General Circulation Model Simulations Research Articles

A Numerical Study of the High Latitudinal Ion‐Neutral Coupling Time Scale Under Disturbed Conditions

AbstractWhen solar wind and interplanetary magnetic field (IMF) disturb, thermospheric winds change accordingly. Among the momentum forces driving high‐latitude thermospheric winds, ion drag is supposed to greatly affect wind variations through ion‐neutral coupling when abrupt and strong changes in ion drifts occur. However, due to the great inertia of thermospheric winds it needs a certain period of time for the wind changes to be prominent both in speed and direction. How long the neutral winds take to change from one steady state to another through the ion‐neutral coupling process is currently still a controversial issue. In this paper, we examine the high latitudinal ion‐neutral coupling time scale based on the Thermosphere Ionosphere Electrodynamics General Circulation Model simulations, which can determine whether wind variations are dominantly driven by ion drag by analyzing the relative contribution of each momentum force. It is found that the spatial variation of ion‐neutral coupling time scale is primarily determined by local electron density, but also varies with neutral density and ion‐neutral collision frequency. Simulations during periods of medium solar activity at ∼250 km altitude show that the ion drag‐dominated region is generally located at the dayside convection inverse boundary and the coupling time scale (e‐folding time) is ∼1 hr when IMF By is the dominant component of the IMF and changes direction. Meanwhile, the southward component of IMF Bz enlarges the ion drag‐dominated region. When IMF Bz is southward with a large magnitude, ion drag‐dominated region is primarily located in the nightside auroral oval with ∼2 hr coupling time scale.

Evaluating Large‐Storm Dominance in High‐Resolution GCMs and Observations Across the Western Contiguous United States

AbstractExtreme precipitation events are projected to increase in frequency across much of the land‐surface as the global climate warms, but such projections have typically relied on coarse‐resolution (100–250 km) general circulation models (GCMs). The ensemble of HighResMIP GCMs presents an opportunity to evaluate how a more finely resolved atmosphere and land‐surface might enhance the fidelity of the simulated contribution of large‐magnitude storms to total precipitation, particularly across topographically complex terrain. Here, the simulation of large‐storm dominance, that is, the number of wettest days to reach half of the total annual precipitation, is quantified across the western United States (WUS) using four GCMs within the HighResMIP ensemble and their coarse resolution counterparts. Historical GCM simulations (1950–2014) are evaluated against a baseline generated from station‐observed daily precipitation (4,803 GHCN‐D stations) and from three gridded, observationally based precipitation data sets that are coarsened to match the resolution of the GCMs. All coarse‐resolution simulations produce less large‐storm dominance than in observations across the WUS. For two of the four GCMs, bias in the median large‐storm dominance is reduced in the HighResMIP simulation, decreasing by as much as 62% in the intermountain west region. However, the other GCMs show little change or even an increase (+28%) in bias of median large‐storm dominance across multiple sub‐regions. The spread in differences with resolution amongst GCMs suggests that, in addition to resolution, model structure and parameterization of precipitation generating processes also contribute to bias in simulated large‐storm dominance.

Thermospheric nitric oxide energy budget during extreme geomagnetic storms: a comparative study

We selected three superstorms (disturbance storm time [Dst] index less than −350 nT) of 2003–04 to study the thermospheric energy budget with a particular emphasis on the thermospheric cooling emission by nitric oxide via a wavelength of 5.3 μm. The nitric oxide radiative emission data are obtained from the Sounding of the Atmosphere by Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite and the thermosphere ionosphere electrodynamic general circulation model (TIEGCM) simulation. Different energy sources for the magnetospheric energy injection and the thermospheric/ionospheric dissipation processes are calculated using empirical formulations, model simulations, and space-borne and ground-based measurements. The Joule heating rates calculated from different sources showed similar variations but significant differences in the magnitude. The nitric oxide cooling power is calculated by zonally and meridionally integrating the cooling flux in the altitude range of 100–250 km. The satellite observed that cooling flux responds faster to the energy input, as compared to the modeled results. The cooling power increases by an order of magnitude during storm time with maximum radiation observed during the recovery phase. Both the satellite-observed and modeled cooling powers show a strong positive correlation with the Joule heating power during the main phase of the storm. It is found that the maximum radiative power does not occur during the strongest storm, and it strongly depends on the duration of the main phase. The model simulation predicts a higher cooling power than that predicted by the observation. During a typical superstorm, on average, a cooling power of 1.87 × 105 GW exiting the thermosphere is estimated by the TIEGCM simulation. On average, it is about 40% higher than the satellite observation.

