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This study investigates the contribution of external forcings on global and regional ocean wave height change during 1961–2020. Historical significant wave height (Hs) produced for different CMIP6 external forcings and preindustrial control conditions following the Detection and Attribution Model Intercomparison Project (DAMIP) are employed. The internal variability ranges are compared with different external forcing scenario. Statistically significant linear trends in Hs computed over regional ocean basins are found to be mostly associated with anthropogenic forcings: greenhouse gas‐only (GHG) and aerosol‐only (AER) forcing. For Hs, GHG signals are robustly detected and dominant for most of the global ocean, except over North pacific and South Atlantic, where AER signals are dominant. These results are supported by multi‐model analysis for wind speed. The remarkable increase in Hs over the Arctic (22.3%) and Southern (8.2%) Ocean can be attributed to GHG induced sea‐ice depletion and larger effective fetch along with wind speed increase.

Along with the mean sea level rise due to climate change, the sea level exhibits natural variations at a large number of different time scales. One of the most important is the one linked with the seasonal cycle. In the Northern Hemisphere winter, the sea level is as much as 20 cm below its summer values in some locations. It is customary to associate these variations with the seasonal cycle of the sea surface net heat flux which drives an upper-ocean thermal expansion creating a positive steric sea level anomaly. Here, using a novel framework based on steric sea level variance budget applied to observations and to the Estimating the Circulation and Climate of the Ocean state estimate, we demonstrate that the steric sea level seasonal cycle amplitude results from a balance between the seasonal sea surface net heat flux and the oceanic advective processes. Moreover, for up to 50% of the ocean surface, surface heat fluxes act to damp the seasonal steric sea level cycle amplitude, which is instead forced by oceanic advection processes. We also show that eddies play an important role in damping the steric sea level seasonal cycle. Our study contributes to a better understanding of the steric sea level mechanisms which is crucial to ensure accurate and reliable climate projections.

As well as having an impact on the background state of the climate, global warming due to human activities could affect its natural oscillations and internal variability. In this study, we use four initial-condition ensembles from the CMIP6 framework to investigate the potential evolution of internal climate variability under different warming pathways for the 21st century. Our results suggest significant changes in natural climate variability, and point to two distinct regimes driving these changes. First, a decrease of internal variability of surface air temperature at high latitudes and all frequencies, associated with a poleward shift and the gradual disappearance of sea-ice edges, which we show to be an important component of internal variability. Second, an intensification of the interannual variability of surface air temperature and precipitation at low latitudes, which appears to be associated with the El Niño–Southern Oscillation (ENSO). This second regime is particularly alarming because it may contribute to making the climate more unstable and less predictable, with a significant impact on human societies and ecosystems.

Satellite observations of significant wave height (Hs) have recently reached 30 years of continuous record. Is this length sufficient to detect the effect of anthropogenically forced climate change on wave height trends? Wave height decadal variability is influenced by a combination of internal variability and forced variability caused by both anthropogenic and natural forcing. Using a statistical model to derive Hs from sea level pressure field and exploiting ERA-5 reanalysis data as well as 80 members of the Community Earth System Model v2 large ensemble, we show that, over the North Atlantic (NA), altimetry-based Hs trends are mostly caused by internal variability. This suggests that Hs changes computed over the satellite era are not yet controlled by anthropogenic climate change. Starting from 1993, the date of emergence, defined as the date when the forced signal becomes dominant over the internal variability, is later than 2050 for Hs in the NA. Plain Language Summary Satellite observations of significant wave height will soon reach 30 years of continuous record. Changes in significant wave height over this period can either be attributed to both anthropogenic or “natural” climate forcing or to the intrinsic variability of the climate system. In this article, we show that the intrinsic variability effect largely dominates the significant wave height changes in the North Atlantic (NA) over the 30 years of satellite observations. We further estimate that the significant wave height changes associated with anthropogenically forced climate change will become dominant over those due to the intrinsic variability after 2050 in most of the NA region.

Global mean sea level rose by 15 mm over June 2014 – May 2016. This rise is 7 mm larger than the 8 mm increase associated with the long-term trend of 4 mm/yr estimated over 2006–2016. Using a combination of satellite gravimetry data and in situ measurements, we find that 20% of this rise is explained by ocean thermal expansion and 80% by an ocean mass increase, the latter being largely correlated with an equivalent terrestrial water storage (TWS) decrease. Half of the global ocean mass increase during that period can be attributed to the South American continent where we find a significant contribution of the TWS over the Amazon basin (5 mm). This TWS change between oceans and continents occurred during two El Nino events: one aborted in 2014–2015 and an extreme event in 2015–2016 which affected precipitation patterns, especially over the equatorial Pacific ocean and over South America. Key Points Global mean sea level rose by 15 mm over June 2014 – May 2016 80% of this rise had a mass origin (12 mm) and 20% had an ocean warming origin (3 mm) The terrestrial water storage change in the Amazon basin (5 mm) contributed to one third of the substantial sea level rise Plain Language Summary Interannual variability of global mean sea level (GMSL) change is linked to natural climate modes of variability such as El Nino Southern Oscillation (ENSO). Over May 2014–June 2016, two consecutive El Nino events (warm phase of ENSO) occurred in the tropical Pacific ocean: one aborted in 2014–2015 and an extreme event in 2015–2016. At the same time, satellite altimetry recorded a GMSL increase of 15 mm. 80% of the rise was due to a global ocean mass increase. Half of the global ocean mass increase is attributed to the South American continent with an exceptional contribution from the Amazon basin (5 mm). Those unusual El Nino events affected the precipitation pattern worldwide, decreasing the TWS in the Amazon basin and therefore leading to an increase of the global mean ocean mass.

