Drivers of exceptionally cold North Atlantic Ocean temperatures and their link to the 2015 European heat wave

Abstract

The North Atlantic and Europe experienced two extreme climate events in 2015: exceptionally cold ocean surface temperatures and a summer heat wave ranked in the top ten over the past 65 years. Here, we show that the cold ocean temperatures were the most extreme in the modern record over much of the mid-high latitude North-East Atlantic. Further, by considering surface heat loss, ocean heat content and wind driven upwelling we explain for the first time the genesis of this cold ocean anomaly. We find that it is primarily due to extreme ocean heat loss driven by atmospheric circulation changes in the preceding two winters combined with the re-emergence of cold ocean water masses. Furthermore, we reveal that a similar cold Atlantic anomaly was also present prior to the most extreme European heat waves since the 1980s indicating that it is a common factor in the development of these events. For the specific case of 2015, we show that the ocean anomaly is linked to a stationary position of the Jet Stream that favours the development of high surface temperatures over Central Europe during the heat wave. Our study calls for an urgent assessment of the impact of ocean drivers on major European summer temperature extremes in order to provide better advance warning measures of these high societal impact events.

Full text

Environ. Res. Lett. 11 (2016)074004 doi:10.1088/1748-9326/11/7/074004
LETTER
Drivers of exceptionally cold North Atlantic Ocean temperatures and
their link to the 2015 European heat wave
Aurélie Duchez
1,3
, Eleanor Frajka-Williams
2
, Simon A Josey
1
, Dafydd G Evans
2
, Jeremy P Grist
1
,
Robert Marsh
2
, Gerard D McCarthy
1
, Bablu Sinha
1
, David I Berry
1
and Joël J-M Hirschi
1
1
National Oceanography Centre, University of Southampton Waterfront Campus, European Way, Southampton SO14 3ZH, UK
2
Ocean and Earth Science, National Oceanography Centre, University of Southampton Waterfront Campus, European Way,
Southampton SO14 3ZH, UK
3
Author to whom any correspondence should be addressed.
E-mail: aurelie.duchez@noc.ac.uk
Keywords: 2015 heat wave, cold Atlantic Ocean anomaly, ocean atmosphere interactions, North Atlantic, ocean variability, air-sea fluxes
Supplementary material for this article is available online
Abstract
The North Atlantic and Europe experienced two extreme climate events in 2015: exceptionally cold
ocean surface temperatures and a summer heat wave ranked in the top ten over the past 65 years. Here,
we show that the cold ocean temperatures were the most extreme in the modern record over much of
the mid-high latitude North-East Atlantic. Further, by considering surface heat loss, ocean heat
content and wind driven upwelling we explain for the first time the genesis of this cold ocean anomaly.
We find that it is primarily due to extreme ocean heat loss driven by atmospheric circulation changes
in the preceding two winters combined with the re-emergence of cold ocean water masses.
Furthermore, we reveal that a similar cold Atlantic anomaly was also present prior to the most extreme
European heat waves since the 1980s indicating that it is a common factor in the development of these
events. For the specific case of 2015, we show that the ocean anomaly is linked to a stationary position
of the Jet Stream that favours the development of high surface temperatures over Central Europe
during the heat wave. Our study calls for an urgent assessment of the impact of ocean drivers on major
European summer temperature extremes in order to provide better advance warning measures of
these high societal impact events.
1. Introduction
The North Atlantic-Europe region experienced a
striking juxtaposition of temperature extremes in
2015 with exceptionally cold ocean surface tempera-
tures developing in the first half of the year and a
major heat wave over Central Europe in the summer
that followed (figure 1(a)). The causes of both of
these events are outstanding research questions. In
this paper, we determine for the first time the causes
of the 2015 cold ocean anomaly. Additionally, we
find a relationship between ocean temperatures and
European heat waves since 1980, including the one
in 2015, which indicates that cold mid-latitude ocean
temperature anomalies are a common precursor to
heat wave events and need to be included amongst
their potential drivers.
The outstanding problem in understanding Eur-
opean heat waves is a reliable identification of the fac-
tors that cause them to develop (Perkins 2015). Several
regional drivers have been put forward including soil
moisture, precipitation deficits and anomalously
warm surface temperatures in marginal seas particu-
larly the Mediterranean (Vautard et al 2005, Fischer
et al 2007, Feudale and Shukla 2011a, Stefanon
et al 2012). Another factor is the North Atlantic Ocean
as it has the potential to modify the atmospheric circu-
lation upstream of Europe prior to the development of
a heat wave through the influence of anomalies in the
temperature of the sea surface (e.g. Brayshaw
et al 2011, Dong et al 2013, Buchan et al 2014). This
precursor role for the Atlantic has received little atten-
tion to date although previous studies have found
some evidence for the influence of sea surface
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temperature (SST)anomalies acting at the same time
as the heat wave (Black et al 2004, Nakamura
et al 2005, Della-Marta et al 2007). In addition, a role
for atmospheric temperature anomalies over the Tro-
pical Atlantic was found by (Cassou et al 2005)
although their study did not consider ocean temper-
ature anomalies.
