Community Earth System Model Large Ensemble Research Articles

Impacts of ENSO on Tropical Pacific Chlorophyll Biomass Under Historical and RCP8.5 Scenarios

AbstractThe present study aims to investigate the influences of El Niño and Southern Oscillation (ENSO) events on chlorophyll biomass in the tropical Pacific under historical and RCP8.5 scenarios with simulations from the Community Earth System Model Large Ensemble (CESM‐LE) project. Large variance in surface chlorophyll concentrations is identified across the Peruvian Upwelling (PU), Equatorial Upwelling (EU), and Western Pacific (WP) regions within the tropical Pacific. Results suggest that responses of surface chlorophyll to ENSO in these regions are governed by different mechanisms. The increasing chlorophyll during La Niña is a consequence of increasing nutrients, influenced by local upwelling systems in the PU and EU regions, while the supplementary nutrients in the WP region arise from the eastern Pacific through stronger surface westward currents. Under RCP8.5 scenario, stronger water warming in the eastern equatorial Pacific leads to a remarkable reduction in the amplitudes of seasonal cycles and interannual variations of chlorophyll in both PU and EU regions. This results in less responsiveness of the chlorophyll biomass to El Niño and moderate La Niña compared to the historical period. However, though warming induces a decrease in chlorophyll concentrations in the WP region, the interannual variations of chlorophyll have shown an improvement in correlation with ENSO events. Meanwhile, despite the small phytoplankton‐dominated community being observed under future scenario, species dominance is likely to shift back to diatoms once extreme La Niña occurs, which is unseen in all El Niño and moderate La Niña cases.

Changes in External Forcings Drive Divergent AMOC Responses Across CESM Generations

AbstractIt has been suggested that the Atlantic meridional overturning circulation (AMOC) in many CMIP6 models is overly sensitive to anthropogenic aerosol forcing, and it has been proposed that this is due to the inclusion of aerosol indirect effects for the first time in many CMIP6 models. We analyze the AMOC response in a newly released ensemble of simulations performed with CESM2 forced by the CMIP5 input data sets (CESM2‐CMIP5). This AMOC response is then compared to the CMIP5‐generation CESM1 large ensemble (CESM1‐LE) and the CMIP6‐generation CESM2 large ensemble (CESM2‐LE). A key conclusion, only made possible by this experimental setup, is that changes in aerosol‐indirect effects cannot explain differences in AMOC response between CESM1‐LE and CESM2‐LE. Instead, we hypothesize that the difference is due to increased interannual variability of anthropogenic emissions. This forcing variability may act through a nonlinear relationship between the surface heat budget of the North Atlantic and the AMOC.

Future changes in North Atlantic winter cyclones in CESM-LE – Part 2: A Lagrangian analysis

Abstract. Future changes in extratropical cyclone structure and dynamics may lead to important impacts but are not yet fully understood. In the first part of this study, we have applied a composite approach together with potential vorticity (PV) inversion to study such changes in the dynamics of North Atlantic cyclones. Here, this is complemented with the help of a Lagrangian perspective, making use of air parcel trajectories to investigate the causes of altered PV anomalies as well as the role that cyclone airstreams play in shaping these changes. Intense cyclones in the extended winter seasons of two periods, 1990–2000 and 2091–2100, are studied in Community Earth System Model Large Ensemble (CESM-LE) simulations, and backward trajectories are calculated from the cyclone area as a basis to construct cyclone-centered composites of Lagrangian tendencies and their projected future changes. Our results show that diabatic processes on a timescale of 24 h shape the cyclones' low-level PV distribution and corroborate that the increasing moisture content along with enhanced ascent in warm conveyor belts leads to amplified latent heat release and larger low- and mid-level PV anomalies near the cyclone center in a warmer climate. In contrast, projected upper-level PV changes are due to a combination of several processes. These processes include cloud diabatic PV changes, anomalous PV advection, and likely also radiative PV generation in the lower stratosphere above the cyclone center. For instance, enhanced poleward advection is the primary reason for a projected decrease in upper-level PV anomalies south of the cyclone center. Warm conveyor belt outflow regions are projected to shift upward, but there is not robust change in the associated upper-level PV anomalies due to compensation between enhanced low-level PV generation and upper-level PV destruction. In summary, our two-part study points to future changes in the relative importance of different processes for the dynamics of intense North Atlantic cyclones in a warming climate, with important consequences for the near-surface wind pattern. In particular, a larger role of cloud diabatic processes is projected, affecting the cyclones through PV production in the lower troposphere. The role of other mechanisms, in particular radiative changes near the tropopause, should be investigated in more detail in future studies.

