Causes and impacts of changes in the Arctic freshwater budget during the twentieth and twenty-first centuries in an AOGCM
The fourth version of the atmosphere-ocean general circulation (AOGCM) model developed at the Institut Pierre-Simon Laplace (IPSL-CM4) is used to investigate the mechanisms influencing the Arctic freshwater balance in response to anthropogenic greenhouse gas forcing. The freshwater influence on the interannual variability of deep winter oceanic convection in the Nordic Seas is also studied on the basis of correlation and regression analyses of detrended variables. The model shows that the Fram Strait outflow, which is an important source of freshwater for the northern North Atlantic, experiences a rapid and strong transition from a weak state toward a relatively strong state during 1990–2010. The authors propose that this climate shift is triggered by the retreat of sea ice in the Barents Sea during the late twentieth century. This sea ice reduction initiates a positive feedback in the atmosphere-sea ice-ocean system that alters both the atmospheric and oceanic circulations in the Greenland-Iceland-Norwegian (GIN)-Barents Seas sector. Around year 2080, the model predicts a second transition threshold beyond which the Fram Strait outflow is restored toward its original weak value. The long-term freshening of the GIN Seas is invoked to explain this rapid transition. It is further found that the mechanism of interannual changes in deep mixing differ fundamentally between the twentieth and twenty-first centuries. This difference is caused by the dominant influence of freshwater over the twenty-first century. In the GIN Seas, the interannual changes in the liquid freshwater export out of the Arctic Ocean through Fram Strait combined with the interannual changes in the liquid freshwater import from the North Atlantic are shown to have a major influence in driving the interannual variability of the deep convection during the twenty-first century. South of Iceland, the other region of deep water renewal in the model, changes in freshwater import from the North Atlantic constitute the dominant forcing of deep convection on interannual time scales over the twenty-first century.
The freshwater supply to the Arctic Ocean plays a key role in controlling the circulation and water mass properties of this ocean. Furthermore, the export of this freshwater toward the convection sites located in the Labrador and Greenland–Iceland–Norwegian (GIN) Seas significantly affects deep water formation (e.g., Manabe and Stouffer 1995; Hakkinen 1999; Jungclaus et al. 2005) and ultimately the Atlantic meridional overturning circulation (AMOC). Excessive freshwater input to the Arctic Ocean and Nordic Seas could thus influence the climate of the North Atlantic area by decreasing the northward ocean heat transport associated with the large-scale AMOC (e.g., Vellinga and Wood 2002; Fichefet et al. 2003). The present study aims at increasing our understanding of the mechanisms of freshwater release from the Arctic Ocean towards sub-Arctic regions on decadal and longer timescales in response to enhanced greenhouse gas conditions. In addition, the interrelation between changes in freshwater transport toward the convection sites and winter deep oceanic convection on interannual time scales is addressed.
During the last few decades, the physical Arctic environment has undergone large changes, as reviewed in recent literature (e.g., Serreze et al. 2000; Moritz et al. 2002; Overland et al. 2004). For example, since the mid-1960s, the annual mean Arctic surface air temperature has increased by approximately 2°C. Since the late 1970s, satellite records show a decreasing linear trend in Arctic sea ice extent at an annual mean rate of about 0.3 × 106 km2 per decade (Cavalieri et al. 2003). Submarine-based observations indicate that the sea ice thickness in deep water areas of the Arctic Ocean has decreased by roughly 1.3 m in average between 1958–1976 and 1993–1997 (Rothrock et al. 1999). River monitoring data show that the average annual discharge of freshwater from the six largest Eurasian rivers to the Arctic Ocean has increased by about 7% from 1936 to 1999, suggesting an increase of precipitation over land in high latitudes of the Northern Hemisphere during that period (Peterson et al. 2002). Hansen et al. (2001) reported that one of the cold, dense overflows across the Greenland–Scotland ridge that feeds the southward branch of the AMOC, has decreased by at least 20% during 1950–2000. Dickson et al. (2002) presented evidence of a rapid and sustained freshening of the system of overflows and entrainment that ventilate the deep Atlantic Ocean over 1965–2000. Curry et al. (2003) found systematic freshening over much of the water column at both poleward ends of the Atlantic Ocean, accompanied by large increases of salinity in the upper water column at low latitudes between the 1950s and the 1990s. Recently, Curry and Mauritzen (2005) estimated that the Nordic Seas and the sub-polar basins were diluted by an extra 19,000 ± 5,000 km3 of freshwater input between 1965 and 1995, with about half of this freshwater being added in the late 1960s at a rate of 2,000 km3 per year during the Great Salinity Anomaly (GSA) event (Dickson et al. 1988).
