Projected changes in the seasonal cycle of the Atlantic meridional heat transport in MPI-ESM

We investigate the effect of a projected reduction in the Atlantic Ocean meridional heat transport (OHT) on changes in its seasonal cycle. We analyze a climate projection experiment with the Max-Planck Institute Earth System Model (MPIESM) performed for the Coupled Model Intercomparison Project phase 5 (CMIP5). In the RCP8.5 climate change scenario, the OHT declines in MPI-ESM in the North Atlantic by 30-50% by the end of the 23rd century. The decline in the OHT is accompanied by a change in the seasonal cycle of the total OHT and its components. We decompose the OHT into overturning 5 and gyre component. For the total OHT seasonal cycle, we find a northward shift of 5 degrees and latitude dependent temporal shifts of 1 to 6 months that are mainly associated with changes in the meridional velocity field. We find that the shift in the OHT seasonal cycle predominantly results from changes in the wind-driven surface circulation which projects onto the overturning component of the OHT in the tropical and subtropical North Atlantic. This leads to latitude dependent shifts of 1 to 6 months in the overturning component. In the subpolar North Atlantic, we find that the reduction of the North Atlantic Deep Water 10 formation in RCP8.5 and changes in the gyre heat transport result in a strongly weakened seasonal cycle with a weakened seasonal amplitude by the end of the 23rd century and thus changes the OHT seasonal cycle in the SPG.


Introduction
Global surface temperatures are projected to warm -depending on the considered climate change scenario -intensively over the next centuries (IPCC, 2013) accompanied by a projected shift in the amplitude and phase of the seasonal cycle of surface air temperatures (Dwyer et al., 2012).In concert, the Atlantic meridional overturning circulation (AMOC) is projected to slow down (Weaver et al., 2012;IPCC, 2013) which can be attributed to a reduction of deep water formation in the North Atlantic, especially in the Labrador Sea and Greenland Sea (Vellinga and Wood, 2002).The associated Atlantic Ocean meridional heat transport (OHT) is also thought to weaken due to the direct linear relation of AMOC and OHT found in observations and model studies (Johns et al., 2011;Msadek et al., 2013).However, it is unclear how climate change along with a projected shift in the seasonal cycle of surface temperatures affects the seasonal cycle of the ocean circulation, and especially of the OHT.Here, we investigate projected changes in the OHT seasonal cycle in a Coupled Model Intercomparison Project phase 5 (CMIP5) climate projection (Taylor et al., 2012) performed in the global coupled Max-Planck Institute Earth System Model (MPI-ESM) .Lozier, 2013).The seasonal coupling between ocean and atmosphere is less understood.Minobe et al. (2010) have shown an atmospheric response to Gulf Stream variability with seasonal variations.When considering also the impact of seasonal variations in the total OHT on European climate, the relation becomes even more complex and thus requires a better understanding of the OHT and its coupling to the atmosphere.
Most of the present understanding stems from model analysis, due to a lack of continuous observations.These observations of the OHT rely on hydrographic snapshots (e.g., Bryan, 1962;Hall and Bryden, 1982;Lavin et al., 1998;Lumpkin and Speer, 2007) or inverse methods (e.g., Macdonald and Wunsch, 1996;Ganachaud andWunsch, 2000, 2003) and give estimates of the time mean OHT of about 1 PW at its maximum at about 20 • N, but do not describe the OHT variability (see also Wunsch, 2005).Further, single hydrographic snapshots may be affected by a seasonal bias due to the predominance of field work during summer.Recently, the two time series of the 26 • N Rapid array and observations at 41 • N have indicated long-term variability and a clear seasonal cycle of the OHT in the North Atlantic (Johns et al., 2011;Hobbs and Willis, 2012).
Model studies led to a better understanding of the dynamics of the seasonal cycle of the OHT.The pioneering study by Bryan (1982a) used a global ocean circulation model forced with observed winds.Bryan pointed out the importance of the winddriven Ekman mass transport and of the associated Ekman heat transport for driving the seasonal variability of the OHT, which was also found in subsequent studies (Sarmiento, 1986;Lee and Marotzke, 1998;Jayne and Marotzke, 2001;Böning et al., 2001;Cabanes et al., 2008;Balan Sarojini et al., 2011;Munoz et al., 2011).Bryan argued that changes in the zonally integrated wind stress, leading to changes in the Ekman mass transport, are balanced by a barotropic return flow.Jayne and Marotzke (2001) provided the theoretical and dynamical justification for Bryan's argumentation, stressing again the important role of the Ekman transport for the seasonal cycle of the OHT.
Traditionally, the OHT is decomposed into a vertical overturning component, which is commonly linked to the large scale overturning, and a horizontal gyre component giving correlations of the zonal deviations of the velocity and temperature field (Bryan, 1962(Bryan, , 1982b;;Bryden and Imawaki, 2001;Siedler et al., 2013).The gyre component is commonly linked to the horizontal gyre circulation and contributions from the eddy field.Previous studies have shown that the overturning component dominates the time mean, as well as the interdecadal variability of the OHT in the tropical and subtropical North Atlantic, whereas the overturning and gyre component contribute about equally to the OHT and its interdecadal variability in the subpolar North Atlantic (e.g., Eden and Jung, 2001).
With this study, we aim to understand how the seasonal cycle of the Atlantic Ocean meridional heat transport is affected by comprises MPIOM for the ocean component and ECHAM6 for the atmospheric component (Marsland et al., 2003;Jungclaus et al., 2013;Stevens et al., 2013).In MPIOM, the horizontal resolution is 1.5 degree on average with 40 unevenly spaced vertical levels (Marsland et al., 2003;Jungclaus et al., 2013).ECHAM6 has a horizontal resolution of T63 and includes 47 vertical levels (Stevens et al., 2013).
For our analysis, we focus on one member in the CMIP5 ensemble and use the historical simulation (1850-2005) extended with the Representative Concentration Pathway RCP8.5 from 2006 to 2300.In RCP8.5, a rising radiative forcing following "business as usual" is applied, which rises to 8.5W/m 2 in the year 2100, and further increases after that (van Vuuren et al., 2011).We focus in this study on long term changes in RCP8.5, comparing the period 1850-1950 for the historical simulation (HIST mean ) to the period 2200-2300 for the RCP8.5 scenario (RCP mean ), where we expect the strongest changes in the North Atlantic Ocean circulation and in the seasonal cycle of the OHT.

