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Was the Great Salinity Anomaly of the 1970S Induced by an Extreme Fram

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Was the Great Salinity Anomaly of the 1970S Induced by an Extreme Fram Was the GreatExample Salinity image Anomaly slide with smallerin the title1970s font Induced by an Extreme Fram Strait Sea Ice GHCNDExport? 1 0.75 Who Kim, Steve Yeager, & Gokhan Danabasoglu 0.5 0.25 2020 CESM Workshop 0 CVCWG Session Cumulative fraction of precipitation 10 20June 17 302020 40 50 Days Example figure with text TRMM 3B42 Thanks: Alper Altuntas (NCAR) 1 0.75 0.5 0.25 0 Cumulative fraction of precipitation 10 20 30 40 50 Days Great Salinity Anomaly of the 1970s 4 I.M. Belkin et al. /Progress in Oceanography 41 (1998) 1–68 (1) • There has been decadal-scale low salinity events in the subpolar North Atlantic (SPNA), first emerging in the sub-Arctic seas, entering the North Atlantic, and moving along the subpolar gyre • The most pronounced event: during the late 1960s and 1970s, called Great Salinity Anomaly (GSA) (2) • Conventional view of GSA (Dickson et al. 1988): 1) Enhanced Fram Strait sea-ice export (FSSIE) in the late 1960s 2) Freshwater anomaly advected to the Labrador Sea, (3) shutting down deep convection during 1969-1971 3) Continued to move following the subpolar gyre and returned back to sub-Arctic seas a decade later Belkin et al. (1998) 2020 CESM Workshop, CVCWG, W. M. Kim ([email protected]) 2 Fig. 1. A scheme by Ellett and Blindheim (1992, Fig. 6) showing transit dates for the minimal values of the “Great Salinity Anomaly” of the 1970s (by Dickson et al., 1988). The Krauss (1986) scheme of circulation has been modified in the Northeast Atlantic by Ellett and Blindheim (1992) after Meincke (1986) and others. T. Koenigk et al.: Variability of Fram Strait sea ice export: causes, impacts and feedbacks in a coupled climate model 25 JUNE 1999 HA¨ KKINEN 1785 Table 2 Same as Table 1 but positive salinity changes using years in the Labrador Sea, there would be a large 23 years with negative ice export anomalies exceeding 1.5 standard deviations influence on the entire thermohaline circulation (Jung- claus et al. 2004, in press). This might happen after large 3). Altogether, the excess ice export through the Den- Great Salinity Anomaly of the 1970sS>1 0.5< S<1 0< S<0.5 S<0 ice export anomalies that were shown by Mikolajewicz D D D D mark Strait was about 900 km3 during the 6-month pe- et al. (submitted) in model simulations with a regional riod, October 1968–March 1969. Timing of this fresh- Number years 13 6 1 3 coupled atmosphere–ocean–sea ice model. % 56.5 26.1 4.3 13.1 water pulse has a strong effect on convection down- The anomalous sea ice export through Fram Strait stream because in late fall–early winter, the new deep affects sea ice concentration in the Greenland (especially waters are formed through densification driven by ther- in the first year after the anomalies, not shown) and the • GSA has received attention because of a potential role of Arcticmeridional-origin overturning couldfreshwater be found in our model. in shuttingLabrador Sea 1–2 years later (Fig. 7 e)). The increase of mal fluxes with freshwater fluxes opposing the process. This agrees well with results of Power et al. (1994) but is sea ice in the Greenland Sea after high ice export events However, besides timing, the intensity of the pulse,down 900 deep convection contrary to findings of Ha¨ kkinen (1999). However, for is due to the direct effect of enhanced ice transport into km3 of freshwater in a few months, is equally important. longer timescales the surface salinity of the Labrador the Greenland Sea (Walsh and Chapman 1990; Wang Also noteworthy is that the Fram Strait ice export was Sea and the meridional overturning are highly positively and Ikeda 2000). In addition, increased ice export is much lower than average for several years before the correlated. It takes several years to consume the reser- associated with the anomalous advection of cold air • Several modeling studies support the conventional view voir of deep water in the Labrador Sea, which is formed from the north into the Greenland Sea. This leads to an 1968 event (H1993), which would make the 1968 export during such 1 year with strong deep convection. Thus, a intensified sea ice formation in winter and less melting in event nearly three times the preceding annual exports. single year with large or without deep convection has no summer. Sea ice cover increases in the Labrador Sea, With this guidance from the limited-area model, the significant influence on the meridional overturning. If because anomalous cold and fresh water is transported simulated ice export volume can be modulated in an salinity anomalies could persist for more than a few into the Labrador Sea after large ice export events. In