Surface Temperature Gradient Research Articles

The West Pacific Gradient tracks ENSO and zonal Pacific sea surface temperature gradient during the last Millennium

Small changes in Pacific temperature gradients connected with the El Niño Southern Oscillation (ENSO) influence the Walker Circulation and are related to global climate anomalies. Therefore, it is of paramount importance to develop robust indices of their past behavior. Here, we reconstruct the difference in sea surface temperature between the west and central Pacific during ENSO, coined the West Pacific Gradient (WPG), based on the Last Millennium Paleo Hydrodynamics Data Assimilation. We show that the WPG tracks ENSO variability and strongly co-varies with the zonal gradient in Pacific sea surface temperature. We demonstrate that the WPG strength is related to significant atmospheric circulation and precipitation anomalies during historical El Niño and La Niña events by magnifying or weakening droughts and pluvials across the Indo-Pacific. We show that an extreme negative WPG coupled to a strong zonal Pacific temperature gradient is associated with enhanced megadroughts in North America between 1400 CE and the late sixteenth century. The twentieth century stands out in showing the most extreme swings between positive and negative WPG conditions over the past Millennium. We conclude that the WPG is a robust index together with ENSO indices to reveal past changes in Pacific zonal sea surface temperature gradient variability.

Dynamics and heat transfer characteristics of isolated liquid film in spray cooling

Although the understanding of spray cooling has significantly improved, knowledge of the heat transfer mechanisms is still limited, and the related models based on experiments are separated from physical phenomena. As an example, while liquid film morphology have been investigated in detail, comparatively little is known about how the liquid films evolve on the surface, owing to the strong interaction and disturbance between atomised droplets and thin films with high turbulence. In this study, the liquid film dynamics of HFE-7000 spray in a two-phase cooling regime on a smooth and unpolished silicon surface were quantitatively captured, including the film morphology, film velocity, wetted area, and contact line length, under different inlet pressures and heat fluxes. Owing to the thermally induced agglomeration with increasing heat flux, the continuous liquid film finally dispersed into an isolated state, which is quantitatively defined as the ‘isolated film regime’ by the critical interval of the wetted area. The velocity of the isolated film under various conditions is a consequence of the radial velocity of the atomised droplets, surface conditions, and surface temperature gradient. Based on the correlation of the radial velocity of atomised droplets, prediction methods for the distribution and surface-averaged velocity of isolated films are proposed.

Colour changing material for the estimation of flue gas radiation on the miniature gas turbine surface

The gas turbine is being used in the applications of the aircraft propulsion system and land-based power generating systems more effectively. The manufacturers should optimise the temperature of the gas turbine engine components to enhance the life span of the components. The present research work concentrates on determining the surface temperature gradient on the fabricated turbine blades using a colour changing paint based on temperature attained on the surface. A calibration database has been created, and the surface temperature has been detected based on the available colour contours on the blade surface using human vision. An image processing algorithm has also been proposed for accurate temperature measurement on the blade surface. The obtained surface temperature using colour changing paint multi-colour change 350-8 has been calibrated with the conventional measurement technique IR thermography for experimental validation. A computational fluid dynamics simulation model of the turbine blade has been simulated to predict the surface temperature of blades using analysis systems fluid dynamics for numerical validation. The experimental and numerical validation results have shown a nominal value of error, which proves that the surface temperature gradient can be easily predicted with the help of temperature indicating paint using the proposed algorithm. The study has been extended further to evaluate the amount of emissive power radiated by the flue gas on the turbine blade surface based on the temperature and the wavelength of the colour obtained for the health monitoring of the blade.

Inside Katabatic Winds Over the Terra Nova Bay Polynya: 1. Atmospheric Jet and Surface Conditions

AbstractThe atmospheric and surface conditions during a late autumnal strong katabatic wind event were quantified using ship and rawinsonde measurements over the Terra Nova Bay coastal polynya. Wind speeds decreased from 35 to 18 ms−1, while the wind direction decreased 18° over the distance from the Nansen Ice Shelf edge out 99 km eastward toward the Ross Sea. Maximum velocity winds (jet cores) at 173, 238, 179 and 144 m elevation were associated with atmospheric boundary layers capped by temperature inversions with bases at 294, 325, 226 and 196 m elevation. The tops of the inversion layers (near 700 m at all locations) also marked the top of the katabatic wind layer. Boundary layer air temperature and specific humidity increased from −31 to −21°C and 0.1 to 0.6 gkg−1, respectively, in response to the warm polynya surface. The air at 15 ± 8 m elevation was saturated with respect to ice, causing supersaturation and snow growth where the air parcels become cooler in the upper atmospheric boundary layer. The surface was characterized by three zones, a fluid zone (open ocean, frazil, shuga and pancake floes), an accumulation zone (fused, rafted and compressed pancake floes) and a young floe zone (large floes). The surface temperature varied from freezing (−1.7°C) in the fluid zone to near air temperature (−20°C) in the floe zone with the largest horizontal surface temperature gradient occurring in the transition between the fluid zone and the accumulation zone, and at the edges of leads in the floe zone.