Diagnosing Mechanisms of Hydrologic Change under Global Warming in the CESM1 Large Ensemble

Abstract Global warming is expected to cause significant changes in the pattern of precipitation minus evaporation (P − E), which represents the net flux of water from the atmosphere to the surface or, equivalently, the convergence of moisture transport within the atmosphere. In most global climate model simulations, the pattern of P − E change resembles an amplification of the historical pattern—a tendency known as “wet gets wetter, dry gets drier.” However, models also predict significant departures from this approximation that are not well understood. Here, we introduce a new method of decomposing the pattern of P − E change into contributions from various dynamic and thermodynamic mechanisms and use it to investigate the response of P − E to global warming within the CESM1 Large Ensemble. In contrast to previous decompositions of P − E change, ours incorporates changes not only in the monthly means of atmospheric winds and moisture, but also in their temporal variability, allowing us to isolate the hydrologic impacts of changes in the mean circulation, transient eddies, relative humidity, and the spatial and temporal distributions of temperature. In general, we find that changes in the mean circulation primarily control the P − E response in the tropics, while temperature changes dominate at higher latitudes. Although the relative importance of specific mechanisms varies by region, at the global scale departures from the wet-gets-wetter approximation over land are primarily due to changes in the temperature lapse rate, while changes in the mean circulation, relative humidity, and horizontal temperature gradients play a secondary role.

Mechanisms for Abrupt Summertime Circumpolar Surface Warming in the Southern Ocean

Abstract In recent years, the Southern Ocean has experienced unprecedented surface warming and sea ice loss—a stark reversal of the sea ice expansion and surface cooling that prevailed over the preceding decades. Here, we examine the mechanisms that led to the abrupt circumpolar surface warming events that occurred in late 2016 and 2019 and assess the role of internal climate variability. A mixed layer heat budget analysis reveals that these recent circumpolar surface warming events were triggered by a weakening of the circumpolar westerlies, which decreased northward Ekman transport and accelerated the seasonal shoaling of the mixed layer. We emphasize the underappreciated effect of the latter mechanism, which played a dominant role and amplified the warming effect of air–sea heat fluxes during months of peak solar insolation. An examination of the CESM1 large ensemble demonstrates that these recent circumpolar warming events are consistent with the internal variability associated with the Southern Annular Mode (SAM), whereby negative SAM in austral spring favors shallower mixed layers and anomalously high summertime SST. A key insight from this analysis is that the seasonal phasing of springtime mixed layer depth shoaling is an important contributor to summertime SST variability in the Southern Ocean. Thus, future Southern Ocean summertime SST extremes will depend on the coevolution of mixed layer depth and surface wind variability. Significance Statement This study examines how reductions in the strength of the circumpolar westerlies can produce abrupt and extreme surface warming across the Southern Ocean. A key insight is that the mixed layer temperature is most sensitive to surface wind perturbations in late austral spring, when the regional mixed layer depth and solar insolation approach their respective seasonal minimum and maximum. This heightened surface temperature response to surface wind variability was realized during the austral spring of 2016 and 2019, when a dramatic weakening of the circumpolar westerlies triggered unprecedented warming across the Southern Ocean. In both cases, the anomalously weak circumpolar winds reduced the northward Ekman transport of cool subpolar waters and caused the mixed layer to shoal more rapidly in the spring, with the latter mechanism being more dominant. Using results from an ensemble of coupled climate simulations, we demonstrate that the 2016 and 2019 Southern Ocean warming events are consistent with the internal variability associated with the Southern Annular Mode (SAM). These results suggest that future Southern Ocean surface warming extremes will depend on both the evolution of regional mixed layer depths and interannual wind variability.