Whether the high-latitude freshening and the reduced overflows observed during the past four decades are a manifestation of a weakening of the AMOC remains very controversial. Indeed, the relationship between the large-scale AMOC and the northern North Atlantic freshening is difficult to infer from current observations because of the overall lack of data and the too short period of measurements compared to timescales of internal variability. Nevertheless, using five transatlantic sections at 25°N, Bryden et al. (2005) suggested recently that the AMOC has slowed down by about 30 percent between 1957 and 2004. This contrasts with the studies of Knight et al. (2005) and Latif et al. (2006) that do not support evidence of a persistent weakening of the AMOC during the last few decades, therefore sustaining some controversy on the current state of the AMOC (Kerr 2005). Using the HadCM3 climate model running over the twentieth century, Wu et al. (2004) reported a deep freshening trend of the Labrador Sea similar to that seen in observations by Dickson et al. (2002), and yet obtained an enhancement of the AMOC instead of a weakening. The situation is however very different over the twenty-first century for which the radiative forcing is much larger than the one experienced over the second half of the past century. In this case models show unequivocally, albeit with various magnitudes, a weakening of the AMOC in response to increasing anthropogenic greenhouse gas forcing (e.g., Houghton et al. 2001; Fichefet et al. 2003; Gregory et al. 2005). It is worth noting that in all models used in the Coupled Model Intercomparison Project (CMIP), the AMOC weakening is caused more by changes in surface heat flux than by changes in surface freshwater flux (Gregory et al. 2005). However, increase in both sea ice melting and river flows are generally found in these climate-change scenarios, leading to an increase in the freshwater storage of the Arctic/sub-Arctic regions. As a result, the high-latitude ocean stratification increases and the water mass transformation becomes much less efficient, thereby contributing to the AMOC weakening.
Although models agree that the northern North Atlantic freshens in response to global warming, the mechanisms driving the changes in freshwater release on decadal or longer timescales from the Arctic Ocean to the Nordic Seas and the sub-polar basin of the North Atlantic are not well understood. On interannual timescales, the influence of freshwater on deep convection in a warming climate has not yet been established. The present analysis is a first step towards understanding these mechanisms. The first objective is to gain an insight into the processes governing the oceanic freshwater export from the Arctic basin to the high latitudes of the northern North Atlantic in response to enhanced greenhouse gas conditions. We especially focus our analysis on the freshwater export at Fram Strait which represents the main source of freshwater for the sub-polar basin of the North Atlantic Ocean. The second objective is to determine whether the mechanisms responsible for the interannual variability of deep winter oceanic convection over the twentieth century are also valid over the twenty-first century. A deliberate emphasis is put on the influence of interannual changes in oceanic freshwater fluxes at boundaries enclosing the convection sites. It should be stressed that this second analysis is based on detrended variables so that the results do not depend on the long-term trends induced by the climate change experienced over the twenty-first century, such as the long-term freshening of the high latitudes for instance. However, the non linear impact of the freshening on the oceanic properties such as stratification or vertical mixing for instance is not removed by this linear detrending. The analysis is based on outputs of simulations performed with an atmosphere-ocean general circulation model for the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC AR4). In this paper, we discuss results from the experiment covering the twentieth century with anthropogenic forcings and the A1B scenario for the twenty-first century, which corresponds to an increase of CO2 until a level of 720 ppm by 2100 as defined by the Special Report on Emission Scenario (SRES).
After a brief description of the coupled climate model and its climatology in Sects. 2 and 3, respectively, the freshwater budget of the Arctic Ocean over the twentieth and twenty-first centuries is presented in Sect. 4. Section 5 deals with the mechanisms influencing the long-term changes in the liquid freshwater transport out of the Arctic Ocean through Fram Strait, while Sect. 6 is devoted to the mechanisms governing the interannual variability of deep oceanic convection over the twentieth and twenty-first centuries. Conclusions are finally given in Sect. 7.
2 Model description and experimental design
We use the fourth version of the AOGCM developed at the ``Institut Pierre Simon Laplace’’, Paris (IPSL-CM4, Marti et al. 2005). This model is made up of the atmospheric general circulation model LMDz (Hourdin et al. 2006), the oceanic general circulation model ORCA (Madec et al. 1998), the thermodynamic–dynamic sea ice model LIM (Fichefet and Morales Maqueda 1997) and the land surface scheme ORCHIDEE (Krinner et al. 2005). Time synchronisation and spatial interpolation of surface fluxes and variables between the atmospheric model and the three other components are ensured through the OASIS coupler (Valcke et al. 2004). There is no flux correction in this model. The fresh water associated with ice sheet melting is drained toward the ocean and does not affect the ice sheet geometry (no dynamics). The atmospheric model has a horizontal resolution of 3.75° in longitude and 2.5° in latitude, with 19 levels along the vertical. The oceanic model has a coarse horizontal resolution of 2° in longitude and latitude, with vertical grid spacing increasing from 10 m in the top 150 m to 500 m at the bottom (31 levels).
The outputs from the experiment covering the twentieth century, which includes only the anthropogenic forcing (observed greenhouse gases and sulphate aerosol concentrations), are first discussed. For the twenty-first century, we use outputs from the SRES A1B climate change scenario which corresponds to a continuous increase of CO2 concentration until a level of about 720 ppm by 2100. A detailed description of those simulations is provided in Dufresne et al. (2005).
