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The Southern Ocean has shown little warming over recent decades, in stark contrast to the rapid warming observed in the Arctic. Along the northern flank of the Antarctic Circumpolar Current, however, the upper ocean has warmed substantially. Here we present analyses of oceanographic observations and general circulation model simulations showing that these patterns—of delayed warming south of the Antarctic Circumpolar Current and enhanced warming to the north—are fundamentally shaped by the Southern Ocean’s meridional overturning circulation: wind-driven upwelling of unmodified water from depth damps warming around Antarctica; greenhouse gas-induced surface heat uptake is largely balanced by anomalous northward heat transport associated with the equatorward flow of surface waters; and heat is preferentially stored where surface waters are subducted to the north. Further, these processes are primarily due to passive advection of the anomalous warming signal by climatological ocean currents; changes in ocean circulation are secondary. These findings suggest the Southern Ocean responds to greenhouse gas forcing on the centennial, or longer, timescale over which the deep ocean waters that are upwelled to the surface are warmed themselves. It is against this background of gradual warming that multidecadal Southern Ocean temperature trends must be understood. Unlike the Arctic, the Southern Ocean has shown little warming. An analysis of observations and numerical simulations suggests that Southern Ocean warming patterns are shaped by meridional overturning more than surface heating.

We study the role of the ocean in setting the patterns and timescale of the transient response of the climate to anthropogenic greenhouse gas forcing. A novel framework is set out which involves integration of an ocean-only model in which the anthropogenic temperature signal is forced from the surface by anomalous downwelling heat fluxes and damped at a rate controlled by a ‘climate feedback’ parameter. We observe a broad correspondence between the evolution of the anthropogenic temperature ( T a n t h r o ) in our simplified ocean-only model and that of coupled climate models perturbed by a quadrupling of CO 2 . This suggests that many of the mechanisms at work in fully coupled models are captured by our idealized ocean-only system. The framework allows us to probe the role of the ocean in delaying warming signals in the Southern Ocean and in the northern North Atlantic, and in amplifying the warming signal in the Arctic. By comparing active and passive temperature-like tracers we assess the degree to which changes in ocean circulation play a role in setting the distribution and evolution of T a n t h r o . The background ocean circulation strongly influences the large-scale patterns of ocean heat uptake and storage, such that T a n t h r o is governed by an advection/diffusion equation and weakly damped to the atmosphere at a rate set by climate feedbacks. Where warming is sufficiently small, for example in the Southern Ocean, changes in ocean circulation play a secondary role. In other regions, most noticeably in the North Atlantic, changes in ocean circulation induced by T a n t h r o are central in shaping the response.

Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) are potent greenhouse gases regulated under the Montreal Protocol and its amendments. Emission estimates generally use constant atmospheric lifetimes accounting for loss via hydroxyl radical (OH) reactions. However, chemistry‐climate models suggest OH increases after 1980, implying underestimated emissions. Further, HCFCs and HFCs are soluble in seawater and could be destroyed through in situ oceanic microbial activity. These ocean sinks are largely overlooked. Using a coupled atmosphere‐ocean model, we show that increases in modeled OH imply underestimated HCFC and HFC emissions by ∼10% near their respective peak emissions. Our model results also suggest that oceanic processes could lead to up to an additional 10% underestimation in these halocarbon emissions in the 2020s. Ensuring global compliance to the Protocol and accurate knowledge of contributions to global warming from these gases therefore requires understanding of these processes. Plain Language Summary Man‐made hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) contribute to global warming, prompting worldwide agreement to control the production of these chemicals. It is important to estimate their emissions to ensure global compliance with the agreed phaseout. But correct emission estimation requires knowledge of different loss pathways. One major loss pathway of halocarbons is through chemical reactions with the atmospheric “scrubber” called OH. OH is difficult to measure and usually assumed to be constant with time. But some models suggest OH has increased, which implies increased emissions to match observed abundances. These halocarbons also dissolve into the oceans, where microbes may also metabolize them, but these processes are not included in current emission estimates. We show that if these halocarbons are being consumed in the oceans, this would also lead to an additional underestimation of human emissions. Confidence in the success in the Montreal Protocol and its Kigali amendment to reduce HFCs will therefore require a better understanding of both OH trends and ocean sinks, along with use of HFC and HCFC measurements. Key Points Increasing OH suggested by Coupled Model Intercomparison Project Phase 6 models can lead to a 5%–7% underestimation in hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HFC) emission estimations in 2005 If there is significant ocean degradation through microbial activity, HCFC, and HFC emissions could be underestimated by up to 10% Our study suggests an uncertainty in the combined contribution to global warming from HCFCs and HFCs up to 15%–20% in the 2020s