Thermal Resistance of Insulated Precast Concrete Sandwich Panels

Many nations are already working toward full implementation of energy efficiency in buildings known as Green Building. In line with this perspective, this paper aims to develop a thermally efficient precast concrete sandwich panels (PCSP) for structural applications. Therefore, an experimental investigation was carried out to determine the thermal resistance of the proposed PCSP using Hotbox method and the results were validated using finite element method (FEM) in COMSOL Multiphysics Software. The PCSP were designed with staggered shear connectors to avoid thermal bridges between the successive layers. The staggered connectors are spaced at 200 mm, 300 mm and 400 mm on each concrete layer, while the control panel is designed with 200 mm direct shear connection. In the experimental test, four (4) panels of 500 mm × 500 mm and 150 mm thick were subjected to Hotbox Test to determine the thermal resistance. The result shows that thermal resistance of the PCSP with staggered shear connection increases with increase in spacing. The PCSP with 400 mm staggered shear connectors indicates the best thermal efficiency with a thermal resistance (R value) of 2.48 m2K/W. The thermal performance was verified by FEA which shows less than 5% error coupled with a precise prediction of surface temperature gradient. This indicates that, with conventional materials, thermal path approach can be used to develop a precast concrete building with better thermal resistant properties. Hopefully, stakeholders in the green building industry would find this proposed PCSP as an alternative energy efficient load bearing panel towards sustainable and greener buildings.

Hydrological Changes Related to ENSO‐Like States During the Last Deglaciation in Central Eastern China

AbstractPresent knowledge of hydrological changes in the East Asian summer monsoon region during the last deglaciation lacks precision, most especially for central eastern China. A hydrological reconstruction based on elemental ratios, such as Rubidium/Strontium (Rb/Sr) and Zirconium/Rubidium (Zr/Rb), and grain‐size distribution for the period of 22.8–7.4 ka was derived from the Dajiuhu basin in central eastern China. The reconstruction shows that there were some small wet‐dry fluctuations within the basin between 22.8 and 17.5 ka, but an abrupt wetting after 17.5 ka, with a maximum occurring around 15.0 ka, then a drying trend until the early Holocene. This pattern parallels the zonal gradient of sea surface temperature (SST) across the tropical Pacific, which suggests that wet conditions in the Dajiuhu basin respond to the decreased SST zonal gradient. A marked decrease in the tropical Pacific SST zonal gradient, seen as an El Niño‐like state, could lead to an anomalous anticyclonic circulation at the Western Pacific and a southward excursion of the Western Pacific Subtropical High, which favors the occurrence of wet conditions in central eastern China. Meanwhile, this El Niño‐like state could sharpen the SST meridional gradient between the tropics and the extratropics, and then shift the upper tropospheric westerlies southward, which in turn induces a prolonged Mei‐yu and more precipitation in central eastern China. Overall, it is concluded that the El Niño‐Southern Oscillation‐like states might be a major forcing for hydrological changes in central eastern China during the last deglaciation.

Forcing Convection to Aggregate Using Diabatic Heating Perturbations.

Tropical deep convection can aggregate into large clusters, which can have impacts on the local humidity and precipitation. Sea surface temperature (SST) gradients have been shown to organize convection, yet there has been little work done to investigate the impact of diabatic heating perturbations in the atmosphere on the aggregation of convection. Here we investigate how anomalous diabatic heating of the atmospheric column, through an idealized aerosol plume, affects the existence and mechanisms of convective aggregation in non‐rotating, global radiative‐convective equilibrium simulations. We show that the aerosol forcing has the ability to increase the degree of aggregation, especially at lower SSTs. Detailed investigation shows that the diabatic heating source incites a thermally driven circulation, forced by the shortwave perturbation. The increase in aggregation is caused in part by this circulation, and in part by the longwave heating anomalies occurring due to the surface convergence of moisture and convection. At higher SSTs, longwave feedbacks are crucial for the aggregation of convection, even with the shortwave heating perturbation. At lower SSTs, convection is able to aggregate with the shortwave perturbation in the absence of longwave feedbacks. These perturbations provide a link to studying the effects of absorbing aerosol plumes on convection, for example during the Indian monsoon season. We argue that, as there is aggregation for plumes with realistic aerosol absorption optical depths, this could be an analogue for real‐world organization in regions with high pollution.

