A new diagnostic for AMOC heat transport applied to the CESM large ensemble

Atlantic time-mean heat transport is northward at all latitudes and exhibits strong multidecadal variability between about 30N and 55N. Atlantic heat transport variability influences many aspects of the climate system, including regional surface temperatures, subpolar heat content, Arctic sea-ice concentration and tropical precipitation patterns. Atlantic heat transport and heat transport variability are commonly partitioned into two components: the heat transport by the AMOC and the heat transport by the gyres. In this paper we compare three different methods for performing this partition, and we apply these methods to the CESM1 Large Ensemble at 34N, 26N and 5S. We discuss the strengths and weaknesses of each method. One of these methods is a new physically-motivated method based on the pathway of the northward-flowing part of AMOC. This paper presents a preliminary version of our method. This preliminary version works only when the AMOC follows the western boundary of the basin. In this context, the new method provides a sensible estimate of heat transport by the overturning and by the gyre, and it is easier to interpret than other methods. According to our new diagnostic, at 34N and at 26N AMOC explains 120% of the multidecadal variability (20% is compensated by the gyre), and at 5S AMOC explains 90% of multidecadal variability.