Sea salt aerosol response to climate change: last glacial maximum, pre-industrial,

Transcription

1 Sea salt aerosol response to climate change: last glacial maximum, pre-industrial, and doubled-carbon dioxide climates To be submitted to GRL, Natalie M. Mahowald 1, Jean-François Lamarque 1, Xue Xi Tie 1, Eric Wolff 2 1 National Center for Atmospheric Research, Boulder, Colorado 80307, USA. 2 British Antarctic Survey, Natural Environment Research Council, Cambridge, UK. Abstract Sea salt aerosols represent a significant fraction of the aerosol optical depth over the oceans, and thus their response to changes in climate represents an important potential feedback on climate. We show model results for sea salt aerosols for last glacial maximum, pre-industrial, current and doubled-carbon dioxide climate model simulations. Our model results suggest that sea salt sources, deposition and loading are not very sensitive to climate change and globally change <5% for these disparate climates. While ice core studies show 2-5-fold changes in sea salt fluxes between last glacial maximum and the current climate, our simulations cannot reproduce these changes, even after including a proposed sea ice source of sea salts. 1.0 Introduction Sea salt aerosols are the leading contributor to the global-mean clear-sky radiative balance over oceans [Haywood et al., 1999] and can modulate cloud radiative properties [O'Dowd et al., 1999]. Ice core records indicate large changes in deposited sea salts, indicating a strong sensitivity of sea salt aerosols to climate [DeAngelis et al., 1997; Petit and al., 1999; Roethlisberger et al., 2002]. Sea salt entrainment into the atmosphere is

2 proportional to surface winds cubed, suggesting that small changes in surface winds can impact the source of this aerosol. Additionally, downwind transport and distributions can be modified by changes in atmospheric transport pathways and precipitation patterns, allowing this aerosol to potentially feedback onto climate. In this study, we estimate changes in the natural aerosol sea salt due to climate change for the last glacial maximum, pre-industrial and doubled-carbon dioxide climates compared to the current climate. 2.0 Model Simulations Slab ocean model simulations of the Community Climate System Model (CCSM3) are conducted for the last glacial maximum (LGM), pre-industrial, current climate and doubled-carbon dioxide climate [Collins et al., submitted; Kiehl et al., submitted; Otto- Bliesner et al., submitted]. These simulations should have the same sensitivity as the fully-coupled simulations on which they are based [Kiehl et al., submitted; Otto-Bliesner et al., submitted]. Once the climate is in equilibrium, 10-year simulations of sea salts are conducted, with the last nine years included in the averages presented here. Details of the sea salt aerosol scheme and comparisons to observations for the current climate are contained in the online supplement. Generally, the annual average comparisons between model and observations show that the model is within a factor of two of observations. For sensitivity studies, we also include a source of sea salt aerosols from new sea ice formation, as postulated by [Wagenbach et al., 1998; Wolff et al., 2003; Kaleschke et al., 2004]. In this case, the only change is where the sea salt aerosols are formed; the source, transport and deposition are all assumed identical to open ocean sea salts aerosols in the

3 model. Only the last 3 years of 4 year integrations are used from these sensitivity studies. We conducted this simulation using both new sea ice formation in a slab ocean model and using all sea ice as the source. Because the slab ocean model does not allow for the movement of ice due to winds or currents, the sources of new sea ice are all on the low latitude edges of the sea ice. This is in contrast to the sources of new ice in the fully coupled version of the model, where the new sea ice formation occurs throughout the sea ice covered regions of the model. Therefore, for the slab ocean model simulations conducted here, a more realistic distribution of new sea ice formation is obtained from using the total sea ice as the source of sea salts, and the ratios at the ice cores are higher (and thus more similar to the observations see Section 3.0). 3.0 Results Model source and loading results for the four climate regimes are shown in Table 1. As is clear from the table, the model shows <5% sensitivity to climate for either the globally integrated source or loading. These changes are similar to the inter-annual variability in the modeled sea salts, or to the changes in sea salts when model resolution changes (Online Supplement). Note that the wet and dry lifetime of sea salts stay roughly constant through all four climates (~0.8 days for all bins together). Table 1: Sea salt response to climate change LGM Pre-industrial Current 2xCO2 Source (Tg/year) Loading (Tg) % Change * Source Loading *Relative to current climate.