3 Climatology of the Arctic Ocean
The mean sea ice velocities computed by the model (not shown) are in reasonable agreement with those derived from the 85.5 GHz Special Sensor Microwave/Imager (SSM/I) data by Emery et al. (1997). The sea ice drift features the Beaufort Gyre, the Transpolar Drift, the divergent motion on the Siberian shelf and the sea ice export through Fram Strait. The simulated large-scale ocean surface circulation (not shown) displays roughly the same features as the mean sea ice drift pattern in the Arctic Ocean, namely the Beaufort Gyre, the Transpolar Drift stream and the East Greenland Current. Both the Bering Strait inflow in the Arctic Ocean (1 Sv) and the Canadian Arctic Archipelago throughflow (0.7 Sv) are close to observational data. In the GIN Seas, a cyclonic gyre is simulated, but the inflow of Atlantic waters into the Barents Sea appears too small (0.17 Sv) compared to observational estimates of Blindheim (1989), which rather give a value of about 3 Sv. This model deficiency is mostly related to the too weak AMOC simulated by the model. The Fram Strait outflow (0.5 Sv) is therefore weaker than the one observed (3 Sv) in order to balance the mass budget of the Arctic Ocean. Consequently, the modeled Arctic Ocean is somewhat isolated from the northern North Atlantic.
4 Simulated Arctic freshwater budget over the twentieth and twenty-first centuries
4.1 Definitions of freshwater storages and fluxes
It should be noted that the calculation of freshwater fluxes is based on monthly outputs which do not allow by themselves to close the total (solid plus liquid) freshwater budget since the eddy fluxes owing to the sub-monthly variability of ocean currents, salinity and sea ice are missing. The computation of these terms would require online calculations during the course of the integration and were not made available for the present study that is based on experiments especially designed for the IPCC AR4 which already produce a large amount of data. This therefore introduces an uncertainty in the modeled oceanic freshwater transport estimates. However, as seen in Fig. 4, the imbalance associated with the absence of these terms (298 km3 year−1 over 1951–2000 and −438 km3 year−1 over 2051–2100) is an order of magnitude smaller than the major sources or sinks of fresh water. The method employed here to estimate the freshwater budget in the Arctic Ocean appears therefore as a suitable approximation.
4.2 Arctic Ocean freshwater budget over 1951–2000
Observation-based estimates of annual mean oceanic freshwater fluxes for the Arctic Ocean relative to a salinity of 34.8 psu, river input and precipitation less evaporation
3200 ± 110
Serreze et al. (2000)
2000 ± 200
Bering Strait inflow
2500 ± 300
Woodgate and Aagaard (2005)
Canadian Arctic Achipelago water outflow
−3200 ± 320
1998–2000 + model
Prinsenberg and Hamilton (2005)
Canadian Arctic Achipelago ice outflow
Prinsenberg and Hamilton (2005)
Fram Strait upper water outflow
−2400 ± 400
Meredith et al. (2001)
Fram Strait deep outflow
500 ± 130
Dickson et al. (2006)
Fram Strait ice outflow
−2300 ± 340
Vinje et al. (1998)
West Spitzbergen Current (Atlantic inflow)
−760 ± 320
Dickson et al. (2006)
Barents Sea Branch (Atlantic inflow)
−340 ± 80
Dickson et al. (2006)
As a percentage of the total freshwater input to the Arctic Ocean (7,486 km3 year−1), the dominant terms are the river runoff (46%), the Bering Strait water inflow (38%), and the net precipitation (15%). A weak liquid freshwater inflow through the Barents Sea (63 km3 year−1, 1% of the total) is simulated while observations reported by Dickson et al. (2006) rather show an annual mean export of 340 km3 year−1 there (Table 1). The magnitude of the river input (3,456 km3 year−1) is close to the value of 3,200 km3 year−1 estimated by Serreze et al. (2006). Through Bering Strait, a sea ice export of 196 km3 year−1 is simulated in contrast to the observations that show an inflow of 400 km3 year−1 (Woodgate and Aagaard 2005). However, the total (i.e., solid plus liquid) freshwater inflow at Bering Strait (2,635 km3 year−1) remains close to observation-based estimates (2,500 km3 year−1, Woodgate and Aagaard 2005). The underestimation of the net precipitation (1,136 km3 year−1), averaged over the Arctic Ocean, compared to the ERA40 reanalysis (2,000 km3 year−1) is the main cause of the lower than observed total inflow (8,500 km3 year−1, Serreze et al. 2006) of freshwater into the Arctic Ocean in the model.
The total freshwater outflow (6,955 km3 year−1) is largely dominated by the transport at Fram Strait (72%), followed by the transport through the Canadian Arctic Archipelago (25%) and the Bering Strait sea ice export (3%). At Fram Strait, the outflow is roughly equally partitioned between the solid and liquid contributions in agreement with observational data (Table 1). The annual mean ice export at Fram Strait (93,400 m3 s−1) compares quite well with data collected by Vinje et al. (1998) and Kwok et al. (2004). Finally, the strong agreement of the simulated liquid freshwater export at Fram Strait (2,453 km3 year−1) with observational-based estimates (Table 1) results from the compensation of two biases: the greater than observed liquid freshwater storage in the Arctic basin combined with the too weak Fram Strait volume flux (0.51 ± 0.44 Sv).