Projected changes in the North Atlantic sea surface temperatures
In concert with the projected warming of surface air temperatures, the sea surface temperatures (SST) are projected to warm globally and also in the North Atlantic sector in RCP8.5 (Fig. 1).A similar "warming hole" signature as found for surface air temperatures (c.f., Drijfhout et al., 2012) is present in the North Atlantic SSTs (Fig. 1) with a stronger warming in polar regions and an area of reduced warming in the SPG (Fig. 1c).Pronounced regional variations of the SST change suggest important changes in the North Atlantic Ocean circulation and its dynamics.The SST front along the Gulf Stream/North Atlantic current path shifts northward and weakens which might also impact the North Atlantic storm track as already shown for the current climate state (e.g.Minobe et al., 2008Minobe et al., , 2010;;Hand et al., 2014).

Projected changes in the North Atlantic horizontal gyre circulation and zonal-mean zonal wind
The area of reduced warming in the eastern SPG indicates changes in the North Atlantic Ocean dynamics and in the gyre circulation (e.g., Drijfhout et al., 2012).The North Atlantic barotropic stream function shows substantial changes in the annual mean pattern (Fig. 2).The barotropic stream function weakens in the subtropical gyre and intensifies in the SPG in RCP8.5.
We identify a northward shift of the subtropical gyre and a northward shift of the boundary between subtropical and subpolar gyre by about 5 degrees between the HIST mean (Fig. 2a) and RCP mean (Fig. 2b) associated with the northward shift of the atmospheric wind field (Fig. 2c).
The zonal-mean zonal wind across the Atlantic indicates considerable changes in the annual mean surface wind field in RCP8.5 (Fig.2c,3).As compared to the HIST mean , the northern Hadley cell slightly expands poleward and equatorward and the Ferrel Earth Syst.Dynam. Discuss., doi:10.5194/esd-2016-25, 2016 Manuscript under review for journal Earth Syst.Dynam.Published: 16 June 2016 c Author(s) 2016.CC-BY 3.0 License.cell shifts poleward in RCP mean in MPI-ESM (Fig. 3a) as in most CMIP5 models (e.g., Hu et al., 2013).As a consequence, the westerlies between 30 • N and 60 • N are shifted poleward in RCP8.5 by about 5 degrees (Fig. 3b,c).This shift resembles a positive NAO anomaly, which is associated with an acceleration of the westerlies over large areas of the SPG (Fig. 3b,c), along with a deceleration of the westerlies between 30 • N -40 • N and a slight intensification of the trade winds south of 30 • N.
In concert with this intensification of the surface wind field the circulation of the SPG strengthens with an increase of the average transport by about 2 Sv, which might be related to changes in heat fluxes in the SPG (e.g.Eden and Willebrand, 2001;Eden and Jung, 2001;Barrier et al., 2014).In particular, the flat-bottom Sverdrup transport in the subpolar gyre indicates only a weak increase of about 0.5 Sv in the gyre strength from HIST mean to RCP mean (not shown), suggesting that changes in the deep circulation are important (Greatbatch et al., 1991).The subtropical gyre shows a weakening in the barotropic streamfunction by about 20 Sv at its maximum at about 30 • N and by about 4 Sv in its mean indicating important changes in the dynamics of the subtropical gyre (Fig. 2).Considering the Sverdrup transport in the subtropical gyre, we find a decrease in the mean by about 1.5 Sv, while the maximum is reduced by roughly 10 Sv.In concert with the northward shift of the ocean circulation in RCP8.5 the North Atlantic current moves further north in RCP8.5.This leads to the simulated changes in the SST front (Fig. 1).