In recent decades, the Arctic has been warming and sea ice disappearing. By contrast, the Southern Ocean around Antarctica has been (mainly) cooling and sea-ice extent growing. We argue here that interhemispheric asymmetries in the mean ocean circulation, with sinking in the northern North Atlantic and upwelling around Antarctica, strongly influence the sea-surface temperature (SST) response to anthropogenic greenhouse gas (GHG) forcing, accelerating warming in the Arctic while delaying it in the Antarctic. Furthermore, while the amplitude of GHG forcing has been similar at the poles, significant ozone depletion only occurs over Antarctica. We suggest that the initial response of SST around Antarctica to ozone depletion is one of cooling and only later adds to the GHG-induced warming trend as upwelling of sub-surface warm water associated with stronger surface westerlies impacts surface properties. We organize our discussion around climate response functions (CRFs), i.e. the response of the climate to step changes in anthropogenic forcing in which GHG and/or ozone-hole forcing is abruptly turned on and the transient response of the climate revealed and studied. Convolutions of known or postulated GHG and ozone-hole forcing functions with their respective CRFs then yield the transient forced SST response (implied by linear response theory), providing a context for discussion of the differing warming/cooling trends in the Arctic and Antarctic. We speculate that the period through which we are now passing may be one in which the delayed warming of SST associated with GHG forcing around Antarctica is largely cancelled by the cooling effects associated with the ozone hole. By mid-century, however, ozone-hole effects may instead be adding to GHG warming around Antarctica but with diminished amplitude as the ozone hole heals. The Arctic, meanwhile, responding to GHG forcing but in a manner amplified by ocean heat transport, may continue to warm at an accelerating rate.

Tropical cyclones instigate an isolated blast of vigorous mixing in the upper tropical oceans, stirring warm surface water with cooler water in the thermocline. Previous work suggests that the frequency, intensity, and lifetime of these storms may be functions of the climate state, implying that transient tropical mixing could have been stronger during warmer equable climates with higher concentrations of carbon dioxide. Stronger mixing of the tropical oceans can force the oceans’ meridional heat flux to increase, cooling tropical latitudes while warming higher ones. This response differs significantly from previous modeling studies of equable climates that used static mixing; coupling mixing to climate changes the dynamic response. A parameterization of mixing from tropical cyclones is developed, and including it leads to a cooling of tropical oceans and a warming of subtropical waters compared with control cases with fixed mixing. The mixing penetration depth regulates the magnitude of the response.

The ocean is a reservoir for CFC-11, a major ozone-depleting chemical. Anthropogenic production of CFC-11 dramatically decreased in the 1990s under the Montreal Protocol, which stipulated a global phase out of production by 2010. However, studies raise questions about current overall emission levels and indicate unexpected increases of CFC-11 emissions of about 10 Gg · yr−1 after 2013 (based upon measured atmospheric concentrations and an assumed atmospheric lifetime). These findings heighten the need to understand processes that could affect the CFC-11 lifetime, including ocean fluxes. We evaluate how ocean uptake and release through 2300 affects CFC-11 lifetimes, emission estimates, and the long-term return of CFC-11 from the ocean reservoir. We show that ocean uptake yields a shorter total lifetime and larger inferred emission of atmospheric CFC-11 from 1930 to 2075 compared to estimates using only atmospheric processes. Ocean flux changes over time result in small but not completely negligible effects on the calculated unexpected emissions change (decreasing it by 0.4 ± 0.3 Gg · yr−1). Moreover, it is expected that the ocean will eventually become a source of CFC-11, increasing its total lifetime thereafter. Ocean outgassing should produce detectable increases in global atmospheric CFC-11 abundances by the mid-2100s, with emission of around 0.5 Gg · yr−1; this should not be confused with illicit production at that time. An illustrative model projection suggests that climate change is expected to make the ocean a weaker reservoir for CFC-11, advancing the detectable change in the global atmospheric mixing ratio by about 5 yr.