4.3 Arctic Ocean freshwater budget over 2051–2100
Figure 4b illustrates the mean annual freshwater budget of the Arctic Ocean in the model over 2051–2100. Compared to 1951–2000, the mean liquid freshwater storage over 2051–2100 (184,000 km3) is increased by 56%, while its mean annual trend (1,691 km3 year−1) is mutiplied by 6. During the same period, the volume of freshwater stored in sea ice is 50% less, with a mean value of 10,000 km3. It should be noted that the trend in sea ice volume, which is the same as the one in solid freshwater storage (since the sea ice salinity is constant), over 2051–2100 is weak (−24 km3 year−1) compared to 2001–2050 (−158 km3 year−1). This interesting behavior might be related to the negative feedback documented by Bitz and Roe (2004). This thermodynamic feedback implies that thinner ice does not need to thin as much than thicker ice to increase its growth rate. As a consequence, assuming that the sea ice volume changes are dominated by sea ice thickness changes rather than sea ice cover changes, the trend in sea ice volume would decrease along with the sea ice pack thickness when this latter is sufficiently small (i.e., from year 2050 onwards in our model). The total inflow (10,842 km3 year−1) and outflow (9,613 km3 year−1) of freshwater are 45 and 38% more than those simulated over 1951–2000, respectively. With the trends in freshwater storage, our results indicate therefore an imbalance of −438 km3 year−1, a small value compared to the dominant sources and sinks of freshwater (see Sect. 4.1 for further discussion).
The freshening of the Arctic Ocean is caused, with similar contributions, by the increases in continental runoff and the freshwater import from the Barents Sea. This enhanced river discharge into the Arctic Ocean (2,035 km3 year−1 = 0.065 Sv, or 59% relative change) is an indication of the increase in the integrated high-latitude net precipitation over continental areas resulting from an intensified hydrological cycle in response to global warming (Wu et al. 2005). The large shift seen in the freshwater import through the northern boundary of the Barents Sea into the Arctic basin (2,020 km3 year−1) results from the combined effect of the freshening in the GIN Seas and the strengthening of the northward volume flux through the Barents Sea. The change in the atmospheric freshwater flux (P-E) represents only a small contribution (358 km3 year−1) to the freshening of the Arctic Ocean. Liquid freshwater export at Fram Strait over the period 2051–2100 is more than twice that simulated over 1951–2000 and appears to dominate the sink of freshwater (−5,398 km3 year−1). Solid freshwater export at Fram Strait (1,691 km3 year−1) is however 35% less than over 1951–2000 in agreement with the net decrease in sea ice volume. The freshwater export through the Canadian Archipelago is enhanced by about 73% compared to 1951–2000, while the volume flux there is reduced by 20%. This indicates that the long-term increase in the freshwater export through the Canadian Archipelago is caused by the freshening of adjacent water masses in the western Arctic Ocean. In contrast, the freshwater import through the Bering Strait is decreased by about 25%. This latter feature is consistent with the freshening of the Arctic Ocean which induces a decrease in the mean meridional pressure gradient between the Pacific and the Arctic Oceans, and therefore weakens the Bering Strait throughflow (Coachman and Aagaard 1966).
5 Processes influencing the long-term changes in the Fram Strait liquid freshwater flux
About 50% of the total volume of freshwater leaves the Arctic Ocean through Fram Strait (Serreze et al. 2006). This gives a particular importance to this freshwater flux in driving the freshwater storage of the Arctic Ocean, as well as the water mass properties in the GIN Seas and to some extent the overflow into the North Atlantic. In addition, this freshwater outflow may also interact with the Atlantic meridional overturning circulation. Observation-based estimates indicate that about half of the freshwater flux there is liquid (Serreze et al. 2006). Over the late twenty-first century however, our analysis suggests that the liquid freshwater export at Fram Strait might represent a significant contribution (75%) to the total Fram Strait freshwater flux (Fig. 4b). It becomes therefore essential to determine the mechanisms that drive this export, and in particular on time scales at which human influence on climate is most likely to occur. In this section, we specifically focus our analysis on the mechanisms influencing the long-term changes in liquid freshwater export at Fram Strait. Our analysis is based on the experiments covering the twentieth century with anthropogenic forcing and the SRES A1B scenario.
5.1 Strengthening of the Fram Strait volume flux
5.2 Weakening of the Fram Strait volume flux
Around year 2080, the model predicts a second transition threshold beyond which the Fram Strait volume flux rapidly weakens (Fig. 6a). There is indeed a competition between the positive feedback described above, that is responsible for the stability of the state in which the Fram Strait volume flux is large, and the impact of the long-term freshening in the GIN Seas. This latter can be measured through the freshwater storage in the GIN Seas which is increased from 8,400 km3 to 19,300 km3 between 1951–2000 and 2051–2100. This surface freshening contributes to the stabilization of the water column and thereby decreases the ability of the deep ocean to release heat to the atmosphere. This leads to a regional cooling over the GIN Seas (Fig. 7d). Surface waters subjected to this atmospheric cooling are no longer able to sink and mix with the warmer deep layers during winter. The annual mean sea surface temperature (SST) in the GIN Seas drops by more than 2°C between 2080 and 2100, while the averaged SAT decreases by about 1°C during the same period. The cooling of the surface waters allows a southward expansion of sea ice in the Barents Sea in winter (Fig. 7e). The resulting reduced insulating effect of sea ice during that period contributes to further drop surface air temperatures. The drop in SAT averaged in the Barents Sea reaches 6°C between 2080 and 2100, much larger than over the GIN seas.
