Fram Strait Ice Export During the Nineteenth and Twentieth Centuries Reconstructed from a Multiyear Sea Ice Index from Southwestern Greenland

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Fram Strait Ice Export during the Nineteenth and Twentieth Centuries Reconstructed from a Multiyear Sea Ice Index from Southwestern Greenland

TORBEN SCHMITH AND CARSTEN HANSEN* Danish Climate Centre, Danish Meteorological Institute, Copenhagen, Denmark

(Manuscript received 8 May 2002, in ®nal form 30 January 2003)

ABSTRACT Historical observations of multiyear ice, called ``storis,'' in the southwest Greenland waters exist from the period 1820±2000, obtained from ship logbooks and ice charts. It is argued that this ice originates in the Arctic Ocean and has traveled via the Fram Strait, southward along the Greenland coast in the East Greenland Current, and around the southern tip of Greenland. Therefore, it is hypothesized that these observations can be used as ``proxies'' for reconstructing the Fram Strait ice export on an annual basis. An index describing the storis extent is extracted from the observations and a linear statistical model formulated relating this index to the Fram Strait ice export. The model is calibrated using ice export values from a hindcast study with a coupled ocean±ice model over the period 1949±98. Subsequently, the model is used to reconstruct the Fram Strait annual ice export in the period 1820±2000. The model has signi®cant skill, calculated on independent data. Based on this reconstruction, it is discussed how time periods with large and small ice export on multidecadal timescales coincide with time periods of cold and warm North Atlantic sea surface temperatures reported by others. This implies that trend studies based on satellite observations should be regarded with some care, since the time period of satellite observations, the last decades, where a particularly strong negative trend is observed in the ice export, is preceded by a time period with a positive trend. The occurrence of ``great salinity anomalies'' (GSAs) is also connected to the multidecadal variability. The GSAs observed in Greenland waters around 1968± 70 and 1980±82 both occurred when the general level of ice export was high. Prior to these there was a long period with generally low ice export and no GSAs, but during an epoch around the turn of the nineteenth century several GSAs occurred. Finally, it is found that the correlation between the Fram Strait ice export and the North Atlantic Oscillation (NAO) index has alternating intervals of signi®cant and nonsigni®cant correlation throughout the period.

1. Introduction land Current (EGC) with minor contributions from gla- ciers and locally formed sea ice. In January or February Multiyear sea ice originating in the Arctic Ocean is the ice pack usually stretches all the way down to Cape being exported through the Fram Strait into the Green- Farewell. From Cape Farewell the EGC bends north- land Sea (a map with relevant names is shown in Fig. westward and continues in the West Greenland Current 1) and represents the largest drain in the freshwater (WGC) along the southwestern coast of Greenland. balance of the Arctic Ocean (Aagaard and Carmack Also, the polar ice, which here has an average thickness 1989). According to the most recent estimate (Vinje et of more than 3 m, (Buch 1991) is carried along the coast al. 1998), the ice volume ¯ux amounts to 2846 km3 yrϪ1 in the WGC in a thin band (Wadhams 2000). The ice on average, but varying between 2046 and 4687 km3 of polar origin is on this coast known as ``storis'' and yrϪ1 during the years 1990±96. The ice export rate varies has its maximum northern extent in midsummer. over the year in a characteristic pulsation with maximum From the Fram Strait to the southwestern coast of in winter (December±April) and minimum in August. Greenland the Arctic sea ice travels a distance of more The bulk of this exported ice is carried southward along the eastern coast of Greenland in the East Green- than 2000 km, giving an average velocity of 0.1±0.2 m sϪ1 for a travel time of 6 months. This is agreement with oceanographic investigations of the EGC, where Ϫ1 * Current af®liation: Royal Danish Administration of Navigation velocities up to 0.5 m s are not unusual (see Bacon and Hydrography, Copenhagen, Denmark. et al. 2002, and references therein). Other types of ice are found in the southwest Green- land waters, namely, the ``westice,'' which is ®rst-year Corresponding author address: Dr. Torben Schmith, Danish Me- teorological Institute, Lyngbyvej 100, DK-2100 Copenhagen é, Den- ice formed in the Baf®n Bay and Davis Strait, and lo- mark. cally formed coastal ice. These ice types have their max- E-mail: [email protected] imum extent during winter and are therefore clearly dis-

