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

Nikolaos Skliris,Robert Marsh,Jan D Zika,Jennifer V Mecking

doi:10.1007/s00382-020-05261-y

Nikolaos Skliris, Robert Marsh + Show 2 more

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https://doi.org/10.1007/s00382-020-05261-y

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Abstract

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° 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.

Highlights

Net evaporation over the Atlantic Ocean provides the source of moisture that is exported to the Pacific, principally via the trade winds and makes the Atlantic more saline than other ocean basins, most notably the Pacific Ocean
The North Atlantic subtropical region shows an overall increased net evaporation but there is no widespread signal as strong net evaporation increases roughly north of 25° N are contrasted with net evaporation decreases south of 25° N
The observational estimate of southward ocean freshwater transport increase at 26.5° N together with increasing net evaporation north of 26.5° N over the last few decades requires a large increase of southward freshwater transport at the Arctic/Atlantic boundary

Summary

Introduction

Net evaporation over the Atlantic Ocean provides the source of moisture that is exported to the Pacific, principally via the trade winds and makes the Atlantic more saline than other ocean basins, most notably the Pacific Ocean. South Wales, Sydney, Australia (FWT) throughout the Atlantic sector. These atmospheric and oceanic freshwater pathways are an intrinsic part of the global water cycle, a central component of the climate system, which is likely to change dramatically under global warming. The ocean salinity 3-D field, a well-measured parameter over the last few decades, is considered to be an integrator of surface freshwater flux change, and it is often used to indirectly estimate long-term changes in the global water cycle (Helm et al 2010; Durack et al 2012; Skliris et al 2016; Zika et al 2018).

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