Variability in North Atlantic heat content and heat transport in a coupled ocean atmosphere GCM

Transcription

1 Climate Dynamics (22) 19: DOI 1.17/s B. Dong Æ R.T. Sutton Variability in North Atlantic heat content and heat transport in a coupled ocean atmosphere GCM Received: 22 May 21 / Accepted: 6 March 22 / Published online: 13 June 22 Ó Springer-Verlag 22 Abstract A coupled ocean atmosphere general circulation model has been used to study the variations of North Atlantic upper ocean heat content (OHC), sea surface temperature (SST) and ocean heat transport (OHT), and the relationships between these three quantities. We find that OHC anomalies, and salinity anomalies, propagate anti-cyclonically around the North Atlantic subtropical gyre. They propagate eastward in midlatitudes and westward in low latitudes. Both the advection of mean temperature by anomalous currents and the advection of temperature anomalies by mean currents are responsible for these zonal propagations. In addition to zonal propagations, upper ocean temperature anomalies propagate southward in the eastern North Atlantic, where subduction plays a dominant role. Variability in the northward OHT in the Atlantic is primarily governed by variability in the ocean circulation rather than variability in temperatures. Fluctuations in OHT are the major cause of anomalies in OHC and SST in the Gulf Stream extension region. This is true both for interannual variability and for decadal variability. On interannual time scales, however, surface fluxes also make a significant contribution. Analysis of the relationships of OHT with OHC and SST suggests that a knowledge of OHT fluctuations could be used to predict variations in OHC, and therefore sea surface temperatures, several years in advance. 1 Introduction In the last century the Atlantic region experienced considerable climate variability, especially on decadal time B. Dong (&) Æ R.T. Sutton Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, United Kingdom swsdong@met.reading.ac.uk scales. The North Atlantic Oscillation (NAO) index exhibited major low-frequency fluctuations, with high values predominating at the beginning and end of the century and low values in the 195s 197s (Hurrell 1995). These fluctuations were associated with large anomalies in storminess, temperature and rainfall, with major impacts on peoples and economies of Europe. Current understanding of these climate fluctuations is limited. Interactions between the oceans and the atmosphere are known to be a major cause of the climate fluctuations on interannual time scales (e.g. El Nino) and may be also a significant factor on decadal time scales (e.g. Latif 1998). Better understanding of the role of the oceans in climate variability is essential to assess the likely range of future climate fluctuations, including the climate response to increasing levels of greenhouse gases. Recent variability in the North Atlantic ocean has been the subject of a considerable number of observation and model based studies. Analyses of historical data have revealed that coherent large-scale temperature anomalies occur in the North Atlantic Ocean on interannual to decadal time scales (Deser and Blackmon 1993; Kushnir 1994; Hansen and Bezdek 1996; Sutton and Allen 1997; Watanabe et al. 1999; Grey et al. 2; Hakkinen 2; Cooper and Gordon 22). Model studies have shown that most of this variability can be explained as part of the ocean response to atmospheric variability (Halliwell 1997, 1998; Visbeck et al. 1998; Seager et al. 2; Eden and Willebrand 21; Eden and Jung 21). In particular, much of the variability in the sea surface temperature (SST) can be accounted for as part of the mixed layer response to variability in surface fluxes (Cayan 1992; Battisti et al. 1995; Seager et al. 2). Forcing by surface flux variations cannot, however, account for all the observed features of Atlantic ocean variability. One of the most intriguing features of the North Atlantic ocean is the occurrence of persistent SST anomalies propagating along preferred paths such as

2 486 Dong and Sutton: Variability in North Atlantic heat content and heat transport that of the Gulf Stream/North Atlantic Current (NAC) (Hansen and Bezdek 1996; Sutton and Allen 1997, Watanabe et al. 1999; Cooper and Gordon 22). The anomalies propagate at speeds of a few cm s 1, much slower than the peak speed in the core of the Gulf Stream. The mechanisms responsible for the formation and propagation of these anomalies are not fully understood, but there has been some recent progress. Visbeck et al. (1998) and Krahmann et al. (21) showed that, in their ocean GCM, upper ocean temperature anomalies are formed off the east coast of North America by anomalous oceanic heat transport divergence with surface fluxes playing a secondary role. The subsequent northeastward propagation is governed by a combination of oceanic advection and interference with out of phase atmospheric forcing in the second half of the pathway. In experiments with idealized atmospheric forcing the propagation speed was found to be determined largely by the forcing frequency. Saravanan et al. (2) investigated decadal variability in midlatitudes using a simple, idealized coupled midlatitude ocean atmosphere model. They suggested that the formation of SST anomalies near the western boundary can be understood in terms of the perturbation advection acting to bring warmer water from lower latitudes, across the strong meridional gradient in SST in the mean state. Away from the strong meridional gradient near the western boundary, thermal anomalies