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The Eocene–Oligocene transition (EOT) was a climate shift from a largely ice-free greenhouse world to an icehouse climate, involving the first major glaciation of Antarctica and global cooling occurring ∼34 million years ago (Ma) and lasting ∼790 kyr. The change is marked by a global shift in deep-sea δ18O representing a combination of deep-ocean cooling and growth in land ice volume. At the same time, multiple independent proxies for ocean temperature indicate sea surface cooling, and major changes in global fauna and flora record a shift toward more cold-climate-adapted species. The two principal suggested explanations of this transition are a decline in atmospheric CO2 and changes to ocean gateways, while orbital forcing likely influenced the precise timing of the glaciation. Here we review and synthesise proxy evidence of palaeogeography, temperature, ice sheets, ocean circulation and CO2 change from the marine and terrestrial realms. Furthermore, we quantitatively compare proxy records of change to an ensemble of climate model simulations of temperature change across the EOT. The simulations compare three forcing mechanisms across the EOT: CO2 decrease, palaeogeographic changes and ice sheet growth. Our model ensemble results demonstrate the need for a global cooling mechanism beyond the imposition of an ice sheet or palaeogeographic changes. We find that CO2 forcing involving a large decrease in CO2 of ca. 40 % (∼325 ppm drop) provides the best fit to the available proxy evidence, with ice sheet and palaeogeographic changes playing a secondary role. While this large decrease is consistent with some CO2 proxy records (the extreme endmember of decrease), the positive feedback mechanisms on ice growth are so strong that a modest CO2 decrease beyond a critical threshold for ice sheet initiation is well capable of triggering rapid ice sheet growth. Thus, the amplitude of CO2 decrease signalled by our data–model comparison should be considered an upper estimate and perhaps artificially large, not least because the current generation of climate models do not include dynamic ice sheets and in some cases may be under-sensitive to CO2 forcing. The model ensemble also cannot exclude the possibility that palaeogeographic changes could have triggered a reduction in CO2.

The effect of tropical cyclone (TC) size on TC-induced sea surface temperature (SST) cooling and subsequent TC intensification is an intriguing issue without much exploration. Via compositing satellite-observed SST over the western North Pacific during 2004–19, this study systematically examined the effect of storm size on the magnitude, spatial extension, and temporal evolution of TC-induced SST anomalies (SSTA). Consequential influence on TC intensification is also explored. Among the various TC wind radii, SSTA are found to be most sensitive to the 34-kt wind radius (R34) (1 kt ≈ 0.51 m s–1). Generally, large TCs generate stronger and more widespread SSTA than small TCs (for category 1–2 TCs, R34: ∼270 vs 160 km; SSTA: −1.7° vs −0.9°C). Despite the same effect on prolonging residence time of TC winds, the effect of doubling R34 on SSTA is more profound than halving translation speed, due to more wind energy input into the upper ocean. Also differing from translation speed, storm size has a rather modest effect on the rightward shift and timing of maximum cooling. This study further demonstrates that storm size regulates TC intensification through an oceanic pathway: large TCs tend to induce stronger SST cooling and are exposed to the cooling for a longer time, both of which reduce the ocean’s enthalpy supply and thereby diminish TC intensification. For larger TCs experiencing stronger SST cooling, the probability of rapid intensification is half of smaller TCs. The presented results suggest that accurately specifying storm size should lead to improved cooling effect estimation and TC intensity prediction.

The duration of the eyewall replacement cycle (ERC) in typhoons is determined by the rate of dissipation of the inner eyewall and intensification of the outer eyewall, which is an important indicator for predicting changes in the intensity and structure of typhoons. Previous studies on ERCs have focused on the internal storm dynamics associated with the interactions between the concentric eyewalls (CEs), but the impacts of the sea surface cooling (SSC) on ERCs remain not adequately investigated. The slow movement of Typhoon Trami results in remarkable SSC. Using a coupled atmosphere-ocean model, the simulation for Trami generates an ERC that matches observations, whereas an unrealistic long-lived ERC is produced in the uncoupled simulation. Numerical simulations suggest that the typhoon-induced nonuniform SSC can not only weaken the typhoon, but can also modulate the duration of the ERCs. The SSC acts like a catalyst for triggering the negative feedback between the surface heat exchange and the circulations of Trami to reduce the energy supply to the inner eyewall more severely where the sea surface temperature (SST) dropped more sharply. The SSC works in concert with the interactions between the CEs to weaken the inner eyewall faster, thus terminating the ERC of Trami rapidly. The results indicate that a better understanding of the modulation effect of SSC is required for the accurate forecast of ERCs.

