Arctic Mediterranean exchanges: a consistent volume budget and trends in transports from two decades of observations
Authors: Østerhus Svein , Woodgate Rebecca , Valdimarsson Héðinn , Turrell Bill , de Steur Laura , Quadfasel Detlef , Olsen Steffen M. , Moritz Martin , Lee Craig M. , Larsen Karin Margretha H. , Jónsson Steingrímur , Johnson Clare , Jochumsen Kerstin , Hansen Bogi , Curry Beth , Cunningham Stuart , Berx Barbara 
Published: April 2019

Abstract. The Arctic Mediterranean (AM) is the collective name for
the Arctic Ocean, the Nordic Seas, and their adjacent shelf seas. Water enters into this
region through the Bering Strait (Pacific inflow) and through the passages across the
Greenland–Scotland Ridge (Atlantic inflow) and is modified within the AM. The modified
waters leave the AM in several flow branches which are grouped into two different
categories: (1) overflow of dense water through the deep passages across the
Greenland–Scotland Ridge, and (2) outflow of light water – here termed surface outflow
– on both sides of Greenland. These exchanges transport heat and salt into and out of
the AM and are important for conditions in the AM. They are also part of the global ocean
circulation and climate system. Attempts to quantify the transports by various methods
have been made for many years, but only recently the observational coverage has become
sufficiently complete to allow an integrated assessment of the AM exchanges based solely
on observations. In this study, we focus on the transport of water and have collected
data on volume transport for as many AM-exchange branches as possible between 1993 and
2015. The total AM import (oceanic inflows plus
freshwater) is found to be 9.1 Sv (sverdrup,
1 Sv =106 m3 s−1) with an estimated uncertainty of 0.7 Sv and has
the amplitude of the seasonal variation close to 1 Sv and maximum import in October.
Roughly one-third of the imported water leaves the AM as surface outflow with the
remaining two-thirds leaving as overflow. The overflow water is mainly produced from
modified Atlantic inflow and around 70 % of the total Atlantic inflow is converted
into overflow, indicating a strong coupling between these two exchanges. The surface
outflow is fed from the Pacific inflow and freshwater (runoff and precipitation), but is
still approximately two-thirds of modified Atlantic water. For the inflow
branches and the two main overflow branches (Denmark Strait and Faroe Bank Channel),
systematic monitoring of volume transport has been established since the mid-1990s, and
this enables us to estimate trends for the AM exchanges as a whole. At the 95 %
confidence level, only the inflow of Pacific water through the Bering Strait showed a
statistically significant trend, which was positive. Both the total AM inflow and the
combined transport of the two main overflow branches also showed trends consistent with
strengthening, but they were not statistically significant. They do suggest, however,
that any significant weakening of these flows during the last two decades is unlikely and
the overall message is that the AM exchanges remained remarkably stable in the period
from the mid-1990s to the mid-2010s. The overflows are the densest source water for the
deep limb of the North Atlantic part of the meridional overturning circulation (AMOC),
and this conclusion argues that the reported weakening of the AMOC was not due to
overflow weakening or reduced overturning in the AM. Although the combined data set has
made it possible to establish a consistent budget for the AM exchanges, the observational
coverage for some of the branches is limited, which introduces considerable uncertainty.
This lack of coverage is especially extreme for the surface outflow through the Denmark
Strait, the overflow across the Iceland–Faroe Ridge, and the inflow over the Scottish
shelf. We recommend that more effort is put into observing these flows as well as
maintaining the monitoring systems established for the other exchange branches.