Nansen Environmental and Remote Sensing Center, Norway WWW.BJERKNES.UIB.NO Sources and sinks for heat and salt in the Arctic Mediterranean via exchanges at the Greenland-Scotland Ridge Helene R. Langehaug Nansen Environmental and Remote Sensing Center, Norway

Outline Motivation: How: Results: Summary Decadal variability (11 yrs filter) in the net amount of heat and salt to the Arctic Mediterranean – what are the sources and sinks? How: Pre-industrial multi-century simulation (600 yrs) by the fully coupled Bergen Climate Model (Otterå et al., 2009) Results: Heat- and salt transports at the Greenland-Scotland Ridge Origin of salinity anomalies at ridge Summary Otterå et al (2010), Nature Geoscience: has studied the climate in the North Atlantic region over the past 600 years using the Bergen Climate Model and the observed temperature evolution. They point to changes in the solar intensity and explosive volcanic eruptions as important causes for climate variations in the North Atlantic during this period.

Arctic Mediterranean Heat and salt anomalies important for: Deep-water formation feeding the Atlantic Overturning Excessive amounts of fresh water could alter the ocean density contrasts that drive the northernmost extension of the Atlantic MOC, diminish its northward heat transport. Nord Polar Meer, Justhus Perthes, See-Atlas 1906

Net ocean heat & salt transport to the Arctic Mediterranean Bergen Climate Model: Net heat: 284 TW Net salt: 6 kT/s (net fw = -180 mSv) Bering Strait Canadian Archipelago Northern Sea Opening Greenland-Scotland Ridge The "net" numbers are then the heat and salt convergence in AM that - for a mean circulation - must be balanced by heat loss to the atmosphere and freshwater input. There should be a freshwater flux to the ocean (due to icemelting/freezing, river runoff, precipitation-evaporation) adding up to 6 kT/s. (But, there is no gain in volume due to runoff – the freshwater is added by changing the salinity in the ocean). A closed budget; you can actually define a real reference salinity (the mean salinity of the total AM outflow). Lancaster Sound! This is in good agreement with the most recent observational estimates (Dickson et al., 2007), where the external freshwater contribution to the entire Arctic Mediterranean is 0.22 Sv, compared to the modelled of 0.16 Sv. Taking into account that Dickson et al. (2007) use present day estimates with the current melting of sea ice, the slight underestimation of the model is in good agreement for a pre-industrial run. Also, the sea ice extent is further to the south in the model (more sea ice). Annual means.

Greenland-Scotland Ridge Iceland-Faroe Ridge Denmark Strait Island-Skottland overstrømmen Faroe-Shetland Channel

Currents at the Greenland-Scotland Ridge – providing the largest heat- and salt transport into the Arctic M. Currents are defined by hydrography

Norwegian Atlantic Current: 7.4 (8.5) Sv East Greenland Current: 2.1 (0.4-2.1) Sv Overflow: 5.7 (6.4) Sv NwAC Overflow EGC Observations from: Olsen et al. (2008), Østerhus et al. (2005), Nilsson et al. (2008).

Heat transport via Greenland-Scotland Ridge Overflow (27 TW) EGC (23 TW) NwAC (308 TW) GSR GSR net heat: 258 TW AM net heat: 284 TW 91% of the ocean heat transport to AM comes via the GSR. So, if no storage in AM, that means the atmosphere should gain 284 TW of heat from the ocean. Østerhus et al. (2005) – 313 TW into the Nordic Seas on the Greenland-Scotland Ridge (compared to our NwAC). This simply reflects that the inflow is the Arctic Mediterranean’s source of salt and heat, while the outflows and the net forcing are the sinks. This is not the case in BCM, where the mean T of the outflows is higher than 0 degrees: Forskjell iforhold til obs. er gjerne størst for PW, da denne er en god del varmere i BCM (Tmiddel=3.3C) enn i obs. OW i BCM har Tmiddel=1.4C. For NwAC, temperature anomalies are small relative to the mean of ~10C, and hence the variability is carried by the volume transport. The case for OW and EGC is very different, as the mean temperatures are much closer to 0, your reference temperature. Regression between AM net ocean heat transport and the heat transport carried by the three different components at the GSR. Peak correlation for annual values; 0.67 (0yr), 0.28 (1yr), 0.23 (-7yr). Filtered values; 0.77 (0yr), 0.26 (3yr), 0.41 (-7yr). Increase in the AM net ocean heat transport: Correspond to an increase in the NwAC’s heat transport (volume transport*). The increase in the Overflow’s heat transport could reflect its correlation with the NwAC’s heat transport (0.51), rather than leading an increase in the AM heat??? But, this peak correlation is relatively small. Correspond to an increase in the EGC’s heat transport (temperature**) 7 yrs after the increase in the AM heat. *NwAC’s heat transport goes as NwAC’s volume transport (corr: 0.94). **EGC’s heat transport follows closely EGC’s temperature (corr: 0.79). Oveflow’s heat transport follows closely Overflow’s temperature (corr: 0.85). std of AM net heat = 17 TW

Densification of the NwAC through heat loss to the atmosphere m/day Long term average (Nov-Apr) of heat flux (W m-2). North Atlantic long term average extended winter (Nov-Apr) convection (colours; in m per day) together with long term average annual barotropic stream function (contours; in Sv). Solid lines indicate anticyclonic and dashed lines cyclonic circulation. Long term average (Nov-Apr) of heat flux (W m-2), where positive values indicate heat lost from the ocean to the atmosphere. Plume amount: amount of dense water that is produced within a day during winter (Nov-Apr), due to hydrostatic instability. (Medhaug et al., in prep)

