The Coherence and Impact of Meridional Heat Transport Anomalies in the Atlantic Ocean Inferred from Observations

Observations of thermosteric sea level (TSL) from hydrographic data, equivalent water thickness (EWT) from GRACE and altimetric sea surface height (SSH) are used to infer meridional heat transport (MHT) anomalies for the Atlantic Ocean. An ``unknown control" version of a Kalman filter in each of eight regions extracts smooth estimates of heat transport convergence (HTC) from discrepancies between the response to monthly surface heat and freshwater fluxes and observed mass and heat content. Two models are used: Model A using only the heat budget for 1993-2010 and Model B using both heat and mass budgets for 2003-2010. Based on the small contributions of mass to SSH, Model A is re-run using SSH in place of TSL to improve temporal resolution data consistency. Estimates of  MHT are derived by summing the HTC from north to south assuming either negligible anomalies at 67N or setting MHT to observed values near 40N.  Both methods show that MHT is highly coherent between 35S and 40N. The former method gives a large drop in coherence north of 40N while the latter method gives a less dramatic drop. Estimated anomalies in MHT comparable to or larger than that recently observed at the RAPID/MOCHA line at 26.5N have occurred multiple times in this 18-year period. Positive anomalies in coherent MHT correspond to increased heat loss in the North Atlantic subtropical gyre demonstrating the feedback of oceanic heat transport anomalies on air-sea fluxes. A correlation of MHT with the Antarctic Oscillation suggests a southern source for the coherent MHT anomalies.