Circulation of Pacific Winter Water in the Western Arctic Ocean

Abstract

Pacific Winter Water (PWW) enters the western Arctic Ocean from the Chukchi Sea; however, the physical mechanisms that regulate its circulation within the deep basin are still not clear. Here, we investigate the interannual variability of PWW with a comprehensive data set over a decade. We quantify the thickening and expansion of the PWW layer during 2002–2016, as well as its changing pathway. The total volume of PWW in the Beaufort Gyre (BG) region is estimated to have increased from 3.48 ± 0.04 × 1014 m3 during 2002–2006 to 4.11 ± 0.02 × 1014 m3 during 2011–2016, an increase of 18%. We find that the deepening rate of the lower bound of PWW is almost double that of its upper bound in the northern Canada Basin, a result of lateral flux convergence of PWW (via lateral advection of PWW from the Chukchi Borderland) in addition to the Ekman pumping. In particular, of the 70‐m deepening of PWW at its lower bound observed over 2003–2011 in the northwestern basin, 43% resulted from lateral flux convergence. We also find a redistribution of PWW in recent years toward the Chukchi Borderland associated with the wind‐driven spin‐up and westward shift of the BG. Finally, we hypothesize that a recently observed increase of lower halocline eddies in the BG might be explained by this redistribution, through a compression mechanism over the Chukchi Borderland.

Analysis of the Beaufort Gyre Freshwater Content in 2003–2018

“…This explanation is in agreement with seasonal changes of freshwater content below 65 m observed in the mooring data where maximum freshwater content was observed in January–February and minimum in June–July when wind‐driven downwelling in the southwest Canada Basin/Chukchi Sea is replaced by upwelling. Zhong et al (, this special issue) investigated changes in the Pacific winter water located between the Pacific summer and Atlantic water layers and showed that under the influence of Ekman pumping and lateral advection, the thickness and freshwater content of this layer increased by approximately 18% over the years 2002–2016.…”

“…This explanation is in agreement with seasonal changes of freshwater content below 65 m observed in the mooring data where maximum freshwater content was observed in January-February and minimum in June-July when wind-driven downwelling in the southwest Canada Basin/Chukchi Sea is replaced by upwelling. Zhong et al (2019, (Steele et al, 2001;Serreze et al, 2006;Timokhov & Tanis, 1997. Instead, the observations indicated that over 2003-2008, there were two seasonal maxima in freshwater content; one in June-July when sea ice thickness reached its minimum (maximum ice melt) and the second in November-January when Ekman pumping was strongest and salt input from ice growth had not yet reached its maximum.…”

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

“…Kelly, Popova, Aksenov, et al, () employ the use of tracers in the Coupled Model Intercomparison Project Phase 5 (CMIP5) model to ascertain Arctic Ocean pathways of pollutants from Siberian Rivers. Thickening, expansion, and redistribution of the Pacific Winter Water layer are quantified by Zhong et al (). Shu et al () investigate future changes of freshwater content under influence of different factors including circulation changes in the future.…”

“…Aksenov et al, 2016;Jones, 2001;McLaughlin et al, 2002;Steele et al, 2004;Timmermans et al, 2014). In this special collection, Spall et al (2018), Zhong et al (2019), Hirano et al (2018), and Hu and Myers (2019) discuss the most recent findings associated with the circulation of the Pacific water layers and factors influencing their dynamics and properties.…”

Deepening of the Winter Mixed Layer in the Canada Basin, Arctic Ocean Over 2006–2017

“…Overall, it is not possible, from our ITP data, to conclude whether the strength of vertical mixing at the mixed layer base changed over 2006-2017. Beyond the conclusion that vertical mixing is not the sole cause of increased mixed layer salinity, the exact mechanism can only be speculated on from these observations. It is possible that the stabilization of the Beaufort Gyre documented in recent studies led to increased mixed layer salinities via a reduced Ekman convergence of freshwater (Dewey et al, 2018;Meneghello et al, 2018;Zhang et al, 2016;Zhong et al, 2018), which made it easier for the "background" level of mechanical mixing to deepen the mixed layer base. Alternatively, the increased mixed layer salinity could have resulted from a net export of freshwater out of the Beaufort Gyre, implying that the Arctic Ocean could increase freshwater export to the North Atlantic should these conditions continue.…”