Analysis of the Beaufort Gyre Freshwater Content in 2003–2018

Abstract

Hydrographic data collected from research cruises, bottom-anchored moorings, driftingIce-Tethered Profilers, and satellite altimetry in the Beaufort Gyre region of the Arctic Ocean document an increase of more than 6,400 km 3 of liquid freshwater content from 2003 to 2018: a 40% growth relative to the climatology of the 1970s. This fresh water accumulation is shown to result from persistent anticyclonic atmospheric wind forcing accompanied by sea ice melt, a wind-forced redirection of Mackenzie River discharge from predominantly eastward to westward flow, and a contribution of low salinity waters of Pacific Ocean origin via Bering Strait. Despite significant uncertainties in the different observations, this study has demonstrated the synergistic value of having multiple diverse datasets to obtain a more comprehensive understanding of Beaufort Gyre freshwater content variability. For example, Beaufort Gyre Observational System (BGOS) surveys clearly show the interannual increase in freshwater content, but without satellite or Ice-Tethered Profiler measurements, it is not possible to resolve the seasonal cycle of freshwater content, which in fact is larger than the year-to-year variability, or the more subtle interannual variations. Plain Language AbstractThe Beaufort Gyre centered in the Canada Basin of the Arctic Ocean is the major reservoir of fresh water in the Arctic. The primary focus of this study is on quantifying variability and trends in liquid (water) and solid (sea ice) freshwater content in this region. The Beaufort Gyre Exploration Program was initiated in 2003 to synthesize results of historical data analysis, design and conduct long-term observations, and to provide information for numerical modeling under the umbrella of the FAMOS (Forum for Arctic Observing and Modeling Synthesis) project. The data collected from research cruises, moorings, Ice-Tethered Profiler observations, and satellite altimetry document an increase of more than 6,400 km 3 of liquid freshwater content from 2003 to 2018, a 40% growth relative to the climatology of the 1970s. This fresh water volume is comparable to the fresh water volume released to the sub-arctic seas during the Great Salinity Anomaly episode of the 1970s. Thus, since the 2000s, the stage has been set for another possible release of fresh water to lower latitudes with accompanying climate impacts, including changes to sea ice conditions, ocean circulation, and ecosystems of the Sub-Arctic similar to the influence of the Great Salinity Anomaly observed in the 1970s.

Understanding Arctic Ocean Circulation: A Review of Ocean Dynamics in a Changing Climate

“…For example, the accumulation of fresh water requires the availability of fresh water (e.g., sea‐ice melt water or river influxes) to coincide with atmospheric forcing that drives Ekman convergence in the surface ocean layer. Proshutinsky et al () show that the dominant contributions to recent freshwater accumulation in the Beaufort Gyre have been Pacific Water inflows through Bering Strait and fresh water from the Mackenzie River; changes to either could yield changes in Beaufort Gyre freshwater content even while the atmospheric forcing remains the same. We revisit changes in Beaufort Gyre fresh water in section .…”

“…Between 1992 and 2012 Arctic Ocean total freshwater content (integrated fresh water relative to a salinity of 34.8) increased at a rate of around 600±300 km 3 year −1 ; about two thirds has been attributed to salinity decreases, with the remainder a result of a thickening of the freshwater layer (Carmack et al, ; Haine et al, ; Rabe et al, ). The most comprehensive in situ hydrographic measurements are from the Beaufort Gyre region where observations indicate an overall increase in total freshwater content by almost 40% since the 1970s (from around 17×10 3 km 3 to 23.5×10 3 km 3 in 2018) (Proshutinsky et al, , ). Such increases are associated with the strengthening of the Beaufort Gyre responding to anticyclonic wind forcing over the Canadian Basin, freshwater accumulation from sea‐ice melt, increasing freshwater flux through Bering Strait, and greater influence of Mackenzie River water (Krishfield et al, ; Proshutinsky et al, , ).…”

“…The most comprehensive in situ hydrographic measurements are from the Beaufort Gyre region where observations indicate an overall increase in total freshwater content by almost 40% since the 1970s (from around 17×10 3 km 3 to 23.5×10 3 km 3 in 2018) (Proshutinsky et al, , ). Such increases are associated with the strengthening of the Beaufort Gyre responding to anticyclonic wind forcing over the Canadian Basin, freshwater accumulation from sea‐ice melt, increasing freshwater flux through Bering Strait, and greater influence of Mackenzie River water (Krishfield et al, ; Proshutinsky et al, , ).…”

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

“…The physical changes that occurred in 1948-2013 were examined by Proshutinsky et al (2015), and the analysis was extended to recent years in this special collection (Proshutinsky, Krishfield, Toole, et al, 2019). Proshutinsky, Krishfield, Toole, et al (2019) show that the last two decades of sustained anticyclonic wind forcing in the region (1997-2018) differ significantly from the climatology shown in Figure 2. In addition, it was found that seasonal change in Beaufort Gyre total freshwater content ranges from 5 to 10% of the total freshwater content (depending on the intensity and sense of the atmospheric circulation); interannual changes can amount to as much as 1,500 km 3 (around 8% of the total on average); decadal changes were approximately 1,000 km 3 per decade prior to 2000; an accelerated rate of increase of about 4,000 km 3 per decade characterizes the period since 2003; during 2003-2008, the Beaufort Gyre geostrophic circulation has intensified (Armitage et al, 2016(Armitage et al, , 2017McPhee, 2013;McPhee et al, 2009); and the baroclinic component of the Transpolar Drift current intensified and shifted toward Canada, accelerating sea-ice drift.…”

Freshening of the western Arctic negates anthropogenic carbon uptake potential

“…Changes in net precipitation/evaporation affect TA through dilution/concentration, but such changes tend to be very localized, and can easily be accounted for by normalizing TA to salinity (Woosley et al 2016). Increases in other freshwater inputs such as river runoff and sea-ice melt would dilute TA, and both are known to be increasing in the western Arctic (Jones et al 2008;Yamamoto-Kawai et al 2009;Newton et al 2013;Carmack et al 2016;Proshutinsky et al 2019;Zhong et al 2019). Surface ocean waters entering the Arctic from the Pacific have a TA around 2200 μmol kg −1 , and deeper Atlantic derived waters are~2300 μmol kg −1 .…”