We discuss the data and methods used for our ana-
lysis in section 2and determine the causes of the 2015
North Atlantic cold anomaly in section 3. In section 4,
we explore the relationship between North Atlantic
Ocean temperature anomalies and European heat
waves with an emphasis on the potential role of ocean
temperature anomalies as a precursor.
2. Data and methods
Data used in this paper were extracted for the period
January 1948–December 2015. The anomalies were
calculated by removing the seasonal cycle from
January 1981 to December 2010, which is a reference
period commonly used by weather services worldwide
to define climatological anomalies. The NCEP-NCAR
reanalysis (Kalnay et al 1996)was used to provide
(monthly and daily)SST and monthly 2 m maximum
air temperature over Europe, monthly net air–sea heat
flux, monthly 850 mb geopotential height and 300 mb
zonal wind speed (for the Jet Stream). Output from the
ERA-Interim reanalysis (Dee et al 2011)has also been
used to support the NCEP/NCAR based evaluation of
the causes of the cold SST anomaly (see supplementary
materials).
Ocean heat content for the time series in
figure 1(d)is a monthly product from the National
Oceanographic Data Center 0–700 m product. It is
derived from profiles in the World Ocean Database
(Boyer et al 2013). Temperature profiles for the volu-
metric analysis are from the Roemmich and Gilson
Argo climatology, and are available monthly through
December 2015 (Roemmich and Gilson 2009). The
box chosen for the Atlantic SST anomaly is: 45°N–60°
N, 40°W–15°W. We have assessed the sensitivity of
our results to the choice of the boxed region for the
ocean cold anomaly and found that between 1955 and
2015 changes in ocean heat content and SST mainly
occur in this region. For alternative North Atlantic
boxes, which encompass the boxed area shown in
figure 1(a), our results do not change significantly (not
shown). The Central European box chosen to estimate
the heat wave temperature is 45°N–53°N, 1°E–35°E.
Variations in SST are governed by the heat balance
in the surface mixed layer of the ocean, which is influ-
enced by surface air–sea heat fluxes (influenced by
wind speed, air temperature, cloudiness and humid-
ity), horizontal advective and diffusive processes in the
Figure 1. (a)2015 JJA sea surface temperature anomalies (over the ocean)and maximum 2 m air temperature anomalies (over land)in
°C(shading). Contours delineate regions exceeding 2 and 2.5 standard deviations (contours). The left box indicates the area chosen to
define the cold Atlantic SST anomaly; the right box indicates the area used to define the maximum 2 m air temperature anomaly. (b)
Anomalies of geopotential height (in m)at 850 mb for JJA 2015. (c)Evolution of JJA SST anomalies (red, averaged over left box)and
maximum 2 m air temperature anomalies (black, averaged over right box). The seasonal cycle for 1981–2010 has been removed. (d)
Ocean heat content anomaly time series (in GJ m
−2
)up to Dec 2015 averaged over the box (45°–60°N, 40°–15°W)and extracted from
the National Oceanographic Data Center (NODC)0–700 m product (blue). Argo profiles represent the main source of data after 2003,
as shown by an Argo-only index smoothed with a 12 month running window (red). Both time series are referenced to the period
2000–2015 and show a stronger reduction since 2013.
2
Environ. Res. Lett. 11 (2016)074004

mixed layer, and entrainment processes at the base of
the mixed layer. The heat budget for the upper-ocean
mixed layer may be written as (Deser et al 2010)
()
()() ()
   
r
=++⋅
++-
T
t
Q
CH UU T
WWTT
H
d
d
.
1
p
b
net geo ek
eek
The first right-hand term in this equation repre-
sents the surface heat flux, the second term the hor-
izontal temperature advection, and the last term the
vertical water exchange. ρ(1027 kg m
−3
)is the density
of seawater, C
p
(3985 J kg
−1
K
−1
), the specific heat
capacity of seawater, H, the mixed layer depth, T, the
mixed layer temperature (equal to the SST),Q
net
, the
net surface heat flux, U
geo,
the geostrophic current
velocity, U
ek
, the Ekman current velocity, W
e
, the ver-
tical entrainment rate, W
ek
, the Ekman pumping velo-
city and T
b
is the temperature of the water at depth that
is entrained into the mixed layer.