High-resolution simulations of decadal climate variability impacts on spring and winter wheat yields in the Missouri River Basin with the Soil and Water Assessment Tool (SWAT)

The Missouri River Basin (MRB) encompasses one of the most important agricultural regions in the world. Three decadal climate variability (DCV) phenomena — the Pacific Decadal Oscillation (PDO), the tropical Atlantic sea surface temperature (SST) gradient variability (TAG), and the West Pacific Warm Pool (WPWP) variability — substantially influence hydro-meteorology and, consequently, spring and winter wheat yields in the MRB as indicated by data from 1961 to 2010. We applied the Soil and Water Assessment Tool (SWAT) to simulate DCV impacts on wheat yields in response to realistic values of the DCV indices in approximately 13,500 hydrologic unit areas covering the MRB. SWAT, driven by scenarios of past hydro-meteorological anomalies associated with positive and negative phases of the PDO and TAG, indicated major impacts on wheat yields, as much as ± 40% of the average in many locations, with smaller impacts of the WPWP variability. SWAT showed much larger wheat yield increases when the positive phase of the PDO and the negative phase of the TAG are superposed, and an equivalent decrease in yields when opposite phases of the two DCV phenomena are superposed. Thus, combined effects of DCV phenomena on wheat yields in the MRB can be dramatic with important consequences for food production and security. The usefulness of this inter-disciplinary study to farmers and other stakeholders for adapting MRB agriculture to DCV, and the applicability of the methodology to other agricultural regions are described. The results’ implications for detection and attribution of climatic change impacts are also described.

Magnitude of nitrate turbulent diffusion in contrasting marine environments

Difficulties to quantify ocean turbulence have limited our knowledge about the magnitude and variability of nitrate turbulent diffusion, which constitutes one of the main processes responsible for the supply of nitrogen to phytoplankton inhabiting the euphotic zone. We use an extensive dataset of microturbulence observations collected in contrasting oceanic regions, to build a model for nitrate diffusion into the euphotic zone, and obtain the first global map for the distribution of this process. A model including two predictors (surface temperature and nitrate vertical gradient) explained 50% of the variance in the nitrate diffusive flux. This model was applied to climatological data to predict nitrate diffusion in oligotrophic mid and low latitude regions. Mean nitrate diffusion (~ 20 Tmol N y−1) was comparable to nitrate entrainment due to seasonal mixed-layer deepening between 40°N–40ºS, and to the sum of global estimates of nitrogen fixation, fluvial fluxes and atmospheric deposition. These results indicate that nitrate diffusion represents one of the major sources of new nitrogen into the surface ocean in these regions.

Mechanisms shaping wind convergence under extreme synoptic situations over the Gulf Stream region

AbstractAir-sea interaction processes over the Gulf Stream have received particular attention over the last decade. It has been shown that sea surface temperature (SST) gradients over the Gulf Stream can alter the near surface wind divergence through changes in the marine atmospheric boundary layer (MABL). Two mechanisms have been proposed to explain the response: the Vertical Mixing Mechanism (VMM) and the Pressure Adjustment Mechanism (PAM). However, their respective contribution is still under debate. It has been argued that the synoptic perturbations over the Gulf Stream can provide more insight on the MABL response to SST fronts. We analyze the VMM and PAM under different atmospheric conditions obtained from a classification method based on the deciles of the statistical distribution of winter turbulent heat fluxes over the Gulf Stream. Lowest deciles are associated with weak air-sea interactions and anticyclonic atmospheric circulation over the Gulf Stream, whereas highest deciles are related to strong air-sea interactions and a cyclonic circulation. Our analysis includes the low and high-resolution versions of the ARPEGEv6 atmospheric model forced by observed SST, and the recently released ERA5 global reanalysis. We find that the occurrence of anticyclonic and cyclonic perturbations associated with different anomalous wind regimes can locally modulate the activation of the VMM and the PAM. In particular, the PAM is predominant in anticyclonic conditions, whereas both mechanisms are equally present in most of the cyclonic conditions. Our results highlight the role of the atmospheric circulation and associated anomalous winds in the location, strength and occurrence of both mechanisms.