are essentially just advected by the mean background flow in the region. A somewhat different perspective on propagating heat content anomalies has been provided recently by Cooper and Gordon (22). They show that propagating anomalies can be induced by anomalous convection in the Labrador Sea. The anomalous convection generates anomalies in Labrador Sea Water (LSW) that propagate southward along the boundary of North America, and then eastward along the NAC. They argue that the timescale of surface anomalies moving along the NAC is determined by the movement of LSW anomalies at intermediate depths. Whatever the propagation mechanism, the existence of propagating temperature anomalies raises the more general issue of what is the nature and role of variations in ocean heat transport (and its divergence) in accounting for variability in the North Atlantic ocean. The heat transport is a fundamental quantity for understanding the role of oceans in climate, and understanding this transport in the Atlantic basin is of special importance because of its key role in the thermohaline circulation. Bjerknes (1964), Kushnir (1994) and Grotzner et al. (1998) all suggested that ocean heat transport variations could play an important role in the Atlantic ocean heat budget on decadal time scales. Curry and McCartney (21) have presented evidence of multidecadal variations in the baroclinic transport of the North Atlantic Current linked to changes in the NAO. The aim of this study is to investigate further the characteristics of variability in upper ocean heat content and SST, and related variability in heat transport, in the North Atlantic ocean. Our approach is to analyze output from a state of the art coupled general circulation model. We seek to address several specific questions: (1) what are the basic characteristics of the variability? 2. What are the mechanisms responsible? In particular, how are persistent ocean heat content anomalies generated and how do they evolve? What is the role of the atmospheric forcing? 3. What is the relationship between variations in oceanic heat transport and variations in ocean heat content and SST? The study is organized as follows. In Sect. 2, the coupled ocean atmosphere model and experiment are described. The model mean climate is briefly summarized in Sect. 3. The variability in upper ocean temperature and salinity is investigated in Sect. 4, and in Sect. 5 we study the variability in ocean heat transport. The processes responsible for initiating the variations in heat content are considered in Sect. 6, and finally discussions and conclusions are drawn in Sect Coupled model and experimental design The model we use is a version of the United Kingdom Hadley Centre global coupled ocean atmosphere general circulation model known as HadCM3, which is described in Gordon et al. (2). The atmospheric model component in HadCM3 is a version of the United Kingdom Meteorological Office (UKMO) unified forecast and climate model run with a horizontal grid spacing of and 19 vertical levels using a hybrid vertical coordinate. The model uses a new radiation scheme developed by Edwards and Slingo (1996), as well as a new land surface scheme (Cox et al. 1999). The detailed description of the model formulation and their performance in a simulation forced by observed SSTs are described in Pope et al. (2). The oceanic component of the model is a 2 level version of the Cox (1984) model on a latitude-longitude grid. The vertical levels are distributed to provide enhanced resolution near to the ocean surface. The two components are coupled once a day. The model does not require flux corrections to maintain a stable climate. The mean climate and its stability in a 1 year control simulation are discussed in Gordon et al. (2). We modified the model in such a way as to restrict atmosphere ocean interactions to regions outside the tropical Pacific and tropical Indian Ocean. Over the tropical Pacific and tropical Indian Ocean (Fig. 1), the SSTs were relaxed to HadCM3 climatology with a time scale of 2.5 days. The main effect of this Fig. 1. The relaxation region where the sea surface temperature is relaxed to the model climatology

3 Dong and Sutton: Variability in North Atlantic heat content and heat transport 487 relaxation is to eliminate the El Nino-Southern Oscillation (ENSO) phenomena and to suppress SST variability over tropical Indian Ocean in the simulation. This allows us to study ocean atmosphere interactions in the Atlantic basin without interference from the tropical Pacific. We performed a control integration of 1 years duration employing this relaxation. The initial state was taken from a 1 year control simulation of the globally coupled model. 3 The mean climate of the simulation Shown in Fig. 2 are the climatological annual mean sea surface temperature, surface salinity and windstress from observations and the model simulation. The observational SST and salinity are based on the Levitus climatology (Levitus 1982) and the windstress is from the ECMWF Reanalysis. Although the model climatology is similar to that observed, there are a few disagreements. The most conspicuous disagreement is that the model simulated SST front over the Gulf Stream is weaker than is observed. Both the zonal and meridional wind stresses are weaker than observed in midlatitudes. The associated underestimate of the windstress magnitude is also seen in the uncoupled model HadAM3, and discussed in Pope et al. (2). Even though the model has several deficiencies in simulating mean SST and currents, the model bias is small compared to