Q
net
is defined as the sum of the sensible and latent
(turbulent)heat fluxes and solar and longwave radia-
tive fluxes. The turbulent energy flux is proportional
to the wind speed and the sea–air temperature (or
humidity)difference, the radiative fluxes are functions
of solar elevation, air temperature, humidity and clou-
diness. Ekman and geostrophic currents contribute to
the heat budget of the mixed layer through horizontal
advection, whereas entrainment and Ekman pumping
alter the SST through vertical advection. A complete
discussion of these terms may be found in texts such as
Pond and Pickard (1983)and Vallis (2006), with
detailed discussion of the surface fluxes in Josey et al
(2013). In order to explain the origin of the 2015 cold
ocean anomaly, we assess the respective contribution
of surface air–sea fluxes, ocean circulation changes
(horizontal temperature advection)and vertical water
exchange (Ekman upwelling)in section 3.
3. Exceptionally cold North Atlantic in 2015
First, we consider the exceptionally cold ocean
surface anomaly that was already in place prior to the
onset of the 2015 heat wave. The SST anomaly field
for June 2015 (figure 2(a)) shows temperatures up to
2°C colder than normal over much of the sub-polar
gyre with values that are the coldest observed for this
month of the year in the period 1948–2015 indicated
by stippling. The cause of this cold anomaly has been
the subject of widespread interest in the media, we
nowshowforthefirsttimethatitcanbeattributedto
a combination of air–sea heat loss from late 2014
through to spring 2015 and a re-emergent sub-
surface ocean heat content anomaly that developed
in preceding years. Prior to the winter of 2014–15, in
November 2014, a localised cold SST feature was
already evident centred at 50°N, 30°W(figure 2(b)).
The subsequent net air-sea heat exchange from
December 2014 to May 2015 (figure 2(c)) is char-
acterised by extreme, basin-wide heat loss in the
band from 55°–65°N resulting from wind speed
anomalies associated with intensification of the
meridional surface pressure gradient.
We have determined the June 2015 SST anomaly
that is expected from air–sea fluxes by taking the
November 2014 SST field as an initial state and adding
the December 2014–May 2015 integrated surface heat
flux forced SST anomaly using net heat flux from the
NCEP/NCAR reanalysis. For the heat flux calculation,
afixed value for the ocean mixed layer depth of 100 m
is adopted which is close to the mean value for the box
in figure 1(a)for this period. A more detailed calcul-
ation would employ time varying MLD but our aim is
to show the potential magnitude of the heat flux rela-
ted signal, for which a fixed value of the MLD over the
period is adequate.
The resulting field (figure 2(d)) is in reasonable
agreement with the observed SST anomaly
(figure 2(a)) particularly in the eastern subpolar gyre.
Thus, the extremely cold surface feature can be lar-
gely explained in terms of the pre-existing Novem-
ber 2014 SST anomaly and the integrated effects of
subsequent surface heat loss from December 2014 to
May 2015. A quantitative measure of the level of
agreement between the estimated (figure 2(d)) and
observed fields (figure 2(a)) is provided by a spatial
correlation coefficient value of 0.77 over the domain
(35°N–63°N, 40°W–10°W)that spans the key fea-
tures of the anomaly field. Note that the estimated
anomaliesarenotasstrongasthoseobservedonthe
southern margin of the cold feature, most noticeably
in the small region 20–30°W, 45–50°N. This could
be due to both the simple assumption of a constant
mixed layer depth as well as to the absence of possi-
ble contributing effects from the ocean circulation
(e.g. changes in the strength of the AMOC and/or of
the subpolar gyre).
We have repeated the calculation of the estimated
SST anomaly using net heat flux and SST data from the
ERA-Interim reanalysis (figure S1 in supplementary
materials)and again find close agreement between the
estimated and observed SST field in June 2015 indicat-
ing that our conclusions are not sensitive to the choice
of heat flux product employed.
The anomalous SST in November 2014 is a re-
emergence of a cold, subsurface anomaly originating
in the winter of 2013–14 (Grist et al 2015). Profiles of
temperatures in the Atlantic show that the cold anom-
aly became noticeable in early 2014 and was persistent
and intense, representing an average cooling of 0.7 °C
throughout the top 700 m in 2015 (figure 3(a)). Some
apparent cooling may result from adiabatic heave
(upwelling or downwelling of waters). To remove this
potential effect, we consider only the volume of water
above the 27.6 kg m
−3
isopycnal (Walin 1982).By
considering the variability of volume in each temper-
ature class, we can determine the amount of water that
3
Environ. Res. Lett. 11 (2016)074004

Figure 2. (a)Observed SST anomaly in June 2015, stippled cells indicate that the June 2015 SST is the coldest in that month over the
period 1948–2015 (b)observed SST anomaly in November 2014, (c)average net heat flux anomaly for December 2014–May 2015,
with corresponding SLP anomaly (contoured, 1 mb intervals, solid lines for positive and zero, dashed lines for negative contours
respectively)and 10 m wind speed anomaly (arrows),(d)estimated SST anomaly in June 2015 obtained by integrating the heat flux
anomaly in (b)over the ocean mixed layer and adding to the November 2014 initial state in (c). Units are °C for SST and Wm
−2
for net
heat flux.