The Atlantic meridional heat transport and its decomposition
Traditionally, the meridional heat transport H is diagnosed from the zonal and vertical integral of the heat flux across an east-west section through the Atlantic (e.g., Hall and Bryden, 1982): with ρ 0 a reference density, c p the specific heat capacity of sea water, H the water depth, x the longitude, y the latitude, z the depth, x E and x W the eastern and western boundaries of the transect, v the meridional velocity and θ the potential temperature in • C.

Impact of the variability of the temperature and velocity field on the OHT
In order to assess the impact of temporal variations in the velocity field and in the potential temperature field, we separate their contributions to the OHT.In a first step we calculate the OHT with a time mean velocity field ([v], Eq. 2), and in a second step with a time mean temperature field ([θ], Eq. 3) over the analyzed periods HIST mean and RCP mean .We consider the time variability of the θ-(v-) field, such that the two contributions can be calculated from [v(x, y, z)] θ(x, y, z, t) dz dx (2) with v the meridional velocity, θ the temperature and [v] and [θ] the time mean of the velocity and temperature ( • C) field over the analyzed periods HIST mean and RCP mean .The two cases correspond to the time mean velocity field advecting the time-dependent temperature field and the time-dependent velocity field acting on the time mean temperature field.Based on this split-up of the OHT we then analyze the impact of the variability in the velocity and temperature field on the seasonal cycle of the OHT.

Overturning and gyre component of the OHT
Well-established is the decomposition of the OHT into contributions from the zonal mean vertical circulation and the horizontal circulation by considering the zonal mean (v, θ) and deviations from the zonal mean (v , θ ) of the meridional velocity and temperature field respectively: v = v + v and θ = θ + θ (e.g., Bryan, 1962Bryan, , 1982b;;Bryden and Imawaki, 2001).This yields for the OHT v(x, y, z) θ(x, y, z) dz dx giving an overturning component H ov and a gyre component H gyre from the horizontal gyre circulation.As the total OHT, both components hold mass balance by definition for a closed basin.Traditionally, the overturning component is related to the zonally averaged vertical-meridional (overturning) circulation and the gyre component is related to the horizontal transport by the large-scale gyres and small-scale eddies.
Further, an Ekman heat transport contribution to the overturning heat transport can be calculated from with τ x the zonal wind stress, f the Coriolis parameter, θ the temperature field averaged zonally and vertically across the section and θ ek the temperature of the Ekman layer following Böning and Hermann (1994).Here, the Ekman heat transport at the surface is assumed to be compensated by a deep return flow.We also assume θ ek to be close to the surface temperature, which yields only small uncertainties (Johns et al., 2011).Williams et al. (2014) analyzed contributions from the overturning, gyre and Ekman heat transport to the heat convergence in the North Atlantic for decadal signals based on perturbation experiments with and without wind.Thus, they avoid the assumption of a uniform return flow as done in Eq.5.Jayne and Marotzke ( 2001) showed the computation of the Ekman heat transport conserves mass only for short time scales of some weeks, but not necessarily for the time mean heat transport, so that we apply the Ekman transport calculation only to the OHT seasonal variability and not to the time mean OHT.
3 Mean changes in the Atlantic meridional overturning circulation and meridional heat transport

AMOC
The mean changes seen in the SSTs, the surface wind field and in the North Atlantic Ocean circulation influence the AMOC and the OHT, which we focus on in the remainder of the study.The AMOC shows significant changes in the time mean from HIST mean to RCP mean (Fig. 4).The AMOC calculated in depth coordinates shows that the northward overturning cell is reduced and shifted to the surface from the HIST mean to RCP mean (Fig. 4a,b).The maximum ψ max of the stream function x W v(x, y, z) dx dz commonly used as an index for the AMOC, is substantially reduced between 30% and 50% in the North Atlantic from HIST mean to RCP mean (Fig. 4a,b;Fig.5a).
Considering the AMOC in density coordinates (Fig. 4c,d) indicates a similar surfaceward shift of the AMOC cell to layers of lower density from HIST mean to RCP mean (Fig. 4c,d).We find only a slight decrease of the wind-driven surface cell in the tropics by about 2 Sv at the maximum, whereas the deep cell is reduced by more than 50% from a maximum of about 24 Sv in HIST mean to about 10 Sv in RCP mean .In RCP8.5, the formation of NADW in the Labrador Sea and the Nordic seas is almost absent for the 2200-2300 period.Instead of deep convection mixing surface water down to the bottom (about 3000m depth in the Labrador Basin and Irminger Basin) in the historical simulation, the maximum mixed layer depth is mostly limited to the upper 1000 meters in RCP8.5 (not shown), which thus directly reduces the deep branch of the AMOC.In addition, the AMOC's weakening is associated with a reduction of the geostrophic volume transport (Fig. 5a).For simplicity, we approximate the maximum geostrophic transport ψ geo by the residual of ψ max and the Ekman transport ψ ek given by with τ x the zonal wind stress at the ocean surface: ψ geo ≈ ψ max − ψ ek .The geostrophic transport is proportional to the zonal cross-basin density gradient which is decreased from HIST mean to RCP mean and thus reduces the AMOC in the North Atlantic (not shown).The Ekman transport indicates only small and local changes from HIST mean to RCP mean that do not contribute significantly to the weakening of the AMOC (Fig. 5a).

