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Formation and Propagation of Great Salinity Anomalies H GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 9, 1473, doi:10.1029/2003GL017065, 2003 Formation and propagation of great salinity anomalies H. Haak,1 J. Jungclaus,1 U. Mikolajewicz,1 and M. Latif 2 Received 5 February 2003; accepted 3 April 2003; published 8 May 2003. [1] North Atlantic/Arctic ocean and sea ice variability for LS is presented self-consistently within the framework of a the period 1948–2001 is studied using a global Ocean global ocean/sea ice model. General Circulation Model coupled to a dynamic/ thermodynamic sea ice model forced by daily NCEP/ NCAR reanalysis data [Kalnay et al., 1996]. Variability of 2. Numerical Experiments Arctic sea ice properties is analysed, in particular the [3] The Max-Planck-Institute ocean model (MPI-OM) is formation and propagation of sea ice thickness anomalies a primitive equation model (z-level, free surface), with the that are communicated via Fram Strait into the North hydrostatic and Boussinesq assumptions made. MPI-OM Atlantic. These export events led to the Great Salinity includes an embedded dynamic/thermodynamic sea ice Anomalies (GSA) of the 1970s, 1980s and 1990s in the model with viscous-plastic rheology following Hibler Labrador Sea (LS). All GSAs were found to be remotely [1979]. For details, see Marsland et al. [2003]. The excited in the Arctic, rather than by local atmospheric particular model configuration has 40 vertical levels, with forcing over the LS. Sea ice and fresh water exports through 20 of them in the upper 600 m. The horizontal resolution the Canadian Archipelago (CAA) are found to be only of gradually varies between a minimum of 20 km in the Arctic minor importance, except for the 1990s GSA. Part of the and a maximum of about 350 km in the Tropics. The model anomalies are tracked to the Newfoundland Basin, where is initialized from Levitus et al. [1998] climatological they enter the North Atlantic Current. The experiments temperature and salinity data and integrated 11 times for indicate only a minor impact of a single GSA event on the the period 1948–2001 using daily NCEP/NCAR reanalysis strength of the North Atlantic Thermohaline Circulation forcing. On a global average NCEP/NCAR downward short (THC). INDEX TERMS: 4207 Oceanography: General: Arctic wave radiation (DSWR) is appr. 10% higher than ERBE and Antarctic oceanography; 4255 Oceanography: General: estimates or ECMWF reanalysis data. A global scaling Numerical modeling; 4215 Oceanography: General: Climate and factor of 0.89 is applied on DSWR to correct for this interannual variability (3309); 4540 Oceanography: Physical: Ice systematic bias in the forcing. In ice-free regions salinity in mechanics and air/sea/ice exchange processes. Citation: Haak, H., the surface layer (0–12 m) is restored towards the Levitus J. Jungclaus, U. Mikolajewicz, and M. Latif, Formation and climatology, with a time constant of 180 days. The second propagation of great salinity anomalies, Geophys. Res. Lett., 30(9), and all subsequent integrations use the conditions at the end 1473, doi:10.1029/2003GL017065, 2003. of the previous run as initial condition. The first two cycles are neglected in the following analysis to account for the 1. Introduction models spinnup. To explore the response of the ocean/sea ice system to low-frequency fluctuations in the atmospheric [2] The Arctic is an important fresh water source for the forcing fields, additional sensitivity experiments were con- North Atlantic, influencing stratification and thus the verti- ducted. In these, a high pass (hp) filter is applied to certain cal circulation in the North Atlantic sinking regions. Small forcing fields. The hp-filter removes climate variability with variations in freshwater supply from the Arctic (via the East timescales longer than one month and maintains daily fluc- Greenland Current (EGC)) into the North Atlantic can alter tuations and the climatological seasonal cycle. The sensitiv- or even prevent convection in the LS for several years. The ity experiments are (1) all forcing fields hp-filtered (HPF GSA of the early 1970s [Dickson et al., 1988] is an example ALL), (2) only wind stress hp-filtered (HPF TAU) and (3) all of such an event. On the other hand, LS winter time deep forcing fields except wind stress hp-filtered (HPF Q + FW). convection is also influenced by the North Atlantic Oscil- lation (NAO) on interannual to decadal timescales [Curry et al., 1998]. Previous modelling studies on this issues were 3. Results based either on stand-alone sea ice models [Hilmer et al., [4] Figure 1 shows the ensemble mean time evolution of 1998], or limited domain coupled ocean/sea ice models Arctic sea ice volume. The