Tracing the Imprint of River Runoff Variability on Arctic Water Mass Transformation

Abstract

The Arctic Ocean receives a net freshwater input from land and from the atmosphere. This flux of freshwater, along with net surface heat loss, acts to transform the water mass properties of inflowing Atlantic and Pacific waters. In this study, model simulations are used to quantify the Arctic water mass transformation in salinity and temperature space, and its explained variance due to variability in the largest freshwater source to the Arctic: river runoff. This explained variance is quantified using a novel tool, the seasonal climate response function, which describes the magnitude and time scale of adjustment to a runoff perturbation at monthly resolution. Using this method, the transient response of Arctic water mass transformation is reconstructed over time scales ranging from several months to a decade. Model simulations with variable runoff indicate a significant explained model variance of several terms contributing to salinity transformation, including diffusion, the formation and melt of sea ice, and a possibly model-dependent surface salinity-restoring term. Most notably, an increase in river runoff strengthens the diffusion of salt and heat, which ultimately leads to an increase in the advective salt and heat import into the Arctic. These results provide evidence for the potential predictability of the Arctic system based on variability in river runoff. Plain Language Summary The outflow of rivers into the Arctic Ocean varies greatly fromyear to year and has a large impact on the waters, which are exchanged between the Arctic and the surrounding oceans. In this study, we use a numerical model to understand how variability in river outflow affects processes in the Arctic, such as the formation and melt of sea ice, and on what time scales. To describe the impact of river outflow on time scales from months to a decade, we present a simple mathematical tool that is compared to simulations from the numerical model. We find that this tool can be used to explain, and possibly predict, variations in Arctic processes based on knowledge of variations in river outflow. Interestingly, we find that an increase in river outflow increases the transport of heat to the Arctic and reduces the sea ice extent.

Introduction to Special Collection on Arctic Ocean Modeling and Observational Synthesis (FAMOS) 2: Beaufort Gyre Phenomenon

“…Other outstanding issues related to Beaufort Gyre freshwater content, which have required modeling approaches, relate to understanding the role of meteoric water (river runoff plus net precipitation; see Lambert et al, 2019, and, and Kelly, Proshutinsky, Popova.et al, (2018), in this special collection) in the observed freshwater content changes, mechanisms for Pacific water transport into the Beaufort Gyre and better understanding of the role of eddies in Beaufort Gyre stabilization (Manucharyan & Isachsen, 2019;, in this special collection).…”

“…The imprint of river runoff variability is examined by Lambert et al (2019) and Brown et al (2019) in this special collection. Grivault et al (2018) evaluate the water transport and volume of fresh water flowing through the straits of the Canadian Arctic Archipelago.…”

Changing freshwater contributions to the Arctic

“…Pemberton and Nilsson (2016) performed runoff sensitivity experiments in an ocean/sea-ice model that enhanced runoff and shallowed the halocline, while also modifying the Atlantic water inflow. Lambert et al (2019) used a climate response function approach to show the diffusion of heat and salt in the Arctic increased under enhanced runoff, eventually leading to enhanced advective imports of salt and heat into the Arctic.…”

Seasonal Variation of Legacy Organochlorine Pesticides (OCPs) From East Asia to the Arctic Ocean

“…For example, ocean currents passing through the Bering Strait make a significant contribution to the input of OCPs to the Arctic Ocean, particularly for β‐hexachlorocyclohexane (β‐HCH) (Li et al, 2002). River runoff carries land‐based OCPs from soils, dry or wet absorption, and snow and ice meltwater into the large Arctic drainage basin, which arguably is mostly the result of prior LRAT (Cai et al, 2012; Lambert et al, 2019; Ma et al, 2018). OCPs from atmospheric, oceanic, and riverine sources thus converge in the Arctic Ocean (Bidleman et al, 2015; Cabrerizo et al, 2018; Hung et al, 2016), with Arctic regions acting as reserves and secondary sources of OCPs (Ma et al, 2016).…”