other coupled GCMs. The simulated Atlantic ocean heat transport (OHT) is shown in Fig. 3, and is broken down into component parts due to the zonal mean meridional overturning circulation (MOC) and the gyre circulation (defined as the deviation from the zonal mean). (Note that this partitioning is not equivalent to a separation between buoyancy and wind forced circulations. For example, the overturning heat transport includes contributions from both shallow wind driven overturning and deep overturning associated with the southward flow of North Atlantic Deep Water.) The mean OHT in the model simulation is very close to that seen in the control simulation of the globally coupled model (Gordon et al. 2). The total OHT across 24.4 N is 1.18 PW with a contribution of 1.5 PW from overturning circulation and.13 PW from gyre circulation. The observation estimates of heat transport across 24 N given in Hall and Bryden (1982) are 1.22 PW with a contribution of 1.28 PW from overturning circulation and.6 PW from gyre circulation. Figure 3 also suggests that heat transport due to the gyre circulation is not important in the tropics and subtropics while it has a significant impact on total heat transport in the subpolar gyre where the east west temperature difference is as large as the temperature difference between the surface and the deep ocean. Fig. 3. Model annual mean Atlantic ocean heat transport in petawatts (1 15 watts, hereafter referred as PW) as a function of latitude 4 The variability of upper ocean temperature and salinity In this section, we examine the variations of upper ocean temperature and salinity in the model simulation. The physical mechanisms responsible for propagating features are discussed. 4.1 Propagating features of heat content and salinity anomalies For the following discussion, we use space-time diagrams to investigate the evolution of ocean heat content (upper 5 m mean temperature, hereafter referred to as OHC) and salinity variations. Shown in Fig. 4 are the anomalous OHC variations along 4.6 N, 3 W, and 15.6 N in the simulation. It can been seen that OHC anomalies propagate eastward along the northern boundary of the subtropical gyre with a phase speed of about cm s 1, which is considerably slower than the peak zonal upper ocean current of cm s 1 at this latitude. Along 15.6 N, westward propagation of heat content anomalies has a speed of cm s 1 close to Fig. 2a, b. Climatological annual mean sea surface temperature in C (contour), surface salinity in psu (grey scale) and windstress in N m 2 (vector). a Observation based on Levitus (1982), b model simulation

4 488 Dong and Sutton: Variability in North Atlantic heat content and heat transport Fig. 4a c. Model annual mean ocean heat content (mean temperature of upper 5 m, hereafter referred as OHC) anomalies ( C) as a function of longitude (latitude) and time. a At latitude 4.6 N, b at longitude 3 W, and c at latitude 15.6 N. Positive anomalies are in solid lines and negative ones in dashed lines the zonal mean current speed of 2.7 cm s 1 (despite a slight drift about.3 C/decade of the ocean heat content). In addition to the zonal propagations, there are southward propagating signals in the eastern subtropical Atlantic along 3 W. The propagation speed is about.3.4 cm s 1, which is very close to the mean current speed of.35 cm s 1. Indeed, the space time evolution of OHC anomalies indicates an apparent continuity of the middle latitude anomalies, the eastern subtropical anomalies, and the subtropical anomalies, around the subtropical gyre. For example, the cold anomalies that start propagating southward along 3 W around year 65 appear to connect to the middle latitude cold anomalies at 4.6 N in years 55 to 67. In years 75 to 9, these anomalies reached 15.6 N and appeared to propagate westward along the southern boundary of the subtropical gyre. The propagating features exist in the upper ocean temperature from depth of about 1 m to about 5 m and they are clearest at a depth of 3 m (not shown). The propagating features bear some similarity to results from analyses of observed SST anomalies (Sutton and Allen 1997; Hansen and Bezdek 1996) and upper ocean temperature anomalies (Grey et al. 2). There is evidence from observations of coherent variations in salinity associated with propagating temperature signals (Hansen and Bezdek 1996; Reverdin et al. 1997). Shown in Fig. 5 are the time evolutions of salinity content (upper 5 m mean) anomalies along 4.6 N, 3 W, and 15.6 N in the simulation. Comparison with Fig. 4 shows that the salinity content anomalies are correlated with the heat content anomalies. Higher heat content anomalies are associated with saltier water and lower heat content anomalies with fresher water. This correspondence between the heat content anomalies and salinity content anomalies implies, firstly that the anomalies are to some extent density compensated, and secondly that latent heat flux anomalies do not play a major role in generating the anomalies. Anomalous evaporation would induce correlations between low temperatures and high salinity rather than low temperatures and low salinity as we observe. In fact, the higher heat content being concurrent with higher salinity is due to the dominant role of the advection of mean temperature (salinity) by anomalous circulation, which brings warmer and saltier water from the south (Sect. 5). 4.2 Mechanisms responsible for propagating features In the previous sections we have shown that, in our model, ocean heat content and salinity anomalies propagate around the North Atlantic subtropical gyre.