The South Atlantic Convergence Zone (SACZ) is an atmospheric system occurring in austral summer on the South America continent and sometimes extending over the adjacent South Atlantic. It is characterized by a persistent and very large, northwest-southeast-oriented, cloud band. Its presence over the ocean causes sea surface cooling that some past studies indicated as being produced by a decrease of incoming solar heat flux induced by the extensive cloud cover. Here we investigate ocean–atmosphere interaction processes in the Southwestern Atlantic Ocean (SWA) during SACZ oceanic episodes, as well as the resulting modulations occurring in the oceanic mixed layer and their possible feedbacks on the marine atmospheric boundary layer. Our main interests and novel results are on verifying how the oceanic SACZ acts on dynamic and thermodynamic mechanisms and contributes to the sea surface thermal balance in that region. In our oceanic SACZ episodes simulations we confirm an ocean surface cooling. Model results indicate that surface atmospheric circulation and the presence of an extensive cloud cover band over the SWA promote sea surface cooling via a combined effect of dynamic and thermodynamic mechanisms, which are of the same order of magnitude. The sea surface temperature (SST) decreases in regions underneath oceanic SACZ positions, near Southeast Brazilian coast, in the South Brazil Bight (SBB) and offshore. This cooling is the result of a complex combination of factors caused by the decrease of solar shortwave radiation reaching the sea surface and the reduction of horizontal heat advection in the Brazil Current (BC) region. The weakened southward BC and adjacent offshore region heat advection seems to be associated with the surface atmospheric circulation caused by oceanic SACZ episodes, which rotate the surface wind and strengthen cyclonic oceanic mesoscale eddy. Another singular feature found in this study is the presence of an atmospheric cyclonic vortex Southwest of the SACZ (CVSS), both at the surface and aloft at 850 hPa near 24°S and 45°W. The CVSS induces an SST decrease southwestward from the SACZ position by inducing divergent Ekman transport and consequent offshore upwelling. This shows that the dynamical effects of atmospheric surface circulation associated with the oceanic SACZ are not restricted only to the region underneath the cloud band, but that they extend southwestward where the CVSS presence supports the oceanic SACZ convective activity and concomitantly modifies the ocean dynamics. Therefore, the changes produced in the oceanic dynamics by these SACZ events may be important to many areas of scientific and applied climate research. For example, episodes of oceanic SACZ may influence the pathways of pollutants as well as fish larvae dispersion in the region.

The relationship between changes in surface air temperature and sea surface temperature is important for understanding past and future climate change. In this study, we use reconstructions and model simulations to investigate the ratio of global mean air versus sea surface temperature change (S) during the Last Glacial Maximum (LGM). The simulated S at the LGM is 1.97 ± 0.22 (1σ), 44 ± 16% greater than under future warming, primarily due to the influence of elevated continental ice sheets. Results reveal that the glacial air‐sea cooling contrast is negatively related to the magnitude of sea surface cooling, consistent with a simple moist static energy theory. This relationship can be used to constrain S, further suggesting a median LGM surface cooling of −5.6°C. These results caution against the use of a fixed S under different climate background and have implications for paleotemperature reconstructions and climate projections.

Using a new measure that relates tropical cyclone (TC)-induced sea surface cooling with the strength of TCs, interannual variations in potential impacts of the upper ocean stratification on TC-induced sea surface cooling associated with the evolution of El Niño/Southern Oscillation (ENSO) are investigated in the northwestern Pacific using an ocean reanalysis product, with a special focus on haline effects. It is found that the haline stratification could suppress the sea surface cooling by as much as 20% to the south of 20°N in the peak typhoon season (July-October), and this contribution is different between their developing years (September-October) and decaying years (July-August). More specifically, the haline effects may vary up to 25% (40%) during the decaying years of El Niño (La Niña). Due to anomalous haline effects, the region to the west of 160°E is susceptible to the sea surface cooling during the developing and decaying years of El Niño, while the cooling could be suppressed in this region during the decaying years of La Niña. Although the effects of haline stratification have been found less important than those of thermal stratification, potential impacts of the upper ocean salinity on TC-induced sea surface cooling associated with the ENSO have been quantitatively estimated for the first time. Since the main focus of this paper is to present the new measure and discuss potential impacts of the upper ocean salinity stratification, further verifications need to be conducted once more observational data is accumulated or through numerical simulations.

