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Title
High-Latitude Ocean and Sea Ice Surface Fluxes: Challenges for Climate Research
Permalink
https://escholarship.org/uc/item/1ts065wc
Journal
Bulletin of the American Meteorological Society, 94(3)
ISSN
0003-0007 1520-0477
Authors
Bourassa, Mark A
Gille, Sarah T
Bitz, Cecilia
et al.
Publication Date
2013-03-01
DOI
10.1175/BAMS-D-11-00244.1
Copyright Information
This work is made available under the terms of a Creative Commons Attribution
License, availalbe at https://creativecommons.org/licenses/by/4.0/
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High latitudes present extreme conditions for the measurement and estimation of
air–sea and ice fluxes, limiting understanding of related physical processes and
feedbacks that are important elements of the Earth’s climate.
HIGH-LATITUDE OCEAN AND SEA
ICE SURFACE FLUXES: CHALLENGES
FOR CLIMATE RESEARCH
by Mark a. bourassa, sarah T. Gille, CeCilia biTz, DaviD Carlson, ivana CeroveCki, Carol anne Clayson,
M
eGhan F. Cronin, Will M. Drennan, Chris W. Fairall, ross n. hoFFMan, GuDrun MaGnusDoTTir,
r
aChel T. Pinker, ian a. renFreW, Mark serreze, kevin sPeer, lynne D. Talley, anD Gary a. WiCk
H
igh-latitude climate
change can manifest
itself in astonishing
ways. Arctic sea ice extent
at the end of the melt season
in September is declining
at a mean rate of 12% per
decade, with record sea-
sonal minima in 2007
and 2012 (Comiso et al.
2008; Shawstack 2012). In
2001/02, the Larsen B Ice
Shelf on the Antarctic
Peninsula collapsed in a
matter of months (Rignot
et al. 2004), and in 2008, the
Wilkins Ice Shelf collapsed
equally quickly (Scambos
et al. 2009). Ocean heat
content is rising rapidly
in high-latitude regions
of both hemispheres (e.g.,
Gille 2002; Karcher et al.
2003; Bindoff et al. 2007;
Purkey and Johnson 2010).
The observed trends are
expected to continue and
are broadly consistent with
projections of anthropogenic climate change reported
in the Intergovernmental Panel on Climate Change
(IPCC) Fourth Assessment Report (AR4) (Randall
et al. 2007). A common element in high-latitude
climate changes is a dependence on surface fluxes;
we focus on the exchange of energy, momentum,
and material between the ocean and atmosphere and
between atmosphere and sea ice (the basic concepts
defining surface fluxes are outlined in “Primer: What
is an air–sea flux?”). Surface fluxes at high latitudes
Fig. 1. Schematic of surface fluxes and related processes for high latitudes.
Radiative fluxes are both SW and LW. Surface turbulent fluxes are stress, SHF,
and LHF. Ocean surface moisture fluxes are P and E (proportional to LHF).
Processes specific to high-latitude regimes can strongly modulate fluxes. These
include strong katabatic winds, effects due to ice cover and small-scale open
patches of water associated with leads and polynyas, air–sea temperature
differences that vary on the scale of eddies and fronts (i.e., on the scale of the
oceanic Rossby radius, which can be short at high latitudes), deep and bottom
water formation, and enhanced freshwater input associated with blowing snow.
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march 2013amErIcaN mETEOrOLOGIcaL SOcIETY
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Primer: What is an air–sea flux?
are important to processes in the ocean (e.g., deep
convection, dynamics of the Southern Ocean and
the Greenland–Iceland–Norwegian Seas, water
mass transformation), the cryosphere (warming of
waters, ice transport, and cloud formation), and the
atmosphere (cloud modification of radiative fluxes,
feedbacks to annular modes), and have variability
associated with a broad range of processes, as depicted
schematically in Fig. 1. However, the magnitude and
variations of surface fluxes at high latitudes are poorly
known, contributing to the present large uncertainty
in estimates of the high-latitude climate of the
ocean and lower atmosphere (e.g., Dong et al. 2007;
Vancoppenolle et al. 2011; Kwok and Untersteiner
2011; Cerovecki et al. 2011, 2013), and limiting our
ability to validate climate models used to project
twenty-first-century climate (e.g., Christensen et al.
2007). Improving our knowledge of high-latitude
surface fluxes will require close collaboration among
meteorologists, oceanographers, ice physicists, and
climatologists, and between observationalists and
modelers, as well as new combinations of in situ
measurements and satellite remote sensing [e.g.,
improvements on ideas discussed by Bourassa et al.