Understanding the mechanisms of regional steric sea level variability is fundamental to understand the regional sea level variability recorded by satellite altimetry for years and to insure that future projections made by climate models are realistic. Here, we first develop a novel method based on steric sea level variance budget that allows to detect the sources and sinks of the variability. Using the “Estimating the Circulation and Climate of the Ocean” (ECCO V4) state estimate, we then show that interannual steric sea level variability is mainly sustained by interannual fluctuating winds via Ekman transport almost everywhere. The damping of the variability is made by both the interannual fluctuating net heat flux from the atmosphere, that largely dominates the atmospheric freshwater fluxes, and the parametrized effect of eddies. It is also found that the parametrized effect of diffusion on the variability is weak in most regions and that, although globally weak, the fluctuations of atmospheric freshwater fluxes are a source of variance close to the Equator in the Pacific ocean. Key Points The main source of interannual steric sea level variance is the advection of the mean density by interannual velocity anomalies. This source of variance is balanced by fluctuating net heat flux from the atmosphere and by the fluctuating eddy induced velocities. The main source of interannual steric sea level variance is mostly sustained by the wind variability via Ekman velocities. Plain Language Summary Since the launch of Topex/Poseidon satellite in August 1992, sea level is routinely measured every ∼10 days at a nearly global coverage. For the first time, satellite altimetry data show that regional sea level trends experience large regional variability compared to its global mean trend. If global mean sea level rise is a direct consequence of the on-going climate change, regional sea level change is more relevant for coastal impacts. Thus, investigating the regional sea level

Long-term changes of wind-generated ocean waves have important consequences for marine engineering, coastal management, ship routing, and marine spatial planning. It is well-known that the multi-annual variability of wave parameters in the North Atlantic is tightly linked to natural fluctuations of the atmospheric circulation, such as the North Atlantic Oscillation. However, anthropogenic climate change is also expected to influence sea states over the long-term through the modification of atmospheric and ocean circulation and melting of sea ice. Due to the relatively short duration of historical sea state observations and the significant multi-decadal variability in the sea state signal, disentangling the anthropogenic signal from the natural variability is a challenging task. In this article, the literature on inter-annual to multi-decadal variability of sea states in the North Atlantic is reviewed using data from both observations and model reanalysis.

A linear model based on the quasigeostrophic equations is constructed in order to predict the vertical structure of Rossby waves and, more broadly, of anomalies resolved by altimeter data, roughly with periods longer than 20 days and with wavelengths larger than 100 km. The subsurface field is reconstructed from sea surface height and climatological stratification. The solution is calculated in periodic rectangular regions with a 3D discrete Fourier transform. The effect of the mean flow on Rossby waves is neglected, which the authors believe is a reasonable approximation for low latitudes. The method used has been tested with an idealized double- gyre simulation [performed with the Miami Isopycnal Coordinate Ocean Model (MICOM)]. The linear model is able to give reasonable predictions of subsurface currents at low latitudes (below approximately 308) and for relatively weak mean flow. However, the predictions degrade with stronger mean flows and higher latitudes. The subsurface velocities calculated with this model using AVISO altimetric data and velocities from current meters have also been compared. Results show that the model gives reasonably accurate results away from the top and bottom boundaries, side boundaries, and far from western boundary currents. This study found, for the regions where the model is valid, an energy partition of the traditional modes of approximately 68% in the barotropic mode and 25% in the first baroclinic mode. Only 20% of the observed kinetic energy can be attributed to free Rossby waves of long periods that propagate energy to the west.

When a source-sink dipole forces a fluid on a beta-plane limited by a western boundary, the linear steady solution can be obtained analytically and consists of zonally elongated gyres that extend west of the forcing and close as western boundary currents. The nondimensional parameter Ro(upsilon) = U/(4 beta a(2)) (with U the zonal velocity of the flow and 2a the distance between the source and sink) is used to characterize the nonlinearity of the flow. When Ro(upsilon) reaches 0.1, the numerical shallow-water solution shows that the configuration with the source to the north of the sink becomes unstable, while the reverse configuration remains steady. Indeed, that reverse configuration remains steady for much larger values of the nonlinearity parameter Ro(upsilon), and begins to share some of the characteristics of a pure inertial circulation. The asymmetry of the stability properties of the two configurations, also found in the laboratory experiments of Colin de Verdiere [Quasigeostrophic flows and turbulence in a rotating homogeneous fluid, 1977], is rationalized herein through the stability properties of the zonal central jet that flows between the source and sink. We consider, in turn, (i) the Kuo's [J. Meteor. 1949, 6, 105-122] zero potential vorticity gradient necessary criteria (valid for an infinite zonal jet), (ii) enstrophy budgets and (iii) linear stability analysis of the mean flow. All three methods point out to the enhanced instability of the westward jet. We show that the transition regime has the characteristics of a super critical Hopf bifurcation.