Search

The influence of greenhouse warming on mesoscale air-sea interactions, crucial for modulating ocean circulation and climate variability, remains largely unexplored due to the limited resolution of current climate models. Additionally, there is a lack of theoretical frameworks for assessing changes in mesoscale coupling due to warming. Here, we address these gaps by analyzing eddy-resolving high-resolution climate simulations and observations, focusing on the mesoscale thermal interaction dominated by mesoscale sea surface temperature (SST) and latent heat flux (LHF) coupling in winter. Our findings reveal a consistent increase in mesoscale SST-LHF coupling in the major western boundary current regions under warming, characterized by a heightened nonlinearity between warm and cold eddies and a more pronounced enhancement in the northern hemisphere. To understand the dynamics, we develop a theoretical framework that links mesoscale thermal coupling changes to large-scale factors, which indicates that the projected changes are collectively determined by historical background wind, SST, and the rate of SST warming. Among these factors, the large-scale SST and its warming rate are the primary drivers of hemispheric asymmetry in mesoscale coupling intensification. This study introduces a simplified approach for assessing the projected mesoscale thermal coupling changes in a warming world. This study shows that greenhouse warming intensifies mesoscale thermal coupling in western boundary currents, primarily driven by large-scale sea surface temperature and its warming rate, with key implications for ocean circulation and climate variability. [ABSTRACT FROM AUTHOR]

Marine heatwaves (MHWs) are prolonged extreme warm water events in the ocean, exerting devastating impacts on marine ecosystems. A comprehensive knowledge of physical processes controlling MHW life cycles is pivotal to improve MHW forecast capacity, yet it is still lacking. Here, we use a historical simulation from a global eddy-resolving climate model with improved representation of MHWs, and show that heat flux convergence by oceanic mesoscale eddies acts as a dominant driver of MHW life cycles over most parts of the global ocean. In particular, the mesoscale eddies make an important contribution to growth and decay of MHWs, whose characteristic spatial scale is comparable or even larger than that of mesoscale eddies. The effect of mesoscale eddies is spatially heterogeneous, becoming more dominant in the western boundary currents and their extensions, the Southern Ocean, as well as the eastern boundary upwelling systems. This study reveals the crucial role of mesoscale eddies in controlling the global MHW life cycles and highlights that using eddy-resolving ocean models is essential, albeit not necessarily fully sufficient, for accurate MHW forecasts. New paper by Bian and colleagues shows that oceanic mesoscale eddies act as a dominant driver of #marine #heatwave life cycles over most parts of the global #ocean. [ABSTRACT FROM AUTHOR]

Submesoscale eddies in the equatorial Pacific induce a pronounced upward heat flux from the subsurface to the surface ocean. Yet their response to greenhouse warming and feedback on the mean‐state change remain unknown. Using a long‐term high‐resolution climate simulation under a high carbon emission scenario, we show that submesoscale eddies in the central and eastern equatorial Pacific weaken, leading to an ∼50% reduction in their upward heat flux at the end of the 21st century, compared to its mid‐20th century level. This reduction is caused by an intensified stratification and a weakened cold tongue thermal front associated with a projected El Niño‐like warming pattern. The reduced vertical submesoscale eddy heat flux plays an important role in the anomalous heat budget of the surface layer over the central and eastern equatorial Pacific under greenhouse warming, acting as negative feedback on the El Niño‐like warming pattern. Plain Language Summary: The central and eastern equatorial Pacific Ocean is abundant in submesoscale eddies, that is, filaments, fronts, and coherent vortices that have a horizontal scale ranging from several tens to hundreds of kilometers. These eddies transport heat from the subsurface to the surface ocean, providing an important heat source for the surface layer. So far, it remains unclear how the submesoscale eddies and their associated vertical heat flux will change under greenhouse warming. Using a long‐term high‐resolution climate simulation, we show that submesoscale eddies in the central and eastern equatorial Pacific will become weaker under a high carbon emission scenario. Their induced vertical heat flux will reduce by 50% at the end of the 21st century compared to its mid‐20th century level. The reduced vertical submesoscale eddy heat flux will retard the sea surface temperature increase in the central and eastern equatorial Pacific under greenhouse warming. Key Points: Submesoscale eddies in the equatorial Pacific and their associated vertical heat flux are weakened under greenhouse warmingThe weakened vertical submesoscale eddy heat flux under greenhouse warming acts as negative feedback on the El Niño‐like warming pattern [ABSTRACT FROM AUTHOR]

Increasing climate model resolution offers multifaceted benefits, such as improving modeled tropical cyclones. However, the extent to which it benefits El Niño‐Southern Oscillation (ENSO) simulation remains unknown. Here, comprehensive information on the sensitivity of ENSO performance to various resolutions is provided, based on a multi‐model and multi‐resolution ensemble of global coupled models. Overall, the reduced model biases of the equatorial sea surface temperature (SST) and precipitation mean state in higher‐resolution models may be attributed to increased oceanic resolution, and thus, better resolved eddy‐driven heat transport. ENSO spatial patterns were reproduced clearer in the eddy‐present models, likely due to the improved mean state and associated surface thermodynamic feedback. However, increasing atmospheric resolution alone deteriorates ENSO asymmetry, which may be due to the degradation of nonlinear atmospheric feedback. It remains challenging to alleviate the SST−shortwave‐flux feedback bias, which is a major source of too‐weak net heat flux feedback, irrespective of model resolution. Plain Language Summary: The impact of model resolution on simulated weather and climate phenomena has received much attention in recent studies. In this study, we used seven global coupled models, at horizontal grid spacings ranging from 250 to 10 km, to assess El Niño‐Southern Oscillation (ENSO) performance in historical simulations from 1950 to 2014. Several metrics regarding the tropical Pacific mean states, ENSO characteristics, feedback processes, and teleconnections were evaluated. We found that in higher resolution models, the model biases of sea surface temperature and precipitation climatology in the equatorial Pacific had reduced, which may be because of the increased resolution of oceanic components. Such improvements would appear to be beneficial for a better representation of the ENSO spatial structure. However, solely refining the atmospheric resolution of climate models substantially degrades the reproduction of ENSO asymmetry and could conflict with the improvements made by the high‐resolution oceanic component. A comprehensive assessment of impacts from various resolutions offers a useful perspective for future model development. Key Points: Higher oceanic resolution improves eddy‐induced heat transport, equatorial Pacific sea surface temperature (SST) and precipitation mean stateEddy‐present models improve El Niño‐Southern Oscillation (ENSO) patterns because of the realistic mean state and associated SST‐net heat flux feedbackSolely increasing the atmospheric resolution of coupled models exacerbates the bias of a too‐low ENSO asymmetry [ABSTRACT FROM AUTHOR]