Fine Cloud Structures on a Hard Snowball Earth

AbstractEarth may have been globally ice‐covered at least two times during the Cryogenian Period (720–635 Ma). Previous studies showed that clouds could strongly warm a hard snowball Earth and promote its deglaciation. However, the understanding of clouds was largely based on coarse‐resolution global simulations with parameterized convection and clouds, or high‐resolution cloud‐resolving simulations with explicit convection and clouds but limited to a small domain without the effects of large‐scale circulation. Here we present the first global non‐hydrostatic high‐resolution (30 km) simulation to investigate snowball clouds and their radiative effects. We show that spatial‐temporal cloud distributions exhibit clear meanders (patterns of curved lines rather than straight lines along the west‐east direction) and strong variabilities, which result from the tight interactions among the Inter‐Tropical Convergence Zone, Hadley cells, and baroclinic instability. The net cloud radiative effect is around 20 W m−2 in the tropics, consistent with previous global general circulation model simulations.

Reduction in the Tropical High Cloud Fraction in Response to an Indirect Weakening of the Hadley Cell

AbstractTropical high cloud cover decreases with surface warming in most general circulation models. This reduction, according to the “stability‐iris” hypothesis, is thermodynamically controlled and linked to a decrease in the radiatively‐driven clear‐sky convergence, when the peak anvil clouds rise because of the rising isotherms. The influence of the large‐scale dynamical changes on the tropical high cloud fraction remains difficult to disentangle from the local thermodynamic influence, given that the mean meridional circulation remains inextricably tied to the local thermodynamic structure of the atmosphere. However, using idealized general circulation model simulations, we propose a novel method to segregate the dynamical impact from the thermodynamic impact on the tropical high cloud fraction. To this end, our investigation primarily focuses on the mechanisms underpinning changes in the high cloud cover in the deep tropics in response to extratropical surface warming, when the tropical sea surface temperatures remain invariant. Net convective detrainment of ice cloud condensates decreases at the peak detrainment region, without a rise in its altitude. We also find that the importance of depositional growth of ice cloud condensates in controlling the high cloud fraction response in the deep tropics varies with altitude.

The Short‐Periodic Response of Equatorial Electrojet to the Temporal Variations of IMF Bz During Geomagnetic Quiet Time

AbstractThe equatorial electrojet (EEJ) is an important and intense ionospheric current, flowing in a narrow band along the dip equator. Using the total electron content from Global Navigation Satellite System, electron density from Swarm, EEJ from Tatuoca station, and the corresponding Thermosphere‐Ionosphere Electrodynamic General Circulation Model simulations, this work investigates the behaviors of quiet‐time EEJ during the temporal oscillations of interplanetary magnetic field (IMF) Bz. It is found that an average relative perturbation of 17.5%, 5.51% and 23.17% could be generated in the quiet time EEJ on August 24 in 2021, 2022, and 2023, respectively. The prominent feature shown in EEJ changes is the short‐periodic oscillations with high frequency and a period of tens of minutes. A power spectrum from wavelet supports that the small‐scale perturbations with high frequency are mainly attributed to the prompt penetration electric field (PPEF). During the temporal oscillations of IMF Bz, a similar spectrum is established between IMF Bz and PPEF. PPEF could be promptly superposed into the zonal equatorial electric field within minutes, producing similar frequent responses in EEJ. The large changes with low frequency are related to the disturbance wind dynamo electric field.