Figure 3. (a)Temperature anomaly in the subpolar gyre (45–60°N, 40–15°W).(b)Observed ocean heat content (OHC)change (in
GJ m
−2
)since 2004 for the volume of water above the 27.6 kg m
−3
isopycnal. Changes predicted from surface fluxes for the same
temperature classes are in dashed. Grey shading highlights the January–June periods in 2013 and 2015. Anomalies in these timeseries
were calculated by subtracting the climatology (the background monthly time series)from January 2004 through December 2013.
4
Environ. Res. Lett. 11 (2016)074004

becomes warmer or cooler and compare that to the
warming or cooling predicted by air–sea heat fluxes
acting over the same region (figure 3(b)). The volu-
metric distribution in temperature classes is deter-
mined following Marshall et al (1993)and Evans et al
(2014)by summing the total volume of grid cells that
lie within each 0.5 °C temperature class using an Argo-
based gridded climatology (Roemmich and Gil-
son 2009, Boyer et al 2013). Transformations of water
across surfaces of constant temperature (isotherms)
are determined by building a series of linear equations
describing the volume change in each temperature
class in terms of the unknown transformations and
solving using a matrix inversion (Evans et al 2014).
These transformations have units of m
3
s
−1
. The
transformations of water across isotherms predicted
by air–sea heat fluxes are determined by integrating
the air–sea fluxes over the surface area of the ocean
occupied by a specific temperature class. Full details of
this method can be found in (Evans et al 2014). For this
region, the anomalous transformations are then inte-
grated over the volume of water shallower than the
27.6 kg m
−3
isopycnal and accumulated in time, and
scaled by density and specific heat capacity to give
units of heat content. The monthly mean (using the
period from 2004 to the end of 2013)is removed from
the time series.
Between January–June 2013 and January–June
2015, the observed cooling of the volume of water
represents an OHC change of 1.1 GJ m
−2
, with the
strongest changes occurring between December 2013
and January 2014 (figure 3(b)). Such a change can
either occur through a transformation of water masses
by air–sea heat fluxes, or via a net increase in the lateral
input of water into colder temperature classes. Esti-
mating the expected OHC change due to air-sea heat
fluxes between these same time periods accounts for
0.7 GJ m
−2
.
We have also determined the respective contrib-
ution from wind driven upwelling (third term in
equation (1)) and ocean circulation changes (second
term in equation (1)) and found them to be minor
terms over this period as we now discuss. Entrainment
of relatively cool water from below the mixed layer is
associated with upwelling across the subpolar gyre
(Marshall et al 1993). Fields of monthly upwelling rate
were computed from the wind stress curl over the box
chosen for the Atlantic SST anomaly. While upwelling
was more intense than usual, the associated heat con-
tent reduction of 0.2 GJ m
−2
(estimated by multi-
plying upwelling by the vertical temperature gradient
between the surface and 700 m)was only a small pro-
portion of the observed 1.1 GJ m
−2
cooling. Under
typical conditions, surface heat losses in the subpolar
regions are balanced by the influx of warm subtropical
waters carried by the ocean circulation. During the
July 2013–June 2015 period, the observed overturning
circulation at 41°N, determined using in situ floats and
satellite altimeters (Willis 2010)was weaker than usual
(12.3 Sv compared to 13.6 Sv over the preceding 10
year period), bringing less heat northward. However,
the reduced northward flow across 41°N is insufficient
to explain the observed heat content changes, nor the
observed changes in volumes of water in different
temperature classes.
In contrast to the behaviour observed in 2015, cir-
culation changes are thought to be important for the
multidecadal to centennial timescale cooling of the
North Atlantic evident in analysis of temperature
trends over the 20th century (Drijfhout et al 2012,
Rahmstorf et al 2015). On these timescales, an Atlantic
warming hole (offset to the south–west from the 2015
anomaly)has been identified. This cooling has been
linked to a potential decline in the AMOC (Drijfhout
et al 2012, Smeed et al 2014, Rahmstorf et al 2015)and
also to the effect of the Atlantic Multi-decadal Oscilla-
tion which in recent years has been changing from a
positive (anomalously warm North Atlantic)to a
negative phase (McCarthy et al 2015, Parker and
Ollier 2016). It is important to distinguish between
this long-term warming hole and the short-term 2015
cold anomaly that is the focus of our study.
