Changing water cycle and freshwater transports in the Atlantic Ocean in observations and CMIP5 models

Observations over the last 40 years show that the Atlantic Ocean salinity pattern has amplified, likely in response to changes in the atmospheric branch of the global water cycle. Observational estimates of oceanic meridional freshwater transport (FWT) at 26.5 degrees N indicate a large increase over the last few decades, during an apparent decrease in the Atlantic Meridional Overturning Circulation (AMOC). However, there is limited observation based information at other latitudes. The relative importance of changing FWT divergence in these trends remains uncertain. Ten models from the Coupled Model Intercomparison Project Phase 5 are analysed for AMOC, FWT, water cycle, and salinity changes over 1950-2100. Over this timescale, strong trends in the water cycle and oceanic freshwater transports emerge, a part of anthropogenic climate change. Results show that as the water cycle amplifies with warming, FWT strengthens (more southward freshwater transport) throughout the Atlantic sector over the 21st century. FWT strengthens in the North Atlantic subtropical region in spite of declining AMOC, as the long-term trend is dominated by salinity change. The AMOC decline also induces a southward shift of the Inter-Tropical Convergence Zone and a dipole pattern of precipitation change over the tropical region. The consequent decrease in freshwater input north of the equator together with increasing net evaporation lead to strong salinification of the North Atlantic sub-tropical region, enhancing net northward salt transport. This opposes the influence of further AMOC weakening and results in intensifying southward freshwater transports across the entire Atlantic.