AMOC changes and impacts

The paper by Vellinga and Wood (2002) demonstrating the widespread and global impacts of an AMOC slowdown was a revelation, and the resemblance of its impacts to the Younger Dryas event convinced me of its relevance to past (and future) climate change. The AMOC rivals the El Nino-Southern Oscillation in its global impacts, but in this case the center of action being in the high latitudes and the impacts felt in the tropical rainband.

Wei Cheng, Cecilia Bitz, and I became interested in unpacking the details of AMOC shutdown and its large-scale climate impacts. The Cheng et al. (2007) paper was the main accomplishment of our collaboration. Amongst its key findings was a detailed energetic analysis that connected the southward ITCZ shift to increased atmospheric norhtward heat transport extending from the southern tropics to the northern high latitudes, the latter to compensate for the loss of the northward ocean heat transport. A little-known but significant result from this study was in showing that the high-latitude cooling from an AMOC shutdown was far greater in an LGM basic state than in a present-day basic state, despite the fact that the ocean heat transport reduction was about the same in both cases. The reason was because sea-ice feedbacks were substantially larger in the LGM case. Chiang et al. (2008) detailed the teleconnection to the tropical Atlantic ITCZ, finding that the atmospheric teleconnection to be the main cause of the ITCZ shift, but that baroclinic ocean adjustments do play a role in altering the North Atlantic SST response. Bitz et al. (2007) also found that AMOC shutdown in the LGM state takes longer to recover than in a present-day basic state, thus prolonging the AMOC impacts.

I worked on AMOC impacts related several paleoclimate scenarios. In Lee et al. (2011), we proposed an atmospheric connection from the high latitude North atlantic to the Southern hemisphere westerlies as an explanation for the the observed Southern Ocean upwelling seen by Anderson et al.(2009) during deglaciation. The connection works through the southward ITCZ shift that weakens the southern hemisphere subtropical jet and strengthens the midlatitude eddy-driven jet. In Rhodes et al. (2015), we explained a curious increase in atmospheric methane during Heinrich stadials to come from a pronounced southward shift in the tropical rainbands that caused more rainfall over southern hemisphere tropical land regions. In Bhattacharya et al. (2017), we explained a dry periods over Mesoamerica during 800-1050CE to result from a long-term cooling of the tropical North Atlantic SST driven by AMOC.

I’ve dabbled a bit with AMOC variations. Cheng et al. (2013) reports on the slowdown of the AMOC in all CMIP5 RCP simulations. In Chiang et al. (2021), we used Maximum Covariance Analysis on a long control simulation to untangle the intrinsic relationship between the AMOC and surface T/S properties over the high latitude North Atlantic.

Bitz, C.M., Chiang, J.C.H., Cheng, W. and Barsugli, J.J., 2007. Rates of thermohaline recovery from freshwater pulses in modern, Last Glacial Maximum, and greenhouse warming climates. Geophysical research letters, 34(7).

Bhattacharya, T., Chiang, J.C. and Cheng, W., 2017. Ocean-atmosphere dynamics linked to 800–1050 CE drying in mesoamerica. Quaternary Science Reviews, 169, pp.263-277.

Cheng, W., Bitz, C.M. and Chiang, J.C.H., 2007. Adjustment of the global climate to an abrupt slowdown of the Atlantic meridional overturning circulation. Geophysical Monograph-American Geophysical Union, 173, p.295.

Cheng, W., Chiang, J.C. and Zhang, D., 2013. Atlantic meridional overturning circulation (AMOC) in CMIP5 models: RCP and historical simulations. Journal of Climate, 26(18), pp.7187-7197.

Chiang, J.C., Cheng, W. and Bitz, C.M., 2008. Fast teleconnections to the tropical Atlantic sector from Atlantic thermohaline adjustment. Geophysical Research Letters, 35(7).

Chiang, J.C.H., Cheng, W., Kim, W.M. and Kim, S., 2021. Untangling the Relationship Between AMOC Variability and North Atlantic Upper‐Ocean Temperature and Salinity. Geophysical Research Letters, 48(14), p.e2021GL093496.

Lee, S.Y., Chiang, J.C., Matsumoto, K. and Tokos, K.S., 2011. Southern Ocean wind response to North Atlantic cooling and the rise in atmospheric CO2: Modeling perspective and paleoceanographic implications. Paleoceanography, 26(1).

Rhodes, R.H., Brook, E.J., Chiang, J.C., Blunier, T., Maselli, O.J., McConnell, J.R., Romanini, D. and Severinghaus, J.P., 2015. Enhanced tropical methane production in response to iceberg discharge in the North Atlantic. Science, 348(6238), pp.1016-1019.