4. The 2015 summer heat wave
In the summer of 2015 following the development of
the cold ocean anomaly, Europe experienced a major
heat wave that has been ranked in the top ten over the
past 65 years (Russo et al 2015). As pointed out by
Meehl and Tebaldi 2004 and Perkins and Alexan-
der 2012, there is no universal definition for heat
waves. The heat waves we consider in this study are
defined as the summers (JJA)during which the highest
2 m maximum air temperatures were recorded over
central Europe in the area shown in figure 1(a).
We now explore whether the development of the
2015 heat wave could have been driven in part by the
cold ocean temperatures that preceded it. First we
consider the circulation pattern associated with the
heat wave. The high temperatures coincided with per-
sistent high and low-pressure systems over Europe
and the central North Atlantic respectively
(figure 1(b)). This atmospheric pressure pattern sub-
jected Central Europe to the influence of subtropical
air masses. Combined with high pressure and summer
insolation, this led to the elevated surface air tempera-
tures that were characteristic of the heat wave
(figure 1(a)). Using NCEP maximum 2 m air tempera-
tures averaged over Central Europe (defined as the
land box in figure 1(a)), we found that the standard
deviation (STD)of Tmax from the 1981–2010 mean is
1.1 °C over the 1948–2015 period. The Tmax anomaly
for summer 2015 is 2.4 °C, which means that averaged
temperatures during that summer were 2.2 STD
higher than the mean Tmax summer value for the
1981–2010 period. For the summer of 2003, the Tmax
anomaly over our chosen region corresponds to a
5
Environ. Res. Lett. 11 (2016)074004

deviation from the mean of 1.5 STD higher and for the
summers 1994 and 1992 the deviations are 1.6 and 1.2
STD, respectively. These results show that 2015 was
the most extreme central European heat wave in the
1980–2015 period considered. Moreover, using a grid-
ded version (E-OBS 11.0)of the European Climate
Assessment & Data (Haylock et al 2008), ECA&D,
www.ecad.eu), Russo et al 2015 also showed that the
summer of 2015 was the most extreme heat wave
recorded in parts of central/eastern Europe and was
classified within the top ten most extreme European
heat waves since 1950.
Could this heat wave have been driven in part by
the cold ocean temperature anomaly that preceded it?
Previous model and observation based studies suggest
that atmospheric circulation changes can develop in
response to SST anomalies (Walin 1982, Sutton and
Mathieu 2002, Nakamura et al 2005, Smeed
et al 2014). To explore whether this is the case in 2015
we compare the daily SST field evolution to the timing
of shifts in the position of the atmospheric Jet Stream
(JS). The SST anomaly develops through winter
2014–15 and amplifies through spring into early sum-
mer preceding the onset of the heat wave (figure 4).
In order to estimate the strength of the JS shift, we
define a measure of the southward JS displacement
that captures the difference in high level (300 mb)
wind speed between adjacent latitude bands north and
south of 50°N: a northern band (NB, 50–65°N)and a
southern band (SB, 35–50°N), in each case for the
longitude range 50–10°W. This parameter indicates
whether the JS is predominantly located to the north
or south in the North Atlantic and is calculated as the
difference in the mean zonal 300 mb wind speed (u
300
)
between the two bands:
()D= -Mean u Mean u .2
JS 300NB 300SB
If the winds are stronger in the north than the
south, Δ
JS
takes positive values (up to 30 m s
−1
). The
value of Δ
JS
becomes negative if the JS has been dis-
placed into the southern band. Six-hourly values of
Δ
JS
have been calculated and smoothed with a 55-
point Parzen filter to produce the time series shown on
figure 4. In late June, Δ
JS
undergoes a clear shift from a
highly variable state, with no persistent positive or
negative values, to a persistent state of negative values
indicating a southward displacement of the JS
(figure 4). This displacement is preceded by a strength-
ening of the meridional SST gradient in early June sug-
gesting that the SST gradient increase leads the
southward JS deflection by several weeks. This is
demonstrated further by the purple line on figure 4,
which shows the difference between the maximum
warm anomaly in the southern band and the mini-
mum cold anomaly in the northern band (called Δ
SST
)
rising sharply from mid-May to a maximum in the
first week of June. The lagged response of the JS posi-
tion to the strong meridional SST gradient indicates a
potential role for the ocean in steering the JS position
although further model experiments are required to
confirm this suggestion. Δ
JS
remains negative until
September indicating that the JS is displaced south-
wards throughout much of the summer. From mid-
July to the beginning of September, the most pro-
nounced cold SST anomalies are located further
southwards (at about 47°N rather than 57°N pre-
viously)which is consistent with a response of the
ocean to the southward shift in the JS.
