Decadal-to-Centennial Variations of Tropical Ocean Currents Over the Next 400 Years Under Increasing Greenhouse Gas Emissions

The NASA Goddard Institute for Space Studies (GISS) E2 system of climate models has three atmospheres, two oceans and the same land dataset with different RCPs. We used salinity, temperature and velocity produced by the “H” and “R” ocean models with the TCADI atmosphere model under RCP4.5 to describe decadal-to-centennial variations of the Equatorial Undercurrent (EUC) and North Equatorial Countercurrent (NECC). In 2006-2015 the H and R EUC transports were 12.2 and 18.4 Sv, respectively, and the 50% difference persisted over decadal and centennial times. Compared to the H EUC transport, the R EUC transport was in better agreement with non-coincident observations and results produced by an ocean general circulation model constrained with observations. The east-west slope of the thermocline along the Equator was about two times larger in the H model compared to the R model, producing an enigma because the strengths of EUC and east-west slope of the thermocline are connected. In the case of the NECC, a near agreement occurred in H and R NECC transports (~ 6.4 Sv). However, the H model produced an upward slope of the thermocline from 5-10°N about 50% larger than that computed with the R model, although both estimates of the north-south slope were less than that observed. On longer time scales, the average 2006-2035 H and R EUC transports were both 1-Sv higher than in 2076-2105, which corresponded to a slowing of the H and R EUCs by 9.3 and 5.6%, respectively. The H and R EUC transports in 2100 remained constant over the following 300 years and the spatial structure of the EUC in 2400 was essentially unchanged from 2000.