᭧ 2003 American Meteorological Society

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following grounds: transport through the Smiths Sound was not possible since the ice ¯oes would then go along the coast of Baf®n and Labrador, and transport along the Siberian coast would take more than three years. Nansen's idea was that an appropriately built ship could also, starting in the East Siberian Sea, traverse the Arctic Ocean, thereby passing close to the North Pole, and exit via the Fram Strait. Since the storis observed on the southwestern coast during spring and summer originates in the Arctic Ocean via the EGC±WGC, we ®nd it meaningful to hypoth- esize that historical storis observations can be related to the Fram Strait ice export. It is the aim of the present work to investigate whether the Fram Strait ice export can actually be reconstructed from the historical storis FIG. 1. Map showing Greenland with surroundings; important names are given. observations. Such a reconstructed series of Fram Strait ice export will have a length of almost two centuries. Changes in the Arctic sea ice conditions have drawn tinguishable from the storis. For completeness, we note attention within recent years. Rothrock et al. (1999) that icebergs also occur here. For a general description described a thinning of the Arctic ice cover, based on of the oceanography of the Greenland coastal waters thickness measurements from submarine-borne instru- including ice conditions, see Nielsen (1928) and Buch ments over the period 1958±97. However, since most (1991). submarine data are from the Canadian Arctic, Holloway That material objects frequently travel the distance and Sou (2002) point to the danger of getting spurious from the Fram Strait, and even from the Siberian coasts, trends due to redistribution of ice masses by wind. Par- to the southwest coast of Greenland is manifested in the kinson et al. (1999) reported on generally decreasing abundant occurrence of Siberian driftwood on this coast. sea ice extents based on satellite passive microwave data Although almost no trees grow on these coasts, the lead- from the period 1978±97. Johannessen et al. (1999) ing ``Saqqaq'' culture had woodworking as one of its found a trend in the composition of the Arctic ice cover activities (Groennov 1996). The missionary Fabricius from satellite measurements toward a smaller fraction (1810) wrote, ``Usually it comes with the drift ice . . . of multiyear ice during the past 20 years. whole trees, roots and all . . .'' In southwest Greenland The signi®cance of these ®ndings should be evaluated there is also a village named ``Nanortalik'' (meaning against the decadal and interdecadal variability of the ``polar bear village''), despite the natural habitat of polar Arctic Ocean circulation including its sea ice conditions bears being the Arctic Ocean. This is another illustration documented by Ikeda (1990), Mysak and Power (1992), of the ice drift: from time to time polar bears travel on Proshutinsky and Johnson (1997), Polyakov and John- ice ¯oes from the Arctic Ocean and make their landing son (2000), and others. Since variations in Arctic sea in southwest Greenland. ice conditions are re¯ected in the Fram Strait ice export Nansen (1897) tells the anecdote of how he got the (HaÈkkinen 1993; Hilmer et al. 1998; KoÈberle et al. 1999; idea for reaching the North Pole and this is an excellent Arfeuille et al. 2000), knowledge of the long-term var- illustration of the general movement of the ice masses iability of the Fram Strait ice export would contribute in the Arctic Ocean. Many attempts to reach the North to the assessment the signi®cance of such ®ndings. Pole by ship via the strait between Greenland and Spits- The freshwater export from the Arctic Ocean, in- bergen (later named Fram Strait) had been carried out cluding the Fram Strait ice export, is not merely a pas- in the 19th century but they were all stopped by a mas- sive indicator of the state of the Arctic climate system, sive ice pack. The American Jeanette expedition was but is also an active component. Increased export of the one of the few attempts to enter the Arctic Ocean via freshwater as occurred during the great salinity anomaly the Bering Strait. However, it was not successful: Jea- (GSA; Dickson et al. 1988) stabilizes the upper water nette sank in the East Siberian Sea, demolished by the column in the Greenland, Iceland, and Labrador Seas. ice masses. Three years later Nansen read a newspaper This leads to increased formation of sea ice and dimin- article by Professor Mohn, telling that identi®able re- ished production of intermediate and deep water masses mains from Jeanette, for example, handwritten docu- through ocean convection. Also, the model study by ments and clothes with names of crew, had been found HaÈkkinen (1999) shows that changes in the freshwater on the southwest coast of Greenland. Professor Mohn ¯uxes from the Arctic Ocean similar to the GSA could argued that these things could only have been trans- change the convection intensity and pattern and thereby ported on ice ¯oes across the Arctic Ocean in what later in¯uence the global thermohaline circulation (THC). was named the transpolar drift stream and, subsequently, The THC possesses enhanced variability on the de- the EGC±WGC. Other routes were discarded on the cadal±interdecadal timescales, as reviewed by Rahms-

