Thread by @ClimateOfGavin

I see that Twitter is excited today about instability, abrupt changes, come backs, and shut downs.

So, time for a mini-thread on the Atlantic meridional overturning circulation (AMOC), predictability and perhaps the Younger Dryas cold reversal?

There is some discussion of potential current and ongoing changes in AMOC at @RealClimate this week realclimate.org/index.php/arch… from @rahmstorf, but I'm going to talk about something a little different, namely, what is the scope for changes in AMOC over the longer-term?

It's well known that GCMs suggest that over the 21st Century under increasing GHGs, the AMOC decreases - mostly because of ocean surface warming decreasing the density. doi.org/10.1175/JCLI-D…

We know too that freshwater inputs from Greenland (and in the past, other NH ice sheets or paleo-lake collapses) can cause relatively abrupt shutdowns in the circulation. A (partial) collapse 8200 yr ago is the most recent example

The poster-child for abrupt change involving AMOC is the multi-century Younger Dryas cooling during the last deglaciation (despite multiple 'alternate' theories that have been suggested over the years).

Many claims have been made for a specific freshwater source that could have triggered the initial slowdown of circulation (via the St. Lawrence, The Mississippi or the MacKenzie - for instance nature.com/articles/natur…)

Whether any of these sources are sufficient either to trigger a collapse, or maintain it for ~1000 years is unclear. Modeling evidence in support of this scenario is 'ambiguous'. But there are lots of uncertainties - so it's certainly not ruled out.

The point to make is that it isn't as strongly quantified as listening to paleo-climate talks might sometimes lead you to believe (I'm not implicating anyone specific here). But it is much better supported than anything else.

What Q's remain?
1) A quantified fresh water history through this period. (W/o an independent input function, getting the 'right' model response is hard)
2) It's easy for models to cool at about the right rate, but v. hard to get them to warm abruptly at the end.

3) How do you maintain an 'off state' for multiple centuries? Most models show the circulation comes back relatively quickly if you turn off the fresh water. Maybe the freshwater lasted centuries? Maybe models are too stable?

Many of these questions remain open because it's hard to do simulations of the whole deglaciation, also the background climate before the YD is different to today or the pre-industrial (for which most simulations have been made), and we don't know the FW forcing well enough.

So, let's look elsewhere. For instance, what are the really long term responses in the GCMs to anthropogenic forcings - way beyond the 2100 or even 2300 horizons we are used to seeing? (eg. longrunmip.org)

Buried in that graphic are some interesting variations... I'll talk about some of the GISS models.

3 kinds of experiment here, abrupt increases to 4xCO2, ramped up CO2 (at 1%/yr) and the standard RCP runs, all run for multiple centuries...

Notice anything?

In at least 3 simulations (w/same model & w/strong forcing >7 W/m2), there's a relatively fast shutdown in the AMOC, a long multi-century period w/o overturning circulation, & then a v. rapid overshoot & recovery. Note that there aren't *any* external freshwater sources here.

In these simulations, some threshold is clearly crossed and - depending a little on the initial conditions and the scenario - the same phenomenon occurs ± a hundred years in timing. Oddly, in a v. similar model (minus only an aerosol indirect effect on clouds), nothing happens.

Looking at the differences btw models that flip and those that don't, it has nothing to do with standard ocean diagnostics of circulation stability (one would predict all models would be stable based on FW budgets).

Rather it depends on the sensitivity of atmospheric fluxes to cooling. In the models that flip, the change in evaporation for a change in SST in the North Atlantic is stronger (which is destabilizing). A big evap decrease when the surface cools decreases salinity & density.

The trigger for the collapse/recovery seems to be the hemispheric balance of temperature. The SH is slow to warm, so sub-polar water density in each hemisphere initially converges, triggering a collapse (and NH cooling). When the NH eventually catches the SH, the AMOC restarts.

But is this really anything like the Younger Dryas?

You be the judge.

Ok - that's enough. If you want more details on these analyses, please check out our new paper in JGR:
agupubs.onlinelibrary.wiley.com/doi/abs/10.102…

Now back to your regularly scheduled chaos.