Mesoscale Dynamics and Eddy Heat Transport in the Japan/East Sea from 1990 to 2010: A Model-Based Analysis

The driving mechanisms of mesoscale processes and associated heat transport in the Japan/East Sea (JES) from 1990 to 2010 were examined using eddy-resolving ocean model simulations. The simulated circulation showed correctly reproduced JES major basin-scale currents and mesoscale dynamics features. We show that mesoscale eddies can deepen isotherms/isohalines up to several hundred meters and transport warm and low salinity waters along the western and eastern JES boundaries. The analysis of eddy kinetic energy (EKE) showed that the mesoscale dynamics reaches a maximum intensity in the upper 300 m layer. Throughout the year, the EKE maximum is observed in the southeastern JES, and a pronounced seasonal variability is observed in the southwestern and northwestern JES. The comparison of the EKE budget components confirmed that various mechanisms can be responsible for the generation of mesoscale dynamics during the year. From winter to spring, the baroclinic instability of basin-scale currents is the leading mechanism of the JES mesoscale dynamics’ generation. In summer, the leading role in the generation of the mesoscale dynamics is played by the barotropic instability of basin-scale currents, which are responsible for the emergence of mesoscale eddies, and in autumn, the leading role is played by instabilities and the eddy wind work. We show that the meridional heat transport (MHT) is mainly polewards. Furthermore, we reveal two paths of eddy heat transport across the Subpolar Front: along the western and eastern (along 138∘ E) JES boundaries. Near the Tsugaru Strait, we describe the detected intensive westward eddy heat transport reaching its maximum in the first half of the year and decreasing to the minimum by summer.

Global contributions of mesoscale dynamics to meridional heat transport

Mesoscale ocean processes are prevalent in many parts of the global oceans and may contribute substantially to the meridional movement of heat. Yet earlier global surveys of meridional temperature fluxes and heat transport (HT) have not formally distinguished between mesoscale and large-scale contributions, or they have defined eddy contributions based on temporal rather than spatial characteristics. This work uses spatial filtering methods to separate large-scale (gyre and planetary wave) contributions from mesoscale (eddy, recirculation, and tropical instability wave) contributions to meridional HT. Overall, the mesoscale temperature flux (MTF) produces a net poleward meridional HT at midlatitudes and equatorward meridional HT in the tropics, thereby resulting in a net divergence of heat from the subtropics. In addition to MTF generated by propagating eddies and tropical instability waves, MTF is also produced by stationary recirculations near energetic western boundary currents, where the temperature difference between the boundary current and its recirculation produces the MTF. The mesoscale contribution to meridional HT yields substantially different results from temporally based “eddy” contributions to meridional HT, with the latter including large-scale gyre and planetary wave motions at low latitudes. Mesoscale temperature fluxes contribute the most to interannual and decadal variability of meridional HT in the Southern Ocean, the tropical Indo-Pacific, and the North Atlantic. Surface eddy kinetic energy (EKE) is not a good proxy for MTF variability in regions with the highest time-mean EKE, though it does explain much of the temperature flux variability in regions of modest time-mean EKE. This approach to quantifying mesoscale fluxes can be used to improve parameterizations of mesoscale effects in coarse-resolution models and assess regional impacts of mesoscale eddies and recirculations on tracer fluxes.

Climate as a complex, self-regulating system

This paper explores whether climate is complicated or complex by examining the performance of a heat engine in the tropical Pacific, the Pacific Ocean heat engine, which is linked to a teleconnected network of circulation and oscillations. Sustained radiative forcing is widely expected to produce gradual change but instead produces step-wise regime shifts. The engine is a heat pump with cold-to-hot circulation maintained by kinetic energy produced by the Coriolis Effect. It is a fundamental response of a coupled ocean-atmosphere system to asymmetric circulation. This paper surveys emergent behaviours in climate models linked to such shifts. It explores how well models represent the heat engine, compares regime changes in models and observations, and examines how geostrophic controls on meridional heat transport set critical boundary conditions. The results reinforce the description of climate as a self-regulating system governed by the principle of least action. Teleconnected steady-state regimes are physically-induced by the need to maintain boundary-limited dissipation rates between the hemispheres, the equator and the poles. A sufficient imbalance of energy at the planetary surface produces regime shifts that switch between slow and fast dissipation pathways. The strength of coupling measured via heat engine characteristics is weaker in models than in the observed climate, failing to distinguish clearly between free and forced modes. The capacity of the coupled ocean-atmosphere system to maintain homeostasis allows Earth’s climate to be classified physically rather than statistically, the basic unit of climate being the steady-state regime.