Unauthenticated | Downloaded 09/28/21 03:11 PM UTC 2784 JOURNAL OF CLIMATE VOLUME 16 torf (1999). This is also seen in long runs with coupled pressure (MSLP) patterns. Vinje (2001) analyzed 135 atmosphere±ocean GCMs without external forcing (La- years of sea ice in the Nordic Seas and related the recent tif 1998), pointing to internal oscillatory modes in the retreat of ice cover to the natural variability over the climate system, being either a truly coupled mode (Tim- long time span. He concluded that the retreat of the sea mermann et al. 1998) or an ocean-only mode (Delworth ice began prior to the warming of the Arctic. He ascribed et al. 1993; Goosse et al. 2002). Weaver et al. (1991) the ice retreat mainly to rising ocean temperatures. observed interdecadal variability in the ocean in long The investigation presented in this paper builds on integrations of an idealized ocean model when there was storis observations from ships plying the southwestern, a large freshwater ¯ux in northern latitudes. If such inhabited coast of Greenland. As we will explain in the modes occur in nature, they may be possible to predict. following section, this record can be extended back to In connection with global warming scenarios, the is- 1820, which is much longer back in time than sea ice sue of the stability of the THC to the climatic changes datasets covering the whole Arctic, for example, the is still discussed. Comparison of the model studies by Global Sea Ice and Sea Surface Temperature data Mikolajewicz and Voss (2000) and by Dixon et al. (GISST2.2) described in Parker et al. (1995). (1999) shows that the relative role of the warming and A systematic study of the storis record was ®rst car- the associated enhanced freshwater ¯uxes to midlati- ried out by Vibe (1967). He was able to explain the tudes in diminishing the convective overturning is de- long-term variation in the catch at southwest Greenland pendent on the model used. What has been largely over- of the ringed seal, the polar bear, and other animals. looked in such studies is the role of the freshwater (liq- Valeur (1976) has presented a statistical analysis for the uid and sea ice) transport in the EGC, as was noted by period 1900±72 with the purpose of ®nding lagged cor- Birch®eld (1993) in a comment on the thermohaline relations suited for prediction purposes but he did not ocean circulation model experiment by Manabe and ®nd any. Stouffer (1988). He pointed out that the role of the EGC This paper is organized as follows. In section 2 the is to advect the net freshwater supply away from the historical storis observations and the calculation of an polar region and into the central North Atlantic. There- annual storis extent index are described, while the re- fore, a properly modeled EGC freshwater transport is construction of the time series of Fram Strait annual ice needed in models used for climate change scenarios. export from this index is done in section 3. The Fram Sea ice conditions have always been important for Strait annual ice export is discussed in section 4. Finally, navigation and other activities in the Arctic societies section 5 contains a summary with main conclusions. and have therefore been recorded in naval logbooks and by of®cials. These observations could contribute valu- 2. Historical ice observationsÐThe storis summer able information from the Arctic region, where mete- extent index orological and oceanographic observations are more or less nonexistent before World War II. Kelly et al. (1987) The extent of the storis on the southwesten coast of related the ``Koch'' Icelandic sea ice index from 1899 Greenland was, and still is, essential for ship connec- onward to sea ice concentration data from the period tions with the population in Greenland and has therefore 1953±77. They found that this index is a proxy for ice been regularly recorded in ships logbooks for almost conditions in the Greenland Sea and they identi®ed 200 years. Speerschneider (1931) aggregated these ob- northerly winds over the Fram Strait in winter and west- servations from the period 1820±96 into monthly de- erly winds north of Iceland in winter and spring being scriptive summaries of length of approximately 15±30 in favor of above-normal sea ice conditions. This anal- written lines. Beginning in the year 1897, the Danish ysis was followed by Walsh and Chapman (1990), who Meteorological Institute compiled the sea ice observa- focused on the GSA and showed that this was better tions into monthly ice charts. An example of May 1910 explained by multiannual accumulation of ice in the is shown in Fig. 2. This example illustrates that the sea Arctic Ocean rather than an ``instantaneous'' enhanced ice conditions along the southwestern coast are fairly ice export due to wind. Mysak et al. (1990) traced anom- well known, while almost no information exists about alies in sea ice extent from the Greenland Sea to the conditions along the major part of the eastern coast. On Labrador Sea with a propagation time of approximately their approach to the southwest Greenland coast from 4 yr. Furthermore, they noted that sea ice anomalies Europe, the captains often followed the same naviga- coincide with temperature and salinity anomalies in the tional strategy (depending on the month) from year to respective basins and, in particular, that the largest one year. This makes us believe in the homogeneity of these coincides with the GSA. Subsequently, Mysak and Pow- observations. er (1992) showed that anomalies in sea ice extent were Valeur (1976) de®ned the storis extent as the north- propagated from the Beaufort Sea via the Beaufort Gyre ernmost position (on the southwest coast) of the storis and the Transpolar Drift to the Greenland Sea. More ice edge, given as a code proportional to the distance recently Venegas and Mysak (2000) analyzed century- from Cape Farewell ``as the crow ¯ies,'' and determined long datasets and identi®ed frequency bands with co- it for each month during 1900±72. An extended series herent variability between sea ice and mean sea level of this storis extent, including the years 1973±94 and