4 For comparison, we show ice core data for changes in sea salts for the LGM compared to the current climate in Table 2 [DeAngelis et al., 1997; Petit and al., 1999; Roethlisberger et al., 2002; Parrenin et al., 2001; Schwander et al., 2001; Many papers looking at sea salt and other aerosol fluctuations use concentration in the ice cores [e.g. DeAngelis et al., 1997] in part due to problems in the derivation of flux measurements [e.g. Meeker et al., 1997]. However the atmospheric deposition of sea salts is proportional to atmospheric loading of sea salts, while the concentration in the core is the atmospheric deposition divided by the ice accumulation rate, which varies strongly during different climates. As shown in Table 2, estimated deposition ratios (LGM/current climate) are between 2-5 while concentration ratios are between 4-12 because of the large decrease in ice accumulation observed in the ice cores in the LGM relative to the current climate. Model results for precipitation can also be compared to observations the model shows a ratio (LGM/current) of precipitation of ~ , a larger decrease than the observations show ( ). The model tends to not be able to show increases in sea salt deposition as observed at the ice cores between the LGM and current climate similar to [Genthon, 1992; Reader and McFarlane, 2003]. As shown in Figure 1a, 1c and 1e, this is not because of a decrease in aerosol deposition in the Southern Ocean in general, but because of the covering of open ocean close to the Greenland and Antarctic coast by sea ice (Figure 1i and 1j show the maximum (blue) and minimum (red) extent of sea ice), and a decrease in transport to the high altitude ice core regions. Figures 1e and 1g show the LGM/current ratios for deposition fluxes and concentrations in the ice cores, which are the most different in the high latitude regions where the ice cores are measured.

5 Comparisons to observed concentrations in the Southern Ocean and the South Pole (online supplement) suggest that the model is over-predicting by a factor of 2-28 current sea salt aerosol concentrations close to Antarctica. Precipitation comparisons show the model does a good job of capturing both large-scale precipitation in the region, and at the Dome C ice core (not shown). The modeled sodium deposition to the Dome C ice core is g/m2/year, while observations suggest g/m2/year [Roethlisberger et al., 2002], showing that the model is a factor of ~30 too high. Problems with transport to Antarctica have been explored in some detail by [Genthon, 1994], although it appears that in our simulations the errors are in both the source (too strong of winds in the Southern Ocean), and the transport/deposition in the current climate. Additionally, there is some uncertainty in the size distribution of sea salt sources. The ratio of sea salts in the LGM to the current climate is sensitive to the size of the aerosol in this model, with the ratio being 0.6,0.6,0.5,0.2 for the size bins 1-4 (4 is the largest size) at Dome C. Most of the deposition to the ice cores is in the smaller size bins; if the size distribution were much smaller than simulated here, it may be possible to increase the modeled sea salt ratio. Additionally, if there were processes that would increase the relative production of small particles in the LGM versus the current climate, this would also impact the modeled ratios. Table 2: LGM/Current ratios in ice cores vs. model results Flux ratio Concentration Ratio Precipitation Ratio Model (New Sea Model (New Sea Ice) Obs Model Ice core Latitude, Longitude Obs, Model Ice) Obs. Model Vostok 1 78S, 107E Dome C 2 75S, 124E GRIP 3 73N, 38W [Parrenin et al., 2001; Petit and al., 1999; Parrenin, personal communication, 2005]

6 2 [Roethlisberger et al., 2002; Schwander et al., 2001] 3 [DeAngelis et al., 1997] Closer analysis of the mean wind speeds and sea ice fraction suggests that the surface winds cubed over ocean regions are slightly larger in the LGM versus the current climate (+8%), but that the open ocean fraction is slightly smaller (-8%), and that these terms roughly cancel and make the LGM sea salt aerosol source approximately the same as today. Additionally, the spatial distribution is roughly similar (as can be seen the LGM/current deposition flux ratios in Figure 1e or in the online supplement). The zonally averaged vertical concentration distribution shows differences larger than 40% in some regions (shown in online supplement). The zonal distribution of optical depth is similar (shown in online supplement) for the 4 different climates. [Wagenbach et al., 1998; Wolff et al., 2003;Kaleschke et al., 2004] hypothesized from current climate measurements that much of the sea salt aerosols being observed and deposited into the ice cores in Antarctica may not be from open ocean wind effects, but rather from new sea ice formation. During the process of new sea ice formation, brine is rejected and frost flowers are formed, and these frost flowers display similar chemical and isotopic signatures to the observed aerosols. Thus, the increase in sea salts observed in the ice cores could be interpreted as increases in sea ice in the LGM versus the current climate. Since the source processes for the sea salt aerosols formed from new sea ice formation is not known, we assume that the process is identical to that from open ocean sea salts in terms of size distribution and wind speed dependence. We then simulate the new sea ice sea salts for the current and LGM climate (using existing sea ice distributions as a proxy for new sea ice, see Section 2.0). Results from these simulations are included in Table 2 and Figures 1b, 1d and 1f. Note that while there is more sea salts