(2010b)].
This article, an outcome of the U.S. Climate
Variability and Predictability (CLIVAR) Working
Group on High Latitude Surface Fluxes Workshop
(www.usclivar.org/hlat.php), describes the scientific
requirements for surface fluxes at high latitudes, which
A
ir–sea fluxes represent the exchange
of energy and material between the
ocean and lower atmosphere. They
include the net fluxes of momentum
(stress) from wind, energy (downward
and reflected shortwave radiation,
downward and emitted longwave radia-
tion, latent heat flux, and sensible heat
flux), and mass (Fig. 1). Mass fluxes
encompass a broad number of variables,
including moisture (precipitation and
deposition, evaporation or sublimation,
and runoff or ice melt) and gases (e.g.,
CO
2
), as well as atmospheric aerosols
(solid or liquid particles), which can, for
example, supply salt to the atmosphere,
provide chlorine that can contribute
to ozone depletion, or deliver iron-
rich dust derived on land to the ocean,
spurring biological growth.
Wind stress, sensible and latent
heat fluxes, gas and aerosol exchange,
and evaporation are classified as
turbulent fluxes. These fluxes depend
on nonlinear, covarying terms, meaning
that random errors in bulk variables
can have a rectified effect, causing
significant errors even in large-scale
averaged fields. Depending on the
space and time scales being inves-
tigated (Fig. 3), these fluxes could
be averaged over a wide range of
surface and meteorological conditions.
Turbulent fluxes on the time scales of
intense storms (roughly one day) can
be very large compared to long-term
averages. Although turbulent fluxes can
be measured directly, they are typi-
cally parameterized (see Table 1) (e.g.,
Curry et al. 2004). At high latitudes,
low-level winds are associated with
intense storms from small (polar lows)
to large (warm-core seclusions) scales,
and they can be enhanced by orography
(e.g., low-level jets around Greenland)
and reduced friction over some types
of ice, leading to intense katabatic
winds (e.g., from the Antarctic conti-
nent), and consequently strong air–sea
momentum exchange along coastlines.
Openings in the ice (i.e., leads and
polynyas) can lead to small-spatial-scale
variations in air–sea turbulent heat
fluxes, with greatly enhanced exchange
of heat over open water (Fig. 1). Small-
scale ocean currents and eddies can
also modify turbulent heat fluxes.
Radiative fluxes at the surface
include downwelling and upwelling
(reflected) shortwave radiation
originating from the sun as well as
downwelling and upwelling longwave
radiation emitted by the atmosphere
and the surface, respectively (e.g.,
Petty 2006) Radiative fluxes exhibit
unique characteristics at high latitudes.
AFFILIATIONS: bourassa, Clayson,* anD sPeer—The Florida State
University, Tallahassee, Florida; Gille, CeroveCki, anD Talley—
University of California, San Diego, La Jolla, California; biTz—
University of Washington, Seattle, Washington; Carlson—British
Antarctic Survey, Cambridge, United Kingdom; Cronin—NOAA/
Pacific Marine Environmental Laboratory, Seattle, Washington;
Drennan—University of Miami, Miami, Florida; Fairall anD WiCk—
NOAA/Earth System Research Laboratory, Boulder, Colorado;
hoFFMan—Atmospheric and Environmental Research, Lexington,
Massachusetts; MaGnusDoTTir—University of California, Irvine, Irvine,
California; Pinker—University of Maryland, College Park, College
Park, Maryland; renFreW—University of East Anglia, Norwich, United
Kingdom; serreze—University of Colorado, Boulder, Colorado
*CURRENT AFFILIATION: Woods Hole Oceanographic
Institution, Woods Hole, Massachusetts
CORRESPONDING AUTHOR: Sarah Gille, Scripps Institution of
Oceanography, University of California, San Diego, 9500 Gilman
Dr., Mail Code 0230, La Jolla, CA 92093-0230
E-mail: sgille@ucsd.edu
The abstract for this article can be found in this issue, following the table
of contents.
DOI:10 .1175 / B AM S - D-11- 0 024 4.1
In final form 22 August 2012
©2013 American Meteorological Society
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we define as including the Arctic/
subarctic Ocean and the Southern
Ocean. We inventory the reasons,
both logistical and physical, why
existing flux products do not meet
these requirements. We conclude
with suggestions for improving high-
latitude flux estimates. Our focus
is on ocean–atmosphere fluxes and
radiative fluxes over high-latitude
seas and sea ice. We do not endeavor
to replicate material in the recent
Snow, Water, Ice, Permafrost in the
Arctic (SWIPA) assessment (AMAP
2011, www.amap.no/swipa/), which
provides an up-to-date description
of surface and lateral fluxes and net
mass changes of the Greenland ice
sheet, and addresses requirements for
measuring carbon fluxes over tundra
and terrestrial permafrost regions.
