NAO scenarios are used as a proxy for climate variability in these regions. A three-dimensional circulation and ecosystem model is adapted for the Western Irish Shelf, and the Spring circulation and bloom dynamics are diagnosed for idealized NAO scenarios. The flow regime and modeled dynamics of the Winter/Spring bloom for the Western Irish Shelf are compared with previous work for the Georges Bank/Gulf of Maine region.; Modeled circulation of the Western Irish Shelf reveals recirculation features over the Porcupine Bank which may be driven by Taylor column physics and are sensitive to variability in mean flow. Analysis of time and length scales for advection highlight that two distinct flow regimes exist over the Porcupine Bank: in the southern Porcupine Bank, advective scales are much larger than Georges Bank, nonlinearity is not important, and the flow can be treated as linear; in the northern Porcupine Bank, advective scales are small and are comparable with Georges Bank, and nonlinearity is important. Taylor column flow formation over the Porcupine Bank in Spring may contribute to isolating nutrient rich water and enabling localized growth of phytoplankton. Variability in physical forcing influences the timing and intensity of the modeled Spring phytoplankton bloom. During a negative NAO period, weaker, variable springtime winds, increased light and warmer water temperatures, contribute to the mixed layer shoaling earlier in the year, and the onset of stratification, supporting favorable conditions for photosynthesis to take place and an earlier Spring bloom. During a positive NAO period, stronger, westerly springtime winds, reduced light and cooler water temperatures, delay shoaling of the mixed layer and the onset of stratification, leading to a later and less intense Spring bloom.