Trait gradients inform predictions of seagrass meadows changes to future warming

Comparing populations across temperature gradients can inform how global warming will impact the structure and function of ecosystems. Shoot density, morphometry and productivity of the seagrass Posidonia oceanica to temperature variation was quantified at eight locations in Sardinia (western Mediterranean Sea) along a natural sea surface temperature (SST) gradient. The locations are spanned for a narrow range of latitude (1.5°), allowing the minimization of the effect of eventual photoperiod variability. Mean SST predicted P. oceanica meadow structure, with increased temperature correlated with higher shoot density, but lower leaf and rhizome width, and rhizome biomass. Chlorophyll a (Chl-a) strongly impacted seagrass traits independent of SST. Disentangling the effects of SST and Chl-a on seagrass meadow shoot density revealed that they work independently, but in the same direction with potential synergism. Space-for-time substitution predicts that global warming will trigger denser seagrass meadows with slender shoots, fewer leaves, and strongly impact seagrass ecosystem. Future investigations should evaluate if global warming will erode the ecosystem services provided by seagrass meadows.

Effects of human footprint and biophysical factors on the body-size structure of fished marine species.

Marine fisheries in coastal ecosystems in many areas of the world have historically removed large‐bodied individuals, potentially impairing ecosystem functioning and the long‐term sustainability of fish populations. Reporting on size‐based indicators that link to food‐web structure can contribute to ecosystem‐based management, but the application of these indicators over large (cross‐ecosystem) geographical scales has been limited to either fisheries‐dependent catch data or diver‐based methods restricted to shallow waters (<20 m) that can misrepresent the abundance of large‐bodied fished species. We obtained data on the body‐size structure of 82 recreationally or commercially targeted marine demersal teleosts from 2904 deployments of baited remote underwater stereo‐video (stereo‐BRUV). Sampling was at up to 50 m depth and covered approximately 10,000 km of the continental shelf of Australia. Seascape relief, water depth, and human gravity (i.e., a proxy of human impacts) were the strongest predictors of the probability of occurrence of large fishes and the abundance of fishes above the minimum legal size of capture. No‐take marine reserves had a positive effect on the abundance of fishes above legal size, although the effect varied across species groups. In contrast, sublegal fishes were best predicted by gradients in sea surface temperature (mean and variance). In areas of low human impact, large fishes were about three times more likely to be encountered and fishes of legal size were approximately five times more abundant. For conspicuous species groups with contrasting habitat, environmental, and biogeographic affinities, abundance of legal‐size fishes typically declined as human impact increased. Our large‐scale quantitative analyses highlight the combined importance of seascape complexity, regions with low human footprint, and no‐take marine reserves in protecting large‐bodied fishes across a broad range of species and ecosystem configurations.

Associated atmospheric mechanisms for the increased cold season precipitation over the Three-River Headwaters region from the late 1980s

AbstractPrecipitation in the Three-River Headwater (TRH) region has undergone significant changes as a result of global warming, which can affect water resources in downstream regions of Asia. However, the underlying mechanisms of the precipitation variability during the cold season (October to April), are still not fully understood. In this study, the daily China gridded precipitation product of CN05.1 as well as the NCEP-NCAR reanalysis are used to investigate the characteristics of the cold season precipitation variability over the TRH region and associated atmospheric mechanisms. The cold season precipitation shows an increasing trend (5.5 mm decade-1) from 1961 to 2014, with a dry-to-wet shift in around the late 1980s. The results indicate that the increased precipitation is associated with the enhanced easterly anomalies over the Tibetan Plateau (TP) and enhanced southeasterly water vapor transport. The enhanced Walker circulations, caused by the gradients of sea surface temperature between the equatorial central-eastern Pacific and Indo-western Pacific in tropical oceans, resulted in strengthened easterly anomalies over the TP and the westward expansion of the anticyclone in the western North Pacific. Meanwhile, the changed Walker circulation is accompanied by a strengthened local Hadley circulation which leads to enhanced meridional water vapor transport from tropical oceans and the South China Sea toward the TRH region. Furthermore, the strengthened East Asia Subtropical Westerly jet may contribute to the enhanced divergence at upper level and anomalous ascending motion above the TRH region leading to more precipitation.

