Increases in the Pacific inflow to the Arctic from 1990 to 2015, and insights into seasonal trends and driving mechanisms from year-round Bering Strait mooring data

Abstract Year-round in situ Bering Strait mooring data (1990–2015) document a long-term increase (∼0.01 Sv/yr) in the annual mean transport of Pacific waters into the Arctic. Between 2002 and 2015, all annual mean transports (except 2005 and 2012) are greater than the previously accepted climatology (∼0.8 Sv). The record-length maximum (2014: 1.2 ± 0.1 Sv) is 70% higher than the record-length minimum (2001: 0.7 ± 0.1 Sv), corresponding to a reduction in the flushing time of the Chukchi Sea (to ∼4.5 months from ∼7.5 months). The transport increase results from stronger northward flows (not fewer southward flow events), yielding a 150% increase in kinetic energy, presumably with impacts on bottom suspension, mixing, and erosion. Curiously, we find no significant trends in annual mean flow in the Alaskan Coastal Current (ACC), although note that these data are only available 2002–2015. Record-length trends in annually integrated heat and freshwater fluxes (primarily driven by volume flux trends) are large (0.06 ± 0.05 × 1020 J/yr; 30 ± 20 km3/yr; relative to −1.9 °C and 34.8 psu), with heat flux lows in 2001 and 2012 (∼3 × 1020 J) and highs in 2007 and 2015 (∼5.5 × 1020 J), and a freshwater range of ∼2300 km3 (2001) to ∼3500 km3 (2014). High-flow year 2015 (volume transport ∼1.1 Sv) has the highest annual mean temperature recorded, ∼0.7 °C, astoundingly warmer than the record-length mean of 0.0 ± 0.2 °C, while low-flow year 2012 (∼0.8 Sv) is also remarkably cold (∼−0.6 °C), likely due to anomalously weak northward flow in January–March, partly driven by anomalously strong southward winds in March. A seasonal decomposition of properties of the main flow shows significant freshening in winter (∼0.03 psu/yr, January–March) likely due to sea-ice changes, but no trend (or perhaps salinization) in the rest of the year. A seasonal warming trend in the strait proper in May and June (∼0.04 °C/yr) is reflected in a trend to earlier arrival (0.9 ± 0.8 days/yr) of waters warmer than 0 °C. Contrastingly, no significant trend is found in the time of cooling of the strait. The strait’s seasonal increasing transport trends (∼0.02 Sv/yr) are largest from May–November, likely due to the large wind-driven variability masking the signal in other months. We show that Ekman set-up of waters along the coast in the strait can explain the strong correlation of the water velocity with along-strait winds (as opposed to across-strait winds). We highlight the strong seasonality of this relationship (r ∼ 0.8 in winter, but only ∼0.4 in summer), which reflects the weak influence of the (seasonally weak) winds in summer. By separating the flow into portions driven by (a) the local wind and (b) a far-field (Pacific-Arctic “pressure-head”) forcing, we find the increase in the Bering Strait throughflow is primarily due to a strong increase in the far-field forcing, not changes in the wind. We propose a higher annual mean transport for the strait for the 2000s, (1.0 ± 0.05 Sv) based on recent flow increases, and present estimated seasonal climatologies for properties and fluxes for the strait and for the ACC. Heat and freshwater seasonalities are strongly influenced by the ACC and stratification. Finally we conclude that year-round in situ mooring are still the only currently viable way of obtaining accurate quantifications of the properties of the Pacific input to the Arctic.

[1]  E. Fahrbach,et al.  Direct measurements of volume transports through Fram Strait , 2001 .

[2]  Thomas J. Weingartner,et al.  The Siberian Coastal Current: A wind‐ and buoyancy‐forced Arctic coastal current , 1999 .

[3]  S. Bacon,et al.  Export of nutrients from the Arctic Ocean , 2013 .

[4]  Richard B. Lammers,et al.  Record Russian river discharge in 2007 and the limits of analysis , 2009 .

[5]  K. Aagaard,et al.  ON THE WATER EXCHANGE THROUGH BERING STRAIT1 , 1966 .

[6]  R. Woodgate,et al.  The 2007 Bering Strait oceanic heat flux and anomalous Arctic sea‐ice retreat , 2010 .

[7]  J. Overland,et al.  Northward flow in the Bering and Chukchi seas , 1987 .

[8]  R. B. Tripp,et al.  Bering Strait the Regional Physical Oceanography , 1975 .

[9]  E. Carmack,et al.  The role of sea ice and other fresh water in the Arctic circulation , 1989 .

[10]  Jinlun Zhang,et al.  Circulation on the central Bering Sea shelf, July 2008 to July 2010 , 2012 .

[11]  J. Grebmeier,et al.  The Pacific Arctic Region: Ecosystem Status and Trends in a Rapidly Changing Environment , 2014 .

[12]  K. Aagaard,et al.  Toward new mass and heat budgets for the Arctic Ocean , 1975 .

[13]  E. Carmack,et al.  Pacific Ocean inflow: Influence on catastrophic reduction of sea ice cover in the Arctic Ocean , 2006 .

[14]  John Woodward,et al.  New grid of Arctic bathymetry aids scientists and mapmakers , 2000 .

[15]  Axel Schweiger,et al.  Evaluation of Seven Different Atmospheric Reanalysis Products in the Arctic , 2014 .

[16]  Thorsten Markus,et al.  Changes in Arctic melt season and implications for sea ice loss , 2014 .

[17]  P. Stabeno,et al.  Flow Patterns in the Eastern Chukchi Sea: 2010–2015 , 2018 .

[18]  M. Jakobsson.  Hypsometry and volume of the Arctic Ocean and its constituent seas , 2002 .

