Rapid barotropic sea level rise from ice sheet melting

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /2011jc007733, 2012 Rapid barotropic sea level rise from ice sheet melting K. Lorbacher, 1 S. J. Marsland, 1 J. A. Church, 2 S. M. Griffies, 3 and D. Stammer 4 Received 6 November 2011; revised 23 April 2012; accepted 24 April 2012; published 6 June [1] Sea level rise associated with idealized Greenland and Antarctic ice sheet melting events is examined using a global coupled ocean sea-ice model that has a free surface formulation and thus can simulate fast barotropic motions. The perturbation experiments follow the Coordinated Ocean-ice Reference Experiment (CORE) version III. All regions of the global ocean experience a sea level rise within 7 8 days of the initialization of a polar meltwater input of 0.1 Sv (1 Sv 10 6 m 3 s 1 ). The fast adjustment contrasts sharply with the slower adjustment associated with the smaller steric sea level evolution that is also connected with melt events. The global mean sea level rises by 9 mm yr 1 when this forcing is applied either from Greenland or Antarctica. Nevertheless, horizontal inter-basin gradients in sea level remain. For climate adaption in low-lying coastal and island regions, it is critical that the barotropic sea level signal associated with melt events is taken into consideration, as it leads to a fast sea level rise from melting ice sheets for the bulk of the global ocean. A linear relation between sea level rise and global meltwater input is further supported by experiments in which idealized melting occurs only in a region east or west of the Antarctic Peninsula, and when melting rates are varied between 0.01 Sv and 1.0 Sv. The results indicate that in ocean models that do not explicitly represent the barotropic signal, the barotropic component of sea level rise can be added off-line to the simulated steric signal. Citation: Lorbacher, K., S. J. Marsland, J. A. Church, S. M. Griffies, and D. Stammer (2012), Rapid barotropic sea level rise from ice sheet melting, J. Geophys. Res., 117,, doi: /2011jc Introduction [2] The purpose of this note is to demonstrate, through global ocean/sea ice simulations, the near instantaneous response of sea level around the globe. As argued by Gower [2010] in his comment on the study of Stammer [2008], we emphasize the dominant role played by the barotropic response of sea level associated with increased ocean mass upon adding ice-melt. Accordingly, the global mean sea level should adjust within a matter of days, through barotropic waves traveling at speeds of order 200 m s 1,to changes in the ocean mass induced by melting land-ice. A complementary emphasis was provided by Stammer [2008], who focused on the slower and smaller steric adjustment of sea level arising from meltwater induced changes in salinity. 1 Centre for Australian Weather and Climate Research, CSIRO-Bureau of Meteorology, Aspendale, Victoria, Australia. 2 Centre for Australian Weather and Climate Research, CSIRO-Bureau of Meteorology, Hobart, Tasmania, Australia. 3 NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA. 4 CEN, Universität Hamburg, Hamburg, Germany. Corresponding author: S. J. Marsland, Centre for Australian Weather and Climate Research, CSIRO-Bureau of Meteorology, Private Bag 1, Aspendale, Vic 3195, Australia. (simon.marsland@csiro.au) Copyright 2012 by the American Geophysical Union /12/2011JC [3] As noted by Gower [2010], differences between the barotropic response and the far slower (roughly 100 times slower) baroclinic/advective response to land-ice melt are particularly relevant for coastal and low-lying regions. Worldwide, all such regions should experience, within the matter of days, sea level rise associated with land-ice melt from either the Greenland or Antarctic ice sheets (or glaciers elsewhere). [4] Gower [2010] pointed out that Stammer s [2008] conclusions that it will take a significant length of time until the Pacific Ocean will feel this extra volume, for example, in form of sea level rise is misleading and dependent on the use of virtual salt fluxes to represent mass gain by the ocean. Stammer [2010] clarified that this statement is concerned only with the (local) steric sea level change and that his study did not consider the mass-induced change. Virtual salt fluxes represent the additional freshwater input by changing the model s salt content but neglects the far greater freshwater mass gain. Here, we investigate the impact of directly representing the mass gain by the ocean results for a rapid global response of sea level. [5] To date there remains a considerable uncertainty about melting rates, the sea level response, and its spatial distribution. It is therefore our aim here to demonstrate the fast global ocean adjustment in response to high-latitude freshwater forcing. The study is based on numerical simulations analogous to Stammer [2008], where we use an ocean model in which the simulated sea level responds directly to the 1of10