The 2015 heat wave results open an interesting
route for research into whether Atlantic temperature
anomalies have also contributed more generally to
extreme European heat waves that have occurred in
the last decades (figure 1(c)shows the timing of these
heat waves). Past analyses have considered the role
played by Mediterranean SST anomalies coinciding
with specific heat waves (Xoplaki et al 2003, Feudale
and Shukla 2011a,2011b).Wefind a potential role for
North Atlantic Ocean SSTs as a precursor of European
Figure 4. Hovmöller diagram of 2015 daily SST anomalies (in °C)averaged over 40–20°W(coloured field). Overlaid is the Jet Stream
displacement measure (Δ
JS
)represented with a solid grey line and smoothed using a 10-day running mean. Also shown is the
difference in between the Southern Band (SB, 35–50°N)maximum SST and Northern Band (NB, 50–65°N)minimum SST (Δ
SST
,
purple dash-dot line).
6
Environ. Res. Lett. 11 (2016)074004

heat waves using a lagged correlation analysis between
May SST for the North Atlantic box (defined in
figure 1(a)) and June–July–August averaged 850 mb
height anomaly for all years from 1948–2015
(figure 5(a)).
Geopotential height over central Europe in sum-
mer (JJA)is significantly anti-correlated with the pre-
cursor SST field in May. The calculation has been
repeated for mid-late summer by omitting June from
the calculation and this reveals an intensification of the
lagged correlation pattern (figure 5(b)). Note that in a
previous study that considered the JJA SST field coin-
cident with heat wave occurrence (Della-Marta et al
2007)an east–west dipole pattern was found with a
different spatial structure from the one we find here
for the precursor May SST field.
To further examine the lagged relationship
between European heat waves and North Atlantic
Ocean temperatures, we carry out a composite analysis
of Atlantic SST anomalies in May and 850 mb geopo-
tential height in June, July and August for the five lead-
ing summer European heat waves (figures 5(c)–(f)).
These five hottest summers were selected from the
time series of maximum temperature (Tmax)and cor-
respond to the years: 1992, 1994, 2003, 2012 and 2015.
They were selected as the years for which the averaged
Figure 5. (a)Correlation between May SST anomaly averaged over the North Atlantic box (defined in figure 1(a)) and June–July–
August (JJA)850 mb geopotential height (GEOP)anomaly (in m)from 1980 to 2015, (b)as (a)but showing the correlation between
May SST and July–August GEOP anomaly. (c)–(f)Composite mean fields determined for the set of 1980–2015 Central European heat
waves (1992, 1994, 2003, 2012, 2015)of (c)May SST anomaly (in °C)and (d)–(f)850 mb geopotential height anomaly (in m)for (d)
June, (e)July and (f)August. The green dotted areas show the 95% significance level calculated using a Monte Carlo method.
7
Environ. Res. Lett. 11 (2016)074004

JJA European temperatures were above a one STD
threshold (figure 1(c)). The European region used to
estimate these heat waves is shown in figure 1(a)(right
box). A Monte-Carlo method (e.g. Duchez et al 2015)
was used to determine the significance levels of the sig-
nals in composite maps produced by averaging over
the five hottest summers (figure 5). The significance
level was calculated from the distribution of composite
values obtained by repeated (1000 times)random
selection of five summers (with replacement)during
the period 1948–2015.
A cold anomaly is present in May (figure 5(c)) in
conjunction and associated with a co-located low-
pressure and an accompanying high-pressure anom-
aly over Europe in subsequent months June–August
(figures 5(d)–(f)). Thus, a coherent picture emerges,
from both the general lagged correlation analysis and
the targeted composite study, of cold North Atlantic
Ocean surface temperatures preceding the atmo-
spheric circulation anomalies that lead to heat waves
over Europe.
Note that this composite analysis has also been
performed for cold summers (not shown)and
although the ocean temperature anomalies are antic-
orrelated with Tmax above Central Europe
(figures 5(a)and (b)), no warm summer anomaly in
the southeast subpolar gyre is associated with these
cold summers. This shows that the anticorrelation
between SST and Tmax is mainly due to the processes
happening during warm summers (i.e. the anomaly
patterns coinciding with cold summers are not the
inverse of the warm summers). Finally we also note
that lagged correlations and composites described in
here do not establish a causal link between the cold
anomaly and European summer extreme tempera-
tures but do show a significant lagged relationship
between these two phenomena which requires a fol-
low-on model study to confirm causality and the
details of the mechanism involved.
5. Discussion and conclusions
The main focus of our study has been two climate
events that occurred in 2015: exceptionally cold North
Atlantic Ocean surface temperatures and a severe
European summer heat wave, and the potential
relationship between them. We have identified the
causes of the cold ocean temperatures by observation
based analysis of the range of possible drivers involved,
including surface heat loss, ocean heat content and
wind driven upwelling. The primary drivers are
extreme ocean heat loss driven by atmospheric circula-
tion changes in the preceding two winters combined
with the re-emergence of cold ocean water masses.