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FIG. 2. An extract from a Danish Meteorological Institute ice chart of May 1910; inset in the lower right corner is the original legend.

1820±99 but mostly covering only the months March± metric, that is, approximately normal, during the sum- August, was published by Fabricius et al. (1995) from mer months, while it is asymmetric during winter, original reports from the southwestern coast of Green- spring, and autumn. land and following Speerschneiders method. This series We now have three arguments for basing a storis sum- was updated to 2001 by A. Rosing-Asvid (2002, per- mer extent index on data from May to July for analysis: sonal communication). 1) the storis extent is maximum and therefore more well The fractional data coverage (fraction of data not de®ned; 2) this choice gives the longest record for anal- missing) of the storis extent de®ned as above is shown ysis; and 3) the statistical distribution is symmetric, that in Fig. 3 for each month. The months May±July have is, approximately normal, which eases the statistical the largest fraction (above 0.9) of data coverage, while analysis. the fraction for some other months is below 0.6. The For these three reasons we will calculate the storis years with missing data for these months are not ran- summer extent index for the period 1820±2001 as fol- domly distributed but are due to systematically missing lows. 1) The time series of storis extent for each of the values in the earlier years of the record. Therefore, by months May, June, and July is standardized by sub- considering summer values only, one gets the longest tracting the long-term average and dividing by the long- record for analysis. term standard deviation. 2) The average of these three In Fig. 4 is shown the median of the monthly storis standardized time series is taken. The storis summer extent and the variability range de®ned as the 0.05 and extent index de®ned in this way has long-term average 0.95 fractiles. One notes the northward advancement of and standard deviation equal to 0 and 1 respectively. the sea ice from January until culmination in June fol- The Storis summer extent index is shown in its full lowed by a rapid retreat until September. This is con- length in Fig. 5. ®rmed by checking the distribution among months of maximum storis extent. It turns out that the storis extent 3. Reconstruction of the Fram Strait ice export culminates in one of the months May, June, or July in approximately 80% of the years. Note also that the sta- As explained earlier, our hypothesis is that the storis tistical distribution of the storis extent tends to be sym- summer extent index is related to the ice export through

