a conspicuous area of cold

While global surface temperatures are increasingly dominated by warm anomalies, a conspicuous area of cold has persisted south of Greenland and Iceland visible at the ocean surface in sea surface temperature observations. The abnormal cold there has been more anomalous than the US northeast winter. While the most recent northern winter was the warmest on record globally, the ocean surface area south of Greenland & Iceland had the lowest temperatures in the 136 year record. How could this be?

glob_map_T_anoms_DJF_2014-2015_med

global map of winter 2014/2015 temperature anomalies.

A new study estimates the Atlantic Meridional Overturning Circulation (AMOC) using the sea surface temperature difference at that cold spot south of Greenland/Iceland with the Northern Hemispheric temperature from NASA GISS instrumental records since the 1880s (Hansen and others 1999) and from coral-based proxies after Sherwood and others (2011) that span years since 500 AD.

Rahmstorf, S., J.E. Box, G. Feulner, M.E. Mann, A. Robinson, S. Rutherford and E.J. Schaffernicht, Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation, Nature Climate Change, 23 MARCH 2015, DOI: 10.1038/NCLIMATE2554

Based on this AMOC reconstruction, the study finds that the slowdown of the Atlantic Meridional Overturning Circulation (AMOC) after 1975

  1. appears unprecedented in the past millennium;
  2. is expected to continue, even intensify through year 2100, as simulated with the MPI-ESM-MR global climate model of the Max Planck Institute in Hamburg (Jungclaus and others, 2013)
  3. may result to a large degree from Greenland melting.

My contribution was my work of 6 years, a 172 year Greenland mass balance reconstruction published in a 3 part series in the Journal of Climate (Box and others 2013; Box, 2013; Box and Colgan, 2013), enabling Greenland melting to be brought more into context of its ocean thermohaline perturbation.

Rahmstorf_et_al_2015_NCC_Fig6

increasing Greenland contribution to dilution of surrounding surface waters, figure in Rahmstorf and others (2015)

Melt from Greenland produces water that is lighter and colder than the sea surface waters. The meltwater is light enough to float above the saltier sea surface waters. Because Atlantic surface waters flow northward (see map below), an increasing ice melt freshwater supply ( blue line in the figure above) may pile up near the sea surface, capping or backing up the Gulf Stream North Atlantic Drift current that a.) delivers warmth to northwestern Europe and b.) is part of a global ocean heat conveyer. While Bamber and others (2012) set the stage with “Recent large increases in fresh- water fluxes from Greenland into the North Atlantic”, the new study more directly quantifies the possible impact from Greenland on the ocean thermohaline circulation.

Atlantic Conveyor - graph by Rahmstorf from PIK 20150317

Atlantic Conveyor after Rahmstorf 1997

Why should we care?

The North Atlantic ocean circulation is an important part of a global ocean circulation that exchanges heat from the equatorial surplus to the poles where the energy is lost by thermal radiation to space. A slowed global oceanic ‘conveyer belt’ may further destabilize our changing global climate. We expect no new ice-age – but major negative effects are possible. The effects could be on global climate, fisheries, or also for example storminess.

Influence on weather?

The study will stimulate discussion and research on how the large area of negative sea surface temperatures anomalies south of Greenland and Iceland may influence European and downstream weather. Given some heat exchange between a warm air mass with an anomalously cold North Atlantic sea surface, some strengthening of winds, lowering of central pressure of cyclonic systems, should result from increased “baroclinic instability” (see for example Holton et al. 1992) arising from an increased temperature difference between sea surface and atmosphere. As the subpolar North Atlantic cools and the atmosphere warms, the physics are set to strengthen cyclonic circulation in warm air masses. Converseley, cold air masses drifting off of N America would be less prone to baroclinic deepening. In any case, the perturbation may be felt not just in that part of the world, but downstream, and like the proverbial butterfly (seagull) flapping its wings, alters atmospheric and oceanic flow, with certain though hard to predict downstream consequences.

Work Cited

Bamber, J., M. van den Broeke, J. Ettema, J. Lenaerts, and E. Rignot (2012), Recent large increases in fresh- water fluxes from Greenland into the North Atlantic, Geophys. Res. Lett., 39, L19501, doi:10.1029/2012GL052552.

Box, J.E., N. Cressie, D.H. Bromwich, J. Jung, M. van den Broeke, J.H. van Angelen, R.R. Forster, C. Miège, E. Mosley-Thompson, B. Vinther, J.R. McConnell. 2013. Greenland ice sheet mass balance reconstruction. Part I: net snow accumulation (1600-2009). J. Climate, 26, 3919–3934. doi:10.1175/JCLI-D-12-00373.1 [2]. Box, J. E. 2013. Greenland ice sheet mass balance reconstruction. Part II: Surface mass balance (1840-2010), J. Climate,Vol. 26, No. 18. 6974-6989.  doi:10.1175/JCLI-D-12-00518.1 Box, J.E., W. Colgan. 2013. Greenland ice sheet mass balance reconstruction. Part III: Marine ice loss and total mass balance (1840–2010). Journal of Climate, 26, 6990–7002. doi:10.1175/JCLI-D-12-00546.1

Curry, R. & Mauritzen, C. 2005. Dilution of the northern North Atlantic Ocean in recent decades. Science 308, 17721774.

Hansen, J., Ruedy, R., Glascoe, J. & Sato, M. GISS analysis of surface temperature change. J. Geophys. Res. 104, 3099731022 (1999).

Holton, J .R. (1992): An Introduction to Dynamic Meteorology, 3rd ed., Academic Press.

Jungclaus, J. H. et al. Characteristics of the ocean simulations in the Max Planck Institute Ocean Model (MPIOM) the ocean component of the MPI-Earth system model. J. Adv. Model. Earth Syst. 5, 422446 (2013).

Rahmstorf, S., J.E. Box, G. Feulner, M.E. Mann, A. Robinson, S. Rutherford and E.J. Schaffernicht, Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation, Nature Climate Change, 23 MARCH 2015 | DOI: 10.1038/NCLIMATE2554

Sherwood, O. A., Lehmann, M. F., Schubert, C. J., Scott, D. B. & McCarthy, M. D. Nutrient regime shift in the western North Atlantic indicated by compound-specific delta N-15 of deep-sea gorgonian corals. Proc. Natl Acad. Sci. USA 108, 10111015 (2011).