Bakker, Pepijn; Prange, Matthias (2018): CESM1.2 atmospheric surface fields for pre-industrial Southern Ocean hosing experiments. PANGAEA, https://doi.org/10.1594/PANGAEA.891414, Supplement to: Bakker, P; Prange, M (2018): Response of the Intertropical Convergence Zone to Antarctic Ice Sheet melt. Geophysical Research Letters, 45(16), 8673-8680, https://doi.org/10.1029/2018GL078659
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We use high-resolution coupled ocean-atmosphere simulations to show that reasonable past melt rates of the Antarctic Ice Sheet can have led to shifts of the ITCZ through large-scale surface air temperature changes over the Southern Ocean. Through sensitivity experiments employing slightly negative to large positive meltwater fluxes we deduce that meridional shifts of the Hadley cell and therewith the ITCZ are, to a first order, a linear response to Southern Hemisphere high-latitude surface air temperature changes and Antarctic Ice Sheet melt rates.
The simulations were performed using the Community Earth System Model version1.2 (CESM1.2), a global climate model that includes interactive atmosphere (CAM4), ocean (POP2), land (CLM4.0; including carbon-nitrogen dynamics), and sea-ice (CICE4) components. For the atmosphere (running with a finite volume dynamical core) and land a horizontal resolution of 0.9° x 1.25° was used with the former having 26 vertical levels. The ocean and sea-ice components use a displaced dipole grid with a nominal horizontal resolution of 1° . The ocean grid has 60 levels.
The AIS contribution to meltwater pulse 1A [Clark et al., 1996, doi:10.1029/96PA01419] is highly uncertain, but in the most thorough attempt thus far to quantify this flux, a mean value of 0.034 Sv was found, with maximum values up to 0.11 Sv for a period of 350 years [Golledge et al., 2014, doi:10.1038/ncomms6107]. The Holocene AIS variability suggested by Bakker et al. [2017, doi:10.1038/nature20582] is 0.048 Sv (1σ). Finally, the future response of the AIS if global warming is to continue in the next centuries again varies widely, ranging from ∼0.2-0.5 Sv for the year 2100 in, respectively the so-called Representative Concentration Pathway (RCP) scenario 4.5 and RCP8.5 [Meinshausen et al., 2011, doi:10.1007/s10584-011-0156-z], and ranging from ∼0.2-0.25 Sv for the year 2500 in again RCP4.5 and RCP8.5, respectively [deConto and Pollard , 2016, doi:10.1038/nature17145]. Taken together, credible past and future rates of AIS meltwater into the Southern Ocean seem to range from slightly negative values, i.e. periods of minor AIS growth [Bakker et al., 2017], to positive values of several tenths of Sverdrups. To assess the impact of such AIS meltwater rates we performed four 200 year long experiments in which different magnitudes of freshwater forcing (FWF), namely -48 mSv, 48 mSv, 100 mSv and 200 mSv, were continuously added to the surface of the Southern Ocean south of 60° S (referred to as PIm48, PI48, PI100 and PI200, respectively). The negative -48 mSv meltwater flux implies that freshwater is removed from the internally calculated freshwater flux that enters the ocean, comprising of precipitation, continental runoff and sea-ice melt. AIS freshwater input is not compensated for elsewhere. All experiments start from the same spinup pre-industrial state and in addition a 200 year long control simulation was performed in which no additional freshwater was added to the Southern Ocean (referred to as 'PI'). For the analyses averages over the last 20 years of each simulation are taken.
There are 5 different experiments, namely a pre-industrial (PI) control experiment (PI.001) and 4 experiments including different amount of freshwater added to the Southern Ocean (namely -48 mSv, 48 mSv, 100 mSv and 200 mSv, referred to as PIm48, PI48, PI100 and PI200, respectively). For every of these experiments 7 atmospheric surface variables are included:
PRECC (Convective precipitation rate (liq + ice); m/s)
PRECL (Large-scale (stable) precipitation rate (liq + ice); m/s)
PS (Surface pressure; Pa)
PSL (Sea level pressure; Pa)
TREFHT (Reference height temperature; K)
U (Zonal wind; m/s)
V (Meridional wind; m/s)
175 data points