UNIQUE CHALLENGES AND
DESIRED ACCURACIES. High-
latitude fluxes differ markedly from
those in temperate regions. As de-
picted in Fig. 1, fluxes are influenced by the pres-
ence of ice, frequent high wind speeds (Fig. 2), low
winter temperatures, both large and small seasonal
temperature ranges, and pronounced variability
on local scales, particularly along sea ice margins
and leads (linear openings in the ice cover) or in
They are not expressed using simple
bulk formulas and are therefore not
included in Table 1. All the radia-
tive fluxes vary with cloud cover and
aerosol amount and characteristics.
For the downwelling shortwave, the
major modulators at high latitudes are
the solar angle and the surface albedo
(i.e., the fraction of incident radiation
that is reflected). Surface albedo varies
strongly between ice, snow, and water,
and this is further complicated by
temporal surface variability during melt
periods and by darkening due to dust
and carbonaceous aerosol deposition.
Downwelling longwave radiation is con-
trolled largely by cloud cover, which is
high at high latitudes, cloud-base height,
which is often low at high latitudes, and
by water vapor concentration, which
is small at high latitudes. The upwelling
longwave radiation depends on surface
skin temperature, which differs widely
between the ice and the open-water
bodies and is not well known in areas
with ice. Furthermore, small changes
in shortwave reflectivity and longwave
emissivity can alter the energy budget
sufficiently to cause substantial growth
or melting of ice.
Net freshwater fluxes into a
volume of the ocean are determined
by oceanic transport, runoff (including
melting land ice), precipitation (P,
which includes rain and snow), and
evaporation (E). The latter two are
often viewed in the combined term
of net precipitation, or P – E. An
important factor for ocean freshwater
and salinity balances in regions with
sea ice is the fractionation of water
and salt in a process called brine
rejection: sea ice is greatly depleted
in salt, and most of the salt enters
the underlying seawater, where
it increases the seawater density.
When the sea ice melts, the resulting
seawater is significantly freshened and
hence lighter. When sea ice, which can
be thought of as seawater of very low
salinity, is transported from one region
to another, an advective freshwater
flux between the regions arises. Ice
and brine formation are modulated
locally by the intermixed areas of open
water, organic slicks, new ice, existing
bare ice, snow-covered ice, and melt
ponds; these interact radiatively
with overlying regions of haze, low
cloud, and clear sky. Furthermore,
deposition of particles (e.g., of black
carbon) and gases (e.g., of mercury)
affect the surface radiative properties
(e.g., albedo). Thus, subtle changes in
heat or momentum fluxes cause, and
respond to, rapid water phase change.
Fig. 2. Frequency of winds exceeding 25 m s
–1
for the QuikSCAT
observations from Jul 1999 through Jun 2009, based on Remote
Sensing Systems’ Ku2001 algorithm. Statistics are computed by
averaging vector winds from the original satellite swath measure-
ments into 0.25° × 0.25° bins. Poleward of roughly 55° neighboring
orbits overlap and only one of the two observations is considered.
Northern Hemisphere extreme winds are associated with topography
(e.g., around Greenland; see Renfrew et al. 2008) and western bound-
ary currents, and occur in boreal winter; Southern Hemisphere
events are more widespread and occur year-around. Locations with
less than approximately 51% of possible observations are plotted as
white, thereby excluding some regions with too much seasonal ice.
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association with mesoscale eddies. As a result, physi-
cal understanding gained in temperate regions is not
necessarily applicable to high latitudes.
The high-latitude environment also poses logisti-
cal challenges. Capturing an annual cycle in fluxes,
for example, requires that instruments function
through long periods of cold polar darkness, often far
from support services, in situations subject to icing
and extreme wave conditions. These logistical chal-
lenges are reflected in a relative paucity of standard
surface and upper-air meteorological data and an
almost complete absence of moored
1
or free-drifting
sensor systems in large areas of the polar oceans,
particularly those covered with seasonal or perennial
ice. Frequent cloud cover in high latitudes restricts the
availability of surface and atmospheric information
derived from visible and infrared (IR) wavelength
satellite observations. Moreover, most current visible
and IR sensors have difficulty distinguishing clouds
from snow or ice cover. This lack of information
as well as uncertainties in the parameterizations
of fluxes reduces the quality of data assimilation
1
The first meteorological mooring in the Southern Ocean was deployed in March 2010, at 47°S, 140°E, by the Australian
Integrated Marine Observing System (Trull et al. 2010). It measures wind, temperature, humidity, atmospheric pressure, solar
radiation, and precipitation but not turbulent fluxes. A second mooring that was deployed in the Agulhas Return Current at
38.5°S, 30°E in November 2010 broke loose from its anchor after less than seven weeks. Similar difficulties occurred in the
northern high latitudes (Moore et al. 2008).
