More Like this

1. Greenhouse-gas forced changes in the Atlantic meridional overturning circulation and related worldwide sea-level change

   https://doi.org/10.1007/s00382-022-06386-y  

   Couldrey, Matthew P. ; Gregory, Jonathan M. ; Dong, Xiao ; Garuba, Oluwayemi ; Haak, Helmuth ; Hu, Aixue ; Hurlin, William J. ; Jin, Jiangbo ; Jungclaus, Johann ; Köhl, Armin ; et al ( August 2022 , Climate Dynamics)

   The effect of anthropogenic climate change in the ocean is challenging to project because atmosphere-ocean general circulation models (AOGCMs) respond differently to forcing. This study focuses on changes in the Atlantic Meridional Overturning Circulation (AMOC), ocean heat content ($$\Delta$$$\Delta$OHC), and the spatial pattern of ocean dynamic sea level ($$\Delta \zeta$$$\Delta \zeta$). We analyse experiments following the FAFMIP protocol, in which AOGCMs are forced at the ocean surface with standardised heat, freshwater and momentum flux perturbations, typical of those produced by doubling$$\hbox {CO}_{{2}}$$${\text{CO}}_2$. Using two new heat-flux-forced experiments, we find that the AMOC weakening is mainly caused by and linearly related to the North Atlantic heat flux perturbation, and further weakened by a positive coupled heat flux feedback. The quantitative relationships are model-dependent, but few models show significant AMOC change due to freshwater or momentum forcing, or to heat flux forcing outside the North Atlantic. AMOC decline causes warming at the South Atlantic-Southern Ocean interface. It does not strongly affect the global-mean vertical distribution of$$\Delta$$$\Delta$OHC, which is dominated by the Southern Ocean. AMOC decline strongly affects$$\Delta \zeta$$$\Delta \zeta$in the North Atlantic, with smaller effects in the Southern Ocean and North Pacific. The ensemble-mean$$\Delta \zeta$$$\Delta \zeta$and$$\Delta$$$\Delta$OHC patterns are mostly attributable to the heat added by the flux perturbation, with smaller effects from ocean heat and salinity redistribution. The ensemble spread, on the other hand, is largely due to redistribution, with pronounced disagreement among the AOGCMs.

    

   more » « less
2. Understanding the Uncertainty in the 21st Century Dynamic Sea Level Projections: The Role of the AMOC

   https://doi.org/10.1029/2018GL080676  

   Chen, Changlin ; Liu, Wei ; Wang, Guihua ( January 2019 , Geophysical Research Letters)

   Climate models show that the largest uncertainties in the 21st century dynamic sea level (DSL) projections are in the high latitudes of the North Atlantic and Southern Oceans. We conduct an intermodel singular value decomposition analysis and find that the DSL uncertainties in these two oceans are both intrinsically connected to the uncertainty in the change of the Atlantic meridional overturning circulation (AMOC). We further conduct a freshwater hosing experiment to show that the AMOC decline not only accounts for the dipole pattern in the DSL change in the North Atlantic but also remotely induces a poleward shift in the Southern Hemisphere westerlies that helps build a belted pattern of DSL change in the Southern Ocean. Our results suggest that reducing the intermodel spread in the change of the AMOC can greatly improve the consistency of DSL projection among models not only in individual basins but over the global ocean.

    

   more » « less
3. Energetic Constraints on the Pattern of Changes to the Hydrological Cycle under Global Warming

   https://doi.org/10.1175/JCLI-D-22-0337.1  

   Bonan, David B. ; Siler, Nicholas ; Roe, Gerard H. ; Armour, Kyle C. ( May 2023 , Journal of Climate)

   Abstract The response of zonal-mean precipitation minus evaporation ( P − E ) to global warming is investigated using a moist energy balance model (MEBM) with a simple Hadley cell parameterization. The MEBM accurately emulates zonal-mean P − E change simulated by a suite of global climate models (GCMs) under greenhouse gas forcing. The MEBM also accounts for most of the intermodel differences in GCM P − E change and better emulates GCM P − E change when compared to the “wet-gets-wetter, dry-gets-drier” thermodynamic mechanism. The intermodel spread in P − E change is attributed to intermodel differences in radiative feedbacks, which account for 60%–70% of the intermodel variance, with smaller contributions from radiative forcing and ocean heat uptake. Isolating the intermodel spread of feedbacks to specific regions shows that tropical feedbacks are the primary source of intermodel spread in zonal-mean P − E change. The ability of the MEBM to emulate GCM P − E change is further investigated using idealized feedback patterns. A less negative and narrowly peaked feedback pattern near the equator results in more atmospheric heating, which strengthens the Hadley cell circulation in the deep tropics through an enhanced poleward heat flux. This pattern also increases gross moist stability, which weakens the subtropical Hadley cell circulation. These two processes in unison increase P − E in the deep tropics, decrease P − E in the subtropics, and narrow the intertropical convergence zone. Additionally, a feedback pattern that produces polar-amplified warming partially reduces the poleward moisture flux by weakening the meridional temperature gradient. It is shown that changes to the Hadley cell circulation and the poleward moisture flux are crucial for understanding the pattern of GCM P − E change under warming. Significance Statement Changes to the hydrological cycle over the twenty-first century are predicted to impact ecosystems and socioeconomic activities throughout the world. While it is broadly expected that dry regions will get drier and wet regions will get wetter, the magnitude and spatial structure of these changes remains uncertain. In this study, we use an idealized climate model, which assumes how energy is transported in the atmosphere, to understand the processes setting the pattern of precipitation and evaporation under global warming. We first use the idealized climate model to explain why comprehensive climate models predict different changes to precipitation and evaporation across a range of latitudes. We show this arises primarily from climate feedbacks, which act to amplify or dampen the amount of warming. Ocean heat uptake and radiative forcing play secondary roles but can account for a significant amount of the uncertainty in regions where ocean circulation influences the rate of warming. We further show that uncertainty in tropical feedbacks (mainly from clouds) affects changes to the hydrological cycle across a range of latitudes. We then show how the pattern of climate feedbacks affects how the patterns of precipitation and evaporation respond to climate change through a set of idealized experiments. These results show how the pattern of climate feedbacks impacts tropical hydrological changes by affecting the strength of the Hadley circulation and polar hydrological changes by affecting the transport of moisture to the high latitudes. 