time series exhibit pronounced [Ha¨kkinen, 1999, Ko¨berle and Gerdes, 2003]. Here a global interannual to multidecadal variability with distinct maxima ocean/sea ice model is used to investigate the North in the mid 1960s, early 1980s and late 1980s. Sea ice Atlantic/Arctic interactions and their impact on the THC. volume increases from appr. 20000 km3 in 1948 by almost In particular, the chain of processes that led to GSAs in the 37% to 27500 km3 in 1966. The ice covered area (not shown) increases at the same time by roughly 8% from 8.9 * 6 2 6 2 10 km to 9.6 * 10 km , while the area-averaged Arctic sea 1 ice thickness (not shown) increases by over 30% from 2.2 m Max-Planck-Institut fuer Meteorologie, Hamburg, Germany. to almost 2.9 m. Sea ice thickness stays at rather high values 2Institut fuer Meereskunde an der Universita¨t Kiel, Germany. for the mid 1960s to mid 1980s. During the last 15 years of Copyright 2003 by the American Geophysical Union. the simulation ice volume drops by appr. 30% from 28000 to 3 6 2 0094-8276/03/2003GL017065 20000 km , ice area by 6% from 9.6 to 9.0 * 10 km and 26 -- 1 26 - 2 HAAK ET AL.: GREAT SALINITY ANOMALIES 1980/81, 1992/93 and 1994/95. All peaks in ice export correspond, with a lag of about two years, to decreases in the Arctic sea ice volume, indicating at least parts of the the ice volume drops are caused by anomalous Fram Strait export events. The sensitivity experiment HPF Q + FW show clearly that the sea ice export is primarily driven by low- frequency variability in the wind forcing throughout the analysed period (Figure 1). Average exports through the CAA via Lancaster Sound (391 ± 134 km3/yr) as well as from Hudson Bay (205 ± 68 km3/yr) are both smaller than one standard deviation of the Fram Strait transports. Anomalous CAA fresh water (liquid + solid) export events are negative during the late 1960s and early 1980s. However, for the late 1980s the CAA export anomaly is positive and amounts to appr. 50% of the corresponding Denmark Strait value. [6] Figure 2 illustrates the formation and propagation of the mid 1960s sea ice thickness anomaly that lead to the large Fram Strait sea ice export event of 1967/68. Anomalous winds led to an anomalous convergence of sea ice transports in the Laptev Sea in 1965/66. This convergence forms an Figure 1. Upper panel shows Arctic sea ice volume (excl. CAA) of control run (ensemble mean, bars indicate ensemble range; solid black) and runs HPF Q + FW (solid blue) and HPF TAU (solid red) and their linear super- position (dashed green). Units are km3. Lower panel shows Fram Strait freshwater export by sea ice and snow from control run (solid black) and from runs HPF Q + FW (blue) and HPF TAU (red) and observational estimates by Vinje et al. [1998]. Units are km3/yr. sea ice thickness by 20% from 2.9 m to 2.3 m (not shown). The sensititivity experiments reveal that the Arctic sea ice volume variability is forced in equal parts by the wind and by thermal + freshwater forcing, respectively. A linear superposition of the sensitivity runs with hp-filtered wind stress and hp-filtered thermal + freshwater forcing reprodu- ces the variability of the control run reasonably well. Arctic sea ice volume variability is mainly caused by sea ice thickness changes, and can be understood as a passive response to the atmospheric bondary conditions. Finally, no obvious long-term trend in Arctic sea ice volume can be seen in the simulation. [5] Fram Strait sea ice exports are also shown in Figure 1. All volume transports are given in units of liquid freshwater. The time-mean freshwater transport by solid sea ice and snow, and its standard deviation with respect to annual values, amounts to 2874 ± 674 km3/yr. The liquid freshwater transport in Fram Strait (calculated with respect to the simulated climatological mean reference salinity of 35.0) is 1597 ± 397 km3/yr, with typical anomalies in the order of 600 to 700 km3/yr. Fram Strait sea ice export in our simulation is consistent with observational estimates of 2790 km3/yr by Aagaard and Carmack [1989] and 2565 ± 600 km3/yr for the 1990s by Vinje et al. [1998] and Vinje [2001]. Uncertainty is appr. 15% for the latter. The simulated variability of the volume transport agrees well with numer- ical experiments by Hilmer et al. [1998], Ko¨berle and Gerdes [2003] and results presented in Dickson et al. [1999]. Fram Figure 2. Ensemble mean sea ice thickness anomaly (left Strait sea ice export is highly variable with individual column) and sea ice transport anomaly (right column, every anomalies reaching more than 60% of the mean value e.g., forth vector plotted) for the years 1965/66, 1966/67 and the maxima in winter 1967/68 (4658 km3/yr) and 1988/89 1967/68.
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