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FIG. 3. Fraction of observed values of northernmost position (coded FIG. 4. Median value and variability range, de®ned as the 5% and proportional to the distance from Cape Farewell) for each month 95% fractiles, respectively, of observed northernmost position (coded during the period 1820±2001. proportional to the distance from Cape Farewell) of the storis ice edge for each month. Basis is data from the period 1820±2001. the Fram Strait. Thus, we face the problem of general lack of information concerning this export. The following sources of information exists. 1) A time series of sea ice volume ¯ux covering the period 1990±96 published by Vinje et al. (1998). In this work, the sea ice thickness is obtained from moored upward-looking sonar instruments, the width of the ice stream obtained from ice charts and the velocity parameterized by the cross-strait pressure gradient, using a relationship between buoy- and synthetic aperture radar (SAR)-derived ice velocity and the pressure gradient. 2) A time series of sea ice area ¯ux covering the period 1978±96 by Kwok and Rothrock (1999), based on maximum spatial correlation technique applied to passive microwave data from satellite-borne instruments. The area ¯ux data are also combined with ice thickness from Vinje et al. (1998) to get an alter- native time series of ice volume ¯ux from the years 1990±96. This time series has a signi®cantly lower volume ¯ux than Vinje et al. (1998). 3) Several hindcast studies by, for example, Arfeuille et al. (2000), Hilmer et al. (1998), and KoÈberle et al. (1999), with a coupled ocean±ice model forced with atmospheric reanalysis ®elds. From the output of such experiments, the Fram Strait sea ice volume ¯ux can be extracted. FIG. 5. The storis summer extent index for each year during the We are not able to determine which of these gives period 1820±2001.

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FIG. 6. Time series of different estimates of Fram Strait ice export. the most correct estimate of the Fram Strait ice export including its interannual variability. Therefore, we gave high priority to a long time period for comparison with the storis summer extent index, since this will imply FIG. 7. Sample cross-correlation function (ccf) between Fram Strait annual ice export and storis summer extent index. Positive lags mean more con®dence in our results. This led us to use a storis summer extent index lagging the Fram Strait annual ice export. hindcast run with a coupled ocean±ice model forced Dotted line is 2␴ uncertainty range of the sample ccf, assuming the with daily wind stress data and monthly data from the theoretical ccf to be zero for all lags. other atmospheric variables taken from the National Centers for Environmental Prediction±National Center for Atmospheric Research (NCEP±NCAR) reanalysis where⌽Јnn andIЈ (n denotes the year) are the departures during the years 1949±97 (KoÈberle et al. 1999; C. KoÈ- of the annual Fram Strait ice export and the storis sum- berle 2001, personal communication). From this exper- mer extent index from their respective long-term av- iment, we have access to the annual totals (August±July) erages, ␷ 0, ␷1, ␷ 2 . . . is the impulse response function, of the Fram Strait ice export. This time series is shown and Nn is a noise term. The identi®cation and ®tting of in Fig. 6 together with the other estimates mentioned such models is described in Box and Jenkins (1976). above. Autocorrelation in theIЈn series will hamper the de- To eliminate the effect of the initial conditions, the termination of the impulse response function. We there- ®rst ®ve years of the hindcast period were discarded so fore calculated the (sample) autocorrelation function of that the years 1954±97 remained. Furthermore, the lin- this variable (not shown) and noted that it was every- ear trend was removed from both the storis summer where within the 2␴ uncertainty range, assuming the extent index and the Fram Strait ice export from the true autocorrelation to be 0 for all lags and therefore hindcast run prior to the analysis. The reason for doing consistent with this hypothesis. On this basis we con- this is that the initial state of the hindcast run cannot cluded that a prewhitening procedure was not required. be assumed to be in equilibrium and therefore will cause In this case, the model (1) is identi®ed and estimated spurious drift of the model. directly from the (sample) cross-correlation function, The procedure for reconstructing the Fram Strait ice shown in Fig. 7. It is seen that for lags greater than 1, export from the storis summer extent index is similar the cross correlation, and therefore the corresponding to the methodology used in other paleoclimatic recon- impulse response, can be assumed to be 0. structions, for example, Mann et al. (1998), Cook et al. Therefore, only the terms ␷ 0 and ␷1 are non 0 con- (2002), and others. In our case we have only one pre- tributions to the impulse response function, proportional dictor (storis summer extent) and one predictand (Fram to the corresponding cross-correlation function esti- Strait ice export) and no preprocessing of predictors, mates. The noise term, which represents all contribu- such as calculation of principal components or screening tions not related to the Fram Strait annual ice volume for signi®cance, is needed. A priori, the Fram Strait ice ¯ux, that is, local atmospheric in¯uence (wind and tem- ¯ux for a given year may depend on the storis summer perature) and uncertainty in observations, can now be extent index for that particular and later years: determined. Having estimated the parameters in (1) we can use ⌽Јn ϭ ␷ 0IЈϩn ␷ 1IЈϩnϩ12␷ IЈϩnϩ2 ´´´ϩ Nn, (1) the storis summer extent index for reconstructing the