Revisiting the Role of Mountains in the Northern Hemisphere Winter Atmospheric Circulation

The impact of global orography on Northern Hemisphere wintertime climate is revisited using the Whole Atmosphere Community Climate Model, WACCM6. A suite of experiments explores the roles of both resolved orography, and the parameterized effects of unresolved orographic drag (hereafter parameterized orography), including gravity waves and boundary layer turbulence. Including orography reduces the extra-tropical tropospheric and stratospheric zonal mean zonal wind, , by up to 80%; this is substantially greater than previous estimates. Ultimately parameterized orography accounts for 60-80% of this reduction; however, away from the surface most of the forcing of by parameterized orography is accomplished by resolved planetary waves. We propose that a catalytic wave-mean-flow positive feedback in the stratosphere makes the stratospheric flow particularly sensitive to parameterized orography. Orography and land-sea contrast contribute approximately equally to the strength of the mid-latitude stationary waves in the free troposphere, although orography is the dominant cause of the strength of the Siberian high and Aleutian low at the surface, and of the position of the Icelandic low. We argue that precisely quantifying the role of orography on the observed stationary waves is an almost intractable problem, and in particular should not be approached with linear stationary wave models in which is prescribed. We show that orography has less impact on stationary waves, and therefore on , on a backwards rotating Earth. Lastly, we show that atmospheric meridional heat transport shows remarkable constancy across our simulations, despite vastly different climates and stationary wave strengths.

Drivers of (non-)linear Eocene surface warming in the DeepMIP ensemble

<p>The early Eocene greenhouse represents the warmest interval of the Cenozoic and therefore provides a unique opportunity to understand how the climate system operates under elevated atmospheric CO<sub>2</sub> levels similar to those projected for the end of the 21st century. Early Eocene geological records indicate a large increase in global mean surface temperatures compared to present day (by ~14°C) and a greatly reduced meridional temperature gradient (by ~30% in SST). However, reproducing these large-scale climate features at reasonable CO<sub>2</sub> levels still poses a challenge for current climate models. Recent modelling studies indicate an important role for shortwave (SW) cloud feedbacks to drive increases in climate sensitivity with global warming, which helps to close the gap between simulated and reconstructed Eocene global warmth and temperature gradient. Nevertheless, the presence of such state-dependent feedbacks and their relative strengths in other models remain unclear.</p><p>In this study, we perform a systematic investigation of the simulated surface warming and the underlying mechanisms in the recently published DeepMIP ensemble. The DeepMIP early Eocene simulations use identical paleogeographic boundary conditions and include six models with suitable output: CESM1.2_CAM5, GFDL_CM2.1, HadCM3B_M2.1aN, IPSLCM5A2, MIROC4m and NorESM1_F. We advance previous energy balance analysis by applying the approximate partial radiative perturbation (APRP) technique to quantify the individual contributions of surface albedo, cloud and non-cloud atmospheric changes to the simulated Eocene top-of-the-atmosphere SW flux anomalies. We further compare the strength of these planetary albedo feedbacks to changes in the longwave atmospheric emissivity and meridional heat transport in the warm Eocene climate. Particular focus lies in the sensitivity of the feedback strengths to increasing global mean temperatures in experiments at a range of atmospheric CO<sub>2</sub> concentrations between x1 to x9 preindustrial levels.</p><p>Preliminary results indicate that all models that provide data for at least 3 different CO<sub>2</sub> levels show an increase of the equilibrium climate sensitivity at higher global mean temperatures. This is associated with an increase of the overall strength of the positive SW cloud feedback with warming in those models. This nonlinear behavior seems to be related to both a reduction and optical thinning of low-level clouds, albeit with intermodel differences in the relative importance of the two mechanisms. We further show that our new APRP results can differ significantly from previous estimates based on cloud radiative forcing alone, especially in high-latitude areas with large surface albedo changes. We also find large intermodel variability and state-dependence in meridional heat transport modulated by changes in the atmospheric latent heat transport. Ongoing work focuses on the spatial patterns of the climate feedbacks and the implications for the simulated meridional temperature gradients.</p>

