humidity, we expect a 7% increase in atmospheric moisture content for every degree of 30 warming of the Earth's lower troposphere (2). Of greatest importance to society, and the focus 31 of this work, is the strength of the regional pattern of evaporation and precipitation (E--P), 32 which in climate models scales approximately with CC, while global precipitation changes more 33 slowly at a rate of 2--3% °C --1 (2, 4). 34An intensification of existing patterns of global mean surface evaporation and precipitation (E-- do not yet have a definitive view on whether the Earth's water cycle has intensified over the 61 past several decades from atmospheric observing networks (12,20). 62It has long been noted that the climatological mean sea surface salinity (SSS) spatial pattern is 63 highly correlated with the long--term mean E--P spatial pattern (21) ( estimates of long--term SSS change (24, 25, 27). Following the "rich get richer" mechanism (7) Table S2). Our examples of the simulations 117 that most closely replicate the observed spatial change and mean patterns (Fig. 1E, H) and 118 those that produce an almost inverse spatial change pattern (Fig. 1F, I) compared to the 119 observed results (Fig. 1D, G), illustrate the range of responses found in CMIP3 (Fig. 1). Some 120 models show similar numbers to those observed (Fig. 1E, H The PA and PC methodology can also be used when considering other variables, such as the 126 surface water flux (E--P). CMIP3 simulations show a relationship between SSS PA and the E--P PA 127 (Fig. 2B). This key result supports the use of SSS PA as a diagnostic of a changing water cycle, 128 and also provides a relationship in which to consider the observed SSS PA for 1950--2000. The 129CMIP3 SSS patterns amplify at twice the rate of E--P patterns (Fig. 2B) Figure S9). 136For both the 20C3M and SRES CMIP3 simulations, we find a clear relationship between the rate 137 of global average surface warming (ΔTa) and the rate of SSS PA and PC strength ( Fig. 2A). 13820C3M simulations in which the warming rate is low (generally those with comprehensive 139 aerosol schemes; contrast diamonds and circles in Fig. S5; Table S1) suggests that SSS patterns intensify with warming at 8% °C --1 ( Fig. 2A), which is half of our 1950--147 2000 observed rate (16% °C --1 ; Fig. 2A). As expected, based on past analyses of CMIP3 (2), the E--148 P PA is also linearly related to surface warming rates (Fig. 2C) surface water flux, near 3.1% °C --1 (Fig. 2D). The stronger SSS PA response to warming and 152 tighter agreement among CMIP3 when compared to that for the E--P PA ( Fig. 2A vs C To independently demonstrate the strong relationship between 50--year salinity change and an 159 enhanced water cycle, the response of an ocean--only model to an idealised 5% E--P pattern 160 increase was explored. We used a version of the MOM3 ocean model, forced with E--P fields 161 obtained from the NCEP reanalysis. A linear trend in E--P was imposed to achieve a 5% increase (which shows that SSS PA increases at twice the rat...

Using over 1.6 million profiles of salinity, potential temperature, and neutral density from historical archives and the international Argo Program, this study develops the three-dimensional field of multidecadal linear change for ocean-state properties. The period of analysis extends from 1950 to 2008, taking care to minimize the aliasing associated with the seasonal and major global El Niñ o-Southern Oscillation modes. Large, robust, and spatially coherent multidecadal linear trends in salinity to 2000-dbar depth are found. Salinity increases at the sea surface are found in evaporation-dominated regions and freshening in precipitationdominated regions, with the spatial pattern of change strongly resembling that of the mean salinity field, consistent with an amplification of the global hydrological cycle. Subsurface salinity changes on pressure surfaces are attributable to both isopycnal heave and real water-mass modification of the temperaturesalinity relationship. Subduction and circulation by the ocean's mean flow of surface salinity and temperature anomalies appear to account for most regional subsurface salinity changes on isopycnals. Broad-scale surface warming and the associated poleward migration of isopycnal outcrops drive a clear and repeating pattern of subsurface isopycnal salinity change in each independent ocean basin. Qualitatively, the observed global multidecadal salinity changes are thus consonant with both broad-scale surface warming and the amplification of the global hydrological cycle.

