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What caused the significant increase in Atlantic Ocean heat content since the mid-20th century?

Sang-Ki Lee, Wonsun Park, Erik van Sebille, Molly Baringer, Chunzai Wang, David Enfield, Steven Yeager, and Ben Kirtman
In Geophysical Research Letters, 2011, volume 38, page L17607, doi:10.1029/2011GL048856.

Abstract

As the upper layer of the world ocean warms gradually during the 20th century, the inter-ocean heat transport from the Indian to Atlantic basin should be enhanced, and the Atlantic Ocean should therefore gain extra heat due to the increased upper ocean temperature of the inflow via the Agulhas leakage. Consistent with this hypothesis, instrumental records indicate that the Atlantic Ocean has warmed substantially more than any other ocean basin since the mid-20th century. A surface-forced global ocean-ice coupled model is used to test this hypothesis and to find that the observed warming trend of the Atlantic Ocean since the 1950s is largely due to an increase in the inter-ocean heat transport from the Indian Ocean. Further analysis reveals that the increased inter-ocean heat transport is not only caused by the increased upper ocean temperature of the inflow but also, and more strongly, by the increased Agulhas Current leakage, which is augmented by the strengthening of the wind stress curl over the South Atlantic and Indian subtropical gyre.

Key figure

agulhasobs

Figure 1: (a) Simulated Atlantic Ocean heat content change in the upper 700m in reference to the 1871-1900 baseline period obtained from the four model experiments. The thick black line in (a) is the observed heat content of the Atlantic Ocean, which is recomputed from Levitus et al. [2009] for the Atlantic basin from 30S to 75N. (b) Simulated heat budget terms for the Atlantic Ocean obtained from EXP_CTR, all referenced to the 1871-1900 baseline period.