Arctic–North Atlantic model by changing the ice shear Hakkinen (1999) 24 T. Koenigk etKoenigk al.: Variability of Framet Straital. sea(2006) ice export: causes, - Composite impacts and feedbacks in analysis a coupled climate using model CGCM viscosity. Thus we can arrive at an idealized simulation Fig. 8 Composite analysis for Fig. 9 Composite analysis for of the GSA and its effects downstream without requiring annual mean 10 m salinity late winter/early spring afull-blownlong-termsimulationwithinterannually anomalies in psu 1 and 2 years (February, March, April) mixed after high (left) and low (rightSalinity) layer depth anomalies in metres MLD varying forcing. The two simulations, GSA1 and GSA2, Artificially enhanced ice exports through Fram Strait 1 and 2 years after high (left) have an anomalous ice mass accumulation in the Green- and low (right) ice exports through Fram Strait land Sea (Fig. 4a) shown against the limited-area model FSSIE ice mass anomaly (shifted in y axis). In H1993 the total ; Wang ice export from the Arctic was 1700 km3 above the 1990 equilibrium. In GSA1 and GSA2, the total ice export 3 from the Arctic is 3355 and 3207 km above the equi- e)). The increase of 7 3 summarises that low librium (of 2664 and 1907 km ,respectively)forthe 2 first year and is close to zero for the following years. , left). In the Greenland Sea, These exports are much larger than in the limited-area 9 ect of enhanced ice transport into model, but most of this ice export does not exit to the ff subpolarJUNE 1999 gyre, as the excess ice export to the LabradorHA¨ KKINEN 1787 ). In addition, increased ice export is Sea is 1170 and 1240 km3 in GSA1 and GSA2, re- , in press). This might happen after large 2000 spectively during the 6-month period covering October– 2004 December of the first year and January–March of the Control The anomalous sea ice export through Fram Strait ects sea ice concentration in the Greenland (especially The fresh water input in the Labrador Sea after large second year. These ice melt anomalies constitute about ff years in theinfluence Labrador on the Sea, entireclaus there thermohaline et circulation al. would (Jung- beice a export anomalies large thatet were al. shown (submitted) by Mikolajewicz incoupled model atmosphere–ocean–sea simulations ice with model. a regional a in the first year afterLabrador the Sea anomalies, 1–2 not shown) years and later the (Fig. Perturbed sea ice in the Greenlandis Sea after due high to ice the export directthe events e Greenland Sea (Walshand and Ikeda Chapman associated with thefrom anomalous the north advection into the ofintensified Greenland sea cold Sea. ice This formation air leads in to wintersummer. an and less Sea melting in ice coverbecause increases anomalous cold in and the freshinto Labrador water the Sea, is Labrador transported Sea after large ice export events. In 50% of the climatological freshwater (P 2 E)fluxto of surface watersdriven with by deeper atmospheric forcing layerindex), (e.g. plays waters, a high role or which as low well.ice is Table NAO- exports, through Framsalinity Strait, changes in that the lead Labrador to Sea. positive ice export events stabilises theocean. local Deep stratification of convection the suppressed is 1–2 years decreased after highdenced or ice by even exports. a This totally considerable isearly reduction spring evi- (February, in March, April) the mixedup late layer to 700 depth winter/ m in theconvection Labrador Sea. depth Compared to of the mean 1,500duced m, by almost mixed half (Fig. layerit depth is decreased is by re- 200 m.the This position mainly reflects of a deep changethere in convection. is a After deepening low in the iceincreased mixed exports, convection layer is depth and obvious thus in an thean Labrador interannual Sea. On timescale, notween significant correlation the be- salinity in the Labrador Sea and the <0 the subpolar gyre (estimated from Rasmusson and Mo >0 S S D D 1996). It will be seen that it is mainly the ice export ) but is <0 part entering the subpolar gyre that influences the ther- 1994 S <0.5 D S D mohaline circulation. Figures 4b,c show these exports ). However, for with the H1993 ice exports. The GSA runs capture well 0.5< are given in standard À 1999 S the variability in ice exports as simulated by the limited- D 0.5 <1 0< À S area model with more realistic wind forcing. D < kkinen ( S ¨ D ) but positive salinity changes using ) 1< 1 left À b. Horizontal circulation changes right 1 >1 0.5< S À D < ) within 3 years after positive Fram Strait ice The excess ice mass from the Greenland Sea enters S S ) ice exports D D the Labrador Sea during the period of deep mixing in ) and low ( right left Same as Table Composite analysis for wintertime, thus disrupting the convective processes that 2020 CESM Workshop, CVCWG, W.
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