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Abstract. The Ocean Model Intercomparison Project (OMIP) is an endorsed project in the Coupled Model Intercomparison Project Phase 6 (CMIP6). OMIP addresses CMIP6 science questions, investigating the origins and consequences of systematic model biases. It does so by providing a framework for evaluating (including assessment of systematic biases), understanding, and improving ocean, sea-ice, tracer, and biogeochemical components of climate and earth system models contributing to CMIP6. Among the WCRP Grand Challenges in climate science (GCs), OMIP primarily contributes to the regional sea level change and near-term (climate/decadal) prediction GCs.OMIP provides (a) an experimental protocol for global ocean/sea-ice models run with a prescribed atmospheric forcing; and (b) a protocol for ocean diagnostics to be saved as part of CMIP6. We focus here on the physical component of OMIP, with a companion paper (Orr et al., 2016) detailing methods for the inert chemistry and interactive biogeochemistry. The physical portion of the OMIP experimental protocol follows the interannual Coordinated Ocean-ice Reference Experiments (CORE-II). Since 2009, CORE-I (Normal Year Forcing) and CORE-II (Interannual Forcing) have become the standard methods to evaluate global ocean/sea-ice simulations and to examine mechanisms for forced ocean climate variability. The OMIP diagnostic protocol is relevant for any ocean model component of CMIP6, including the DECK (Diagnostic, Evaluation and Characterization of Klima experiments), historical simulations, FAFMIP (Flux Anomaly Forced MIP), C4MIP (Coupled Carbon Cycle Climate MIP), DAMIP (Detection and Attribution MIP), DCPP (Decadal Climate Prediction Project), ScenarioMIP, HighResMIP (High Resolution MIP), as well as the ocean/sea-ice OMIP simulations.

Understanding observed changes to the global water cycle is key to predicting future climate changes and their impacts. While many datasets document crucial variables such as precipitation, ocean salinity, runoff, and humidity, most are uncertain for determining long-term changes. In situ networks provide long time series over land, but are sparse in many regions, particularly the tropics. Satellite and reanalysis datasets provide global coverage, but their long-term stability is lacking. However, comparisons of changes among related variables can give insights into the robustness of observed changes. For example, ocean salinity, interpreted with an understanding of ocean processes, can help cross-validate precipitation. Observational evidence for human influences on the water cycle is emerging, but uncertainties resulting from internal variability and observational errors are too large to determine whether the observed and simulated changes are consistent. Improvements to the in situ and satellite observing networks that monitor the changing water cycle are required, yet continued data coverage is threatened by funding reductions. Uncertainty both in the role of anthropogenic aerosols and because of the large climate variability presently limits confidence in attribution of observed changes.

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a b s t r a c tWe provide an assessment of sea level simulated in a suite of global ocean-sea ice models using the interannual CORE atmospheric state to determine surface ocean boundary buoyancy and momentum fluxes. These CORE-II simulations are compared amongst themselves as well as to observation-based estimates. We focus on the final 15 years of the simulations (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007), as this is a period where the CORE-II atmospheric state is well sampled, and it allows us to compare sea level related fields to both satellite and in situ analyses. The ensemble mean of the CORE-II simulations broadly agree with various global and regional observation-based analyses during this period, though with the global mean thermosteric sea level rise biased low relative to observation-based analyses. The simulations reveal a positive trend in dynamic sea level in the west Pacific and negative trend in the east, with this trend arising from wind shifts and regional changes in upper 700 m ocean heat content. The models also exhibit a thermosteric sea level rise in the subpolar North Atlantic associated with a transition around 1995/1996 of the Atlantic Oscillation to its negative phase, and the advection of warm subtropical waters into the subpolar gyre. Sea level trends are predominantly associated with steric trends, with thermosteric effects generally far larger than halosteric effects, except in the Arctic and North Atlantic. There is a general anticorrelation between thermosteric and halosteric effects for much of the World Ocean, associated with density compensated changes.Published by Elsevier Ltd.

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