We have also examined whether the cold ocean
temperatures may have contributed to the develop-
ment of the 2015 heat wave. In this case we have not
been able to establish causality, as that would require a
complex model analysis that considers the various
possible factors advanced previously as potential dri-
vers (including soil moisture, precipitation and Tropi-
cal Atlantic atmospheric warming)in addition to the
cold ocean precursor. Nevertheless, our results have
established that similar cold Atlantic anomalies were
present prior to the onset of the most extreme Eur-
opean heat waves back to 1980 indicating that it is a
common factor in their development. Furthermore, in
the case of 2015, we suggest that the ocean anomaly,
and the resulting strong meridional SST gradient
could have initiated a propagating Rossby wave train
causing a stationary Jet Stream position that favoured
the development of high pressure and temperature
extremes over Central Europe during the heat wave.
However, this is still a hypothesis to be tested.
Our study calls for an urgent assessment of the
impact of ocean drivers on major European summer
temperature extremes in order to fully elucidate the
driving mechanisms involved and consequently pro-
vide better advance warning measures of these high
societal impact events.
Acknowledgments
AD is beneficiary of an AXA Research Fund postdoc-
toral grant. JJ-MH, SAJ, JPG, and BS are supported by
the UK Natural Environment Research Council
(NERC)National Capability funding. AD is partly
funded by the UK-China Research and Innovation
Partnership Fund through the Met Office Climate
Science for Service Partnership (CSSP)China as part
of the Newton Fund. JJ-MH is partly funded by CSSP
and the NERC project ODYSEA (grant number: NE/
M006107/1). JPG is partly funded by the project
DYNAMOC (grant number NE/M005097/1). GDM
is supported by the NERC RAPID-WATCH pro-
gramme. EFW was supported by a Leverhulme
Research Fellowship. RM acknowledge the support of
various NERC grants. DGE is supported by a NERC
Studentship Award at the University of Southampton.
DB is supported by a NERC grant NE/J020788/1. We
also thank the three anonymous reviewers for their
thoughtful comments that significantly helped to
improve the manuscript.
References
Black E, Blackburn M, Harrison R G, Hoskins B J and Methven J
2004 Factors contributing to the summer 2003 European
heatwave RMETS 59 217–23
Boyer T P et al 2013 World Ocean Database 2013—NOAA Atlas
NESDIS 72 ed S Levitus and A V Mishonov, technical edn
(Silver Spring, MD: National Oceanographic Data Center
Ocean Climate Laboratory)(ftp://ftp.nodc.noaa.gov/pub/
WOD/DOC/wod_intro.pdf)
Brayshaw D J, Hoskins B and Blackburn M 2011 The basic
ingredients of the North Atlantic storm track. Part I: Land–
sea contrast and orography J. Atmos. Sci. 66 2539–58
Buchan J, Hirschi J J-M, Blaker A T and Sinha B 2014 North Atlantic
SST anomalies and the cold North European weather events
8
Environ. Res. Lett. 11 (2016)074004

of winter 2009/10 and December 2010 Mon. Weather Rev.
142 922–32
Cassou C, Terray L and Phillips A S 2005 Tropical Atlantic influence
on European heat waves J. Clim. 18 2805–11
Dee D P et al 2011 The ERA-Interim reanalysis: configuration and
performance of the data assimilation system Q. J. R. Meteorol.
Soc. 137 553–97
Della-Marta P M, Luterbacher J, von Weissenfluh H, Xoplaki E,
Brunet M and Wanner H 2007 Summer heat waves over
western Europe 1880–2003, their relationship to large-scale
forcings and predictability Clim. Dyn. 29 251–75
Deser C, Alexander M A, Xie S-P and Phillips A S 2010 Sea surface
temperature variability: patterns and mechanisms Annu. Rev.
Mar. Sci. 2115–43
Dong B, Sutton R T, Woollings T and Hodges K 2013 Variability
of the North Atlantic summer storm track: mechanisms
and impacts on European climate Environ. Res. Lett. 8
034037
Drijfhout S, van Oldenborgh G J and Cimatoribus A 2012 Is a
decline of AMOC causing the warming hole above the North
Atlantic in observed and modeled warming patterns? J. Clim.
25 8373–9
Duchez A, Courtois P, Hirschi J J-M, Harris E, Josey S, Kanzow T,
Marsh R and Smeed D 2015 Potential for seasonal prediction
of Atlantic sea surface temperatures using the RAPID array at
26
◦
NClim. Dyn. 46 3351–70
Evans D G, Zika J D, Naveira Garabato A C and Nurser A J G 2014
The imprint of Southern Ocean overturning on seasonal
water mass variability in Drake passage J. Geophys. Res. Ocean.