Associated with this strong regional cooling is an increase of the winter mean SLP over the GIN Seas (1 hPa change, Fig. 7f) that tends to weaken the cyclonic circulation in the GIN Seas as well as the Fram Strait outflow (Fig. 6) and the Barents Sea oceanic heat transport (Fig. 8). The weakening of the latter further amplifies the initial expansion of sea ice and thus closes the feedback loop. The decrease in the Fram Strait volume flux over 2080–2100 induces a decrease in the freshwater transport out of the Arctic Ocean. As a result, the freshwater storage dramatically increases in the Arctic Ocean during that period, the drop in SSS averaged over the Arctic Ocean being about 2.5 psu by 2100 compared to 2001–2080.
Schaeffer et al. (2004) also obtained a regional cooling south of Spitzbergen between 2060 and 2100 in response to the SRES A1B scenario. The area delineated by the decrease in SAT in their model coincides with the pattern of change in convection depth and expansion of sea ice. In our model however, the pattern of change in SAT is centered over the Barents Sea (Fig. 7d), and coincides clearly with the area of expansion of sea ice (Fig. 7e), the convection changes being located southward in the interior of the GIN Seas. This indicates that the regional cooling in our model is mostly ascribed to the insulating effect of sea ice in winter rather than to a reduced ocean heat loss induced by a collapse of convection.
Those results more or less suggest that there are two stable oscillatory states for the Fram Strait volume (and thus freshwater) flux in the model because of those positive and negative feedbacks. Furthermore, the coarse resolution of our model does not allow to represent all currents, so that we reproduce only strong and weak modes, while in reality, the shape of the transition between these stable states could be more continuous. Although those changes do not formally proove causal relationships, they suggest the possibility that the long-term freshening in the GIN Seas could lead to strong regional cooling in a warming climate, through reorganization of horizontal ocean currents and heat transports in agreement with Russell and Rind (1999) and Schaeffer et al. (2002, 2004).
6 Freshwater influence on the interannual variability of deep water renewal
The main objective of this section is to determine whether the processes governing the interannual variability of deep winter oceanic convection in the GIN Seas and South of Iceland over the twentieth century are also valid over the twenty-first century. Special emphasis is placed upon the influence of interannual freshwater fluxes at boundaries enclosing the convection sites. We use detrended timeseries of annual mean freshwater fluxes and March mixed layer depth which is used as a proxy for convection depth. The analysis is performed separately over 1901–1980 and 2021–2080. These peculiar periods have been chosen in order to avoid the periods corresponding to the regime shifts described in Sects. 5.1 and 5.2. The anomalies discussed represent the difference between the simulated field and the linear trend calculated at each grid point over these two periods.
6.1 GIN Seas
6.1.1 twentieth century
Figure 9a shows that the correlations between the March mixed layer depth anomalies in the GIN Seas and annual mean salinity anomalies (averaged in the upper 100 m and over the East Greenland Current, EGC) are smaller than 0.3 for all lags between −10 and +10 years. This indicates that freshwater anomalies are not the direct cause of convection depth anomalies during that period. Furthermore, the maximum of the mixed layer depth in the GIN Seas tends to occur about 2 years after a minimum freshwater import from the North Atlantic (Fig. 10a). This is perfectly consistent with the idea that water mass formation in the GIN Seas results from the cooling of warm and salty waters originating from the North Atlantic. On the other hand, the convection depth reaches its maximum about 2 years after the maximum of the southward sea ice and liquid freshwater fluxes at Fram Strait, with a correlation of 0.4 (Fig. 10a). Indeed, almost all sea ice passing through Fram Strait melts close to Iceland over the twentieth century. The subsequent freshwater anomalies are further advected to the North Atlantic and therefore weakly influence the deep convection that takes place in the other parts of the GIN Seas. This is consistent with the regression of winter mean (DJF) SLP upon the timeseries of annual mean Fram Strait outflow (Fig. 11a) which is characterized by a maximum over Greenland (significance 95%). This pattern does not promote eastward advection of freshwater anomalies transported at Fram Strait toward the eastern part of the GIN Seas where deep convection takes place. As a consequence, the convection depth anomalies are not directly related to the influence of the freshwater transport at Fram Strait, but rather to the atmospheric circulation anomalies as discussed below. The spatial structure of this SLP pattern is similar to the dipole anomaly identified by Wu et al. (2006) in the NCEP-NCAR reanalysis dataset. One center is over northern Eurasia and the Arctic marginal seas, and the other is over Canada and Greenland. These authors argued that this dipole anomaly is more important than the Arctic Oscillation for transport of freshwater and ice out of the Arctic Ocean. This is in direct agreement with our results.
The anomalous winter mean atmospheric circulation related to one standard deviation change in convection depth features stronger than usual north-easterly winds in the northern part, and westerly winds in the southern part of the GIN Seas (Fig. 12a). This brings cold air from the Greenland ice sheet and the West Siberian shelf and induces large heat loss in the ice-free regions of the GIN Seas, therefore promoting deep convection there. Furthermore, this atmospheric pattern tends to increase the southward sea ice export through the Fram Strait and explains the positive correlation between sea ice export and mixed layer depth at small negative lags (Fig. 10a).