7 being deposited in the LGM versus the current over the sea ice region, it is not being transported efficiently inland, and the modeled deposition ratios from the new sea ice formation are higher than from open ocean sea salts, but lower than observed in the ice cores, even if all the sea salts come from new sea ice formation (Table 2). It may be that the new sea ice formation of sea salts is dependent on processes not included in our model, such as temperature, open ocean fraction or other processes [Kaleschke et al., 2004]. Also note that while the concentration ratios in ice cores may be more robust than deposition ratios in ice cores [e.g. Meeker et al., 1997], the ice core concentration ratios in the model over the high latitude land regions are quite different than those in the model over the neighboring ocean region, suggesting that interpreting the concentration ratios in ice cores as regional signals is not straightforward. The source of sea salts to the atmosphere is a function of winds to the 3.41 power, and this is calculated every time step in our CCSM3 simulations. We would like to test the sensitivity of our results to the model used for the simulation, but only monthly mean winds are available in the PCMDI archive of IPCC models at the time we obtained the data [ We found in our CCSM3 simulations that the monthly averaged source of sea salts is moderately to highly correlated on a temporal and spatial basis with the monthly mean winds to the 3.41 (not shown), suggesting that just looking at monthly mean winds can give us a rough estimate of the response to climate of sea salt aerosol in other models. Looking at model responses to pre-industrial control compared against in the 20 th century simulations we obtain the following changes in source strength from current to pre-industrial: -3.8, 1.8, 7.3, -0.7 and 1.6%

8 change for the GFDL (Geophysical Fluid Dynamics Laboratory), GISS_AOM (Goddard Institute of Space Studies), GISS_E_H, GISS_E_R and MIROC (Model for Interdisciplinary Research on Climate) models, respectively. For the future scenarios (using from the SRESA1B scenario), we obtain a change of 7.1, 8.7, 13.2, 2.5 and 11.2 for the GFDL, GISS_AOM, GISS_E_H, GISS_E_R and MIROC models, respectively. Both sets of changes are larger than those seen in our CCSM simulations (analyzed the same way, the current to pre-industrial change in the CCSM is 0.7%, while the current to future change is 1.6%). For the pre-industrial change, the models do not agree on whether the change will be an increase or decrease, but all models predict <7% change. For the future scenarios, all models have an increase between % of sea salt source, except the CCSM3, which has the 2% decrease shown above. This suggests that results could be sensitive to model simulations, but that changes in sea salt aerosols in the last and next 100 years could be less than 15%. Of course, this sensitivity study using other model results is very coarse, and simulations should be conducted with other models. 4.0 Summary and Conclusions We show model results of sea salt aerosol sensitivity to climate for the last glacial maximum, pre-industrial, and doubled-carbon dioxide climate using the slab ocean model version of the CCSM3. The source of these natural aerosols is proportional to surface wind cubed, but surprisingly show <5% globally integrated change between different climates, although the changes are larger in some regions. This is due to small changes in globally integrated surface winds, and changes in sea ice extent. Sensitivity studies using

9 winds from 5 different IPCC models suggest that the mean ocean winds to the 3.41 have changed less than 7% since pre-industrial times, and in the future change less than 15%. Observations of sea salt changes in the ice cores between the last glacial maximum and current climate cannot be matched in this model. Some improvement in the model/data comparison is achieved if the generation of sea salt aerosols from new sea ice formation is included, but the ratios are still considerably underestimated. The model/data discrepancies may be due to a) deficiencies in the sea salt source, assumed size distribution and deposition in the model, b) deficiencies in the surface wind changes in the CCSM3 in high latitudes, c) deficiencies in the model transport from the coastal ocean to the high altitude locations of the ice cores or d) problems in the interpretation of the ice core data. In order to better understand the response of natural aerosols to climate change, more simulations should be conducted using different modeling frameworks. Acknowledgements We thank Dennis Savoie and the AERONET network PIs for making available the in situ sea salt aerosol concentrations and sun photometer measurements used in this study in the Online Supplement. We thank Sam Levis, David Fillmore, Masaru Yoshioka, Paul Ginoux, Chao Luo, Bette Otto-Bliesner, Jim McCaa and Alexandra Smirnov for contributions to this paper. We acknowledge the international modeling groups for providing their data for analysis, the Program for Climate Model Diagnosis and Intercomparison (PCMDI) for collecting and archiving the model data, the JSC/CLIVAR Working Group on Coupled Modelling (WGCM) and their Coupled Model