Global contributions of mesoscale dynamics to meridional heat transport

Mesoscale ocean processes are prevalent in many parts of the global oceans, and may contribute substantially to the meridional movement of heat. Yet earlier global surveys of meridional heat transport (MHT) have not formally distinguished between mesoscale and large-scale contributions, or have defined eddy contributions based on temporal rather than spatial characteristics. This work uses spatial filtering methods to separate large-scale (gyre and planetary wave) contributions from mesoscale (eddy, recirculation, and tropical instability wave) contributions to MHT by extending beyond a previous effort for the North Atlantic Ocean. Overall, mesoscale temperature fluxes produce a net poleward MHT at mid-latitudes and equatorward MHT in the tropics, thereby resulting in a net divergence of heat from the subtropics. Mesoscale temperature fluxes are often concentrated near the energetic currents at western boundaries, and the temperature difference between the boundary current and its recirculation determines the direction of the mesoscale temperature flux. The mesoscale contribution to MHT yields substantially different results from temporally-based eddy contributions to MHT, with the latter contributed substantially by gyre and planetary wave motions at low latitudes. Mesoscale temperature fluxes contribute the most to interannual and decadal variability of MHT in the Southern Ocean, the tropical Indo-Pacific, and the North Atlantic. Surface eddy kinetic energy (EKE) is not a good proxy for mesoscale temperature flux variability in regions with the highest time-mean EKE, though it does explain much of the temperature flux variability in regions of modest time-mean EKE. This approach to quantifying mesoscale fluxes can be used to improve parameterizations of mesoscale effects in coarse-resolution models, and assess regional impacts of mesoscale eddies and recirculations on tracer fluxes.

The interpretation of temperature and salinity variables in numerical ocean model output, and the calculation of heat fluxes and heat content

The 2010 international thermodynamic equation of seawater, TEOS-10, defined the enthalpy and entropy of seawater, thus enabling the global ocean heat content to be calculated as the volume integral of the product of in situ density, ρ, and potential enthalpy, h0 (with reference sea pressure of 0 dbar). In terms of Conservative Temperature, Θ, ocean heat content is the volume integral of ρcp0Θ, where cp0 is a constant isobaric heat capacity. However, several ocean models in CMIP6 (as well as all of those in previous Coupled Model Intercomparison Project phases, such as CMIP5) have not been converted from EOS-80 (Equation of State - 1980) to TEOS-10, so the question arises of how the salinity and temperature variables in these models should be interpreted. In this article we address how heat content, surface heat fluxes and the meridional heat transport are best calculated in these models, and also how these quantities should be compared with the corresponding quantities calculated from observations. We conclude that even though a model uses the EOS-80 equation of state which expects potential temperature as its input temperature, the most appropriate interpretation of the model's temperature variable is actually Conservative Temperature. This interpretation is needed to ensure that the air-sea heat flux that leaves/arrives-in the atmosphere is the same as that which arrives-in/leaves the ocean. We also show that the salinity variable carried by TEOS-10 based models is Preformed Salinity, while the prognostic salinity of EOS-80 based models is also proportional to Preformed Salinity. These interpretations of the salinity and temperature variables in ocean models are an update on the comprehensive Griffies et al. (2016) paper that discusses the interpretation of many aspects of coupled model runs.