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Climate-driven changes in high-elevation forest distribution and reductions in snow and ice cover have major implications for ecosystems and global water security. In the Greater Yellowstone Ecosystem of the Rocky Mountains (United States), recent melting of a high-elevation (3,091 m asl) ice patch exposed a mature stand of whitebark pine (Pinus albicaulis) trees, located ~180 m in elevation above modern treeline, that date to the mid-Holocene (c. 5,950 to 5,440 cal y BP). Here, we used this subfossil wood record to develop tree-ring-based temperature estimates for the upper-elevation climate conditions that resulted in ancient forest establishment and growth and the subsequent regional ice-patch growth and downslope shift of treeline. Results suggest that mid-Holocene forest establishment and growth occurred under warm-season (May-Oct) mean temperatures of 6.2 °C (±0.2 °C), until a multicentury cooling anomaly suppressed temperatures below 5.8 °C, resulting in stand mortality by c. 5,440 y BP. Transient climate model simulations indicate that regional cooling was driven by changes in summer insolation and Northern Hemisphere volcanism. The initial cooling event was followed centuries later (c. 5,100 y BP) by sustained Icelandic volcanic eruptions that forced a centennial-scale 1.0 °C summer cooling anomaly and led to rapid ice-patch growth and preservation of the trees. With recent warming (c. 2000-2020 CE), warm-season temperatures now equal and will soon exceed those of the mid-Holocene period of high treeline. It is likely that perennial ice cover will again disappear from the region, and treeline may expand upslope so long as plant-available moisture and disturbance are not limiting. [ABSTRACT FROM AUTHOR]

The decadal to centennial scale climate variability of the past 4000 years consists of colder and warmer periods, potentially initiated by fast varying external forcing, or the lack thereof. These alternating cold and warm periods are most clearly visualized by the waxing and waning of glaciers in the Northern Hemisphere. However, these cold and warm periods are neither spatially nor temporally consistent, and using these defined periods to interpret local variations in climate and society could prove difficult. Here, we use two global earth system models, as well as available proxy reconstructions to examine to which extent the defined warm and cold periods of the last 4000 years before the industrial period are reflected in the climate for Northern, Southern, and Central Europe. We find that on regional scales, the relative role of internal variability appears more pronounced compared to the externally forced signal, thereby decreasing the forcing signal in local climate records. In addition, one model suggests that the climate variability for Northern Europe follows the external forcing more closely than the climate of Central and especially Southern Europe, while the other model shows rather ambiguous results. This study illustrates that periods defined by glacier advances and archaeology have to be carefully used in the interpretation of past events at a local scale, as it is likely that internal variability dominates on such scales. [ABSTRACT FROM AUTHOR]

Tropical Instability Waves (TIWs) have been shown to modulate upper ocean mixing. However, previous studies on the modulation of TIW related mixing are based on small numbers of TIWs and have not considered temporal variability, which can lead to discrepancies in the findings. In this study, using a 12‐year simulation carried out with a comprehensive, global, high‐resolution ocean model, we present evidence of seasonally modulated shear instabilities at TIW fronts in the Atlantic Ocean that reach down to the thermocline depth, potentially inducing mixing below the mixed layer depth. We find that, regardless of whether TIWs are present earlier in the year, frontal instabilities and potential mixing primarily occur in boreal summer, coinciding with a vertical shear maximum between the mean zonal currents. We argue that in the Atlantic Ocean, vertical shear at TIW fronts does usually not suffice to cause frontal instabilities below the mixed layer depth. Instead, the background shear needs to be sufficiently large in addition to TIW shear, to overcome the stability. The background shear in turn varies seasonally and is strongly driven by the variability of the northern branch of the South Equatorial Current (nSEC). As such, the variability of the nSEC strongly contributes to the generation and modulation of instabilities at TIW fronts that reach below the mixed layer depth and have the potential to induce mixing. Our results highlight the importance of seasonal variability when studying TIW impacts and their effect on mixing. Plain Language Summary: Tropical Instability Waves (TIWs) create alternating warm and cold patterns at the ocean's surface, leading to sharp temperature fronts. Past studies have shown that mixing can occur along these fronts, but how this mixing changes over time hasn't been explored. Our research focuses on how instabilities below the surface at TIW fronts vary over time. Studying such instabilities helps to better understand the processes that can impact the regional climate. We analyzed over 10 years of data from a detailed ocean model and found that instabilities at TIW fronts in the Atlantic Ocean can extend down to 100 m and follow a clear seasonal cycle. These instabilities are most common in summer when the difference in zonal current speeds is greatest. We discovered that shear at TIW fronts alone isn't enough to cause these deep instabilities; the overall background shear also needs to be strong. This background shear changes with the seasons, mainly influenced by the northern branch of the South Equatorial Current (nSEC). Thus, the nSEC plays a crucial role in modulating instabilities at TIW fronts and potential ocean mixing. Key Points: Tropical instability waves exhibit shear instabilities at the front that reach the thermocline depth and follow a distinct seasonal cycleThe seasonality of frontal shear instabilities results from a superposition of shear from the instability wave and background current shearVariability in the northern South Equatorial Current dominates the modulation of shear instabilities at Tropical Instability Wave fronts [ABSTRACT FROM AUTHOR]

The anthropogenically intensified greenhouse effect has caused a radiative imbalance at the top of the atmosphere during the industrial period. This, in turn, has led to an energy surplus in various components of the Earth system, with the ocean storing the largest part. The land contribution ranks second with the latest observational estimates based on borehole temperature profiles, which quantify the terrestrial energy surplus to be 6 % in the last 5 decades, whereas studies based on state-of-the-art climate models scale it down to 2 %. This underestimation stems from land surface models (LSMs) having a subsurface that is too shallow, which severely constrains the land heat uptake simulated by Earth system models (ESMs). A forced simulation of the last 2000 years with the Max Planck Institute ESM (MPI-ESM) using a deep LSM captures 4 times more heat than the standard shallow MPI-ESM simulations in the historical period, well above the estimates provided by other ESMs. However, deepening the LSM does not remarkably affect the simulated surface temperature. It is shown that the heat stored during the historical period by an ESM using a deep LSM component can be accurately estimated by considering the surface temperatures simulated by the ESM using a shallow LSM and propagating them with a standalone forward model. This result is used to derive estimates of land heat uptake using all available observational datasets, reanalysis products, and state-of-the-art ESM experiments. This approach yields values of 10.5–16.0 ZJ for 1971–2018, which are 12 %–42 % smaller than the latest borehole-based estimates (18.2 ZJ). [ABSTRACT FROM AUTHOR]