5 Dong and Sutton: Variability in North Atlantic heat content and heat transport 489 Fig. 5a c. Model annual mean upper ocean salinity (mean salinity of upper 5 m) anomalies (psu) as a function of longitude (latitude) and time. a At latitude 4.6 N, b at longitude 3 W, and c at latitude 15.6 N. Positive anomalies are in solid lines and negative ones in dashed lines In addition, there is downward propagation in the eastern subtropical gyre (not shown). What are the physical mechanisms responsible for these propagating features? We have already seen that the eastward propagation speed of OHC anomalies at the northern boundary of the subtropical gyre is slower than the upper ocean current. This implies that the propagation speed is not set purely by the advection speed of the upper ocean. To elucidate the processes involved we now analyze the relative contributions to the temperature (or OHC) tendency along different sections of mean advection V rt. Krahmann et al. (21) used the same approach to study propagation along the North Atlantic Current in their ocean model. They found that both terms made significant contributions to the total tendency but that propagation was dominated by mean advection. Figure 6 shows the total tendency of the OHC, and tendencies due to ocean circulation variation and mean advection at 4.6 N. The plots are restricted to the longitude band where coherent propagation is seen. In line with Krahmann et al. (21) both terms, V rt and V rt, make a significant contribution to the total tendency. There is in fact a substantial degree of cancellation between the two terms. This cancellation can be explained if the heat content anomalies are generated by changes in ocean circulation and then advected downstream (Saravanan et al. 2; Krahmann et al. 21). The residual tendency (not shown), including contributions due to surface heat flux, entrainment, Ekman transport, and diffusion effect, has a similar magnitude as the adjustment and advection terms shown in Fig. 8b and c. It does not, however, show propagating features. An important feature of Fig. 6 is that the propagation is not restricted to the mean advection term. The advection by anomalous currents also shows eastward propagation. This result disagrees with that of Krahmann et al. (2), but is consistent with the findings of Cooper and Gordon (22). It suggests that, as in Cooper and Gordon (22) the propagation we see is substantially controlled by anomalous water masses at intermediate depths, the anomalous currents being in geostrophic balance with these water masses. At 15.6 N, both terms, V rt and V rt, again make a significant contribution to the total OHC tendency and to the propagation (not shown). Further, V rt was broken into the advection due to geostrophic current and the advection due to Ekman current. We find that the advection of mean temperature by anomalous geostrophic current dominates, while advection of mean temperature by anomalous Ekman current plays a negligible role and does not show any

6 49 Dong and Sutton: Variability in North Atlantic heat content and heat transport Fig. 6a c. Model annual mean OHC tendency ( C year 1 )asa function of longitude and time at latitude 4.6 N. a Total tendency, b tendency due to advection of mean heat content byr anomalous gyre circulation 1 H H V rt dz (gyre adjustment), and c tendency due to advection of anomalous heat content R by mean gyre circulation 1 H H V rt dz (advection). Positive anomalies are in solid lines and negative ones in dashed lines propagation. These results suggest that the westward propagation of heat content anomalies at this latitudes is governed by a combination of Rossby wave propagation and mean advection. The Rossby waves are most probably excited by variations in windstress curl (Anderson and Gill 1975). Figure 7 shows the total temperature tendency and the tendencies due to V nablat and V rt at ocean depth of 31 m along longitude 3 W. In general, the variations in V rt and total tendency are very similar. This is especially true in latitude band 15 N to 35 N, suggesting that the advection of anomalous temperature by the mean current in the eastern part of the subtropical North Atlantic gyre is the dominant process for the subsurface temperature variations (the residual tendency is also smaller than the advection term). Salinity tendency shows a similar feature, suggesting advection of anomalous salinity by mean current is responsible for southward propagation of salinity anomalies in the eastern region of the subtropical gyre. The ventilated thermocline theory (Luyten et al. 1983) provides a dynamical framework for interpreting key aspects of the behaviour of thermal and salinity anomalies in the eastern subtropical North Atlantic. It is the subduction of anomalies by the mean circulation that is the main processes responsible for equatorward and downward propagation seen in this region. 5 Variability in meridional ocean heat transport In this section, the variability of the meridional oceanic heat transport (OHT) is analyzed in the simulation and possible mechanisms behind its variation are elucidated. 5.1 Ocean heat transport variations By decomposing the meridional velocity v and ocean temperature T into time-averaged parts v and T, and deviations from the time average v and T, the northward ocean heat transport can be written as qc p Z H Z k W þ qc p Z H þ qc p Z H vtacos/dkdz ¼ qc p Z k W Z k W Z H Z k W v Tacos/dkdz þ qc p vtacos/dkdz Z H Z k W vt acos/dkdz v T acos/dkdz ð1þ