Enhanced ENSO-Unrelated Summer Variability in the Indo–Western Pacific under Global Warming

Abstract El Niño–Southern Oscillation (ENSO) is an important but not the only source of interannual variability over the Indo–western Pacific. Non-ENSO forced variability in the region has received recent attention because of the implications for rainy-season prediction. Using a 35-member CESM1 Large Ensemble (CESM-LE) and 30 CMIP6 models, this study shows that the ensemble means project intensified interannual variability for precipitation, low-level winds, and sea level pressure under global warming, associated with the enhanced large-scale anomalous anticyclone (AAC) over the tropical northwestern (NW) Pacific after the ENSO signal is removed. A decomposition based on the column water vapor budget reveals that enhanced precipitation variability is due to the increased background specific humidity. The resultant anomalous diabatic heating intensifies the AAC, which further strengthens the precipitation anomalies. Over the tropical NW Pacific, the wind-induced evaporative cooling on the southeastern flank of the AAC is countered by the increased shortwave radiation due to the strengthened precipitation reduction. Tropospheric temperature anomalies in the ensemble means show no significant change, suggesting no apparent change of the interbasin positive feedback between the AAC and northern Indian Ocean SST. Intermodel analysis based on CMIP6 reveals that models with a larger increase in ENSO-unrelated precipitation variability over the NW Pacific are associated with stronger background warming in the eastern equatorial Pacific, due to the modulated Walker and Hadley circulations.

The ENSO-induced South Pacific Meridional Mode

Previous studies have investigated the role of the Pacific meridional mode (PMM), a climate mode of the mid-latitudes in the Northern and Southern Hemisphere, in favoring the development of the El Niño Southern Oscillation (ENSO). However little is known on how ENSO can influence the development of the PMM. Here we investigate the relationship between ENSO and the South Pacific Meridional Mode (SPMM) focusing on strong SPMM events that follows strong El Niño events. This type of events represents more than 60% of such events in the observational record and the historical simulations of the CESM Large ensemble (CESM-LE). It is first shown that such a relationship is rather stationary in both observations and the CESM-LE. Our analyses further reveal that strong SPMM events are associated with a coastal warming off northern central Chile peaking in Austral winter resulting from the propagation of waves forced at the equator during the development of El Niño events. The time delay between the ENSO peak (Boreal winter) and this coastal warming (Austral winter) can be understood in terms of the differential contribution of the equatorially-forced propagating baroclinic waves to the warming along the coast. In particular, the difference in phase speeds of the waves (the high-order mode the wave the slower) implies that they do not overlap along their propagation south of 20°S. This contributes to the persistence of warm coastal SST anomalies off Central Chile until the Austral summer following the concurrent El Niño event. This coastal warming is favorable to the development of strong SPMM events as the South Pacific Oscillation become active during that season. The analysis of the simulations of the Coupled Intercomparison Project phases 5 and 6 (CMIP5/6) indicates that very few models realistically simulate this ENSO/SPMM relationship and associated oceanic teleconnection.

Warm conveyor belts in present-day and future climate simulations – Part 2: Role of potential vorticity production for cyclone intensification