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Ã x␷ for veri®cation (⌽n being the reconstructed value in the cross-validation version of the model). In order to test the signi®cance of these results, we performed Monte Carlo simulations for determining the

critical values in the distribution of ␤c and ␤␷ under the hypothesis of these being 0 (no skill of the model). This gave a 99.9% critical value equal to Ϫ0.05, implying that our results are signi®cantly different from zero on a high level. The veri®cation skills of our model also compare in magnitude to what is obtained by dendro- climatological reconstructions (e.g., Cook et al. 2002). We inspected the sample autocorrelation function of the estimated noise time series and the sample cross- correlation function between the storis summer extent index and the noise, from the cross-validation version of the model. Presence of auto- or cross correlation is a hint of an inadequate model, but we did not see any of this. An independent validation of our method is to compare our reconstruction with the one given in Kwork and Rothrock (1999), which is based on an entirely different data material, namely, imagery from space- borne passive microwave sensors. The correlation co- FIG. 8. Fram Strait annual ice export for the period 1820±2000, reconstructed from the storis summer extent index. The 2␴ uncertainty ef®cient between their and our reconstruction over the range is shown in shade. overlapping period 1979±96 is 0.59. The chance of getting this or a larger value by chance is about 10Ϫ2, assuming no serial correlation. Fram Strait summer extent index during the period 1820±2000. This reconstruction is shown in Fig. 8. 4. Discussion and conclusions Due to the relatively short training period, we use a cross-validation technique described in Michaelsen a. Climatological value of the Fram Strait ice export (1987) for veri®cation of the model. In this method, one From our reconstruction over the period 1820±2000 data point is omitted at a time (or a few if autocorrelation we deduce the climatological value of the Fram Strait is present) and the model is calibrated on the remaining annual ice export to 3237 km3 yrϪ1. Earlier estimates data, that is, almost all data points. The reconstructed are tabeled in Rothrock et al. (2000) and observational value corresponding to the omitted data point is then estimates vary between 1600 and 5000 km3 yrϪ1, while calculated from this model. When all data points have estimates based on ocean modeling vary between 1400 been omitted in turn, a reconstructed series where each and 3300 km3 yrϪ1. Thus, there is a large scatter in the value is independent of the corresponding value in the estimates. All of these estimates are based on the recent original data has been obtained. In this way, the model 50 years of data or less. is calibrated and veri®ed on (almost) the full dataset. By avoiding the division into ``calibration period'' and ``veri®cation period'' a better utilization of data is b. Multidecadal variability achieved. An example of using cross validation in a Mann et al. (1995) reported on a multidecadal oscil- reconstruction is Kaas et al. (1996). lation in proxy climate data during ®ve centuries, and As the main diagnostics for our model we use ``re- Stocker and Mysak (1992) give a broad review of cen- solved variance,'' which we calculate in two variants, tury timescale variability seen in high-resolution proxy namely, (mainly tree ring and ice core) data. Deser and Black- (⌽Ϫ⌽Ã )2 mon (1993), Mann and Park (1994), Schlesinger and ͸ nn ␤c ϭ 1 Ϫϭ0.30 Ramankutty (1994), and others identi®ed a multidecadal (⌽Ϫ⌽)2 ͸ n mode centered in the North Atlantic in observed surface temperature data with a surface warming in the 1920s for calibration (⌽Ã being the reconstructed value in the n and 1930s and a cooling in the 1950s and 1960s. This calibration version of the model) and corresponds to a decrease respective increase in ice ex- (⌽Ϫ⌽Ã x␷ )2 port out of the Fram Strait (Fig. 8). One straightforward ͸ nn ␤␷ ϭ 1 Ϫϭ0.23 hypothesis is that the temperature/salinity anomalies (⌽Ϫ⌽)2 ͸ n originate in the Arctic Ocean are exported through the

