Enhanced Atlantic subpolar gyre variability through baroclinic threshold in a coarse resolution model

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Abstract

Direct observations, satellite measurements and paleo records reveal strong variability in the Atlantic subpolar gyre on various time scales. Here we show that variations of comparable amplitude can only be simulated in a coupled climate model in the proximity of a dynamical threshold. The threshold and the associated dynamic response is due to a positive feedback involving increased salt transport in the subpolar gyre and enhanced deep convection in its centre. A series of sensitivity experiments is performed with a coarse resolution ocean general circulation model coupled to a statistical-dynamical atmosphere model which in itself does not produce atmospheric variability. To simulate the impact of atmospheric variability, the model system is perturbed with freshwater forcing of varying, but small amplitude and multi-decadal to centennial periodicities and observational variations in wind stress. While both freshwater and wind-stress-forcing have a small direct effect on the strength of the subpolar gyre, the magnitude of the gyre's response is strongly increased in the vicinity of the threshold. Our results indicate that baroclinic self-amplification in the North Atlantic ocean can play an important role in presently observed SPG variability and thereby North Atlantic climate variability on multi-decadal scales. ©Author(s)2012.

Figures

  • Fig. 1. Equilibrium threshold behaviour of Atlantic SPG volume transport in response to constant surface freshwater flux. The SPG exhibits a threshold behaviour at 15 mSv freshwater forcing strength where it intensifies by more than 40 %. The inlay shows its response in the entire forcing interval of ± 200 mSv.
  • Fig. 2. Difference between the strong (15 mSv forcing) and weak (control) SPG circulation. Panels (a, b) show temperature, (c, d) salinity, (e, f) density and (g) heatflux. Panels (a, c, e) show averages over the upper 1000 m, black flowlines in panels (a, c, e, g) show upper 1000 m strong state velocities. Panel (h) shows weak upper 1000 m velocities as flowlines in red. Panels (b, d, f) show zonal averages between 37.5◦ and 18.75◦W with density anomalies as contours and the approximate gyre centre as a grey line.
  • Fig. 3. The feedback loop that leads to the self-amplification of the SPG transport as described in the text. The stronger SPG imports more warm and saline water from the North Atlantic Current; thus increasing the surface salinity in the subpolar region, which fosters winter convection and thereby leads to lower temperatures at the SPG centre. Thus increasing the centre-rim density contrast and thus geostrophic SPG transport.
  • Fig. 4. Time series of SPG strength showing enhanced variability near the dynamical threshold due to the baroclinic feedback. Left panels show the response to sinusoidal freshwater forcing with 20 mSv amplitude and a period of 100 yr, right panels to NCEPNCAR surface wind stress anomaly. Red curves are with constant freshwater off-set of 20 mSv and black curves are without off-set. Narrow upper two panels show the time-varying forcing. The large panels show from top to bottom SPG strength, density variations, density variations due to salinity alone and density variations due to temperature alone. Averages are taken between 37.5◦ W–18.75◦ W and 45◦ N–60◦ N in the upper 1000 m. In both the cases of freshwater and wind forcing the SPG variability is significantly weaker than observed when the system is far from the threshold (black curves). In the vicinity of the dynamical threshold of Fig. 1, the SPG variability is drastically increased. The main density signal is produced by temperature variations. Major salinity changes are restricted to the surface layers, but play a key role in the process of self-amplification (see Fig. 3).
  • Fig. 5. Upper and middle panels: mean SPG strength and its variability for different constant off-set in surface freshwater flux. Different lines correspond to the forcing amplitudes as denoted in the legend. Lower panels: Wind induced variability of the SPG for different constant freshwater off-sets. The dashed vertical line marks the position of the equilibrium threshold at 15 mSv (compare to Fig. 1). The amplitude of SPG variations increases when approaching the threshold for both time-varying freshwater and wind stress forcing. This is best seen for the 100 yr period freshwater forcing, but still present on the shorter time scales (50 yr period and wind stress forcing). Even close to the threshold the system’s response to NCEP wind stress variability is significantly smaller than its response to centennial scale freshwater variations. The lower panels have a different y-scale.
  • Fig. 6. Mean SPG strength and its variability for varying perturbation amplitude corresponding to Fig. 5, but for periods of 25 and 200 yr. Different lines correspond to different forcing amplitudes as denoted in the legend.
  • Fig. 7. Sea level pressure (a) and sea surface height (b) difference of strong minus weak SPG state with 100 yr period sinusoidal freshwater forcing of 20 mSv amplitude.

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CITATION STYLE

APA

Mengel, M., Levermann, A., Schleussner, C. F., & Born, A. (2012). Enhanced Atlantic subpolar gyre variability through baroclinic threshold in a coarse resolution model. Earth System Dynamics, 3(2), 189–197. https://doi.org/10.5194/esd-3-189-2012

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