7 Dong and Sutton: Variability in North Atlantic heat content and heat transport 491 Fig. 7a c. Model annual mean ocean temperature tendency ( C year 1 ) as a function of latitude and time at longitude 3 W at depth 31 m. a Total tendency, b tendency due to advection of mean temperature by anomalous gyre circulation V rt, and c tendency due to advection of anomalous temperature by mean gyre circulation V rt. Positive anomalies are in solid lines and negative ones in dashed lines where q is a constant reference density, c p the specific heat for constant pressure, H the depth of ocean, a the Earth radius, / the latitude, k W and are the longitude of western and eastern boundaries of the ocean basin. The variability of the annual mean OHT is shown in Fig. 8, together with the contributions due to variations in the ocean circulation (with climatological temperature. i.e. v T team in Eq. 1) and variations in ocean temperature (with climatological ocean circulation. i.e. vt term in Eq. 1). The contribution to total heat transport variations by the co-variation of anomalous current and anomalous temperature (i.e., v T term in Eq. 1) is negligible (not shown). It can be seen that the OHT variation due to ocean circulation variation dominates that due to variation in ocean temperature in tropics and subtropics (note this is for zonal mean heat transport). The OHT anomalies ranges from.2 PW to.2 PW corresponding to about 2% to 3% of the model climatological heat transport depending on latitude. There are several interesting aspects of the OHT variations. First, the variations in the tropics and subtropics show a high degree of coherence with latitude. Secondly, the dominant frequency in OHT differs between low latitudes and mid-to-high latitudes. In lower latitudes, most of the variability is on multi-annual time scales. In mid-high latitudes, lower frequency variations predominate (this has been verified by spectral analysis, not shown). An alternative way to partition the OHT is to split the meridional velocity v and ocean temperature T into depth-averaged parts [v] and [T], and the deviations from the depth average v* and T *. Then the northward OHT can be written as qc p Z H Z k W þ qc p Z H vta cos / dk dz ¼ qc p Z k W Z k W H½vŠ½T Ša cos / dk v T a cos / dk dz ð2þ The first and second terms in the right hand side of Eq. 2 represent the barotropic and baroclinic heat transport respectively. The results (not shown) indicate that variations in baroclinic transport dominate in the tropics and subtropics. However, variations in the barotropic transport play a significant role in middle and high latitudes.

8 492 Dong and Sutton: Variability in North Atlantic heat content and heat transport Fig. 8a c. Latitude-time evolution of the ocean heat transport anomalies (PW) in Atlantic. a Total transport, b transport due to variation of current, and c transport due to variation of temperature. Positive anomalies are in solid lines and negative ones in dashed lines 5.2 Processes responsible for variability of ocean heat transport Fluctuations in OHT may arise from processes that are internal to the ocean, or in response to atmospheric fluctuations. Contributions to the latter component can arise from windstress-driven variations in Ekman transport, from buoyancy flux-driven variations in the thermohaline circulation and from windstress curl-driven gyre variations. The contributions of these different processes to the total variance of OHT are discussed by in a separate study (Dong and Sutton 21). Consequently we will not consider them in detail here, but simply summarise the main results. The importance of the different processes varies (not surprisingly) with timescale. This is illustrated in Fig. 9. Panel a shows the time series of a subtropical OHT index, defined as the average Atlantic OHT (computed from Fig. 8a) over the latitude band 1 N 35 N. This time series is highly correlated with the first principle component of Atlantic OHT fluctuations (see Dong and Sutton 21), with a correlation coefficient of.9. Also shown in panel a is the time series of the estimated Ekman contribution to the total OHT. The two time series are quite well correlated (r =.61) and have similar variance. The agreement is, however, substantially improved when the time series are high-pass filtered (by subtracting a 5 year running mean; Fig. 9b). The correlation rises to.77, illustrating the result of Dong and Sutton (21) that Ekman processes are the dominant contribution to interannual variations of Atlantic OHT. The low pass OHT variability is also shown in Fig. 9b. This time series is highly correlated (r =.96) with PC1 of low pass OHT discussed by Dong and Sutton (21). They show that variations on these (decadal) time scales are influenced both by buoyancy-driven meridional overturning and the windstress curl-driven gyre circulations. 6 Relationship between the variations in ocean heat transport, ocean heat content and sea surface temperature In this section, the relationship between the variability of ocean heat transport and the variability of upper ocean heat content/temperature is considered. To investigate this relationship an OHC and SST indices are defined over the midlatitude region 3 N 45 N, 8 W 4 W in the North Atlantic. This is a region where the interannual variance of OHC has a maximum. It lies to the north of the region where the subtropical OHT index is