Methyl chloroform (MCF) is an ozone‐depleting substance used as an industrial solvent. Its primary sink is oxidation by the hydroxyl radical (OH), making it a key tracer for estimating atmospheric oxidative capacity. Following Montreal Protocol regulations, MCF emissions declined rapidly after the 1990s. However, the recent atmospheric MCF decay suggests persistent emissions and/or declining OH (contradicting chemistry‐climate models projecting increasing or stable OH). The air‐sea exchange of MCF has been poorly constrained due to limited observations and simplified ocean representations. We simulate oceanic MCF fluxes using a modern ocean reanalysis and validate with depth‐resolved observations. Results suggest the ocean has shifted from a net sink to a net source around 2005, outgassing 0.5 Gg yr−1 in the 2010s (up to 30% of inferred MCF emissions). This ocean outgassing is an order of magnitude larger than previous estimates, and accounts for up to a third of the model‐observation discrepancy in OH.

The Arctic Ocean’s Beaufort Gyre (BG) is a wind-driven reservoir of relatively fresh seawater, situated beneath time-mean anticyclonic atmospheric circulation, and is covered by mobile pack ice for most of the year. Liquid freshwater accumulation in and expulsion from this gyre is of critical interest due to its potential to affect the Atlantic meridional overturning circulation and due to the importance of freshwater in modulating vertical fluxes of heat, nutrients and carbon in the ocean, and exchanges of heat and moisture with the atmosphere. Here, we investigate the hypothesis that wind-driven sea ice transport into/from the BG region influences the freshwater content of the gyre and its variability. To test this hypothesis, we use the results of a coordinated climate response function experiment with four ice-ocean models, in combination with targeted experiments using a regional setup of the MITgcm, in which we rotate the surface wind forcing vectors (thereby changing the ageostrophic component of these winds). Our results show that, via an effect on the net thermodynamic growth rate, anomalies in sea ice transport into the BG affect liquid freshwater adjustment. Specifically, increased ice import increases freshwater retention in the gyre, whereas ice export decreases freshwater in the gyre. Our results demonstrate that uncertainty in the ageostrophic component of surface winds, and in the dynamic sea ice response to these winds, has important implications for ice thermodynamics and freshwater. This sensitivity may explain some of the observed inter-model spread in simulations of Beaufort Gyre freshwater and its adjustment in response to wind forcing.

In this study, we present a new modeling framework and a large ensemble of climate projections to investigate the uncertainty in regional climate change over the United States (US) associated with four dimensions of uncertainty. The sources of uncertainty considered in this framework are the emissions projections, global climate system parameters, natural variability and model structural uncertainty. The modeling framework revolves around the Massachusetts Institute of Technology (MIT) Integrated Global System Model (IGSM), an integrated assessment model with an Earth System Model of Intermediate Complexity (EMIC) (with a two-dimensional zonal-mean atmosphere). Regional climate change over the US is obtained through a two-pronged approach. First, we use the IGSM-CAM framework, which links the IGSM to the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM). Second, we use a pattern-scaling method that extends the IGSM zonal mean based on climate change patterns from various climate models. Results show that the range of annual mean temperature changes are mainly driven by policy choices and the range of climate sensitivity considered. Meanwhile, the four sources of uncertainty contribute more equally to end-of-century precipitation changes, with natural variability dominating until 2050. For the set of scenarios used in this study, the choice of policy is the largest driver of uncertainty, defined as the range of warming and changes in precipitation, in future projections of climate change over the US.

The large-scale consequences of diapycnal mixing location are explored using an idealized three-dimensional model of buoyancy-forced flow in a single hemisphere. Diapycnal mixing is most effective in supporting a strong meridional overturning circulation (MOC) if mixing occurs in regions of strong stratification, that is, in the low-latitude thermocline where diffusion causes strong vertical buoyancy fluxes. Where stratification is weak, such as at high latitudes, diapycnal mixing plays little role in determining MOC strength, consistent with weak diffusive buoyancy fluxes at these latitudes. Boundary mixing is more efficient than interior mixing at driving the MOC; with interior mixing the planetary vorticity constraint inhibits the communication of interior water mass properties and the eastern boundary. Mixing below the thermocline affects the abyssal stratification and upwelling profile but does not contribute significantly to the meridional flow through the thermocline or the ocean's meridional heat transport. The abyssal heat budget is dominated by the downward mass transport of buoyant water versus the spread of denser water tied to the properties of deep convection, with mixing of minor importance. These results are in contrast to the widespread expectation that the observed enhanced abyssal mixing can maintain the MOC; rather, they suggest that enhanced boundary mixing in the thermocline needs to be identified in observations.