119 7987–8010
Feudale L and Shukla J 2011a Influence of sea surface temperature
on the European heat wave of 2003 summer: I. An
observational study Clim. Dyn. 36 1691–703
Feudale L and Shukla J 2011b Influence of sea surface temperature
on the European heat wave of 2003 summer. Part II: a
modeling study Clim. Dyn. 36 1705–15
Fischer E M, Seneviratne S I, Vidale P L, Lüthi D and Schär C 2007
Soil moisture–atmosphere interactions during the 2003
European summer heat wave J. Clim. 20 5081–99
Grist J P, Josey S A, Jacobs Z L, Marsh R, Sinha B and Sebille E V
2015 Extreme air–sea interaction over the North Atlantic
subpolar gyre during the winter of 2013–14 and its sub-
surface legacy Clim. Dyn. 46 4027
Haylock M R, Hofstra N, Klein Tank A M G, Klok E J, Jones P D and
New M 2008 A European daily high-resolution gridded data
set of surface temperature and precipitation for 1950–2006
J. Geophys. Res. 113 D20119
Josey S A, Gulev S and Yu L 2013 Exchanges through the ocean
surface Ocean Circulation and Climate: A 21st Century
Perspective ed G Siedler, S Griffies, J Gould and J Church, vol
103 2nd edn (New York: Academic)pp 115–40
Kalnay E et al 1996 The NCEP/NCAR 40-year reanalysis project
Bull. Am. Meteorol. Soc. 77 437–71
Marshall J C, Williams R G and Nurser A J G 1993 Inferring the
subduction rate and period over the North Atlantic J. Phys.
Oceanogr. 23 1315–29
McCarthy G, Haigh I, Hirschi J, Grist J and Smeed D 2015 Ocean
impact on decadal Atlantic climate variability revealed by sea-
level observations Nature 521 508–10
Meehl G a and Tebaldi C 2004 More intense, more frequent, and
longer lasting heat waves in the 21st century Science 305
994–7
Nakamura M, Enomoto T and Yamane S 2005 A simulation study of
the 2003 heatwave in Europe J. Earth Simulator 255–69
Parker A and Ollier C D 2016 There is no real evidence for a
diminishing trend of the Atlantic meridional overturning
circulation J. Ocean Eng. Sci. 130–5
Perkins S E 2015 A review on the scientific understanding of
heatwaves—their measurement, driving mechanisms, and
changes at the global scale Atmos. Res. 164–165 242–67
Perkins S E and Alexander L V 2012 On the measurement of heat
waves J. Clim. 26 4500–17
Pond S and Pickard G L 1983 Introductory Dynamical Oceanography
2nd edn (Oxford: Butterworth-Heinemann)
Rahmstorf S, Box J E, Feulner G, Mann M E, Robinson A,
Rutherford S and Schaffernicht E J 2015 Exceptional
twentieth-century slowdown in Atlantic Ocean overturning
circulation Nat. Clim. Chang. 51–6
Roemmich D and Gilson J 2009 The 2004–2008 mean and annual
cycle of temperature, salinity, and steric height in the global
ocean from the Argo Program Prog. Oceanogr. 82 81–100
Russo S, Sillmann J and Fischer E M 2015 Top ten European
heatwaves since 1950 and their occur- rence in the future
Environ. Res. Lett. 10 124003
Smeed D a et al 2014 Observed decline of the Atlantic meridional
overturning circulation 2004–2012 Ocean Sci. 10 29–38
Stefanon M, D’Andrea F and Drobinski P 2012 Heatwave
classification over Europe and the Mediterranean region
Environ. Res. Lett. 7014023
Sutton R and Mathieu P-P 2002 Response of the atmosphere–ocean
mixed-layer system to anomalous ocean heat-flux
convergence Q. J. R. Meteorol. Soc. 128 1259–75
Vallis G K 2006 Atmospheric and Oceanic Fluid Dynamics
(Cambridge: Cambridge University Press)p 745
Vautard R, Honoré C, Beekmann M and Rouil L 2005 Simulation of
ozone during the August 2003 heat wave and emission
control scenarios Atmos. Environ. 39 2957–67
Walin G 1982 On the relation between sea-surface heat flow and
thermal circulation in the ocean Tellus 34 187–95
Willis J K 2010 Can in situ floats and satellite altimeters detect long-
term changes in Atlantic Ocean overturning Geophys. Res.
Lett. 37 L06602
Xoplaki E, Gonzalez-Rouco J F, Luterbacher J and Wanner H 2003
Mediterranean summer air temperature variability and its
connection to the large-scale atmospheric circulation and
SSTs Clim. Dyn. 20 723–39
9
Environ. Res. Lett. 11 (2016)074004