Upper-ocean bias development and the Atlantic meridional overturning circulation in OMIP simulations

Large upper-ocean biases in the North Atlantic have plagued global climate models for decades. The largest of these anomalies are associated with a poor representation of the Gulf Stream and North Atlantic Current. This issue stems from the ocean model, not from other parts of the climate model. Here, we use the Ocean Model Intercomparison Project (OMIP) simulations to evaluate the development of upper-ocean biases and their consequences for the Atlantic meridional overturning circulation (AMOC). We have calculated AMOC in density coordinates for as many OMIP simulations as possible, finding a wide range of AMOC strengths, consistent with past work. Next, AMOC mean state in the last OMIP cycle is compared to upper-ocean biases. We find that models with a greater upper-ocean cold bias in the Northwest Corner region have northward surface flow along denser isopycnals, southward deep flow on denser isopycnals, and a stronger subpolar overturning streamfunction. From there, we document how the surface biases and AMOC adjust in the first cycle of OMIP1 and OMIP2 simulations. The initial evolution of upper-ocean biases may identify features that contribute to model spread in the AMOC state.