In addition, when convection is strong, the GIN Seas are characterized by an anomalous cyclonic circulation in agreement with the wind forcing and the large-scale geostrophic adjustment to density increase associated with convection (not shown). This ocean circulation anomaly tends to increase both the inflow of salty Atlantic waters in the GIN Seas and the southward liquid freshwater flux at Fram Strait. The sea ice export at Fram Strait also increases since it is essentially controlled by local wind forcing (Arfeuille et al. 2000; Vinje 2001; Koenigk et al. 2005). This explains the almost out-of-phase relationship between correlations depicted in Fig. 10a. The results described here strongly support the idea that interannual variability of deep water renewal in the GIN Seas during the twentieth century is driven by the anomalous atmospheric cooling of anomalously salty surface waters.
The coarse resolution of the model could have an influence on the representation of the processes described above as it does not allow accurate representation of the lateral interactions between the East Greenland Current (EGC) and the remaining interior of the GIN Seas. Nevertheless, it should be mentioned that our results are consistent with those from other coarse resolution coupled atmosphere-ocean models (e.g., Goosse et al. 2002). They are also consistent with the conclusions of Mauritzen and Hakkinen (1997), who showed, using a coupled ice-ocean model having a higher resolution that the present one, that the sea ice export at Fram Strait does not significantly influence convection in the GIN Seas.
6.1.2 Twenty-first century
In contrast to the twentieth century, the convection depth anomalies in the GIN Seas during the twenty-first century are strongly positively correlated (0.88, significant at the 99% level) at lag −1 year (convection lags) with the upper salinity anomalies averaged over the EGC (Fig. 9b), and at zero lag with the upper salinity anomalies averaged in the eastern part of the GIN Seas. This indicates that, for this period, salinity anomalies transported in the EGC could induce changes in deep water mass formation or amplify changes caused by other mechanisms. This is confirmed by the correlation between the convection in the GIN Seas and the lateral exchanges of liquid freshwater between the EGC and the interior of the GIN Seas (not shown). The convection is maximum 1 year after the minimum freshwater import from the EGC (correlation −0.6, significance 95%). Furthermore, an increase of deep water formation in the GIN Seas tends to occur a few years after a decrease of the solid and liquid freshwater export at Fram Strait during the twenty-first century, suggesting that the freshwater anomalies transported at Fram Strait propagate over a few years in the EGC before being exported in the interior of the GIN Seas. This idea is supported by the fact that the winter mean atmospheric circulation associated with a stronger than usual Fram Strait volume flux (Fig. 11b) displays westerly winds in the northern part of the EGC. The correlation reaches 0.35 south of Spitzpergen (significance 90%). This wind forcing promotes lateral exchanges of properties between the EGC and the interior of the GIN Seas over the twenty-first century. Note that no statistically significant correlations are found at small negative lags (convection lags) between the convection and the sea ice import from the EGC into the interior of the GIN Seas (not shown).
The SLP anomaly related to warm conditions in the Barents Sea (Fig. 14) is also associated with southward advection of cold air from the central Arctic that tends to weaken the effect of freshwater on deep convection since salinity effects on deep convection overwhelm those associated with temperature. In addition, this anomalous atmospheric circulation tends to increase the oceanic heat transport in the Barents Sea, as well as the northward advection of warm air from the North Atlantic. This further amplifies the initial SAT anomaly centered over the Barents Sea (arrow 6, Fig. 13) through direct air-sea heat exchanges and intermediate sea ice processes (i.e., insulating effect in winter).
It should be mentioned that we cannot prove from our study if the northward oceanic heat transport into the Barents Sea and the subsequent warming of the the atmosphere are able to influence the SLP around Spitzbergen. Further sensitivity experiments are needed to assess the robustness of our results. However, the winter mean SLP pattern associated to one deviation standard change in Barents Sea SAT (Fig. 14) bears some resemblance with those obtained by Alexander et al. (2004) and Magnusdottir et al. (2004) who analyzed the response of the atmospheric circulation to realistic perturbations of SST and sea ice extent using stand-alone atmospheric general circulation models.
To summarize, the interannual variability of convection in the GIN Seas over the twenty-first century results from the interplay of two major feedbacks: a positive one between the Barents Sea SAT and the Barents Sea oceanic and atmospheric heat transports (arrows 2, 3 and 6, Fig. 13), and a negative one between convection and the freshwater inflow through Fram Strait and from the North Atlantic (arrows 1–5, Fig. 13). The former one was also recognized for the first time by Renssen et al. (2002) in the model of intermediate complexity ECIBilt-CLIO. These authors showed that this positive feedback plays an essential role in the response of the system to a freshwater pulse in the North Atlantic Ocean, and could explain abrupt cooling events such as the one that occured 8.2 kyr ago over Greenland. Using the same model, Goosse et al. (2003) found that this mechanism lead to the formation of large sea ice volume anomalies. Bengtsson et al. (2004) suggested that such a positive feedback could explain the Arctic warming observed in the mid twentieth century. Finally, Goosse and Holland (2005) identified this positive feedback in the second version of the Community Climate System Model (CCSM2) under present-day conditions with no changes in anthropogenic forcing.
