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.
Accepted December 2017.
Published, January 2018.
Citation:
Woodgate, R.A., 2018, 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, Progress in
Oceanography, 160, 124-154, doi:10.1016/j.pocean.2017.12.007.
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
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: 1.2 +- 0.1Sv) is 70% higher
record-length minimum (2001: 0.7 +- 0.1Sv), 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 (0.03 +- 0.02degC/yr; 0.01 +-
0.01psu/yr) are only just significant) are large (0.06 +- 0.05x1020J/yr;
30 +- 20km3yr; 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 0.0
+- 0.2degC, 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 (0.9 +-
0.8days/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 (1.0 +- 0.05Sv) 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.