examPles: surface fluxes from a climate research PersPective
S
urface flux products are widely used
in the fields of oceanography, glaciol-
ogy, sea ice dynamics, and atmospheric
dynamics. Science questions address
time scales from less than an hour to
decades or longer, resulting in a diver-
sity of accuracy requirements. Here we
provide a few examples.
From a long-term climate change
perspective. Over the last few
decades, a number of aspects of the
climate system have changed substan-
tially. In the ocean, observed long-term
warming trends from 1993 to 2003 can
be explained by a mean energy flux into
the ocean of just 0.86 ± 0.12 W m
−2
(Hansen et al. 2005). For sea ice, a
1 W m
−2
flux imbalance equates to a
10-cm ice melt in a year, a significant
fraction of the ice budget. Basin-scale
changes in ocean salinity associated
with global change correspond to small
changes in air–sea freshwater flux on
the order of 0.05 practical salinity
unit (psu) decade
–1
(Boyer et al. 2005)
concentrated in the top 200 m. This is
equivalent to a change in liquid P – E
of 3 cm yr
−1
. Similarly, North Atlantic
freshwater flux anomalies sufficient
to slow deep convection (Curry and
Mauritzen 2005) derive from river
runoff and ice melt, and are equivalent
to P – E of almost 1 cm yr
−1
over the
area of the Arctic and North Atlantic.
These climate change signals of
O(1 W m
−2
) for heat and O(1 cm yr
−1
)
for freshwater are far below any
currently estimated observational accu-
racy globally or in polar regions, even
in averaged estimates computed from
many independent samples. Hence,
long-term changes in these fluxes
are more effectively diagnosed by
observing the ocean temperature and
salinity changes as integrators of heat
and freshwater fluxes (e.g., Hansen
et al. 2005; Levitus et al. 2005; Boyer
et al. 2005, 2007; Domingues et al.
2008; Wunsch et al. 2007; Hosoda
et al. 2009; Levitus et al. 2009; Durack
and Wijffels 2010). Capabilities of
current observing systems should not
be a deterrent to efforts at improve-
ment; significant scientific gains could
be made if the uncertainty in heat
and freshwater flux estimates (as
crudely estimated by the spread in
modern products, cf. Fig. 5) could be
improved by an order of magnitude and
if available products were consistently
released with high-quality uncertainty
and bias information.
From an open-ocean circulation
perspective. When sea ice is not
involved, the open-ocean circulation
is driven primarily by wind stress curl
patterns that deform sea level and
thermocline fields, and by heat and
moisture fluxes that alter water den-
sity. Since the curl patterns are caused
in large part by zonal and meridional
variations in the wind direction (e.g.,
easterly trades in the tropics, west-
erly jet stream at midlatitudes), from
the ocean circulation perspective it is
necessary to resolve not only the wind
stress magnitude but also its direction.
Water density and hence circulation
are also modified by ventilation of the
mixed layer through air–sea heat and
freshwater fluxes. After the water
mass is subducted into the interior
ocean, its properties remain relatively
unchanged as it circulates through
the global ocean. The high-latitude
formation of ocean bottom water is a
critical component of the global ocean
circulation. At high latitudes, surface
cooling produces deeper mixing and
ventilation. Salinity becomes a major
factor controlling density where tem-
peratures approach the freezing point.
Thus, the analysis of high-latitude
ocean processes depends on accurate
surface heat and freshwater fluxes,
including freshwater fluxes linked to
ice formation, export, and melt. For
example, buoyancy gain by excess pre-
cipitation and buoyancy loss by ocean
heat loss are apparently of comparable
importance in estimating Subantarctic
Mode Water formation, which domi-
nates the upper ocean just north of
the Antarctic Circumpolar Current
(Cerovecki et al. 2013). Calculation
of surface water mass transformation
rates from air–sea fluxes requires
accurate and unbiased fluxes. Using
the best available data products to
evaluate the oceanic mixed-layer heat
budget, Dong et al. (2007) found that
the zonally averaged imbalance can be
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