2 input of meltwater associated with idealized land-ice melt scenarios. [6] Cazenave and Llovel [2010] estimated an additional global freshwater flux to the ocean due to melting ice sheets of GT yr 1 since 1992, with enhanced contribution of 200 GT yr 1 since These rates correspond to mm yr 1 and 0.56 mm yr 1 of global mean sea level rise, respectively [Bindoff et al., 2007]. Estimates by Shepherd and Wingham [2007] are 20% higher, where the global mean sea level rise signal is 0.5 mm yr 1 from Greenland and 0.21 mm yr 1 from the combination of East and West Antarctica. To date, the maximum highlatitude ice sheet melting rate was estimated by Rignot et al. [2011] to be in 2006, and corresponds to a global mean sea level rise of mm yr 1. Riva et al. [2010] estimate a contribution of 1.0 mm yr 1 global mean sea level rise due to the total ice and water mass loss from the continents over the period The contributions from Greenland and Antarctica are in addition to contributions from melting land-ice in other parts of the world. Over recent decades these ice sheet contributions exceed the global mean sea level increase due to thermal expansion of the upper 700 m of the water column of 0.52 mm yr 1 [Domingues et al., 2008]. The importance of mass gain for sea level rise is consistent with the fact that for a given amount of heat, a much greater sea level rise results from melting land-ice rather than expanding the ocean. (Trenberth and Fasullo [2010] estimate a factor of [see also Church et al., 2011a]). Thus on the longer term, climate change may well result in substantially larger mass contribution than thermal expansion contribution to sea level rise. We focus in this paper exclusively on the impacts of changing ocean mass on sea level rise, so do not consider quantitatively the steric expansion effects associated with adding freshwater to the ocean. [7] The remainder of this note consists of the following Sections. In section 2, we detail the model configuration, and highlight key differences from the model used by Stammer [2008]. The sea level response to meltwater perturbation experiments is described in section 3, and we offer discussion and concluding remarks in section The Model and Experimental Design [8] The perturbation experiments follow the recommendation of the Coordinated Ocean-ice Reference Experiment (CORE) version III proposed by Gerdes et al. [2006]. Specifically, we add a freshwater perturbation of magnitude 0.1 Sv (1 Sv 10 6 m 3 s 1 ) to the ocean in a region surrounding Greenland, with this amount of water equivalent to 3150 GT yr 1 of ice melt (equivalent to 8.7 mm yr 1 global mean sea level rise). An analogous configuration is chosen for the Antarctic melt event, with 0.1 Sv of meltwater spread evenly around the Antarctic continent. Figure 1 illustrates the additional melt applied in the Greenland and Antarctic perturbation experiments. Both melt patterns are reflective of those used by Stammer [2008]. However, the total glacial freshwater flux used in the simulations by Stammer [2008] was close to 800 GT yr 1 (compare also Stammer et al. [2011]), which is equivalent to roughly a 2 mm yr 1 global sea level increase and is about 1/4 of what is used here. Later, we will show that the results scale linearly with the magnitude of the mass contribution. [9] We use the CORE-I normal year forcing product of Large and Yeager [2009] for our control and perturbation simulations. The control simulation follows the set-up detailed in Griffies et al. [2009]. The perturbation simulations are identical, except for the addition of idealized glacial meltwater. Differences between the meltwater perturbation simulations and the control provide the sea level response associated with enhanced melting. Following Kopp et al. [2010], the sea level as defined here incorporates the mass loading from sea ice, as we are only concerned with the net sea level associated with the mass of both liquid and solid water in the ocean. We focus on sea level differences between the control experiment, consisting of the unmodified CORE runoff, and the modified Greenland (termed here GLISM, Green Land Ice sheet Melting) and Antarctic (termed here AAISM, Antarctic Ice sheet Melting) runoff experiments. [10] The model used in this study is the Australian Climate Ocean Model (AusCOM) which is the ocean-ice component of the global coupled atmosphere-ocean general circulation model within the Australian Community Climate and Earth System Simulator (ACCESS). AusCOM incorporates the Geophysical Fluid Dynamics Laboratory Modular Ocean Model (MOM4p1) [see Griffies, 2009] and the Los Alamos National Laboratory CICE4.0 sea ice model [Hunke and Lipscomb, 2008]. The French CERFACS PRISM 2 5 OASIS3 coupler (OASIS3.25) [Valcke, 2006] mediates the interaction between the ocean and sea ice. The ocean model includes a representation of barotropic gravity waves, with such waves responding directly to the use of a water flux surface boundary condition as detailed by Griffies et al. [2001]. These two model features are important for this study. [11] AusCOM s ocean and ice models are configured horizontally with an orthogonal curvilinear grid. North of 65 N the model discretization employs the tripolar grid of Murray [1996]. Elsewhere, the model has a 1 resolution in longitude, and is nominally of 1 resolution in latitude with the following two refinements: first, an equatorial refinement to 1 3 between 10 S and 10 N; and secondly, a Mercator (cosine dependent) meridional grid spacing for the Southern Hemisphere, ranging from 0.25 at 78 S to 1 at 30 S. AusCOM has 50 vertical levels with 20 levels in the top 200 m. A volume-conserving (Boussinesq) as well as a mass-conserving (non-boussinesq) version of AusCOM are used to detail how mass perturbations affect the global sea level. Further details of AusCOM can be found in Bi and Marsland [2010]. [12] Global mean sea level h evolves according to t h ¼ Q m hri V A t r h i hri ; where Q m is the sea surface mass flux (in kg m 2 s 1 ), r the global mean in situ density, V the global ocean volume and A the global ocean surface area. Global mean sea level changes from mass addition in a Boussinesq formulation do not include the second term on the right hand side of equation (1), which is the global steric contribution ð1þ 2of10