Abstract. Warm conveyor belts (WCBs) are strongly ascending, cloud- and precipitation-forming airstreams in extratropical cyclones. The intense cloud-diabatic processes produce low-level cyclonic potential vorticity (PV) along the ascending airstreams, which often contribute to the intensification of the associated cyclone. This study investigates how climate change affects the cyclones' WCB strength and the importance of WCB-related diabatic PV production for cyclone intensification, based on present-day (1990–1999) and future (2091–2100) climate simulations of the Community Earth System Model Large Ensemble (CESM-LE). In each period, a large number of cyclones and their associated WCB trajectories have been identified in both hemispheres during the winter season. WCB trajectories are identified as strongly ascending air parcels that rise at least 600 hPa in 48 h. Compared to ERA-Interim reanalyses, the present-day climate simulations are able to capture the cyclone structure and the associated WCBs reasonably well, which gives confidence in future projections with CESM-LE. However, the amplitude of the diabatically produced low-level PV anomaly in the cyclone centre is underestimated in the climate simulations, most likely because of reduced vertical resolution compared to ERA-Interim. The comparison of the simulations for the two climates reveals an increase in the WCB strength and the cyclone intensification rate in the Southern Hemisphere (SH) in the future climate. The WCB strength also increases in the Northern Hemisphere (NH) but to a smaller degree, and the cyclone intensification rate is not projected to change considerably. Hence, in the two hemispheres cyclone intensification responds differently to an increase in WCB strength. Cyclone deepening correlates positively with the intensity of the associated WCB, with a Spearman correlation coefficient of 0.68 (0.66) in the NH in the present-day (future) simulations and a coefficient of 0.51 (0.55) in the SH. The number of explosive cyclones with strong WCBs, referred to as C1 cyclones, is projected to increase in both hemispheres, while the number of explosive cyclones with weak WCBs (C3 cyclones) is projected to decrease. A composite analysis reveals that in the future climate C1 cyclones will be associated with even stronger WCBs, more WCB-related diabatic PV production, the formation of a more intense PV tower, and an increase in precipitation. They will become warmer, moister, and slightly more intense. The findings indicate that (i) latent heating associated with WCBs (as identified with our method) will increase, (ii) WCB-related PV production will be even more important for explosive cyclone intensification than in the present-day climate, and (iii) the interplay between dry and moist dynamics is crucial to understand how climate change affects cyclone intensification.

European heatwaves in present and future climate simulations: a Lagrangian analysis

Abstract. Heatwaves are prolonged periods of anomalously high temperatures that can have devastating impacts on the environment, society and economy. In recent history, heatwaves have become more intense and more numerous over most continental areas, and it is expected that this trend will continue due to the ongoing global temperature rise. This general intensification may be modified by changes also in the underlying thermodynamical and dynamical processes. In order to study potential changes in heatwave characteristics and dynamics, we compare Lagrangian backward trajectories of airstreams associated with historic (1991–2000) and future (2091–2100) heatwaves in six different European regions. We use a percentile-based method (Heat Wave Magnitude Index daily) to identify heatwaves in a large ensemble of climate simulations (Community Earth System Model Large Ensemble (CESM-LE) with 35 members). The simulations have been forced by historical representative concentration pathways (RCPs) up to 2005 and by the RCP8.5 scenario afterwards. In general, we find that air parcels associated with heatwaves are located to the east or inside the respective regions 3 d prior to the events. For future heatwaves, the model projects a north-/northeastward shift of the origin of the air masses in most study regions. Compared to climatological values, airstreams associated with heatwaves show a larger temperature increase along their trajectory, which is connected to stronger descent and/or stronger diabatic heating when the air parcels enter the boundary layer. We find stronger descent associated with adiabatic warming in the northern, more continental regions and increased diabatic heating in all regions (except of the British Isles) in the simulated future climate. The enhanced diabatic heating is even more pronounced for heatwaves over continental regions. Diabatic temperature changes of near-surface air are driven by sensible heat fluxes, which are stronger over dry soils. The amplified diabatic heating associated with future heatwaves may thus be explained by an additional drying of the land surface.

Consistent Timing of Arctic Permafrost Loss Across the CESM1 Large Ensemble

AbstractThe Arctic is especially vulnerable to climate change and is warming faster than the global average. Changes in this region pose a heightened threat due to the immense amount of carbon frozen as organic material in the soil. When permafrost thaws, organic material decays and releases as greenhouse gases into the atmosphere, contributing to further warming. A better understanding of the processes that influence permafrost degradation is needed to inform climate adaptation and mitigation planning. This study assesses changes to Arctic permafrost across 35 ensemble member simulations from the Community Earth System Model 1 Large Ensemble Project, under the Representative Concentration Pathway 8.5 21st century scenario. Most Arctic near‐surface permafrost is lost by 2100, but timing varies across regions and with soil depth. Internal climate variability, represented by differences between ensemble members, has a constrained influence on degradation timing due to relative consistency of summer temperature increases and insulation by winter snow cover.