6.2 Region located south of Iceland
6.2.1 twentieth century
Figure 9c shows that the convection depth anomalies south of Iceland are negatively correlated with the upper salinity anomalies averaged in the Irminger Sea (the western part of the box located south of Iceland shown in Fig. 1) with a correlation of −0.2 at zero lag. These correlations are not significant, suggesting that the freshwater content of the Irminger Sea does not influence the convection South of Iceland. This idea is supported by the correlations between changes in annual mean freshwater exports through the Denmark Strait and interannual convection depth anomalies (Fig. 10c). The convection depth reaches a maximum about 1 year after the maximum sea ice and liquid freshwater exports at Denmark Strait indicating that both solid and liquid freshwater fluxes are not the direct cause of the convection anomalies south of Iceland. This contrasts with the studies of Mauritzen and Hakkinen (1997), Hakkinen (1999) and Holland et al. (2001) who argued that convection in the North Atlantic is very sensitive to ice export through the Denmark Strait and its subsequent melting. The convection site in those modelling studies however is located close to the southern tip of Greenland, a region which is directly affected by sea ice export through the Denmark Strait. In contrast, the deep-convection site located south of Iceland in our model is too far eastward of the EGC to be directly influenced by sea ice export through the Denmark Strait.
In addition, the convection tends to be deep when the upper oceanic layers south of Iceland and east of the Irminger Sea are anomalously salty, with a correlation of 0.5 at zero lag (significant at the 90% confidence level, Fig. 9c). This is consistent with the correlation between the convection depth anomalies and the changes in freshwater import from the North Atlantic, which is strongly negative at zero lag (−0.8, Fig. 10c). Furthermore, the correlation between the convection anomalies and the northward volume flux from the North Atlantic into the convection area peaks at 0.8 at zero lag. These features are in agreement with the regression of the winter mean SLP anomalies upon the timeseries of the annual mean convection depth anomalies (Fig. 12c). The associated wind forcing induces a northward advection of salty waters originating from the North Atlantic. Finally, the changes in upper salinity in this region are mainly related to changes in the upper ocean circulation rather than to variations in the large-scale AMOC. Indeed, the maximum upper salinity averaged in the top 100 m tends to occur 2 years after a minimum of the AMOC index with a correlation of -0.3 that is not statistically significant.
In summary, years characterized by strong convection South of Iceland during the twentieth century are associated with unusual advection of cold air from the central Arctic. This process could act as a preconditionning for deep convection. During winter, when the convection anomalies are largest, stronger than usual southerlies induce enhanced advection of salty waters from the mid-latitudes of the North Atlantic Ocean and promote thereby deep water formation.
6.2.2 Twenty-first century
Similarly to the GIN Seas, years characterized by strong convection south of Iceland during the twenty-first century tend to be associated with anomalously salty conditions (Fig. 9d). The correlations are however much higher than during the twentieth century, suggesting that salinity changes could drive the convection depth anomalies during that period. Figure 10d illustrates the correlations between the maximum mixed layer depth and the freshwater exchanges at the Denmark Strait and with the North Atlantic. Similarly to the twentieth century, the convection is weak 2 years after the maximum freshwater import from the North Atlantic (correlation −0.6, significance 95%). This is perfectly consistent with the combination of the two following features: the winter mean atmospheric circulation associated with strong convection (Fig. 12d) which is characterized by north-easterly winds around the convection area, and the low salinity tongue in the mid-latitudes of the North Atlantic Ocean (Fig. 16). Thus, these features support the idea that positive (negative) convection depth anomalies south of Iceland during the twenty-first century result from the weaker (stronger) than usual northward advection of fresh waters originating from the northwestern part of the North Atlantic. This differs from the variability over the twentieth century. In this case, positive convection anomalies are associated with stronger than usual northward advection of salty waters originating from the mid-latitudes of the North Atlantic Ocean.
Outputs from IPSL-CM4 have been used to investigate the processes influencing the long-term changes in the freshwater balance of the Arctic Ocean and the mechanisms governing the interannual variability of deep winter oceanic convection over the twentieth and twenty-first centuries. The analyses have been performed on the basis of simulations performed for the IPCC AR4, namely the experiment covering the twentieth century, which includes only anthropogenic forcings, and the SRES A1B scenario over the twenty-first century.
The model predicts that the increases in continental runoff and liquid freshwater import from the North Atlantic through the Barents Sea constitute the major sources of freshening for the Arctic Ocean over the twenty-first century, while freshwater exports through the Fram Strait and the Canadian Arctic Archipelago represent the major sinks. In an effort to increase our understanding of the mechanisms influencing the freshwater release from the Arctic Ocean to the North Atlantic Ocean in a warming climate, we have further analysed the long-term evolution of Fram Strait liquid freshwater flux over the twentieth and twenty-first centuries. Two significant transitions in this freshwater export are found. The first one occurs between 1990 and 2010. We suggest that it is caused by a regional positive feedback in the atmosphere-sea ice-ocean system initiated by the retreat of the sea ice cover in the Barents Sea over the late twentieth century. Over the late twenty-first century, the model predicts a second transition threshold beyond which the Fram Strait volume and freshwater fluxes rapidly weaken. The long-term freshening of the GIN Seas, which indirectly affects the magnitude of the overall above-mentioned positive feedback (through reorganization of deep convection and ocean currents in the GIN seas) is pointed out as the main contender to explain this large shift. The magnitudes of the long-term changes in the Fram Strait volume flux simulated by the model are roughly 2 Sv. These are comparable to the monthly mean fluctuations reported by Fahrbach et al. (2001) using current meter moorings between 1997 and 1999 in Fram Strait. The timing of the first transition (between 1990 and 2010), depends on the timing of the retreat of the winter mean sea ice cover in the Barents Sea resulting from the warming of the Arctic climate. The timing of the second transition (between 2080 and 2100) is intimately related to the establishement of a strong halocline in the GIN Seas, which is itself influenced by vertical oceanic mixing processes, changes in sea ice, local greenhouse warming and modifications of atmospheric and oceanic transports.