9 Dong and Sutton: Variability in North Atlantic heat content and heat transport 493 Fig. 9a, b. Time series of subtropical OHT anomalies (PW) over (1 N 35 N) and estimated Ekman heat transport anomalies over the same latitudes. a Is for the total OHT and b for the high and low frequency anomalies Fig. 11. a Normalized time series of the high frequency subtropical OHT anomalies, and midlatitude OHC anomalies over (3 N 45 N, 8 W 4 W), b lead-lag correlation coefficients (solid ) between OHC and OHT anomalies, and those (dashed ) between OHC and surface heat flux anomalies, and c linear regression coefficients of OHC anomalies to tendency ( C year 1 ) due to anomalous OHT convergence (solid ) and tendency ( C year 1 ) due to anomalous surface heat fluxes (dashed ). d same as b but for SST. e Linear regression coefficients of SST tendency ( C year 1 ) to tendency ( C year 1 ) due to anomalous OHT convergence (solid ) and tendency due to anomalous surface heat fluxes (dashed )in upper 5 m Fig. 1a, b. Time series of the midlatitude OHC anomalies ( C) over the region (3 N 45 N, 8 W 4 W) for total, high frequency, and low frequency variations. a Based on the model simulation and b based on the Levitus data (Levitus et al. 1994) defined. Following a peak in the northward OHT over the subtropical Atlantic it is anticipated that positive OHC anomalies, and therefore SST anomalies, will develop in the OHC index region associated with anomalous OHT convergence. Figure 1 shows the midlatitude OHC anomalies for the model simulation and from observations based on Levitus (1994). The midlatitude OHC anomalies range from.6 C to.6 C in the model simulation and.35 C to.35 C in the observations. Note that the observations are shown only for the 3 year period from 196 to 199. The OHC anomalies in both the model simulation and observations show interannual and decadal time scale variabilities. We separate OHC

10 494 Dong and Sutton: Variability in North Atlantic heat content and heat transport Fig. 12a d. Differences (positive composite minus negative composite) of high frequency ocean heat content and temperature in the model simulation based on the midlatitude OHC index. Years when high frequency OHC index anomaly is greater (less) than 1.2 ( 1.2) of its standard deviation are chosen for the positive (negative) composite. a OHC ( C), b longitude-depth cross section of temperature ( C) at 4.6 N, c latitude-depth cross section of temperature ( C) at 5 W, and d SST ( C). Positive anomalies are in solid lines and negative ones in dashed lines anomalies into high frequency and low frequency variations in a similar manner as for the OHT. The time series of both low frequency and high frequency variations are also shown in Fig. 1. The decadal and multi-decadal variations are clearly seen in the low frequency variability with a magnitude similar to the total OHC anomalies. We have seen that both the subtropical OHT and midlatitude OHC vary on interannual and decadal time scales. How are the fluctuations in these variables related? To what extent are variations in subtropical OHT responsible for variations in the midlatitude OHC and SST? What is the role of local air sea surface fluxes? Are the relationships time scale dependent? We consider interannual variations first. Shown in Fig. 11a are the time series of the normalized high frequency subtropical OHT and midlatitude OHC anomalies. As expected, enhanced subtropical OHT is followed by positive OHC anomalies. This is clearly illustrated in Fig. 11b. The correlation coefficient between the OHC and OHT index is approximately.5 when the OHT leads by 1 year. Somewhat surprisingly there is also a statistically significant (negative) correlation between the time series when OHT lags by one year, i.e. when OHC leads. We believe that this feature is associated with the advection of heat content anomalies around the subtropical gyre that was discussed in Sect. 4. For example, when positive heat content anomalies that develop in the OHC index region are advected eastward and then southward around the subtropical gyre this can produce a southward (i.e. negative) contribution to the subtropical OHT. Figure 11b also shows the correlation between the OHC fluctuations and variations in the local surface heat flux. The asymmetric structure of the cross-correlation is consistent with the surface fluxes both forcing and, subsequently, damping OHC anomalies. To quantify the relative contributions of anomalous surface heat fluxes and OHT convergence to forcing OHC variations, linear regressions of the OHC tendency against the OHC tendency due to OHT convergence and surface heat fluxes are computed. The regression coefficients are shown Fig. 11c. At lag the tendency due to OHT convergence is about three times that due to surface fluxes, illustrating that variations in ocean heat transport are the dominant process governing the development of OHC anomalies in the midlatitude region on interannual timescales. Figure 11d and 11e show the results of repeating the correlation and regression analyses with OHC replaced by SST. The OHC index and the SST index are, in fact, highly correlated (not shown) with a correlation coefficient of.89. Hence it is not surprising that the results of the SST analyses are similar to the results for OHC, and indicate an important role for ocean dynamics (i.e. variability in OHT) in SST variability. Figure 11e shows

