Holocene Upper Atlantic Temperature Trend: AMOC Variability or Anomalous Heat Uptake

Whether and how the Atlantic Meridional Ocean Circulation (AMOC) has changed during the last 11,000-years is uncertain. Yet, the AMOC influences the distribution of heat, salinity, and nutrients, and variations in its strength can affect global climate, with ecological impacts. Initial data suggest the tropical Atlantic, at about 1000 meters water depth, cooled from approximately 11,000 to 5,000-years ago.

Other evidence also hints similar cooling in this region during this time. Changes in the strength of the AMOC influence how heat is distributed within the deep ocean and may have caused the observed cooling trend. Temperature of the deep ocean is also affected by temperature changes in the regions where surface waters sink into the deep ocean.

These surface temperatures may change due to global temperature change or due to regional climate changes at high latitudes (e. g. winds, sea ice distribution). The goal of this study is to generate data to confirm the observed cooling trend and to assess whether the trend is due to cooling of high latitude waters flowing to these sites or to changes in AMOC, or to both of these mechanisms.

Identifying the cause of this cooling has important implications for understanding Holocene climate and variability. A post-doctoral scientist will participate in this project.

Initial results suggest an early – middle Holocene cooling in the western tropical north Atlantic, at a depth (about 950 m) where Antarctic Intermediate Water (AAIW) is the dominant water mass. This cooling began at approximately 11 kyBP and culminated in near-modern bottom water temperatures (BWT) by about 7-5 kyBP. Hypotheses to explain this cooling include (1) a strengthening Atlantic Meridional Overturning Circulation (AMOC) in response to a decrease in the input of deglacial freshwater to the North Atlantic, or (2) cooling in one or more of the source waters to the site.

Researchers will confirm this cooling by reconstructing BWT at several shallow tropical sites within modern AAIW depths, and test the alternative hypotheses with measurements from other subtropical sites within AAIW and northern source waters. A suite of marine sediment cores from the north, south, and tropical Atlantic, and ranging from 300 to 1100 meters water depth will be used to reconstruct the spatial pattern of temperature change and help distinguish between the two likely causes of the temperature trend, a high-latitude cooling in waters flowing to the tropical Atlantic, or a strengthening of the AMOC during this time period.

As model simulations do not predict a cooling at these sites over this time period, confirming and understanding the cause of this cooling may help identify where models or model set-up can be improved.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.