A curious case of the Indian Summer Monsoon 2020: The influence of Barotropic Rossby Waves and the monsoon depressions

The seasonal summer monsoon rainfall over India has substantially depended on the synoptic-scale systems such as monsoon lows and depressions. India has received above-average rainfall during the 2020 summer monsoon season. Total 12 Low Pressure Areas (LPAs) formed in the north Indian Ocean during summer 2020 (in JJAS season). The significance of this monsoon season is that August 2020 received the highest all-India rainfall in the past 44 years since 1976. This is accompanied by around 50% of the total seasonal LPAs formed in August 2020, none of which intensified into a monsoon depression (MDs). This study attempts to understand the characteristic features of monsoon rainfall during August 2020 and explore the plausible mechanisms behind the LPAs not intensifying/concentrating as MDs. It is noted that the anomalous warming over the Northern Parts of the Arabian Sea (NPAS) resulted in increased convection over this region in August 2020, as a result, strong low-level convergence is observed. In addition to this convergence, strong northwesterly winds emanating from central Asia merged with the enhanced cross-equatorial monsoon flow. However, this strong flow over the Arabian Sea sheared/dissociated into two branches: one extending up to northwest (NW) India along the monsoon trough, another one diverging into an anticyclone over the south Bay of Bengal (SBOB), which reduced the horizontal shear there (Barotropic Instability). This anticyclone strength over the SBOB and its westward shift is determined by the western north Pacific (WNP) anticyclone. Our analysis suggests that due to the poor barotropic instability over the head BOB, LPAs could not develop into MDs. Additionally, upper level (200 hPa) barotropic Rossby wave in August 2020 remains stationary over South Central Asia and retrogressed with a northeast to southwest orientation. It determined the path of movement of the low-level disturbance beneath and affected the all-India rainfall by virtue of enhanced rainfall over NW & Western Ghats (WG) regions. The interplay of the barotropic Rossby wave alongside low-level anticyclone over the WNP accompanied by local conditions caused the above normal rainfall over India in August 2020, even though there are adverse dynamical conditions. We have also verified these mechanisms in Community Earth System Model Large Ensemble (CESM-LE) model simulations, analysis shows that model has a limited skill to simulate the changes in the monsoon rainfall and associated circulation and failed to capture the mid-latitude circulation impact on ISM rainfall.

Atmospheric Contributions to the Reversal of Surface Temperature Anomalies Between Early and Late Winter Over Eurasia

AbstractObservations have shown subseasonal reversal of temperature anomalies between early and late winter over Eurasia, which is distinct from the seasonal mean condition. Based on the reanalysis data, the 1800‐year control simulation and the 40‐member ensemble simulations in 1920–2100 from the Community Earth System Model (CESM) Large Ensemble (CESM‐LE), this study reveals that the reversal of surface air temperature (SAT) anomalies between early and late winter is one of the dominant and intrinsic features of the Arctic‐Eurasian winter climate. Such a reversal is characterized by “colder Arctic, warmer Eurasia” in December (January–February) and “warmer Arctic, colder Eurasia” in January–February (December). Robust climate dynamic processes associated with the reversal of SAT anomalies, including subseasonal reversals of anomalies in the Ural blocking, midlatitude westerlies, and stratospheric polar vortex, are found in both reanalysis data and CESM simulations, indicating the important role of internal atmospheric variability. Further analysis reveals that the reversal of Ural blocking anomalies in late December can be a potential precursor for the reversal of SAT anomalies in late winter. The reversal of midlatitude westerly wind anomalies associated with the Ural blocking can affect upward propagation of planetary‐scale waves especially with wavenumber 1, subsequently promoting the contribution of stratospheric polar vortex to the reversal of SAT anomalies in late winter over the Arctic‐Eurasian regions. Such a troposphere‐stratosphere pathway triggered by the perturbation of tropospheric circulations is confirmed by the CESM‐LE simulations, and it may be useful for the prediction of subseasonal reversal of SAT anomalies.

