Investigation of IPCC AR4 coupled climate model North Atlantic mode water formation

The formation of mode waters in the North Atlantic was examined in the suite of ocean models that comprise the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3). We constructed model climatologies for 1980–1999 from the 20th century simulations, and compared their mode water properties (temperature, salinity, formation rate, volume, turnover time, heat content) with data. In these models, we found biases in both the properties of the mode waters and their formation rates. For Subpolar Mode Water (SPMW), property biases principally involved salinity errors; additionally, some models form SPMW in an anomalous region west of the British Isles, shifting the source location of waters entering the overturning cell and altering the Nordic Seas’ involvement in the Meridional Overturning Circulation. For Subtropical Mode Water (STMW), property biases involved both salinity and temperature errors, while positioning of heat and water fluxes relative to the Gulf Stream and northwest Sargasso Sea influenced STMW formation rate. Deficiencies in STMW formation rate and volume produced a turnover time of 1–2 years, approximately half of that observed; these variations in mode water bulk properties imply variation in ocean heat storage and advection, and hence deficiencies in all the models’ abilities to adequately respond to changes in climatic forcing.

► Calculated WMTR for North Atlantic mode water formation in IPCC AR4 models.
► WMTR calculations showed wide variations in amounts of mode waters.
► Models showed errors in SPMW and STMW water mass properties.
► Errors in formation rates, properties, and locations were found.
► WMTR errors lead to differences in heat stored/advected and circulation patterns.