OHT
Similar to the AMOC, the RCP8.5 scenario reveals considerable changes in the associated OHT.For RCP mean , the OHT the OHT shows regionally varying patterns with a seasonal amplitude declining from the equator the pole and phase changes between the tropical, subtropical and subpolar North Atlantic (Fig. 6).The most obvious change in the OHT from the HIST mean to RCP mean is the reduction of the mean heat transport, which appears in almost all months (Fig. 6a,b).Since the changed seasonal cycle is superimposed on the strong reduction of the OHT, we consider in the following analysis anomalies of the seasonal cycle relative to the annual mean at every latitude (Fig. 6c,d).
The seasonal anomalies indicate changes in space and time in the OHT seasonal cycle from the HIST mean to RCP mean (Fig. 6c,d).The OHT seasonal cycle pattern shows a northward shift by about 5 degrees following the general northward shift of the atmospheric jet and the gyre circulation in RCP8.5.We also find a latitude dependent temporal shift of 1 to 6 months of the minima and maxima of the seasonal cycle that can not be fully explained by the northward shift of the pattern.The temporal shift appears to be different between the tropical, subtropical and subpolar North Atlantic.Especially latitudes along the gyre boundaries between the tropical and subtropical North Atlantic (at about 20 • N) and the subtropical and subpolar North Atlantic (at about 40 • N) indicate significant phase shifts of 4 to 6 months that mostly result from the northward shift here.
In addition, we find changes in the seasonal amplitude in RCP mean which also depend on latitude and are partly influenced by the northward shift.Between 30 • N-40 • N, the seasonal cycle generally exhibits an intensification in the amplitude, whereas the seasonal amplitude between 40 • N-50 • N is influenced mostly by the northward shift.As an example for the subtropical and subpolar gyre, the OHT seasonal cycle is shown at 30 • N and 45 • N from the HIST mean to RCP mean (Fig. 6e-f) showing prominent changes in the amplitude, the phase and the general seasonality of the OHT.

Zonal-mean zonal wind and Ekman heat transport
The seasonal cycle of the zonal-mean zonal wind indicates a seasonal maximum of the atmospheric westerly jet in winter and meridional shifts of the position of the jet from summer to winter in HIST mean (Fig. 8; shown is the full zonal-mean zonal velocity field).Especially in the tropical Atlantic, the seasonality of the wind field is strongly affected by the seasonal migration of the ITCZ (e.g., Schneider et al., 2014).Between HIST mean and RCP mean the zonal wind undergoes changes in amplitude and position of the jet with associated temporal changes in the seasonal cycle (Fig. 8a,b; c.f. Lu et al., 2014).We The seasonal cycle of the Ekman heat transport indicates a weakening in the seasonal cycle in the tropical North Atlantic with a decrease in the seasonal amplitude by about 50% from HIST mean to RCP mean (Fig. 8e-h).In the subtropical gyre, we find a dominant influence of the northward shifted westerlies on the Ekman heat transport.The Ekman heat transport in the SPG shows -in contrast to the subtropical gyre -relatively small changes in terms of the amplitude resulting in a slight strengthening in summer and a weakening in winter (Fig. 8e,f).As an example, the Ekman heat transport seasonal cycle is shown at 30 • N and 45 • N (Fig. 8g,h) indicating the influence of the northward shifted pattern.
The changes in the seasonal amplitude of the Ekman heat transport come in concert with a temporal shift of the seasonal minima and maxima (Fig. 8e-h).The Ekman heat transport in the tropical North Atlantic undergoes a 1-2 months temporal shift to later months.In the southern part of the subtropical gyre (about 20 • N-30 • N), we find the largest temporal shift of the seasonal maximum and minimum of 2-6 months to later months (Fig. 8e).In the northern part, the maximum is shifted by 1-2 months, as is the minimum.The subpolar gyre region shows only small changes in the Ekman heat transport seasonal cycle (1-2 months), while a latitude-dependent larger shift of about 5 months is identified for the maximum at about 40 • N due to the northward shift of the pattern along the gyre boundary (e.g., Fig. 8f).The shift considerably changes the seasonal cycle of the Ekman heat transport depending on latitude, closely following the seasonal cycle of the surface wind.therefore reveal clear similarities to the changes in the seasonal cycle of the total OHT (Fig. 6).We find a similar northward shift of the seasonal cycle pattern by about 5 degrees -suggesting a relation to the surface wind field -and comparable changes to the OHT in the seasonal amplitude with a 2-4 months shift of the minimum and maximum in the subtropical gyre and up to 6 months shift in the subpolar gyre.This close relation shows that changes in the seasonal cycle of the overturning component drive the changes in the seasonal cycle of the total OHT in both the subtropical and subpolar gyre (Fig. 8a,b).Similarly, the overturning component determines changes in the seasonal amplitude of the total OHT, with a reduction in the seasonal amplitude in the tropics and a slight increase of the amplitude between 30 • N and 45 • N.
In RCP mean (Fig. 8c,d), the gyre component reveals a slight intensification of the seasonal amplitude in tropical latitudes, while no significant changes in the seasonal amplitude occur in the subtropical and subpolar gyre.Important changes for the gyre component's seasonal cycle take place at about 40 • N where the gyre boundary is situated in the model.We find a northward shift in the seasonal cycle pattern in the subpolar gyre following the northward shift in the barotropic stream function and the zonal-mean zonal wind (Fig. 2) with the seasonal cycle in the subpolar gyre covering latitudes north of 40 • N in HIST mean while the seasonal cycle covering latitudes north of 45 • N in RCP mean (Fig. 8c,d).
The comparison of the changes in the OHT, the overturning component (Fig. 8a,b) and the Ekman heat transport reveals that changes in the Ekman heat transport (Fig. 8e,f) can explain a large part of the changes in the seasonal cycle of the OHT and overturning component: on the one hand by the Ekman heat transport's seasonal cycle contributing to the overturning component, on the other hand earlier studies have shown effects from wind stress on the vertical motion (heaving and shoaling) of isopycnals (Köhl, 2005;Chidichimo et al., 2010;Kanzow et al., 2010).Thereby, the surface wind stress might change the interior geostrophic flow and hence the heat transport and its variability.Overall, changes in the seasonal cycle are predominantly driven by changes in the ocean's surface and upper ocean, as also found in the seasonal cycle of the temperature transport in potential density coordinates (appendix A), indicating changes in the surface and intermediate circulation.