Inverse Columnar-Equiaxed Transition (CET) in 304 and 316L Stainless Steels Melt by Electron Beam for Additive Manufacturing (AM)

According to Hunt’s columnar-to-equiaxed transition (CET) criterion, which is generally accepted, a high-temperature gradient (G) in the solidification front is preferable to a low G for forming columnar grains. Here, we report the opposite tendency found in the solidification microstructure of stainless steels partially melted by scanning electron beam for powder bed fusion (PBF)-type additive manufacturing. Equiaxed grains were observed more frequently in the region of high G rather than in the region of low G, contrary to the trend of the CET criterion. Computational thermal-fluid dynamics (CtFD) simulation has revealed that the fluid velocity is significantly higher in the case of smaller melt regions. The G on the solidification front of a small melt pool tends to be high, but at the same, the temperature gradient along the melt pool surface also tends to be high. The high melt surface temperature gradient can enhance Marangoni flow, which can apparently reverse the trend of equiaxed grain formation.

Cold water temperatures define the poleward range limits of south American fiddler crabs

Temperature increase due to climate change has caused shifts in the range distribution of several organisms globally. In coastal intertidal environments most organisms have an amphibious life cycle and their poleward range limits may be delimited by their thermal tolerance during the pelagic larval stages. Fiddler crabs are key species in intertidal environments and their early larval stages occur in coastal waters. We evaluated the mean and monthly minimum sea surface temperature (SST) gradient over the South American coast and compared it to the minimum and maximum critical thermal limits (CTmin and CTmax) of the first larval stage of eight fiddler crab species to assess whether temperature delimits their distributional ranges. We found a clinal decrease in mean SST of 0.28 °C per latitudinal degree along the distribution of fiddler crabs in South America. Cold tolerance differed among the larvae of fiddler crab species, which corresponds to the latitudinal temperature gradient observed in their poleward range limits. Thus, our results suggest that cold water temperature can define the poleward range limits of South American fiddler crabs. The CTmax cannot explain the northern poleward range limits of the fiddler crabs. Fiddler crabs larvae showed a similar tolerance to high temperatures (mean 40.5 °C) regardless of species (except Leptuca uruguayensis) and these are higher than environmental temperatures observed in South America. We also observed an increase in mean SST of 0.9 °C in the last 37 years. This increase in water temperature may explain the recent poleward range expansion of one South American fiddler crab species, Leptuca cumulanta. Therefore, we showed that differential thermal responses at the early larval stage have consequences on the geographic range limits of the fiddler crab species. Our findings allow us to hypothesize that fiddler crabs’ poleward range limits may expand to higher latitudes in the future due to global warming.

Spatial modeling of rainfall patterns and groundwater on the coast of northeastern Brazil

The objective of this study was to characterize the patterns of monthly rainfall in the mesoregions of Zona da Mata and Metropolitan of Recife in the state of Pernambuco, Brazil, based on geostatistical techniques and multivariate analysis, in addition to establishing rain flow strategies in urban centers and alerting of possible impacts on local aquifers. The meteorological data are from a 20-year time series (2000–2019). Maps of the projection of aquifers, hydrochemical and hydrogeological for the study region were established. The pluviometric data were submitted to descriptive, geostatistical, and multivariate analysis. The Gaussian model was the one that best fitted (R2 ≥ 0.84). The Atlantic interhemispheric sea surface temperature gradient together with Easterly Wave Disturbances, Intertropical Convergence Zone, sea breezes circulations, and trade winds interfered in the dynamics of coastal rains, especially on the Northeast Brazilian coast. The hydrogeological formation of groundwater on the coast, of the porous type, justified the higher flows of groundwater associated with the greater rainfall. Kriging maps were consistent in the spatialization of rains on the coast of Pernambuco and, thus, making it possible to assess the dynamics of rains and the creation of socio-economic strategies, to minimize the impacts during the period of heavy rains and major floods.