3 Figure 1. Surface volume input of freshwater (in m 3 s 1 ) per model grid point associated with the mass loss of polar ice sheets and in addition to the CORE river-runoff for (a) the Greenland and (b) the Antarctic run. Following Gerdes et al. [2006] and Stammer [2008] glacial meltwater is added to model grid cells over a region within 300 km of the coast. The total area is km 2 and km 2 for Greenland and Antarctica, respectively. The net volume transport of freshwater in the colored area is 0.1 Sv. associated with changes in the global mean density. In our non-boussinesq formulation the contribution of the global steric term to sea level changes amounts to 2% at most of the total global mean sea level rise in the first month (not shown). Given that both the Boussinesq and non-boussinesq model formulations show nearly identical patterns and magnitudes of sea level change for the perturbation simulations considered in this note, we present results only from the non-boussinesq version. [13] For the purposes of this note, an important formulation difference between the ocean model used by Stammer [2008] and AusCOM is the representation of freshwater fluxes at the ocean surface. Stammer [2008] formulates the freshwater flux across the ocean surface as a virtual salt flux, Q S, which is calculated according to Q S ¼ Q m S ref ; ð2þ 3of10

4 where Q m = P E + R + I is the net freshwater flux (in kg m 2 s 1 ), comprised of precipitation P, evaporation E, river runoff R and ice sheet melt I (Q m > 0 if the liquid ocean gains freshwater). S ref is a reference salinity set to 35 psu. In addition a relaxation of the model surface salinity toward climatological surface salinity fields was used in Stammer [2008]. [14] In contrast, AusCOM introduces freshwater directly to the ocean by treating the surface freshwater flux (Q m )asa change in ocean mass while maintaining a constant ocean salt content. The mass change in turn modifies ocean volume, with this change radiated globally, within days, through barotropic gravity waves (manifesting in sea level and vertically integrated velocity) that travel at roughly 200 m s 1 in the deep ocean. Both the water flux boundary condition and the associated barotropic response are explicitly represented in AusCOM. To avoid drift in sea level apart from the response to freshwater from idealized glacial melt, I, we follow an approach previously used in forced ocean models that use real freshwater fluxes [e.g., see Griffies et al., 2009, Appendix B.3]. Each time step we apply a global correction to any imbalance in global net freshwater mass input to the ocean by P E + R + SI, where SI is the ocean-ice freshwater mass flux arising from sea-ice melting or freezing. This is not strictly conservative for sea level in our freshwater flux formulation, since any drift in total global sea-ice volume will manifest as a very small equivalent change in global mean sea level. Also, as was the case for Stammer [2008], we restore the model surface salinity toward climatological surface salinity fields. The surface salinity restoring can lead to an underrepresentation of the halosteric contribution to regional sea level rise in the North Atlantic under a Greenland melting scenario [Stammer et al., 2011]. [15] Models forced with the virtual salt flux, Q S, change the salt content of the model ocean while keeping the mass constant. Hence, virtual salt flux models omit the rapid barotropic response found in the water flux models and in the real world. For sea level impact studies such as considered in the present paper, differences between virtual salt flux and water flux models are fundamental. However, as we will conclude below, for long-term sea level adjustment studies (seasonal and longer timescales), the virtual salt flux models are sufficiently accurate as long as the additional mass flux effect is added separately. Some remaining shortcomings of this approach are pointed out by Huang [1993], Griffies et al. [2001, 2005], and Yin et al. [2010]. 