   more » « less
4. Mean State AMOC Affects AMOC Weakening through Subsurface Warming in the Labrador Sea

   https://doi.org/10.1175/JCLI-D-22-0464.1  

   Lin, Yuan-Jen ; Rose, Brian E. J. ; Hwang, Yen-Ting ( June 2023 , Journal of Climate)

   While most models agree that the Atlantic meridional overturning circulation (AMOC) becomes weaker under greenhouse gas emission and is likely to weaken over the twenty-first century, they disagree on the projected magnitudes of AMOC weakening. In this work, CMIP6 models with stronger climatological AMOC are shown to project stronger AMOC weakening in both 1% ramping CO₂and abrupt CO₂quadrupling simulations. A physical interpretation of this result is developed. For models with stronger mean state AMOC, stratification in the upper Labrador Sea is weaker, allowing for stronger mixing of the surface buoyancy flux. In response to CO₂increase, surface warming is mixed to the deeper Labrador Sea in models with stronger upper-ocean mixing. This subsurface warming and corresponding density decrease drives AMOC weakening through advection from the Labrador Sea to the subtropics via the deep western boundary current. Time series analysis shows that most CMIP6 models agree that the decrease in subsurface Labrador Sea density leads AMOC weakening in the subtropics by several years. Also, idealized experiments conducted in an ocean-only model show that the subsurface warming over 500–1500 m in the Labrador Sea leads to stronger AMOC weakening several years later, while the warming that is too shallow (<500 m) or too deep (>1500 m) in the Labrador Sea causes little AMOC weakening. These results suggest that a better representation of mean state AMOC is necessary for narrowing the intermodel uncertainty of AMOC weakening to greenhouse gas emission and its corresponding impacts on future warming projections.

    

   more » « less
5. 100 Years of Progress in Understanding the Dynamics of Coupled Atmosphere–Ocean Variability

   https://doi.org/10.1175/AMSMONOGRAPHS-D-18-0025.1  

   Battisti, David S. ; Vimont, Daniel J. ; Kirtman, Benjamin P. ( October 2019 , Meteorological Monographs)

   In situ observation networks and reanalyses products of the state of the atmosphere and upper ocean show well-defined, large-scale patterns of coupled climate variability on time scales ranging from seasons to several decades. We summarize these phenomena and their physics, which have been revealed by analysis of observations, by experimentation with uncoupled and coupled atmosphere and ocean models with a hierarchy of complexity, and by theoretical developments. We start with a discussion of the seasonal cycle in the equatorial tropical Pacific and Atlantic Oceans, which are clearly affected by coupling between the atmosphere and the ocean. We then discuss the tropical phenomena that only exist because of the coupling between the atmosphere and the ocean: the Pacific and Atlantic meridional modes, the El Niño–Southern Oscillation (ENSO) in the Pacific, and a phenomenon analogous to ENSO in the Atlantic. For ENSO, we further discuss the sources of irregularity and asymmetry between warm and cold phases of ENSO, and the response of ENSO to forcing. Fundamental to variability on all time scales in the midlatitudes of the Northern Hemisphere are preferred patterns of uncoupled atmospheric variability that exist independent of any changes in the state of the ocean, land, or distribution of sea ice. These patterns include the North Atlantic Oscillation (NAO), the North Pacific Oscillation (NPO), and the Pacific–North American (PNA) pattern; they are most active in wintertime, with a temporal spectrum that is nearly white. Stochastic variability in the NPO, PNA, and NAO force the ocean on days to interannual times scales by way of turbulent heat exchange and Ekman transport, and on decadal and longer time scales by way of wind stress forcing. The PNA is partially responsible for the Pacific decadal oscillation; the NAO is responsible for an analogous phenomenon in the North Atlantic subpolar gyre. In models, stochastic forcing by the NAO also gives rise to variability in the strength of the Atlantic meridional overturning circulation (AMOC) that is partially responsible for multidecadal anomalies in the North Atlantic climate known as the Atlantic multidecadal oscillation (AMO); observations do not yet exist to adequately determine the physics of the AMO. We review the progress that has been made in the past 50 years in understanding each of these phenomena and the implications for short-term (seasonal-to-interannual) climate forecasts. We end with a brief discussion of advances of things that are on the horizon, under the rug, and over the rainbow.

    

   more » « less