cc: sfbtett@meto.gov.uk , k.briffa@uea.ac.uk , t.osborn@uea.ac.uk date: Tue, 06 Feb 2001 23:47:47 +0000 from: "Tim Osborn" subject: sea level in SOAP to: plao@geo.vu.nl Orson, I late and probably incomplete reply to your various questions about the sea level package in SOAP. (1) Going back to your e-mail of 1 Feb. Yes, the global/hemispheric picture is of monotonic cooling from 1000 AD to 1900 AD, then rapid warming. So you might well ask why glacier melting began earlier, and why sea levels seemed to rise suddenly with little lag after 1900 AD. One answer might be that some records/regions, especially circum-North Atlantic reconstructions, indicate more inter-century climate variability, with more marked Little Ice Age and Medieval Warm Period phases, with perhaps notably cooler conditions during the 1600 and 1800s, with a warmer 1700s between. Perhaps the 1600s were coolest of all, then glacier melting began during the warmer 1700s? Or is that too early. Also, there's seasonality to consider (winters showed an upward trend prior to 1900 I believe), plus (for the glacier question) precipitation variability (e.g. Scandinavian glaciers have been growing recently rather than losing mass, due to NAO-related enhanced winter precipitation). As I understand it, the models are capable of producing regional differences in sea level change, related to circulation changes. So the thermal expansion part of sea level should show regionally different results. However, I think the glacier-melt models will be run off-line (i.e., after completion of the climate simulations). If this is the case, then the glacier-melt part of sea level cannot influence ocean salinity and hence ocean density and circulation. That would be a disadvantage of course, as glacier-melt would then have to be added uniformly over the globe. Unless Simon tells me otherwise! (2) Your e-mail of 2 Feb. Thanks for the rationale text - that'll be useful. The model to be used (HadCM3) for the past 500 years is also one that is used for future sea level and climate scenarios. So its relevant to test it. We will have one simulation 1500-2000 forced by natural forcing (solar, volcanic and maybe the small orbital changes). Then a second simulation 1750-2000 that will have natural and anthropogenic forcings (greenhouse gases, sulphate aerosols, and maybe stratospheric ozone and maybe land-use changes). For the overall project, a second model (run in Hamburg, ECHAM) will have a natural forcings run from 1000-2000, plus a natural+anthropogenic forcings for 1750-2000. They have offered their model output for the sea level workpackage too, though I'm not sure how easily it can be used to force the Hadley Centre's glacier melt model (Simon - you'll have to advise here...do you just need monthly fields and temp and precip? I guess they'll produce thermal expansion component anyway, since there model probably doesn't have a rigid lid and will automatically predict regional sea level). Model adaptation is not really possible within this project, so differences between paleo and simulated sea level will raise more questions for future work - but also perhaps provide a range of variation from different sources. The HOLSMEER project does not involve such a model-data comparison, so we should indeed stress that in the SOAP proposal. (3) Your e-mail of 6 Feb. Q1. As outlined above, the model has capability of producing regional sea level (Simon, please correct if I'm wrong, though I'm fairly sure about this), though how realistic this is I'm not sure, especially if glacier melt isn't allowed to be added regionally and affect salinity & circulation (not sure how big a term it would be in the salinity budget, given the fairly small amounts of climate change in the past 500 years, especially under natural forcings only. So we can certainly look at variations within the Atlantic, at a fairly coarse scale though. Q2. Links between simulated multi-decadal/century climate patterns and simulated sea level variability can be looked at within the model data. If these are internally-generated climate patterns (e.g., multi-decadal North Atlantic Oscillation anomalies), then these wouldn't neccessarily coincide with the timing of such variability in the real world, but would still be useful to find out how sea level responds to such patterns. If they're forced externally, then they may also have happened in the real world (a number of caveats here), and so could be searched for. Q3. HadCM3 will be forced as described above. The difference between the natural and the natural+anthropogenic runs will be useful to estimate how much sea level rise we would have got anyway, due to coming out of the Maunder-minimum-induced cool period, compared to what is observed and compared to what the anthropogenic forcing adds. If the model and paleo sea levels show no similarity at all, then I guess we will just have to present them as two equally possible results (or with unequal possibilities if we can gauge uncertainty ranges on each approach). All questions you raise, regarding ocean-climate links etc., can actually be included in the proposal as things that this workpackage can start to address. It sounds like there's scope for lots of work. If you need to expand on the budget and number of months previously suggested then that should be fine, though I remember you didn't want to commit to too much. If not, then we'll just have to make it clear that we will just make a first attempt at this model-data comparison and a first attempt at answering some questions, but won't achieve them all. Apologies to all for what's turned into rather a long e-mail. I'm away Wednesday, but back Thursday. You can cc replies to Keith if you need anything clarified while I'm away on Wednesday. Regards Tim