Discussion
The changes in the mean climate state of the North Atlantic and a projected reduction in the AMOC and OHT in MPI-ESM come in concert with changes in the seasonal cycle of the OHT.Bryan (1982a) and subsequent studies have shown that the Ekman (heat) transport is responsible for a large fraction of the seasonal variability of the overturning heat transport and thus of the total oceanic OHT.We have shown that under climate change the overturning heat transport constitutes the prominent factor for the OHT seasonal cycle on the one hand, and that the overturning heat transport is also the most important term  (Dwyer et al., 2012;Donohoe and Battisti, 2013;Dwyer et al., 2014).
Most prominent among the atmospheric changes with climate change is the expansion of the Hadley cell and the associated northward shift of the ITCZ and the mid-latitude westerlies (Sun et al., 2013;Lu et al., 2014).But the exact mechanism leading to the shift of the ITCZ and the westerlies is still not fully understood and under discussion (Seidel et al., 2008), especially in CMIP5 models where the problem of a double ITCZ occurs in some models (Hwang and Frierson, 2013;Christensen et al., 2013).As shown by Hu et al. (2013)  But changes in the surface winds and wind stress may be model dependent and may differ in detail, i.e. some models do not project a northward shift of the westerlies directly at the surface and in the associated surface wind stress.Thus, the proposed mechanism for changes in the seasonal cycle of the oceanic OHT by the Ekman heat transport and the associated changes in the geostrophic velocity field might differ between individual models used for the CMIP5 multi-model ensemble and might require a similar analysis in other CMIP5 models.
The strong decrease of the mean overturning heat transport leading to the 30-50% decrease in the OHT suggests that either the reduced meridional temperature gradient requires less heat to be transported to the poles or that a compensation mechanism must be at work, bringing additional heat from the equator to the poles to obtain a closed heat budget.In MPI-ESM, the atmosphere compensates the decrease in the meridional ocean heat transport, implying an increased atmospheric heat transport (not shown), as also suggested by Rose and Ferreira (2012).A deeper analysis of the atmospheric compensation and changes in the atmospheric heat transport is needed, but is beyond the scope of our study.
The advection of heat by the ocean determines ocean heat storage rates and is an important factor for air-sea heat exchange (Dong et al., 2007), and thus for carrying heat to the North Atlantic sector and especially towards the European continent.By the changed ocean and heat transport dynamics, the surface air-sea heat fluxes are presumably exposed to changes regarding areas of heat flux divergences and convergences and thus of heat exchange and also shifts in the seasonal cycle of surface heat fluxes, which might affect the climate over Europe.
In agreement with other studies (e.g., Gregory et al., 2005), the cooling associated with the decline of the OHT and the AMOC is smaller than the radiative heating of the atmospheric temperatures due to global warming.This yields an overall increase in surface temperature in the North Atlantic sector.Hence, it is difficult to clearly separate the effect of the reduced ocean heat with σ 2 the potential density referenced to 200 dbar, v the meridional velocity, θ the potential temperature in • C. For every density class, the temperature transport is integrated between the depth of the upper and lower limit of that density class given by the depth of the respective isopycnal z(x, y, σ 2i ) and z(x, y, σ 2i+1 ).For the temperature transport, the unit PWT is used to make clear the difference of the temperature transport to the mass balanced OHT.Even though the temperature transport does not hold mass-balance, it is an appropriate choice for the calculation of the heat flux associated with the individual water masses.But for the full integral which is the sum of the individual components of T and gives the OHT, mass is conserved.In contrast to Talley (2003), we use σ 2 as density.
Through the relation of the density, in particular of the zonal density gradient, to the geostrophic transport of the AMOC by the thermal wind relation we expect to find changes in the vertical structure where water mass properties and the potential density changes.For the definition of individual water masses, we therefore perform a regression analysis for eastern boundary fields, western boundary fields and the zonal mean fields of θ, S and σ 2 on the AMOC at 26 • N for HIST mean and RCP mean individually for annual mean values of θ, S and σ 2 .The regression analysis then enables us to identify main water masses based on changes in the vertical profiles of the regression profile of θ, S and σ 2 on the AMOC (not shown) following Baehr et al. (2007).
Based on the regression analysis, we subdivide the temperature transport into four layers with fixed potential density ranges with water masses associated with the surface circulation, an intermediate layer, North Atlantic Deep Water (NADW, including parts of the lower Labrador Sea Water=LSW, Denmark Strait Overflow Water=DSOW and Iceland-Scotland Overflow Wa-ter=ISOW) and abyssal waters from the Antarctic Bottom Water (AABW) (see table 1).The temperature-salinity diagrams reveal changes in the water mass properties from HIST mean to RCP mean with warmer and saltier waters for surface and intermediate layers in RCP mean than in HIST mean yielding layers of lighter density in RCP mean (Fig. 10).Since we find changes in the density classes and the associated water mass characteristics between HIST mean and RCP mean , the water mass definitions differ between the HIST mean and RCP mean and the individual water masses are therefore determined separately.In RCP8.5, the deep water formation in the North Atlantic is significantly reduced, leading to a change in the water mass distribution.It is not convenient anymore to define a traditional North Atlantic Deep Water, which is why the density classes used to define the individual water masses differ between HIST mean and RCP mean .A finer separation of individual water masses is not feasible in the model.For each water mass with the respective density range, we then calculate the temperature transport following Eq.