The early Cretaceous was cold but punctuated by warm snaps resulting from episodic volcanism

The Cretaceous is characterized as a greenhouse climate from elevated atmospheric carbon dioxide concentrations, transgressive seas, and temperate ecosystems at polar paleolatitudes. Here we test the hypothesis that the early Cretaceous was a cold climate state with a new Aptian atmospheric carbon dioxide record from the C3 plant proxy and early Cretaceous sea level curve from stable oxygen isotopes of belemnites and benthic foraminifera. Results show that carbon dioxide concentrations were persistently below 840 ppm during the Aptian, validating recent General Circulation Model simulations of ice sheets on Antarctica at those concentrations. In addition, sea level was estimated to be within the ice sheet window for much of the early Cretaceous prior to the Albian. This background state appears to have been episodically interrupted by Large Igneous Province volcanism followed by long-term carbon burial from weathering. We hypothesize that the early Cretaceous was largely an icehouse punctuated by warm snaps.

Measurements of Titan’s Stratospheric Winds during the 2009 Equinox with the eSMA

Saturn’s moon Titan possesses stratospheric zonal winds that places it among a sparse class of planetary bodies known to have superrotation in their atmospheres. Few measurements have been made of these speeds in the upper stratosphere, leaving their seasonal variations still not well understood. We examined observations made with the extended Submillimeter Array in 2009 March (L s = 355°) and 2010 February (L s = 5°), shortly before and after Titan's northern spring equinox. Cassini observations and atmospheric models find equinoctial periods to be especially dynamic. Zonal wind calculations, derived from the Doppler frequency shift of CH3CN near 349.4 GHz, yielded speeds of 128 ± 27 m s−1 in 2009 and 209 ± 48 m s−1 in 2010. We estimated the measured emission to originate from vertical altitudes of 336−88+112 km, equivalent to pressures of 3.8−3.4+19.2 Pa, commensurate with Titan’s upper stratosphere/lower mesosphere. This suggests a possible increase in zonal speeds during this period. The results are then compared to those from previous Cassini-inferred and direct-interferometric observations of winds, as well as general circulation model simulations, to form a more complete picture of the seasonal cycle of stratospheric zonal winds.

Modelling water isotopologues (1H2H16O, 1H217O) in the coupled numerical climate model iLOVECLIM (version 1.1.5)

Abstract. Stable water isotopes are used to infer changes in the hydrological cycle for different climate periods and various climatic archives. Following previous developments of δ18O in the coupled climate model of intermediate complexity, iLOVECLIM, we present here the implementation of the 1H2H16O and 1H217O water isotopes in the different components of this model and calculate the associated secondary markers deuterium excess (d-excess) and oxygen-17 excess (17O-excess) in the atmosphere and ocean. So far, the latter has only been modelled by the atmospheric model LMDZ4. Results of a 5000-year equilibrium simulation under preindustrial conditions are analysed and compared to observations and several isotope-enabled models for the atmosphere and ocean components. In the atmospheric component, the model correctly reproduces the first-order global distribution of the δ2H and d-excess as observed in the data (R=0.56 for δ2H and 0.36 for d-excess), even if local differences are observed. The model–data correlation is within the range of other water-isotope-enabled general circulation models. The main isotopic effects and the latitudinal gradient are properly modelled, similarly to previous water-isotope-enabled general circulation model simulations, despite a simplified atmospheric component in iLOVECLIM. One exception is observed in Antarctica where the model does not correctly estimate the water isotope composition, a consequence of the non-conservative behaviour of the advection scheme at a very low moisture content. The modelled 17O-excess presents a too-important dispersion of the values in comparison to the observations and is not correctly reproduced in the model, mainly because of the complex processes involved in the 17O-excess isotopic value. For the ocean, the model simulates an adequate isotopic ratio in comparison to the observations, except for local areas such as the surface of the Arabian Sea, a part of the Arctic and the western equatorial Indian Ocean. Data–model evaluation also presents a good match for the δ2H over the entire water column in the Atlantic Ocean, reflecting the influence of the different water masses.