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Fram Strait and ®nally show up as North Atlantic SST anomalies. In this context it is interesting that Delworth et al. (1997) pointed to the important role of the EGC in multidecadal variability observed in a coupled atmosphere±ocean model study. c. Secular changes Calculated from our reconstruction, the ``secular'' decrease in Fram Strait ice export over the period 1820± 2000, calculated directly as the trend in our reconstruction of the annual export, is Ϫ2.8 Ϯ 0.9 km3 yrϪ1, which is 0.09 Ϯ 0.03% yrϪ1,or16Ϯ 5% over the 181-yr- long period of reconstruction. Rothrock et al. (1999) reported dramatic changes in the ice draft in the Arctic Ocean of some 40% over a 40-yr period from the 1950s to 1990s and Wadhams and Davis (2000) supplied further evidence of such a re- duction. In the light of our analysis, we must warn FIG. 9. Correlation coef®cient between annual Fram Strait ice ex- against drawing too severe conclusions regarding sec- port and winter NAO index in a running 19-yr window. Dotted line is 2␴ uncertainty range. ular changes based on short time series. As an illustration of this, the secular decrease in Fram Strait ice export over the period 1965±95, calculated directly as the trend unforced coupled model integration in coarse resolution in our reconstruction of the annual export, is Ϫ13.6 km3 (2.8Њϫ2.8Њ in the Arctic areas) and found that the yrϪ1, which is 0.4% yrϪ1, or 13% over the 30-yr period. correlation coef®cient between modeled NAO winter However, the period preceding the year 1965 was char- index and Fram Strait ice ¯ux was only 0.10 calculated acterized by an upward trend in the ice export (see Fig. over the 300 yr. 8). Therefore, a part of the negative trend over the period Our reconstruction gives the opportunity to investi- 1965±95 could be due to the multidecadal variability. gate the relationship based on observables in a long- term perspective. We calculated the correlation coef®- cient between storis summer advance and the NAO win- d. Relationship with NAO ter index (Jones et al. 1997) over the period 1823±2000 The North Atlantic oscillation (NAO) is presently a and got 0.17, which supports the conclusion of Jung candidate for explaining the variability of almost any and Hilmer (2001). We also calculated the correlation climatic or climate-related quantity in the North Atlantic coef®cient in a 19-yr running window, shown in Fig. 9 region, for instance, the Fram Strait ice and freshwater and supplementing Fig. 11 in Jung and Hilmer (2001). ¯ux [``the NAO appears to exert a signi®cant control The correlation coef®cient is seen to have an intermit- on the export of ice and freshwater from the Arctic to tent character with signi®cant values around 1980±90, the open Atlantic'' (WCRP 1998)]. Findings by Kwok but also earlier around 1940±50 and 1850±60. This casts and Rothrock (1999), who found a correlation coef®- doubt on the hypothesis of enhanced greenhouse effect cient of 0.66 between NAO index and wintertime (De- being the cause for the recent increase in correlation cember±March) Fram Strait ice ¯ux from remote sens- coef®cient. ing data over the period 1978±96, supports this conclusion, as well as work by Alexeev et al. (1997), who got e. Great salinity anomalies a correlation coef®cient of 0.77 for the period 1976±96 for a Fram Strait ice ¯ux series based on a relationship The reconstructed Fram Strait ice volume ¯ux has a between the sea ice ¯ux and the local meridional pres- local maximum in 1968 of 4219 km3 yrϪ1 (Fig. 8). This, sure gradient across the strait. together with observed salinity minima northeast of Ice- Hilmer and Jung (2000) demonstrated, based on sea land in 1968 (Malmberg 1973) and at Fylla Bank in ice ¯uxes from a hindcast run with an ocean model 1970 (Buch and Stein 1987), ®ts into the picture of a forced with NCEP±NCAR reanalyses, that the relation- ``great salinity anomaly'' (Dickson et al. 1988), origi- ship breaks down when longer time spans are consid- nating in the Arctic Ocean as a sea ice anomaly but ered: they get a correlation coef®cient between ice ex- melting during the summer and propagating to west port and NAO index of 0.7 over the period 1978±97 Greenland as a salinity/temperature anomaly in 1±2 yr. but only 0.06 over the period 1958±77. This ``change Belkin et al. (1998) discussed the more recent GSA in link between NAO and Fram Strait ice export'' is of the 1980s, compared it with the GSA of the 1970s, what Dickson et al. (2000) name the ``GSA paradox.'' and concluded that, while the GSA of the 1970s had its Jung and Hilmer (2001) analyzed data from 300-yr-long origin in the Arctic Ocean, the GSA of the 1980s orig-