11 Dong and Sutton: Variability in North Atlantic heat content and heat transport 495 Fig. 13. a Normalized time series of the low frequency subtropical OHT anomalies, and midlatitude OHC anomalies over (3 N 45 N, 8 W 4 W), b lead-lag correlation coefficients (solid ) between OHC and OHT anomalies, and those (dashed ) between OHC and surface heat flux anomalies, and c linear regression coefficients of OHC to tendency ( C year 1 ) due to anomalous OHT convergence (solid ) and tendency ( C year 1 ) due to anomalous surface heat fluxes (dashed ). d Same as b but for SST. e Linear regression coefficients of SST tendency ( C year 1 ) to tendency ( C year 1 ) due to anomalous OHT convergence (solid ) and tendency due to anomalous surface heat fluxes (dashed )in upper 5 m the linear regressions of SST tendency to the tendency due to anomalous OHT and that due to anomalous surface heat flux (assumed to be absorbed in the upper 5 m of ocean). As is expected, the relative role of surface heat flux in SST tendency is larger in comparison with its role in OHC tendency. Nevertheless the OHT convergence is still the dominant influence on SST in the region considered. The spatial structure of ocean temperature anomalies that is associated with this interannual variability is shown in Fig. 12. The temperature anomalies are constructed as follows. We classified a particular year as a positive (negative) OHC year if the midlatitude OHC anomaly exceeds +1.2 ( 1.2) of its interannual standard deviation, and then formed composites by averaging each set of years. Anomalies were constructed as the differences between the positive composite and the negative composite. The OHC pattern shows a tripole structure with, as expected, positive OHC anomalies over the western midlatitudes, with maximum of 1. C centred on (4 N, 5 W). North and south of these positive anomalies are negative anomalies of smaller magnitude. The sea surface temperature anomalies show similar structures (Fig. 12d). The cross sections (Fig. 12b, c) indicate that maximum temperature anomalies occur at a depth of 5 1 m, decaying gradually down to around 5 m in the middle and high latitudes. We now consider the lower frequency variability in OHC and OHT. Figure 13 shows the normalized time series of the low frequency subtropical OHT, midlatitude OHC and SST anomalies (Fig. 13a) and their crosscorrelation (Fig. 13b). The large positive correlation between the OHT and OHC when the former leads indicates the dynamical effect of OHT variability on OHC variability. The highest correlations occur when the subtropical OHT leads by 3 5 years. The weak negative correlations between the OHC and surface heat flux indicate that on decadal time scales the surface heat fluxes act to damp the OHC anomalies. The linear regression of OHC tendency against the OHC tendency due to low frequency OHT convergence and low frequency surface flux variations (Fig. 13c) confirms the overwhelmingly dominant role of ocean dynamics in governing low frequency variability of OHC in the western midlatitude Atlantic. Figure 13d, e shows the results of similar analyses of the relationships between low frequency OHT, surface heat flux, and SST. As for the higher frequency variability surface fluxes are seen to be a more significant factor for SST than for OHC, but variability in OHT is still the dominant factor. As for OHC, the correlation analysis indicates that SST fluctuations lag those in OHT by 3 5 years. This lag may be associated with the time for anomalous currents to advect the mean temperature gradient. As for the interannual variability, the structure of temperature anomalies associated with the low frequency variability is derived from a composite analysis, the results of which are shown in Fig. 14. By comparison with the interannual variability there are several distinct features. Firstly, the maximum OHC (Fig. 14a) anomalies occur in an extended band along the Gulf Stream region and the path of the North Atlantic Current although this feature is less clear in the SST anomalies (Fig. 14d). Secondly, the temperature anomalies extend to greater depths, down to 1 m (not shown), both in middle and high latitudes and in the subtropics. These differences illustrate the fact that different ocean mechanisms govern the fluctuations in the two frequency bands, with thermohaline processes in particular being a much more significant factor on lower frequencies. The mechanism of variability in the thermohaline circulation, and the impacts on OHC, SST and the atmosphere

