The Role of Ocean Meridional Transports in Subpolar Gyre Steric Height Decadal Variability.
Howe, Nicola; Haines, Keith
University of Reading, UNITED KINGDOM
The convergence and divergence of ocean meridional transports of heat (OHT) and salt (OST) are important factors in the decadal variability of ocean heat (OHC) and salt (OSC) content in the North Atlantic, which in turn contribute to variability in local sea level due to steric height. We use the EN3v2a gridded salinity and temperature from the UK Met Office combined with the AVISO combined product sea surface height to investigate the variability of OHC, OSC and steric height in the observational record.
The meridional transports are commonly deconstructed into a contribution from the gyre and the Atlantic meridional overturning circulation (AMOC). In an eddy-permitting ocean model (the same base model as the reanalysis used in this study) forced with historical surface meteorological fields Grist et al., 2010 showed that the contributions to OHT tend to vary with opposite signs at the southern boundary of the subpolar gyre. The consequence being that large anomalies in subpolar gyre OHC are only observed when the contributions do not balance. We examine the University of Reading Ocean Reanalysis which uses the same base model as Grist et al., 2010. Comparisons are made between the steric height and OHC anomalies in the subpolar gyre in the observations and those related to meridional transports in the reanalysis.
Also in this study we use both heat and salt transports to compare four CMIP5 coupled climate models; HadCM3, CCSM4, GISS(Hycom) and CNRM. Of the four, only HadCM3 shows a strong anti-correlation between the gyre and AMOC contributions to meridional transports at the gyre boundary. The cause of this balance in HadCM3 is found to be related to the strength and extension of the subpolar gyre. We will present results from a comparison of the relationship between steric height in the subpolar gyre and meridional transports in each model and address the questions of why the balance is model dependent and what implications this may have for monitoring ocean transports at the gyre boundary in the real ocean.