A1 and the corresponding seasonal cycles.
The temperature transport for the individual water masses confirms that the northward heat transport is mostly confined to the surface layer in the tropical and subtropical North Atlantic in HIST mean and RCP mean (Fig. 11).The intermediate water temperature transport increases from the subtropical to the subpolar gyre and dominates the total OHT between 40 In the intermediate layer the temperature transport also indicates a relevant contribution to the OHT seasonal cycle (Fig. 12 e-f).
In the tropical and subtropical North Atlantic, the seasonal cycle of the intermediate water is mainly opposite to the seasonal cycle of the surface layer in both HIST mean and RCP mean and thus partly compensates the seasonal cycle in the surface layer.
From HIST mean to RCP mean , the pattern shows shifts in the seasonal cycle of about 1 month to later months, but no clear northward shift as in the surface layer.The seasonal cycle in the subpolar gyre indicates general phase shift of up to 6 months from HIST mean to RCP mean with a shift of the maximum from summer to winter between approximately 40 • N to 50 • N and a shift of the maximum from winter to spring between 50 • N and 60 • N.
In the NADW (Fig. 12 g-h), substantial changes occur resulting from changes in the water mass formation in the North Atlantic.
In the HIST mean , the formation of NADW is present and leads to a seasonal cycle in the temperature transport of the NADW giving an important contribution especially in the subpolar gyre.In RCP mean the seasonal cycle is weakened in the remaining temperature transport of the NADW with a decrease of the seasonal amplitude, thus showing a surface-ward shift of the processes acting on the OHT seasonal cycle especially in the subpolar gyre.
The AABW seasonal cycle is generally weak and thus does not significantly contribute to the full OHT seasonal cycle (Fig. 12 i-j).Still, we find a seasonal cycle in HIST mean .In RCP mean we find changes in the seasonal cycle with a northward shift of the pattern and also latitude dependent temporal shifts.These changes in the AABW might result from changed dynamics in the Southern Ocean also influencing the global ocean circulation, which we do not focus on in this study and thus need further analysis.
global warming and what determines potential changes in the OHT seasonal cycle.For our analysis, we use a CMIP5 climate change projection performed in MPI-ESM, with a focus on the climate change scenario RCP8.5.We aim to identify changes in the seasonal cycle of OHT and its sources.To analyze different physical mechanisms that contribute to the changes in the seasonal cycle, we analyze the individual contributions to the total OHT on seasonal time scales.Therefore, we decompose the OHT into gyre-and overturning component, related to the horizontal gyre circulation and to the overturning circulation in the North Atlantic and consider changes in the wind-driven Ekman heat transport.2 Model and Methods 2.1 The CMIP5 climate change scenario RCP8.5 in MPI-ESM We analyze climate projection experiments of the CMIP5 ensemble (Taylor et al., 2012) performed in the coupled Max-Planck-Institute Earth System model in low resolution configuration (MPI-ESM-LR) integrated from 1850 to 2300.MPI-ESM-LR mean of the v-(θ-) field but consider the full spatial variations of the respective field together with the full spatial and temporal Earth Syst.Dynam.Discuss., doi:10.5194/esd-2016-25,2016 Manuscript under review for journal Earth Syst.Dynam.Published: 16 June 2016 c Author(s) 2016.CC-BY 3.0 License.
shows a pronounced weakening by 30-50% from about 1.2 PW to about 0.8 PW between 10 • N and 30 • N and from about 0.8 Earth Syst.Dynam.Discuss., doi:10.5194/esd-2016-25,2016   Manuscript under review for journal Earth Syst.Dynam.Published: 16 June 2016 c Author(s) 2016.CC-BY 3.0 License.PW to about 0.4PW between 40 • N and 55 • N by the end of the 23rd century (Fig.5b).The reduction in the total OHT in the subtropical North Atlantic can be attributed almost entirely to a reduction in the overturning heat transport, while changes in the gyre component are comparably small.Only in the SPG, the gyre component also indicates a substantial weakening, so that both the overturning and the gyre component contribute to the reduction in the total heat transport in the subpolar North Atlantic.The reduction of the overturning heat transport can be attributed to a reduction of the geostrophic contribution to the AMOC (Fig.5a) and the associated reduction of the zonally-averaged geostrophic meridional velocity field.4 Changes in the seasonal cycle of the Atlantic meridional heat transport 4.1 The total OHT To assess the response of the seasonal cycle of the OHT to a changing climate in RCP8.5, we first analyze the latitude dependent seasonal cycle of the total OHT before focusing on the seasonal cycle of individual OHT components.The seasonal cycle of