The Seasonal Prediction of the Exceptional Yangtze River Rainfall in Summer 2020

During summer 2020, the Yangtze River basin suffered from extreme Meiyu rainfall and catastrophic flooding in June and July. This study explores the seasonal predictability and associated dynamical causes for this extreme Yangtze River rainfall event, based on forecasts from the Met Office GloSea5 operational forecast system. The forecasts successfully predicted above-average rainfall over the Yangtze River basin, which arose from the successful reproduction of the anomalous western North Pacific subtropical high (WNPSH). Our results indicate that both the Indian Ocean warm sea surface temperature (SST) and local WNP SST gradient were responsible for the westward extension of the WNPSH, and the forecasts well captured these tropical signals. We explore extratropical drivers but find a large model spread among the forecast members in the meridional displacements of the East Asian mid-latitude westerly jet (EAJ). The forecast members with an evident southward displacement of the EAJ, favored more extreme Yangtze River rainfall. However, the forecast Yangtze River rainfall anomaly was weaker compared to that observed and no member showed such strong rainfall. In observations, the EAJ displayed an evident acceleration in summer 2020, which could lead to a significant wind convergence in the lower troposphere around the Yangtze River basin, and favor more Meiyu rainfall. The model forecast failed to well reproduce these processes. This difference implies that the observed enhancement of the EAJ intensity gave a large boost to the Yangtze River rainfall, hindering a better forecast of the intensity of the event and disaster mitigation.

Investigation of the response of the Aegean Sea to the Etesian wind forcing

The response of the Aegean Sea to seasonal forcing by the Etesian winds is investigated, based on Copernicus products for weather forecasting (winds) and satellite information (sea-surface temperature (SST) and chlorophyll-α (chl-a) concentration), as well as fisheries records from the Hellenic Statistical Authority. To that end, a site-specific response criterion for upwelling intensity has hereby been developed, based on the zonal SST gradient between eastern and western Aegean. Combined use of the above criterion with lagged cross-correlation between wind, zonal surface temperature gradient, surface chl-a concentration and local fish landings reveals a well-hidden (until now) signal of chl-a increase in response to intensification of wind forcing, as well as a relation between fish landings and upwelling events, challenging the conclusions of previous studies suggesting that the Aegean Sea is immune to summer coastal upwelling, due to the great depth of its nutricline.

Simulation and Experimental Study of the Preheating Mould Temperature Distribution in Semi-Solid Diecasting

The mould temperature distribution has a great influence on the semi-solid diecasting. In the present study the temperature distribution of a plane-shaped mould was investigated by using the method of numerical simulation and experiment. The results showed that the preheating mould temperature field was affected by three important simulation parameters, the heat transfer coefficient hoil between the heat transfer oil and the mould, the heat transfer coefficient hair between the mould and the air, and the heat transfer coefficient hcontact between the mould core and the mould frame. The simulation results showed that (1) with the increase of hoil, the overall mould temperature imcreased; (2) with the increase of hair, the overall mould temperature decreased, while the surface temperature gradient of mould frame grad T-f and the temperature difference between the mould core and the mould frame ∆T increased; (3) With the increase of hcontact, ∆T decreased and the temperature of mould frame increased. When the heat oil temperature Toil=290°C, the heat transfer coefficients were optimized as hoil=500Wm-2K-1, hair=7Wm-2K-1, and hcontact=1000Wm-2K-1 according to the experimental results. The average temperature difference between the simulation result and the experimental result was 3.45°C, and the average relative error was 1.73%.

Reduced High-Latitude Land Seasonality in Climates with Very High Carbon Dioxide

AbstractObservations of warm past climates and projections of future climate change show that the Arctic warms more than the global mean, particularly during winter months. Previous work has attributed this reduced Arctic land seasonality to the effects of sea ice or clouds. In this paper, we show that the reduced Arctic land seasonality is a robust consequence of the relatively small surface heat capacity of land and the nonlinearity of the temperature dependence of surface longwave emission, without recourse to other processes or feedbacks. We use a General Circulation Model (GCM) with no clouds or sea ice and a simple representation of land. In the annual mean, the equator-to-pole surface temperature gradient falls with increasing CO2, but this is only a near-surface phenomenon and is not caused by the change in total meridional heat transport, which is virtually unaltered. The high-latitude land has about twice as much warming in winter than in summer, whereas high-latitude ocean has very little seasonality in warming. A surface energy balance model shows how the combination of the smaller surface heat capacity of land and the nonlinearity of the temperature dependence of surface longwave emission gives rise to the reduced seasonality of the land surface. The increase in evaporation over land also leads to winter amplification of warming over land, although amplification still occurs without it. While changes in clouds, sea ice, and ocean heat transport undoubtedly play a role in high-latitude warming, these results show that enhanced land surface temperature warming in winter can happen in their absence for robust reasons.