10 Intercomparison Project (CMIP) and Climate Simulation Panel for organizing the model data analysis activity, and the IPCC WG1 TSU for technical support. The IPCC Data Archive at Lawrence Livermore National Laboratory is supported by the Office of Science, U.S. Department of Energy. The manuscript benefited from the comments of two anonymous reviewers and Peter Hess. Bibliography Collins, W., et. al., The community climate system model: CCSM3, Journal of Climate, submitted. DeAngelis, M., J.P. Steffensen, M. Legrand, H. Clausen, and C. Hammer, Primary aerosol (sea salt and soil dust) deposited in Greenland ice during the last climatic cycle: Comparison with east Antarctic records, Journal of Geophysical Reserach, 102 (C12), , Genthon, C., Simulations of desert dust and sea-salt aerosols in Antarctica with a general circulation model of the atmosphere, Tellus, 44B, , Genthon, C., Antarctic climate modeling with general circulation models of the atmosphere, Journal of Geophysical Research, 99 (D6), 12,953-12,961, Haywood, J., V. Ramaswamy, and B. Soden, Tropospheric aerosol climate forcing in clear-sky satellite observations over the oceans, Science, 283, , Kaleschke, L., A. Richter, J. Burrows, O. Afe, G. Heygster, J. Notholt, A. Rankin, H. Roscoe, J. Hollwedel, T. Wagner, and H.-W. Jacobi, Frost flowers on sea ice as a source of sea salt and their influences on tropospheric halogen chemistry, Geophysical Research Letters, 31, L16114, doi: /2004gl020655, 2004.

11 Kiehl, J., C. Shields, J. Hack, and W.Collins, The climate sensitivity of the Community Climate System Model: CCSM3, Journal of Climate, submitted. Meeker, L., P. Mayewski, M. Twickler, S. Whitlow, and D. Meese, A 110,000-year history of change in continental biogenic emissions and related atmospheric circulation inferred from the Greenland Ice Sheet Project Ice Core, Journal of Geophysical Research, 102 (C12), 26, , Otto-Bliesner, B., E. Brady, G. Clauzet, R. Tomas, S. Levis, and Z. Kothavala, Last glacial maximum and holocene climate in CCSM3, Journal of Climate, submitted. O'Dowd, C., J. Lowe, M. Smith, and A. Kaye, The relative importance of non-seasalt sulphate and sea-salt aerosol to the marine cloud condensation nuclei population: An improved multi-component aerosol-cloud droplet parameterization, Q. J. R. Meteorol. Soc., 125, , Parrenin, F., J. Jouzel, C. Walbroeck, C. Ritz, and J. Barnola, Dating the Vostok ice core by an inverse method, Journal of Geophysical Research-Atmospheres, 106 (D- 23), , Petit, J.R., and et. al., Climate and atmospheric history of the past 420,000 years from the Vostok Ice core, Antarctica, Nature, 399, , Reader, C., and N. McFarlane, Sea-salt aerosol distribution during the Last Glacial Maximum and its implications for mineral dust, Journal of Geophysical Research, 108 (D8), 4253: doi /2002/JD002063, Roethlisberger, R., R. Mulvaney, E. Wolff, M. Hutterli, M. Bigler, S. Sommer, and J. Jouzel, Dust and sea salt variability in central East Antarctica (Dome C) over the

12 last 45 kyrs and its implications for southern high-latitude climate, Geophysical Research Letters, 29 (20), 1963, doi: /2002gl015186, Schwander, J., J. Jouzel, C.U. Hammer, J.-R. Petit, R. Udisti, and E.W. Wolff, A tentative chronology for the EPICA Dome Concordia ice core, Geophysical Research Letters, 28 (22), , Wagenbach, D., F. Ducroz, R. Mulvaney, L. Keck, A. Minikin, M. Legrand, J.S. Hall, and E.W. Wolff, Sea-salt aerosol in coastal Antarctic regions, Journal of Geophysical Research, 103 (D9), 10,961-10,974, Wolff, E., A. Rankin, and R. Roethlisberger, An ice core indicator of Antarctic sea ice production?, Geophysical Research Letters, 30 (22), 2158, doi: /2003GL018454, 2003.

13 Figure 1a) Current climate deposition of sea salts (annual average) in g/m2/year, b) Current climate deposition of sea salts from the new sea ice source only, c) LGM deposition, d) LGM deposition from new sea ice source only, e) LGM/current ratio for deposition fluxes (Figure 1c/Figure 1a), f) LGM/current ratio for deposition fluxes from new sea ice source only (Figure 1d/Figure 1b), g. LGM/current ratio for concentration in integrated precipitation, equivalent to ice core concentration ratio (Figure 1c/Figure 1a/Figure 1k), h. similar to g., but for new sea ice source (Figure 1d/Figure 1b/Figure 1k), i) Current climate model sea ice distributions, with maximum extent shown in blue, and minimum extent shown in red (above 0.2 fraction sea ice), j. LGM sea ice distributions (similar to i), and k) LGM/current precipitation.

14