4. 1 . 1
Contributions from the seasonal variability in the temperature and velocity fieldTo identify whether changes in the seasonal cycle of the velocity field or in the temperature field dominate the changes seen in the total OHT, we consider the OHT with a time-mean velocity field [v] (Eq.2) allowing for temporal -also seasonal-variability in the potential temperature field and a time-mean temperature field [θ] allowing for temporal variability in the velocity field Earth Syst.Dynam.Discuss., doi:10.5194/esd-2016-25,2016 Manuscript under review for journal Earth Syst.Dynam.Published: 16 June 2016 c Author(s) 2016.CC-BY 3.0 License.(Eq.3), so that the non-time-mean component provides the seasonal variability only.The OHT based on [v] (Fig.7a,b) reveals a reduced seasonality compared to the full OHT seasonal variability, especially in the tropical and subtropical North Atlantic.The changes in the seasonal cycle from HIST mean to RCP mean are rather small.The OHT based on [θ] (Fig.7c,d) reproduces the bulk of the total OHT seasonal cycle and also the changes in the seasonal cycle from HIST mean to RCP mean .This clearly indicates that the strongest changes in the OHT seasonal cycle mostly result from changes in the meridional velocity field, whereas the overall warming of the ocean temperatures plays a less important role in directly changing the OHT seasonal cycle via the temperature field.
find a seasonally dependent shift and expansion of the Hadley cell and a northward shift of the Ferrel cell.During winter, the westerlies are shifted northward by about 5 degrees from HIST mean to RCP mean .In contrast to the changes in winter, we find a general broadening of the westerlies during summer in RCP mean , corresponding to a southward shift of the trade wind regime by about 2 degrees and a poleward shift of the maximum westerlies for RCP mean (Fig.8a,b).Changes of the zonal wind during summer lead to reduced easterly winds over the subtropical gyre, reduced westerlies between 40 • N and 50 • N and enhanced westerlies north of 50 • N during summer (Fig.8a,b).
Earth Syst.Dynam.Discuss., doi:10.5194/esd-2016-25,2016   Manuscript under review for journal Earth Syst.Dynam.Published: 16 June 2016 c Author(s) 2016.CC-BY 3.0 License.4.1.3Overturning and gyre heat transportThe overturning and gyre component show that similar to the time mean and long term variability the overturning component dominates the OHT seasonal cycle in the subtropical North Atlantic Fig.8a,b), while the gyre component gains influence in the subpolar gyre (Fig.8c,d).The changes in the seasonal cycle of the overturning component from HIST mean to RCP mean leading to the changes in the OHT seasonal cycle on the other hand.These changes in the overturning heat transport are mostly wind-driven by the Ekman heat transport mostly confined to the upper layers of the ocean and might also be associated with 10 Earth Syst.Dynam.Discuss., doi:10.5194/esd-2016-25,2016 Manuscript under review for journal Earth Syst.Dynam.Published: 16 June 2016 c Author(s) 2016.CC-BY 3.0 License.changes in the geostrophic interior flow from a wind-driven heaving and shoaling of the isopycnal slope, as shown for the AMOC seasonal cycle in observations (Kanzow et al., 2010), as well as changes in the water mass characteristics (appendix A).Changes in the Ekman transport and the associated vertical Ekman velocities change the isopycnal slope and thus the geostrophic velocity field.Overall the seasonal cycle of the OHT largely adjusts to a changed seasonality of the atmospheric circulation and the zonal wind in RCP8.5.Similar changes in the seasonal cycle for extreme climate change scenarios have also been found in other atmospheric variables such as surface temperatures and precipitation almost all CMIP5 models show a trend of poleward expansion of the Hadley cell in the RCP4.5 and RCP8.5 scenarios for the period 2006 to 2100.Hu et al. also show that the CMIP5 historical simulations underestimate the trend in the poleward expansion of the Hadley cell represented by reanalysis data for the preceding decades, although it is unclear whether the trend is anthropogenically forced or whether the models capture the natural variability and extent of the Hadley cell correctly.
• N and 55 • N in HIST mean and between 40 • N and 70 • N in RCP mean , reflecting the outcropping of the intermediate layer around 45 • N. The NADW contributes with a southward (negative) temperature transport to the total OHT in the subtropical gyre, representing a return flow at depth and thus partially compensates the surface intensified temperature transport in HIST mean and RCP mean .In HIST mean , the temperature transport of the NADW changes to northward (positive) transports in the subpolar gyre, signifi-13 Earth Syst.Dynam.Discuss., doi:10.5194/esd-2016-25,2016 Manuscript under review for journal Earth Syst.Dynam.Published: 16 June 2016 c Author(s) 2016.CC-BY 3.0 License.cantly increases north of 50 • N and dominates the total OHT.Here, the NADW reaches the surface with outcropping isopycnals and thus includes both, the northward flow at the surface and the southward flow at depth and determines the total OHT in the northern SPG.In RCP mean the temperature transport of the NADW is significantly reduced in the subpolar North Atlantic and yields southward temperature transports in the whole North Atlantic.This reflects, that the deep water formation in the North Atlantic is significantly reduced and the isopycnals of the NADW do not outcrop anymore in the subpolar gyre.The temperature transport of the intermediate water shows only little changes, but replaces and even intensifies the northward temperature transport of the NADW in the subpolar gyre in RCP mean .The AABW shows only small transports in the North Atlantic in both HIST mean and RCP mean .A2 Seasonal cycle in the temperature transport in potential density coordinatesWhen analyzing the seasonal cycle of the temperature transport in potential density coordinates we find a strong seasonal cycle in the temperature transport in the surface layer (Fig.12 c-d) in both HIST mean and RCP mean with seasonal amplitudes of about 3 PW and 2 PW, respectively.Between HIST mean to RCP mean the seasonal cycle pattern in the surface layer shifts significantly northward in the tropical and subtropical North Atlantic and thus alters the seasonal cycle between 20 • N to 30 • N with temporal shifts of 4 to 6 months in the minimum and maximum.Further, the seasonal cycle in the surface layer generally intensifies in the subpolar gyre in RCP mean .The surface layer seasonal cycle can be assumed to be mostly wind-driven in the tropical North Atlantic and the subtropical gyre, so that the seasonal cycle also closely follows the Ekman heat transport seasonal cycle.

