They Came From the Pacific: How Changing Arctic Currents Could Contribute to an Ecological Regime Shift in the Atlantic Ocean

Abstract

The Arctic Ocean is rapidly changing. With warming waters, receding sea ice, and changing circulation patterns, it has been hypothesized that previously closed ecological pathways between the Pacific and Atlantic Oceans will be opened as we move toward a seasonally ice-free Arctic. The discovery of the Pacific diatom Neodenticula seminae in the Atlantic suggests that a tipping point may have already been reached and this "opening up" of the Arctic could already be underway. Here, we investigate how circulation connectivity between the Pacific and Atlantic Oceans has changed in recent decades, using a state-of-the-art high-resolution ocean model and a Lagrangian particle-tracking method. We identify four main trans-Arctic pathways and a fifth route that is sporadically available with a shorter connectivity timescale. We discuss potential explanations for the existence of this "shortcut" advective pathway, linking it to a shift in atmospheric and oceanic circulation regimes. Advective timescales associated with each route are quantified, and seasonal and interannual trends in the main four pathways are discussed, including an increase in Fram Strait outflow relative to the Canadian Archipelago. In conclusion, we note that while tipping points for ecological connectivity are species dependent, even the most direct routes require multiannual connectivity timescales.Plain Language Summary With a warming Arctic Ocean, it has been suggested that the ocean currents that connect the Pacific to the Atlantic may change. This could have potential biological consequences, including bringing Pacific species of plankton to the Atlantic. We investigate how the pathways bringing Pacific water to the Atlantic have changed, identify a pathway that takes less time that other routes to bring waters from Pacific to the Atlantic (but that is only occasionally available), and note that even the shortest timescales are over 2 years.

Significant variability of structure and predictability of Arctic Ocean surface pathways affects basin-wide connectivity

“…There are two surface velocity datasets and two-particle tracking algorithms used in this paper. The majority of the paper is based on surface velocity simulated with a global 1/12°resolution ocean-sea ice dynamical model, NEMO (Nucleus for European Modelling of the Ocean), forced by atmospheric reanalysis, with offline particle tracking using ARIANE, which has been used extensively with NEMO model data [48][49][50][51][52][53][54][55][56][57][58][59] . Supplementary Fig.…”

“…The global model simulation, identified as ORCA0083-N06, has been used for many other studies 50,[66][67][68] , and specifically for the Arctic 69 , where it has also been validated against observations [48][49][50] . The model is forced by Drakkar Forcing Set 5.2, for the full model run covering 1958-2015 70 .…”

“…The efficiency comes from employing an analytical solution for the trajectory within each model grid cell, based on the discretised form of the volume conservation equation for that grid structure 72,73 . Here, we follow the approach of previous studies [48][49][50] and use ARIANE on the 5-day-mean output of NEMO/ORCA0083-N06. Note that all the explicit subgrid-scale diffusive parameterisations cited in LIM2 and OPA contribute to the 5-day-mean velocity but that ARIANE is purely advective and does not add any additional subgrid-scale effects, such as simulated diffusion.…”

Hotspots of Dense Water Cascading in the Arctic Ocean: Implications for the Pacific Water Pathways

“…Both the global and regional model configurations are extensively validated in the Arctic against observations of hydrography, satellite dynamical topography and ocean circulation, and sea ice; see Luneva et al (2015), Aksenov et al (2016), Armitage et al (2016), and Kelly et al (2018, 2020) and Supporting Information S1.…”

“…The experiment design is the same as that used by Kelly et al (2018). While having the advantage of being computationally fast with a good level of numerical accuracy in representing pathways of the water masses, the off-line particle tracking methods nevertheless do not include vertical mixing (see, e.g., Kelly et al, 2020, for further discussion and references). However, using the full 3-D velocity field allows us to track details of the mean vertical motion, which are key for the present study.…”