Response of moist and dry processes in atmospheric blocking to climate change

Weather extremes are often associated with atmospheric blocking, but how the underlying physical processes leading to blocking respond to climate change is not yet fully understood. Here we track blocks as upper-level negative potential vorticity (PV) anomalies and apply a Lagrangian analysis to 100 years of present-day (∼2000) and future (∼2100, under the RCP8.5 scenario) climate simulations restarted from the Community Earth System Model–Large Ensemble Project runs (CESM-LENS) to identify different physical processes and quantify how their relative importance changes in a warmer and more humid climate. The trajectories reveal two contrasting airstreams that both contribute to the formation and maintenance of blocking: latent heating in strongly ascending airstreams (moist processes) and quasi-adiabatic flow near the tropopause with weak radiative cooling (dry processes). Both are reproduced remarkably well when compared against ERA-Interim reanalysis, and their relative importance varies regionally and seasonally. The response of blocks to climate change is complex and differs regionally, with a general increase in the importance of moist processes due to stronger latent heating (+1 K in the median over the Northern Hemisphere) and a larger fraction (+15%) of strongly heated warm conveyor belt air masses, most pronounced over the storm tracks. Future blocks become larger (+7%) and their negative PV anomaly slightly intensifies (+0.8%). Using a Theil–Sen regression model, we propose that the increase in size and intensity is related to the increase in latent heating, resulting in stronger cross-isentropic transport of air with low PV into the blocking anticyclones. Our findings provide evidence that moist processes become more important for the large-scale atmospheric circulation in the midlatitudes, with the potential for larger and more intense blocks.

Impact of Internal Climate Variability on Wintertime Surface Air Temperature Trends Over Eurasia in the CESM1 Large Ensemble

AbstractThis study investigates the role of internal climate variability on the wintertime surface air temperature (SAT) trend over Eurasia during 1970–2004 based on outputs from the Community Earth System Model Large Ensemble Project. The SAT trends vary significantly in both spatial pattern and magnitude among ensemble members, indicating the essential role of internal climate variability on SAT trends. The leading modes of the spread of SAT trends are extracted via inter‐member empirical orthogonal function (EOF) analysis of the SAT trend patterns among 40 ensemble members. The first EOF mode shows widespread warming over northern Eurasia and slight cooling southward of 40°N during its positive phase. The second EOF mode displays a zonal dipole, with one center located in Europe and the other of the opposite sign over Siberia. The Arctic Oscillation and Scandinavian patterns are key dynamical factors associated with the two dominant modes, respectively. A dynamical adjustment method is further employed to decompose the internal climate variability into those related to large‐scale atmospheric circulation and residuals. The spread of SAT trends among ensemble members is primarily associated with the various large‐scale atmospheric circulation and second by the residual component that is closely tied to diverse snow cover trends over Eurasia, especially over northern Eurasia. These results may help understand the relationship between internal climate variability and winter SAT trends over Eurasia via dynamical and thermodynamical processes.

Future changes in North Atlantic winter cyclones in CESM-LE – Part 1: Cyclone intensity, potential vorticity anomalies, and horizontal wind speed

Abstract. Strong low-level winds associated with extratropical cyclones can have substantial impacts on society. The wind intensity and the spatial distribution of wind maxima may change in a warming climate; however, the involved changes in cyclone structure and dynamics are not entirely clear. Here, such structural changes of strong North Atlantic cyclones in a warmer climate close to the end of the current century are investigated with storm-relative composites based on Community Earth System Model Large Ensemble (CESM-LE) simulations. Furthermore, a piecewise potential vorticity inversion is applied to associate such changes in low-level winds to changes in potential vorticity (PV) anomalies at different levels. Projected changes in cyclone intensity are generally rather small. However, using cyclone-relative composites, we identify an extended wind footprint southeast of the center of strong cyclones, where the wind speed tends to intensify in a warmer climate. Both an amplified low-level PV anomaly driven by enhanced diabatic heating and a dipole change in upper-level PV anomalies contribute to this wind intensification. On the contrary, wind changes associated with lower- and upper-level PV anomalies mostly compensate for each other upstream of the cyclone center. Wind changes at upper levels are dominated by changes in upper-level PV anomalies and the background flow. Altogether, our results indicate that a complex interaction of enhanced diabatic heating and altered non-linear upper-tropospheric wave dynamics shape future changes in near-surface winds in North Atlantic cyclones.