Figure 2 .aFigure 4 .
Figure 2. The barotropic streamfunction (Sv = 10 6 m 3 s −1 ) in (a) the historical simulation (1850-1950) and (b) RCP8.5 (2200-2300) for the time mean over the respective period.The thick black line shows the zero contour in the historical simulation.Contour interval: 5 Sv.(c) Zonal mean zonal wind (at 1000 hP a) averaged over the North Atlantic region (90 • W to 10 • E ) for the historical simulation (black) and RCP8.5 (red) indicating the northward shift of the westerlies.

Figure 7 .
Figure 7.The Atlantic meridional heat transport seasonal cycle (in P W ) in the historical simulation (1850-1950, (a) and (c)) and RCP8.5 (2200-2300, (b) and (d)) related to the variability in the temperature field (upper panels) and to variability in the velocity field (lower panels).Shown are anomalies relative to the annual mean at every latitude.Colour interval: 0.02 P W .

Figure 8 .
Figure 8.The zonal-mean zonal wind (ms −1 ) over the North Atlantic averaged from 10 • E to 90 • W and the associated Ekman heat transport seasonal cycle (P W ). (a-b) Vertical profile of the zonal wind for historical conditions (1850-1950, black contours) and RCP8.5 (2200-2300).Contour interval in a and b: 1 m/s.(c-d) Seasonal cycle of the surface wind at 30 • N and 45 • N for historical conditions (1850-1950, black) and RCP8.5 (2200-2300, red).(e-f)Seasonal cycle the Ekman heat transport (in P W ) in the historical simulation (1850-1950, left panel) and RCP8.5 (2200-2300, right panel).(g-h) Seasonal cycle of the Ekman heat transport at 30 • N and 45 • N for historical conditions (1850-1950, black) and RCP8.5 (2200-2300, red).Shown are anomalies relative to the annual mean at every latitude.Contour interval in e and f: 0.02 P W .

Figure 9 .Figure 10 .
Figure 9.The seasonal cycle of (a-b) the overturning component and (c-d) the gyre component (in P W ) in the historical simulation (1850-1950, left panel) and RCP8.5 (2200-2300, right panel).Shown are anomalies relative to the annual mean at every latitude.Contour interval: 0.02 P W .

Figure 11 .Figure 12 .
Figure 11.(a) Time-mean temperature transport in the surface layer (red) and intermediate layer (yellow, in P W T ) compared to the total OHT (black) and (b) time-mean temperature transport in the North Atlantic Deep Water (NADW, magenta) and Antarctic Bottom Water (AABW, blue) (in P W T ) compared to the total OHT.The historical simulation (1850-1950) is shown by solid lines, RCP8.5 (2200-2300) by dashed lines.

Table 1 .
Definition of water masses