3. Results [16] We start the meltwater perturbation experiments from a simulation integrated for 50 years using the CORE normal year forcing protocol [Griffies et al., 2009]. Although this protocol omits climate relevant interannual forcing as well as the global climate change signal, we expect that the time mean sea level pattern should reflect the climatological pattern found in the observations. We thus compare in Figure 2 the mean sea level from year 50 of the AusCOM control simulation to the time mean sea level from the AVISO (Archiving, Validation and Interpretation of Satellite Oceanographic data; com/duacs). The simulated and observed sea level patterns share much in common. For example, the sea level difference between the North Pacific and North Atlantic (averaged between N) is 48 cm in the observations and 46 cm in AusCOM. There are, however, significant differences in the sea level minimum for convection sites in the Labrador and Irminger Seas, where the local minima are around 20 cm less than observed. Also, the maximum amplitude at the center of the North Pacific s central subtropical gyre exceeds the observed by 20 cm. We are uncertain whether to attribute such differences to model shortcomings or limitations of the normal year forcing. For our purposes, we consider the time mean sea level in AusCOM to offer a sufficiently realistic starting point for examining the sea level signal associated with meltwater perturbations. [17] Figure 3 shows the sea level response to an additional net meltwater of 0.1 Sv entering the ocean surrounding Greenland. The barotropic signal propagates around the world in just a few days. Initially the signal travels as a boundary Kelvin wave southward in the North Atlantic, crosses the basin (partly at the equator) as basin modes and then travels along the eastern boundary, radiating Rossby waves that communicate the signal into the interior. The signal enters the Pacific Ocean through the Indian Ocean and partly through Bering Strait, adjusting the Pacific in a similar manner as it was when spreading across the Atlantic. To some degree the pattern of a global barotopic sea level adjustment is reflective of the mostly steric adjustment discussed by Stammer [2008], as well as Hsieh and Bryan [1996] and Bryan [1996]. However, the barotropic response is about 200 times faster. Through this barotropic response the bulk of the global ocean experiences, within a few days, a sea level rise. Nevertheless the global ocean is not adjusted after a few weeks and inter-basin gradients remain, even for the fast component. [18] Figure 4 considers the early response in sea level to Antarctic melt. Adjustment starts with a sea level rise within and south of the Antarctic Circumpolar Current (ACC), exciting a boundary Kelvin wave, propagating northward along the east coast of South America toward the equator, where it crosses the Atlantic to the eastern side and subsequently propagates poleward in both hemispheres. Thereafter, the barotropic signal propagates globally in a similar fashion to that described above for the Greenland signal. After one week, the bulk of the global ocean experiences a sea level rise. However, the sea level rise remains most pronounced (and above the global mean value) in the basin where glacial meltwater is added, especially around the source of the freshwater, partly because of the sizable local steric response there. [19] Time series of hourly sea level differences between the first month of the control run and the perturbation experiments are shown in Figure 5. In general, the barotropic response of sea level rise due to additional freshwater mass gain is seen in all regions of the global ocean. For the first month the linear trend for the global mean sea level is 8.6 mm yr 1 for both the Greenland and Antarctic melting experiments. [20] The area-averaged response for the various regions considered in Figure 5 are quantified in Table 1. In particular, we note the response in the Western Pacific box, which includes many of the low-lying Pacific Islands. This region corresponds to the region under consideration in Australia s 4of10

5 Figure 2. (a) Pattern of observed time averaged sea level (relative to the global-averaged sea level) (in cm) from Archiving, Validation and Interpretation of Satellite Oceanographic data (Aviso, year ). (b) Corresponding simulated annual mean (year 50) sea level (in cm) in the control simulation. In this study, we show sea level time series from the following regions: (1) the box identifies a Western Pacific box, the region under consideration in the Pacific Climate Change Science Program (PCCSP) (20 S 25 N, W), (2) the area southward of 35 S (black solid line) defines the Southern Ocean, (3) the North Pacific and (4) the North Atlantic ranges from the equator to 65 N, whereby the North Atlantic area does not include the Mediterranean Sea, Hudson Bay and the English Channel and (5) the Indian Ocean excluding the Red Sea but including the Indonesian Throughflow. 5of10

