Summary of Woodgate 2017

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

Rebecca A Woodgate

Applied Physics Laboratory, University of Washington

Submitted to Progress in Oceanography, June 2017


Highlights   Abstract     Figures     Tables
Paper (downloadable)


HIGHLIGHTS
- The Bering Strait inflow to the Arctic increased from 2001 (~0.7Sv) to 2014 (~1.2Sv)
- This is due to increasing far-field, pressure-head forcing, not local wind changes
- Concurrently heat and freshwater fluxes strongly increased (3-5x1020J, 2300-3500km3)
- Seasonal data show winter freshening, pre-summer warming, summer/fall flow increase
- We present a new climatology (1Sv) for the strait, including seasonality for heat and freshwater



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Polar Programs  PLR-1304052
Part of  the AON (Arctic Observing Network)






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Please contact Rebecca Woodgate (woodgate@apl.washington.edu) for use of any of this material

Abstract  

Year-round in situ Bering Strait mooring data (1990-2015) document a long-term increase (~0.01Sv/yr whole record, ~0.02Sv since 2000) in the annual mean transport of Pacific waters into the Arctic.  Between 2002 and present (2015), all annual mean transports (except 2005 and 2012) are greater than the previously accepted climatology (~0.8Sv).  The record-length maximum (2014: 1.20.1Sv) is 70% higher record-length minimum (2001: 0.70.1Sv), corresponding to a ~1/4year reduction in the flushing time of the Chukchi Sea (to ~4.5months from ~7.5months).  The transport increase results from stronger northward flows (not fewer southward flow events); the velocity distribution's annual mode ranges from <25cm/s to >40cm/s, a 60% increase in speed and a 150% increase in kinetic energy, a metric which scales with the flow's impacts on bottom suspension, mixing and erosion. 

            Record-length trends in annual mean heat and freshwater fluxes (primarily driven by volume flux trends, since warming/freshening trends (0.030.02degC/yr; 0.010.01psu/yr) are only just significant) are large (0.060.05x1020J/yr; 3020km3yr; relative to -1.9degC and 34.8psu), with heat flux lows in 2001 and 2012 (~3x1020J) and highs in 2007 and 2015 (~5.5 x1020J), and a freshwater range of ~2300km3 (2001) to ~3500km3 (2014).  High-flow year 2015 (~1.1Sv) has the highest annual mean temperature recorded, ~0.7degC, astoundingly warmer than the record-length mean of 0.00.2degC, while low-flow year 2012 (~0.8Sv) is also remarkably cold (~-0.6degC), likely due to anomalously weak northward flow in January-March, partly driven by anomalously strong southward winds in March.

            A seasonal decomposition shows significant freshening in winter (~0.03psu/yr January-March) likely due to sea-ice changes, but no trend (or perhaps salinization) in the rest of the year, consistent with the Alaskan Coastal Current (ACC) which shows no significant trends of warming, freshening or flow increase in the available data (2002-2015).  A seasonal warming trend in the strait proper in May and June (~0.04degC/yr) is reflected in a trend to earlier arrival (0.90.8days/yr) of waters warmer than 0degC.  Contrastingly, no significant trend is found in the time of cooling of the strait.  The strait's seasonal increasing transport trends (~0.02Sv/yr) are largest from May-November, likely due to the large wind-driven variability in other months masking the signal. 

            A correlation analysis is used to separate the flow into portions driven by (a) the local wind and (b) a far-field (Pacific Arctic "pressure-head") forcing.  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.  Over the 25 years of data, we find much variability, but no significant trend in the wind or the wind-driven component.  Most notably, however, we find the increase in the Bering Strait throughflow is due to a strong increase in the pressure-head forcing of the flow, consistent through most of the year, reflecting the naturally longer timescales of the far-field forcing of the flow. 

            Considering 2003-2015 data, we propose a higher annual mean transport for the strait (1.00.05Sv) based on recent flow increases (not methodology changes) 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, both of which are still poorly quantified.  We estimate a maximum seasonal range of heat and freshwater fluxes as 0-40TW and 0.05-0.14Sv. 

            Finally we consider the predictability of the throughflow properties and future measurement requirements for the strait, concluding 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. 

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Polar Science Center, University of Washington, 2017

Figures
  For details, see paper

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Tables
  For details, see paper
Table 1
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Table 2
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Polar Science Center, University of Washington, 2017

We gratefully acknowledge financial support for this work from the National Science Foundation (NSF).

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