Causes of the Arctic’s Lower-Tropospheric Warming Structure

AbstractArctic amplification has been attributed predominantly to a positive lapse rate feedback in winter, when boundary layer temperature inversions focus warming near the surface. Predicting high-latitude climate change effectively thus requires identifying the local and remote physical processes that set the Arctic’s vertical warming structure. In this study, we analyze output from the CESM Large Ensemble’s twenty-first-century climate change projection to diagnose the relative influence of two Arctic heating sources, local sea ice loss and remote changes in atmospheric heat transport. Causal effects are quantified with a statistical inference method, allowing us to assess the energetic pathways mediating the Arctic temperature response and the role of internal variability across the ensemble. We find that a step-increase in latent heat flux convergence causes Arctic lower-tropospheric warming in all seasons, while additionally reducing net longwave cooling at the surface. However, these effects only lead to small and short-lived changes in boundary layer inversion strength. By contrast, a step-decrease in sea ice extent in the melt season causes, in fall and winter, surface-amplified warming and weakened boundary layer temperature inversions. Sea ice loss also enhances surface turbulent heat fluxes and cloud-driven condensational heating, which mediate the atmospheric temperature response. While the aggregate effect of many moist transport events and seasons of sea ice loss will be different than the response to hypothetical perturbations, our results nonetheless highlight the mechanisms that alter the Arctic temperature inversion in response to CO2forcing. As sea ice declines, the atmosphere’s boundary layer temperature structure is weakened, static stability decreases, and a thermodynamic coupling emerges between the Arctic surface and the overlying troposphere.

Dynamical Downscaling of Near‐Term (2026–2035) Climate Variability and Change for the Main Hawaiian Islands

AbstractNatural climate variability poses a limit to the confidence in regional climate projections, particularly for the near‐term. Therefore, the next decades are of importance for regional impact studies and the development of regional adaptation plans. This article presents results from an ensemble of regional climate model simulations (years 1996–2005 and 2026–2035) that are used to examine the effects of both anthropogenic forcing and natural variability associated with the Pacific Decadal Oscillation (PDO) on near‐term climate projections for the Hawaiian Islands. The Community Earth System Model Large Ensemble (CESM‐LE) is used in conjunction with the Weather Research and Forecasting (WRF) model for downscaling. The climate responses to the PDO and anthropogenic forcing are isolated and analyzed separately. In response to anthropogenic forcing, significant increases in surface air temperature, of ∼0.8 K, are projected at low elevations. Stronger warming, of up to 1.3 K, is seen at higher elevation areas. Future climate simulations show significant increases in wet season rainfall, of ∼10%–20%, along the windward slopes of Big Island and Maui. Rainfall patterns during the positive PDO phase are projected to reverse in sign, leading to drier conditions, by ∼10%–30%, at many locations. Future climate simulations show daily rainfall extremes will increase, by up to ∼10%–15%, at many locations. Daily temperature extremes are also projected to increase significantly, by up to 1.4 K. Overall, results indicate that natural variability will continue to contribute to uncertainty in near‐term rainfall projections for the Hawaiian Islands, masking the forced signal.