Freshwater transport via Greenland-Scotland Ridge Overflow (35 psu) EGC (34.2 psu) NwAC (35.7 psu) GSR GSR net fw: 190 mSv (south) AM net fw: 180 mSv (out) There should be a freshwater flux to the ocean (due to icemelting/freezing, river runoff, precipitation-evaporation) adding up to 6 kT/s. (But, there is no gain in volume due to runoff – the freshwater is added by changing the salinity in the ocean). With a ref. salinity of 34.9 the salt transports correspond to these freshwater transports: NwAC_fw=170 mSv. Overflow_fw=20 mSv. EGC_fw=40 mSv Sutherland & Pickart (2008): ~55 mSv (EGC&EGCC) CAA: 70 mSv Curry et al. (2010): ~116 mSv %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Go quickly through this slide, since the salt transports are a reflection of the volume transports. NwAC/Overflow/EGC salt transport correlations in your notes are trivial as salinity anomalies are two orders of magnitude lower than the mean; salinity anomalies in individual currents only carry "weight" when the (appropriate) reference salinity has been subtracted. Regression between AM net ocean salt transport and the salt transport carried by the three different components at the GSR. Peak correlation at zero lag for annual values; 0.35,0.33, 0.24. Filtered values at lag -1; 0.42, 0.36, 0.36 (the peak at positive lag is higher). Increase in the AM net ocean salt transport: Correspond to an increase in the NwAC’s salt transport (volume transport*). The increase in the Overflow’s/EGC’s salt transport (volume transport*) is most probably a reflection of the high correlation between the Overflow/EGC and the NwAC (0.70/0.88). *NwAC’s/Overflow’s/EGC’s salt transport goes as its volume transport (corr: 0.998). std of AM net salt = 0.8 kT/s

Sources for salinity anomalies at the Greenland-Scotland Ridge Fresher Polar Water due to? More saline Atlantic Water due to? Since: EGC_fw & sPW: corr -0.50, 0yr. Fresher PW, more EGC freshwater transport southwards (increase in AM salt convergence). NwAC_fw & sAW: corr 0.46,1yr. More saline AW, more salt transport northwards. Look at salinity anomalies propagation.

Propagation of salinity anomalies from the subtropics -6yr -3yr 0yr +3yr Whereas heat is actively exchanged with the atmosphere, salt is a component that is approximately conserved in the ocean. Correlation between anomalies in the mixed layer salinity and the salinity of the Atlantic Inflow to the Nordic Seas: 6 years prior to maximum in the salinity of the Atlantic Inflow, 3 years prior to the maximum, at the same time as the maximum in the salinity of the Atlantic Inflow, and 6 yrs after maximum. sAW & sPW: corr 0.41, 0yr. Changes seen from observations: Warm, saline influences were clearly building in the eastern Subpolar Basins in our last time frame (1998 to 2002; fig. S3). There were also indications of higher salinities in the Atlantic inflow to the eastern Nordic Seas. See also: Häkkinen and Rhines, 2009. The signal in the Subpolar Gyre/LabSea need to be more investigated…Does it contribute to the salinity of Atlantic Water at entrance of the Nordic Seas? Correlation with the salinity of the Atlantic Water (NwAC)

Propagation of salinity anomalies from the Nordic Seas -10yr -5yr 0yr +5yr Correlation with the salinity of the Polar Water (EGC)

Summary + AM salt convergence + NwAC heat/volume transport 91% Hansen and Østerhus 2000 Fresher Polar Water + AM salt convergence More saline Atlantic Water 0.39, 2 yr 0.58, 1 yr + NwAC heat/volume transport + AM heat convergence 0.77, 0 yr + EGC heat transport (temperature) 0.41, ~7 yr 91% Decadal variability in the Arctic Mediterranean budget due to variability in the exchange and properties at the Greenland-Scotland Ridge. Changes seen from observations: The freshening of the entire system of overflow and entrainment that ventilates the deep North Atlantic has already been shown to have taken place at a remarkable, if not quite steady, rate of 0.010 to 0.015 p.s.u. per decade over the past four Decades. In: Dickson, R. R., Yashayaev, I., Meincke, J., Turrell, W., Dye, S. & Holfort, J. Rapid freshening of the Deep North Atlantic over the past four decades. Nature 416, 832–837 (2002). That freshening has been attributed to some combination of enhanced wind-driven exports of ice or fresh water from the Arctic, increased net precipitation rates, and elevated volumes of continental runoff from melting ice. During this time, the flow has been measured to be 3 Sv at each location, with little indication of sustained changes despite steadily declining salinities in the NSOW.

Arctic Mediterranean budget 0.31/0.28 (Medhaug et al., in prep)

Variability in the exchanges across the Greenland-Scotland Ridge Correlation between NwAC and Overflow: 0.70, at zero lag Correlation between NwAC and EGC: 0.88, at zero lag 11 yrs low-pass filter

Salinity at Greenland-Scotland Ridge EGC (34.2psu) NwAC (35.7psu) GSR Less saline Polar Water More saline Atlantic Water  AM net ocean salt transport increases Regression between AM net ocean salt transport and the salinity of the Polar Water (EGC)/Atlantic Water (NwAC). Peak correlation at; 0.58 and -0.39. std of AM net salt = 0.8 kT/s