6 Figure 3. Pattern of sea level response (in mm) at noon of days 1 to 6. The patterns are computed as difference between the Greenland meltwater perturbation experiment (GLISM) and the control experiment. They are mapped at the following hours after initiation of the Greenland melt event: 12, 36, 60, 84, 108 and 132 hours. Note that the contour interval is nonlinear. 6of10

7 Figure 4. Same as Figure 3 but for the Antarctic melting experiment (AAISM). 7of10

8 Figure 5. Time series of hourly sea level response (in mm) to a freshwater perturbation of 0.1 Sv for selected regions of the global ocean (see Figure 2 for definition of the regions). Time series are defined as the sea level difference between perturbation and control experiments: (a) Greenland (GLISM) and (b) Antarctica (AAISM). Pacific Climate Change Science Program (PCCSP, Program.html). 4. Discussion and Conclusions [21] Examination of idealized Greenland and Antarctic ice sheet melting events using the Australian Climate Ocean Model (AusCOM) show that all regions of the global ocean experience a sea level rise within 7 8 days after the initialization of a meltwater input of 0.1 Sv. To examine the robustness of our results, we conducted accompanying melt experiments in which the Antarctic meltwater was input over a region immediately east of the Antarctic Peninsula, and a separate experiment just over a region immediately west of the Antarctic Peninsula, with the West Antarctic Peninsula simulation more realistic given present concerns about the stability of the ice sheet in this region [Shepherd and Wingham, 2007]. The resulting spatial and temporal adjustments of sea level (not shown) do not differ significantly from the perturbation experiment considered around the whole of Antarctica, indicating sea level anomalies are propagated rapidly around the Antarctic continent as seen in observations and high resolution models [Hughes et al., 2003; Killworth and Nanneh, 1994]. [22] We also examined the degree to which the global mean sea level signal scales according to the magnitude of the freshwater input. For Greenland and Antarctic melting experiments having freshwater inputs of 0.01 Sv and 1.0 Sv, we found that the relative sea level rise scaled proportionately with respect to the 0.1 Sv simulations. We also note that the amplitude of the steric signal of sea level rise (regionally varying and around one-order of magnitude smaller than the barotropic signal) has a linear response to the amount of applied glacial meltwaters added (not shown). [23] To understand the impact for low-lying coastal and island regions, it is critical that the mass-induced sea level signal from meltwater events is included, as well as any regional steric response. This signal leads to the rapid (on the order of days) barotropic rise in global sea level from meltwater perturbations, and is critically important for coastal regions. The far longer-timescale (decadal) steric signals considered by Stammer [2008] are generally much smaller than the barotropic signal. This point is highlighted in Figures 6a and 6b, where we show that on the 50 year timescale the model s sea level has risen by approximately 0.45 m in all ocean basins for both the GLISM and AAISM experiments which is substantially larger than the steric signal. After scaling the present results (this study uses a freshwater forcing four times larger than Stammer [2008]) and adding the dominant mass induced component to the Stammer et al. [2011] results, the sum of the steric and massinduced changes in both studies are similar. Consequently, while high latitude meltwater events will impact sea level in the far field (including Tropical Pacific Islands) within a matter of days, the results of the present study show it is possible to add the mass-induced component of sea level rise off-line when using ocean models that do not explicitly represent the barotropic signal (because they use the virtual salt flux boundary conditions). Note that the sea level rise associated with the increase of mass of the ocean is not uniform (even in the steady state) because the changing mass distribution changes the gravitational field and the Earth s surface loading and the elastic response of the Earth [Mitrovica et al., 2011]. These regional distributions need to Table 1. Regionally Averaged Rate of Rise in Sea Level (mm yr 1 ) Over the First Month of the Greenland, GLISM, and Antarctic, AAISM, Meltwater Perturbation Experiments, Relative to the Control Simulation a Region GLISM AAISM North Atlantic Southern Ocean Indian Ocean North Pacific Western Pacific a See Figure 2 for definitions of the regions. 8of10

9 Figure 6. Time series of annual mean sea level response (in mm) for selected regions of the global ocean, analogous to Figure 5: (a) GLISM and (b) AAISM (comparable to Stammer s [2008] Figure 7). Temporal linear trends for the global means in both simulations are 9 mm yr 1. be combined with the steric signals and glacial isostatic adjustment to get the full regional distribution of relative sea level change [Kopp et al., 2010; Slangen et al., 2011; Church et al., 2011b]. [24] Acknowledgments. This work was supported by the NCI National Facility at the ANU. 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