Unauthenticated | Downloaded 09/28/21 03:11 PM UTC 2790 JOURNAL OF CLIMATE VOLUME 16 inated in the Labrador Sea. Their main argument for in general; and with Peter Thejll about the statistical this was that no sea ice anomaly was observed in the methods. Comments from editor Michael Mann and north Icelandic waters in the 1980s. Our results, and three reviewers helped improve the manuscript. The also the results by KoÈberle et al. (1999) and Kwok and NCEP±NCAR atmospheric reanalysis data were kindly Rothrock (1999), show enhanced Fram Strait ice export provided by National Center for Atmospheric Research, in the beginning of the 1980s, supporting the view that Boulder, CO, while the NAO index was kindly provided both GSAs originated in the Arctic Ocean. by the Climatic Research Unit, University of East An- We believe that the confusion occurred because the glia, Norwich, United Kingdom. This work was sup- sea ice extent depends on both the salinity of the upper ported by Nordic Council of Ministers under Contracts water and on the cooling from the atmosphere. In the FS/HFj/KAa/X96002: ``atmospheric circulation related late 1960s, the atmosphere was in the negative NAO to oscillations in sea-ice and salinity.'' phase and, therefore, the freshwater anomaly in the Greenland Sea was manifested in the sea ice extent due to cooling. In the beginning of the 1980s the atmosphere REFERENCES was in the positive NAO phase and the freshwater was Aagard, K., and E. C. Carmack, 1989: The role of sea ice and other not manifested as an increase in the sea ice extent in fresh water in the Arctic ocean. J. Geophys. Res., 94, 14 485± the Greenland Sea. 14 498. Before the GSA of the 1970s the ice export was Alexeev, G. V., O. I. Myakoshin, and N. P. Smirnov, 1997: Variability around 3500 km3 yrϪ1 or lower for a long time span, of sea ice transport through Fram Strait (in Russian). 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