The urban thermal environment is impacted by changes in urban landscape patterns resulting from urban expansion and seasonal variation. In order to cope effectively with urban heat island (UHI) effects and improve the urban living environment and microclimate, an analysis of the heating effect of impervious surface areas (ISA) and the cooling effects of vegetation is needed. In this study, Landsat 8 data in four seasons were used to derive the percent ISA and fractional vegetation cover (FVC) by spectral unmixing and to retrieve the land surface temperature (LST) from the radiative transfer equation (RTE). The percent ISA and FVC were divided into four different categories based on ranges 0–25%, 25–50%, 50–75%, and 75–100%. The LST with percent ISA and FVC were used to calculate the surface heating rate (SHR) and surface cooling rate (SCR). Finally, in order to analyze the heating effect of ISA and the cooling effect of vegetation, the variations of LST with SHR and SCR were compared between different percent ISA and FVC categories in the four seasons. The results showed the following: (1) In summer, SHR decreases as percent ISA increases and SCR increases as FVC increases in the study area. (2) Unlike the dependence of LST on percent ISA and FVC, the trends of SHR/SCR as a function of percent ISA/FVC are more complex for different value ranges, especially in spring and autumn. (3) The SHR (heating capacity) decreases with increasing percent ISA in autumn. However, the SCR (cooling capacity) decreases with increasing FVC, except in summer. This study shows that our methodology to analyze the variation and change trends of SHR, SCR, and LST based on different ISA and FVC categories in different seasons can be used to interpret urban ISA and vegetation cover, as well as their heating and cooling effects on the urban thermal environment. This analytical method provides an important insight into analyzing the urban landscape patterns and thermal environment. It is also helpful for urban planning and mitigating UHI.

The diurnal variability of mixed layer (ML) overturning instabilities remains poorly understood due to the challenge in capturing their rapid evolutions across large spatiotemporal ranges. Using high‐resolution data from 52 gliders in the South China Sea, we examine the diurnal modulations of ML overturning instabilities. The results of the 3‐month field observation show that negative potential vorticity occupies ∼16% of the ML and facilitates several types of forced overturning instabilities, especially symmetric instability (SI). Surface heat fluxes are identified to primarily modulate the diurnal variability of these overturning cells, where nighttime surface cooling is found to energize SI with an ∼2‐hr phase lag. As a result, over 60% of forced submesoscale overturning cells tend to restratify the ML at night. These findings quantitatively highlight the modulation of diabatic atmospheric forcing in submesoscale restratification, which should be considered in submesoscale parameterizations of ocean and climate models. Plain Language Summary The diurnal cycle of submesoscale motions has been explored in high‐resolution simulations and observations, but the understanding for the variability of associated overturning instability remains limited. Here, we present new observational evidence of the diurnal variability of forced overturning instabilities using high‐resolution data from a 3‐month glider array observation. The results show that this diurnal variability is primarily modulated by surface heat fluxes rather than wind friction. Among the identified instabilities, symmetric instability is significantly energized by the surface cooling and tends to drive stronger submesoscale restratification at night. Given their frequent occurrence in the upper ocean, enhanced submesoscale vertical exchange due to nighttime surface cooling may play an important role in oceanic mixed layer restratification, potentially modulating the climate system and marine ecosystem. Key Points New evidence for the diurnal variability of mixed layer (ML) overturning instabilities is provided by a glider array observation Surface heat fluxes are found to primarily modulate the diurnal variability of ML overturning cells in the realistic ocean More than 60% of observed submesoscale overturning cells are energized by nighttime surface cooling and act to restratify the ML