The mechanisms governing the interannual variability of deep convection in the Nordic Seas have then been examined over the twentieth and twenty-first centuries on the basis of correlation and regression analyses of detrended variables. This does not allow us to demonstrate causal effects or to assess the relative contributions of various processes in a particular phenomenon. Additional sensitivity experiments will thus be needed to disentangle the feedbacks discussed in this paper. This could be addressed by specifying SST/sea ice anomalies in the Barents Sea with respect to the base state of the twentieth and twenty-first centuries and to compare the responses to the SLP patterns presented in Figs. 12 and 14. However, we were able to show that the mechanisms of interannual variability of deep convection in the GIN Seas and South of Iceland differ fundamentally between the twentieth and twenty-first centuries. The difference is caused by the dominant influence of freshwater over the twenty-first century.
More specifically, the interannual variability of deep convection in the GIN Seas over the twentieth century is driven by the anomalous atmospheric cooling of anomalously warm and salty waters originating from the North Atlantic which is in agreement with observations. Over the twenty-first century however, modifications in the southward freshwater export through Fram Strait, combined with changes in the northward inflow of fresh waters from the North Atlantic, are seen to play a major role (Fig. 13). Conversely, changes in deep convection in the GIN Seas affect the Fram Strait freshwater export through alterations of SLP around Spitzbergen during the twenty-first century. South of Iceland (i.e., the second convection site in the model), years characterized by strong convection during the twentieth century are associated with unusual salty conditions. Stronger than usual southerlies in this region during winter induce enhanced advection of salty waters from mid-latitudes of the North Atlantic Ocean, thereby promoting deep water formation. In contrast, years characterized by strong convection South of Iceland during the twenty-first century are associated with weaker than usual northward advection of fresh waters originating from a low salinity tongue in the northwestern part of the North Atlantic Ocean.
Although the large-scale atmospheric variability, such as the North Atlantic Oscillation (NAO), may influence the exchanges between the Arctic and the Atlantic Oceans, it does not seem to have a significant impact on the long-term or interannual variability of climate variables simulated in the GIN-Barents Seas region. Instead, the mechanisms are related to regional processes involving interactions in the atmosphere-sea ice-ocean system. For instance, the correlations between changes in convection in the GIN Seas, Fram Strait volume flux or Barents Sea and SLP are not statistically significant in the mid-latitudes of the North Atlantic Ocean, therefore dismissing a potential influence of the NAO. In addition, no statistically significant correlations betweeen these variables and the large-scale AMOC could be found. In contrast, the proximity of the region located south of Iceland with the mid-latitudes of the North Atlantic Ocean makes the interannual changes in convection in this region much more influenced by the NAO.
It should be stressed that our analysis is based on the results of one experiment performed with a particular model. This has the risk that our results are model dependent. The robustness of the mechanisms proposed here need therefore to be reexamined using other climate models. A major drawback of our model is the absence of convection in the Labrador Sea. This model deficiency could have an impact on the reliability of the Arctic climate change simulated by the model during the late twenty-first century. For example, Schaeffer et al. (2004) showed that the location of present-day convection sites in the ocean is crucial for the probability and strength of a regional cooling signal under global warming. The internal variability could also have a strong influence on the timing of abrupt regional cooling in the GIN Seas (Schaeffer et al. 2002), which is responsible for the weakening of the Fram Strait outflow between 2080 and 2100 in our model. Ensemble experiments are therefore needed to assess the probability of such events over the late twenty-first century in our model. In addition, our model does not include a comprehensive Greenland ice sheet model that has yet been shown to have a significant influence on the climate of the late twenty-first century (Fichefet et al. 2003). Because of all these above-mentioned limitations and biases, our results need to be reexamined using other models simulating a realistic present-day climatology.
Nevertheless, several aspects of our results are not is disagreement with previous published results obtained with different coupled models suggesting that similar mechanisms operate. This study suggests that the oceanic freshwater fluxes could drive the changes of North Atlantic deep water formation at interannual time scales during the twenty-first century, and that rapid and strong transitions in the freshwater export out of the Arctic Ocean through Fram Strait could be triggered by a moderate reduction of the sea ice cover in the Barents Sea or by a development of a strong halocine in the GIN Seas, an issue that must be kept in mind when analysing model results and observations in that region.
Didier Swingedouw kindly provided several Ferret routines for the analysis. We would like to thank Penny Ajani for having made wording suggestions on a final draft of this paper. H. Goosse is Research Associate with the Belgian National Fund for Scientific Research. This work was conducted within the European project ENSEMBLES (ENSEMBLE-based Predictions of Climate Changes and their Impacts) and the Action Concertée Incitative Changement Climatique et Cryosphère funded by the French Ministry of Research.
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