12 496 Dong and Sutton: Variability in North Atlantic heat content and heat transport Fig. 14a d. Composite low frequency pentad ocean heat content and temperature differences (year mean minus year mean) based on the model simulation. a OHC ( C), b longitude-depth cross section of temperature ( C) at 4.6 N, c latitude-depth cross section of temperature ( C) at 5 W, and d SST ( C). Positive anomalies are in solid lines and negative ones in dashed lines are currently undergoing and the results will be reported in an another study. 7 Discussion and conclusions A coupled ocean atmosphere general circulation model has been used to study the variability in the North Atlantic of upper ocean heat content (OHC), sea surface temperature (SST), and ocean heat transport (OHT), with particular interest in the relationship between these three variables. We have seen that ocean heat content anomalies and salinity anomalies propagate anti-cyclonically, following the mean currents of the subtropical gyre. The propagating features identified in the simulation are similar to those shown by Grey et al. (2) based on analysis of observations. In our model simulations the ocean heat content anomalies are correlated with salinity anomalies, with higher heat content corresponding to saltier water and lower heat content corresponding to fresher water, suggesting a direct role of ocean dynamics (anomalous advection) in the generation of anomalies. The mechanisms that govern the propagation of heat content anomalies depend on the region. Advection by the mean circulation is important in all parts of the North Atlantic, but advection by anomalous currents is a significant factor only in the zonal (rather than meridional) propagation of anomalies. In low latitudes Rossby wave activity gives rise to westward propagation. The more surprising eastward propagation that is seen in the Gulf Stream/North Atlantic Current region (Sutton and Allen 1997) appears to involve the propagation at intermediate depths of anomalous water masses that are of sufficient magnitude and scale to induce significant anomalies in the geostrophic currents (Cooper and Gordon 22). Note that this mechanism was not seen in the study of Krahmann et al. (21). Variability in the northward transport of heat (OHT) in the Atlantic is primarily governed by variability in the ocean circulation rather than variability in temperatures. This is particularly true in low latitudes and for interannual time scales. Variations in temperatures have more influence on the decadal variability of OHT, which accounts for a larger fraction of the total variance at high latitudes (i.e. in the subpolar gyre). The interannual variability of OHT is dominated by Ekman processes, while variations in the thermohaline and gyre circulations are more important on lower frequencies (Dong and Sutton 21). Fluctuations in OHT are the major cause of anomalies in OHC and SST in the Gulf Stream extension region. This is true both for interannual variability and for decadal variability. As expected, surface flux variability is more important for SST than for OHC, and is more important on interannual than on decadal time scales, but in all cases fluctuations in OHT are the

13 Dong and Sutton: Variability in North Atlantic heat content and heat transport 497 dominant factor. In the case of the interannual variability of SST the result found here for the Gulf Stream region may be contrasted with the fact that, over most of the Atlantic basin, surface fluxes are the dominant influence (e.g. Battisti et al. 1995). The dominance of ocean dynamics on decadal time scales is in line with the early suggestion of Bjerknes (1964), and with more recent results (e.g. Grotzner et al. 1998). Figure 13b and d raises the possibility that a knowledge of OHT fluctuations could be used to predict variations in OHC and SST several years in advance. If the SST variations were to impact the atmosphere then there could be applications to climate prediction. There is however need for a great deal of further work. While the basic mechanisms responsible for variations in Atlantic ocean heat transport are now known, many of the details remain to be clarified. The nature of the interactions between fluctuations in the wind-driven gyre circulation and fluctuations in the thermohaline circulation is one area where progress is urgently needed. Perhaps even more important is the need to improve understanding of how the coupled ocean atmosphere system responds to variations in ocean heat transport. Here we have focused on just one aspect: upper ocean heat content. The wider issues of climate impacts and the possibility of climate predictions will be a major focus of our future work. Acknowledgements This work was supported by the EC SINTEX project under contract ENV4-CT The computing time was provided by the UGAMP, which is funded by the UK Natural Environment Research Council. We would like to thank two anonymous reviewers for their helpful comments. References Anderson DLT, Gill AE (1975) Spin-up of a stratified ocean with application to upwelling. 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