Typhoons, fueled by warm sea surface waters, heighten concern as they increasingly interact with frequent Marine Heatwaves (MHWs) in a changing climate. Typhoon Hinnamnor (2022) weakened and re‐intensified as it approached the Korean Strait, interacting with an underlying MHW in the northern East China Sea (nECS). In‐situ observations and reanalysis products revealed a significant increase in latent heat loss from the nECS during the MHW period, contributing to the typhoon re‐intensification. Strong sea surface wind forcing with the typhoon enhanced vertical mixing and upwelling, resulting in a pronounced (0.90°C) sea surface cooling after the typhoon passage, facilitating MHW disappearance with reduced thermal stratification. During MHWs, increased background stratification increases temperature oscillations associated with semidiurnal internal tides. Furthermore, post‐typhoon changes in stratification weakened semidiurnal internal tides due to unfavorable conditions for generation from a nearby source. These findings highlight the importance of continuous time‐series observations to monitor interactions among climatic extremes. Plain Language Summary Typhoons, powered by warm ocean waters, are causing more concern as they increasingly interact with frequent episodes of extremely warm sea conditions known as Marine Heatwaves (MHWs) in a changing climate. This study focuses on Typhoon Hinnamnor in 2022, which went through a weakening and then strengthened as it moved to the Korean Strait and encountered an MHW in the northern East China Sea (nECS). By using in‐situ data collected in the nECS and additional data analysis, we discovered a significant increase in heat loss from the nECS during the MHW, contributing to the intensification of typhoon. The powerful winds from the typhoon caused enhanced mixing and cooling of the sea surface after it passed, helping to cause the disappearance of the MHW and reduce the layering of temperatures in the ocean. During MHW, strong layering strengthens the temperature oscillation linked with the semidiurnal internal tides in the ocean. After typhoon passage there is a decrease in the layering in the ocean, thus weakening the internal tide. The study emphasizes the importance of continuous observations to understand and monitor these interactions in our changing climate. Key Points Typhoon Hinnamnor (2022) re‐intensified after interacting with the underlying Marine Heatwave (MHW) in the East China Sea Typhoon wind‐driven mixing caused the disappearance of the underlying MHW Stratification change accompanied by MHW, and typhoon reduced the local activities of semidiurnal internal tides

Tropical cyclones (TCs) induce substantial upper‐ocean mixing and upwelling, leading to sea surface cooling. In this study, we explore changes in TC‐induced cold wakes along the United States (U.S.) Southeast and Gulf Coasts during 1982–2020. Our study shows a significant increase in TC‐induced sea surface temperature (SST) cooling of about 0.20°C near the U.S. Southeast Coast over this period. However, for the U.S. Gulf Coast, trends in TC‐induced SST cooling are insignificant. Analysis of the large‐scale oceanic environments indicate that the increasing TC‐induced cold wakes near the Southeast coast have been predominantly caused by the cooling of subsurface waters in that region. This upper‐ocean change is attributed to the enhancement of surface pressure gradient across land‐sea boundary and the associated increase in alongshore winds over there. Further analysis with climate models reveals the important role of anthropogenic forcings in driving these changes in the atmospheric circulation response along the U.S. Southeast Coast. Plain Language Summary Tropical cyclones (TCs) can induce strong mixing in the upper ocean and upwelling of cold, nutrient‐rich water from subsurface to the surface. This process can induce significant sea surface cooling and enhance primary productivity. Utilizing satellite observations, we found evidence that the sea surface cooling associated with TCs has been increasing significantly in the near‐shore regions of United States (U.S.) Southeast Coast. However, corresponding trends in the U.S. Gulf Coast are not significant. Here we show that the enhanced TC‐induced SST cooling along the U.S. Southeast Coast has occurred in response to changes in the atmospheric circulation associated with global warming. These findings shed light on the potential changes in the interactions between TCs and the upper ocean conditions under a non‐stationary climate. Key Points The tropical cyclone induced sea surface temperature cooling has significantly increased near the U.S. Southeast Coast in recent decades The increasing trend in TC‐induced cold wake is primarily attributed to subsurface cooling driven by the change in atmospheric circulation Climate models suggest greenhouse gases are likely responsible for the atmospheric circulation changes