The relationships between the seasonal mean storm-track anomalies and sea surface temperature anomalies (SSTAs) in the midlatitude Southern Hemisphere (SH) on the decadal time scale are investigated using the lagged maximum covariance analysis (MCA). It shows that the coupling between storm-track anomalies and SSTAs is most prominent in austral summer (January–March, JFM). Firstly, large cold SSTAs in the western South Indian Ocean (SIO), corresponding to the strengthened SST front on the equator side, substantially intensify the storm-track activity manifested by the increased low-level poleward transient eddy heat flux over the entire SH. The coherent intensification of the atmospheric baroclinicity and the subtropical jet associated with such strengthened SST front provides baroclinic energy for the growth of synoptic eddies. Further, the intensified storm-track activity induces large cold SSTAs in the South Atlantic (SA), the SIO and the subtropical South Pacific (SP), primarily via anomalous net surface heat fluxes. The mean SST advection by the anomalous Ekman current also contributes to the cold SSTAs in the SA, which is related to the anomalous westerlies induced by the anomalous storm-track activity. [ABSTRACT FROM AUTHOR]

Change in the extratropical wintertime-mean mixed layer has been widely studied, given its importance to both physical and biogeochemical processes. With a focus on the south of the Kuroshio Extension region where the mixed layer is deepest in March, this study shows that variation of the synoptic-scale extreme mixed layer depth (MLD) is a better precursor than the monthly mean (or nonextreme) MLD for change in the subtropical mode water formation in spring, based on the NCEP Climate Forecast System Reanalysis (1979–2010). It is found that the extreme MLD events are attributable to the accumulation of excessive surface cooling driven by the synoptic storms that characterize cold-air outbreaks. Particularly, the difference between the extreme and nonextreme MLD is primarily related to differences in the cumulative synoptic heat flux anomalies, while a change in the preconditioning upper-ocean stratification contributes almost equally to both cases. Relative contributions of oceanic and atmospheric forcing to the interannual variation of the extreme MLD are quantified using a bulk mixed layer model. Results show comparable contributions: the preconditioning stratification change accounts for ~44% of total variance of the extreme MLD, whereas the convective mixing by surface heat flux and the mechanical stirring by wind stress account for ~35% and ~13%, respectively. In addition, both the reanalysis and observational data reveal that the extreme and nonextreme MLD has been shallowed significantly during 1979–2010, which is accounted for by the strengthened stratification due to the enhanced ocean surface warming by the Kuroshio heat transport. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) is a key player in the air‐sea interaction, influencing storm tracks, atmospheric circulation, and climate modes. Although prevailing theories attribute variations of large‐scale SST to atmosphere forcing and ocean internal dynamics, we find that sea surface heat flux anomalies induced by mesoscale eddies exert significant influences on the upper‐ocean heat budget in the Kuroshio‐Oyashio extension region. Despite making nearly no contribution to the net heat exchange at the air‐sea interface, the eddy‐induced heat flux anomalies weaken the thermal stratification in the upper ocean and result in pronounced sea surface cooling. The underlying dynamics is the efficient dissipation of eddy potential energy by eddy‐induced heat flux anomalies. This makes the conversion of eddy potential energy to eddy kinetic energy significantly reduced, corresponding to a weaker eddy‐induced restratification flux. The finding complements the existing theories on large‐scale SST dynamics and has important implications for understanding extratropical climate variability. Plain Language Summary: Sea surface temperature (SST) plays a fundamental role in the air‐sea interactions. At large scales (~1,000 km), it is traditionally thought that the atmospheric forcing drives the midlatitude SST variability. At mesoscales (~100 km), it is an ocean‐driven scenario where pronounced SST anomalies carried by ocean eddies exert an imprint on the atmospheric boundary layer. In this study, we find that such ocean mesoscale‐atmosphere (OME‐A) interactions have a significant influence on the large‐scale SST in the Kuroshio‐Oyashio extension region, complementing the existing views on the large‐scale SST dynamics. This is because the eddy‐induced heat flux anomalies damp the SST anomalies and thus available potential energy of eddies. Correspondingly, the conversion of eddy available potential energy to kinetic energy is significantly reduced in presence of OME‐A interactions, resulting in less heat transported from the subsurface to the surface region. Key Points: Eddy‐induced surface heat flux anomalies act to dissipate the eddy potential energyEnhanced dissipation of eddy potential energy reduces its conversion to eddy kinetic energy, weakening the eddy restratification fluxThe reduced eddy restratification flux weakens the thermal stratification in the upper ocean and results in sea surface cooling [ABSTRACT FROM AUTHOR]