A New Framework for Identifying and Investigating Seasonal Climate Extremes

AbstractPrevious studies have recognized the societal relevance of climatic extremes on the seasonal time scale and examined physical processes leading to individual high-impact extreme seasons (e.g., extremely wet or warm seasons). However, these findings have not yet been generalized beyond case studies since at any specific location only very few seasonal events of such rarity occurred in the observational record. In this concept paper, a pragmatic approach to pool seasonal extremes across space is developed and applied to investigate hot summers and cold winters in ERA-Interim and the Community Earth System Model Large Ensemble (CESM-LENS). We identify spatial extreme season objects as contiguous regions of extreme seasonal mean temperatures based on statistical modeling. Regional pooling of extreme season objects in CESM-LENS then yields considerable samples of analogs to even the most extreme ERA-Interim events. This approach offers numerous opportunities for systematically analyzing large samples of extreme seasons, and several such analyses are illustrated. We reveal a striking co-occurrence of El Niño to La Niña transitions and the largest ERA-Interim midlatitude extreme summer events. Moreover, we perform a climate model evaluation with regard to extreme season size and intensity measures and estimate how often an extreme winter like the cold North American 2013/14 winter is expected anywhere in midlatitude regions. Furthermore, we present a large set of simulated analogs to this event, which makes it possible to study commonalities and differences of their underlying physical processes. Finally, substantial but spatially varying climatological differences in the size of extreme summer and extreme winter objects are identified.

Identifying the evolving human imprint on heat wave trends over the United States and Mexico

Changes in frequency, duration and intensity of three heat wave (HW) types (compound, daytime, and nighttime) over the United States (U.S.) and Mexico during the second half of the 20th-century are investigated using the Community Earth System Model Large Ensemble (CESM-LE). The individual role of anthropogenic aerosols and greenhouse gases (GHGs), as well as the contribution from internal variability (IV), are identified and contrasted by means of the CESM-LE single forcing experiments during two periods: 1950–1975, when North American aerosol emissions peaked, and 1980–2005, when aerosol emissions declined. HW changes are strongly affected by anthropogenic forcing. During 1950–1975, aerosols, via both aerosol-radiation and aerosol-cloud interactions, dominate the decreasing trends in compound HWs over the central U.S., the daytime HWs in large parts of the domain and the nighttime HWs over Mexico. Conversely, all three HW types are considerably more frequent (2 HWs summer−1 decade−1), longer-lasting (with increases of up to 2 days HW−1 decade−1 in some regions) and more intense (e.g., C HW−1 decade−1 in compound HWs) across large regions of the domain during the 1980–2005 period. The results show that the decline in aerosol emissions and the continuous rise in GHGs lead to widespread warming and subsequent circulation adjustments, contributing to the positive HW trends. The contribution of IV is large during 1950–1975 (over 60% in most areas), and considerably reduced later on. This study provides a comprehensive picture of the role of anthropogenic forcing and IV on the marked HW changes in the recent decades and their underpinning physical mechanisms.

Detecting Climate Signals in Southern Ocean Krill Growth Habitat

Climate change is rapidly altering the habitat of Antarctic krill (Euphausia superba), a key species of the Southern Ocean food web. Krill are a critical element of Southern Ocean ecosystems as well as biogeochemical cycles, while also supporting an international commercial fishery. In addition to trends forced by global-scale, human-driven warming, the Southern Ocean is highly dynamic, displaying large fluctuations in surface climate on interannual to decadal timescales. The dual roles of forced climate change and natural variability affecting Antarctic krill habitat, and therefore productivity, complicate interplay of observed trends and contribute to uncertainty in future projections. We use the Community Earth System Model Large Ensemble (CESM-LE) coupled with an empirically derived model of krill growth to detect and attribute trends associated with “forced,” human-driven climate change, distinguishing these from variability arising naturally. The forced trend in krill growth is characterized by a poleward contraction of optimal conditions and an overall reduction in Southern Ocean krill habitat. However, the amplitude of natural climate variability is relatively large, such that the forced trend cannot be formally distinguished from natural variability at local scales over much of the Southern Ocean by 2100. Our results illustrate how natural variability is an important driver of regional krill growth trends and can mask the forced trend until late in the 21st century. Given the ecological and commercial global importance of krill, this research helps inform current and future Southern Ocean krill management in the context of climate variability and change.