[19]  Aixue Hu,et al.  Role of the Bering Strait in the thermohaline circulation and abrupt climate change , 2007 .

[20]  E. Carmack,et al.  Varieties of shallow temperature maximum waters in the Western Canadian Basin of the Arctic Ocean , 2001 .

[21]  G. Garrison,et al.  Satellite and Oceanographic Observations of the Warm Coastal Current in the Chukchi Sea , 1984 .

[22]  J. Toole,et al.  Mechanisms of Pacific Summer Water variability in the Arctic's Central Canada Basin , 2014 .

[23]  D. Nof,et al.  The Bering Strait's grip on the northern hemisphere climate , 2004 .

[24]  Craig M. Lee,et al.  An assessment of the Arctic Ocean in a suite of interannual CORE-II simulations. Part III: Hydrography and fluxes , 2016 .

[25]  G. Johnson,et al.  Some controls on flow and salinity in Bering Strait , 2006 .

[26]  P. Stabeno,et al.  Coupled wind-forced controls of the Bering–Chukchi shelf circulation and the Bering Strait throughflow: Ekman transport, continental shelf waves, and variations of the Pacific–Arctic sea surface height gradient , 2014 .

[27]  Jinlun Zhang,et al.  Sea Ice Response to Atmospheric and Oceanic Forcing in the Bering Sea , 2010 .

[28]  K. Aagaard,et al.  Revising the Bering Strait freshwater flux into the Arctic Ocean , 2005 .

[29]  Michael P. Meredith,et al.  The large‐scale freshwater cycle of the Arctic , 2006 .

[30]  E. Lewis,et al.  The practical salinity scale 1978 and its antecedents , 1980 .

[31]  Craig M. Lee,et al.  An assessment of the Arctic Ocean in a suite of interannual CORE-II simulations. Part I: Sea ice and solid freshwater , 2016 .

[32]  U. Schauer,et al.  Problems with estimation and interpretation of oceanic heat transport – conceptual remarks for the case of Fram Strait in the Arctic Ocean , 2009 .

[33]  E. Curchitser,et al.  Wind‐induced reorganization of the Bering shelf circulation , 2012 .

[34]  Cynthia Suzanne Travers.  Quantifying Sea-Ice Volume Flux using Moored Instrumentation in the Bering Strait , 2012 .

[35]  Thomas J. Weingartner,et al.  Observed increases in Bering Strait oceanic fluxes from the Pacific to the Arctic from 2001 to 2011 and their impacts on the Arctic Ocean water column , 2012 .

[36]  Thomas J. Weingartner,et al.  Direct measurements of transport and water properties through the Bering Strait , 1995 .

[37]  Thomas J. Weingartner,et al.  Monthly temperature, salinity, and transport variability of the Bering Strait through flow , 2005 .

[38]  Aixue Hu,et al.  Bering Strait throughflow and the thermohaline circulation , 2005 .

[39]  C. Garrett,et al.  Geostrophic Control of Fluctuating Barotropic Flow through Straits , 1984 .

[40]  A. Stigebrandt.  The North Pacific: A Global-Scale Estuary , 1984 .

[41]  M. Spall.  Circulation and water mass transformation in a model of the Chukchi Sea , 2007 .

[42]  E. Carmack,et al.  Beaufort Gyre freshwater reservoir: State and variability from observations , 2009 .

[43]  Thomas J. Weingartner,et al.  Interannual changes in the Bering Strait fluxes of volume, heat and freshwater between 1991 and 2004 , 2006 .

[44]  Julienne C. Stroeve,et al.  Variability, trends, and predictability of seasonal sea ice retreat and advance in the Chukchi Sea , 2016 .

[45]  Jinlun Zhang,et al.  On the Flow Through Bering Strait: A Synthesis of Model Results and Observations , 2014 .

[46]  F. Muller‐Karger,et al.  CO2 cycling in the coastal ocean. II. Seasonal organic loading of the Arctic Ocean from source waters in the Bering Sea , 1997 .

[47]  Jinlun Zhang,et al.  Seasonality and long‐term trend of Arctic Ocean surface stress in a model , 2014 .

[48]  J. Morison,et al.  Circulation of summer Pacific halocline water in the Arctic Ocean , 2004 .

[49]  J. Grebmeier,et al.  Shifting patterns of life in the Pacific Arctic and sub-Arctic seas. , 2012, Annual review of marine science.

[50]  Jinlun Zhang,et al.  Reconstruction and analysis of the Chukchi Sea circulation in 1990-1991 , 2010 .

[51]  Frank Kauker,et al.  Arctic Ocean basin liquid freshwater storage trend 1992–2012 , 2014 .

[52]  C. Pease.  Eastern Bering Sea Ice Processes , 1980 .

[53]  K. Aagaard,et al.  A year in the physical oceanography of the Chukchi Sea: Moored measurements from autumn 1990–1991 , 2005 .

[54]  D. Martinson,et al.  Role of the upper ocean in the energy budget of Arctic sea ice during SHEBA , 2009 .

[55]  K. Stafford,et al.  A Synthesis of Year-Round Interdisciplinary Mooring Measurements in the Bering Strait (1990-2014) and the RUSALCA Years (2004-2011) , 2015 .

[56]  R. Schmitt,et al.  Transport of freshwater by the oceans , 1992 .

[57]  E. Fahrbach,et al.  Arctic warming through the Fram Strait: Oceanic heat transport from 3 years of measurements , 2004 .

[58]  D. Nof,et al.  The Exhaust Valve of the North Atlantic , 2004 .

[59]  G. Bigg,et al.  Impact of flow through the Canadian Archipelago and Bering Strait on the North Atlantic and Arctic circulation: An ocean modelling study , 2002 .