Machine Learning Emulation of Subgrid‐Scale Orographic Gravity Wave Drag in a General Circulation Model With Middle Atmosphere Extension

AbstractGravity wave parameterizations contribute to uncertainties in middle atmosphere modeling. To investigate the potential for using machine learning to represent atmospheric gravity waves and the impact of implementing such schemes in a general circulation model (GCM), we train a random forest (RF) emulator on outputs from an existing complex parameterization scheme for orographic gravity wave drag (GWD). The performance of the RF emulator is then evaluated with a focus on stratospheric climatology and variability in climate simulations from the middle atmosphere resolving Beijing Climate Center Atmospheric Circulation Model. In offline tests, the predicted orographic GWD by the RF agrees remarkably well with the target GWD throughout the troposphere and the middle atmosphere. The RF emulator can reproduce the observed climatology of zonal‐mean wind and air temperature in the GCM simulation, as well as its target scheme. Compared to the target orographic GWD parameterization scheme, the RF emulator can reproduce the breakdown of the polar vortex in the Southern Hemisphere stratosphere. This study demonstrates the feasibility of using machine learning to emulate parameterized orographic GWD for modeling the stratosphere with a GCM.

Statistical downscaling of high-resolution precipitation in India using convolutional long short-term memory networks

Abstract Statistical downscaling of the General Circulation Model (GCM) simulations are widely used for accessing climate changes in the future at different spatiotemporal scales. This study proposes a novel Statistical Downscaling (SD) model established on the Convolutional Long Short-Term Memory (ConvLSTM) Network. The methodology is applied to obtain future projection of rainfall at 0.25° spatial resolution over the Indian sub-continental region. The traditional multisite downscaling models typically perform downscaling on a single homogeneous rainfall zone, predicting rainfall at only one grid point in a single model run. The proposed model captures spatiotemporal dependencies in multisite local rainfall and predicts rainfall for the entire zone in a single model run. The study proposes a Shared ConvLSTM model providing a single end-to-end supervised model for predicting the future precipitation for entire India. The model captures the regional variability in rainfall better than a region-wise trained model. The projected future rainfall for different scenarios of climate change reveals an overall increase in the rainfall mean and spatially non-uniform changes in future rainfall extremes over India. The results highlight the importance of conducting in-depth hydrologic studies for different river basins of the country for future water availability assessment and making water resource policies.

Response Time of Joule Heating Rate and Nitric Oxide Cooling Emission During Geomagnetic Storms: Correlated Ground‐Based and Satellite Observations

AbstractJoule heating is the primary source of the high latitude thermospheric energy dissipation during geomagnetic storm period. The nitric oxide (NO) emission at 5.3 μm accounts for the majority of Joule heating energy due to its radiative nature. It is the dominating cooling agent above 100 km that effectively regulates the thermospheric temperature. We studied the relationship between the response time of NO cooling emission and Joule heating rate during geomagnetic storm periods by using the NO emission observations by Sounding of Atmosphere using Broadband Emission Radiometry (SABER) onboard NASA’s Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite and the Joule heating rate measured by the European incoherent scatter (EISCAT) radar over Tromsø (geographic:69.59°N, 19.22°E), Norway, and the thermosphere‐ionosphere‐electrodynamics general circulation model (TIEGCM) simulation. We selected seven geomagnetic storms during which there were continuous measurements of the Pederson conductivity and electric fields when the TIMED/SABER satellite was in the northview mode. The TIEGCM gives a fair description of NO cooling. However, it was found that the Joule heating rates obtained with the TIEGCM often do not show good agreement with those from EISCAT observations. The Joule heating rate peaks with 3–7 hr of storm' onset. The NO cooling flux takes longer time to respond to the energy input during storm period. The fastest response of the NO cooling emission to the Joule heating rate is found during the strongest storm. The weakest storm does not have the longest response time. No correlation between the response time of NO cooling flux with respect to the Joule heating rate and storm’s intensity was observed.

Oceanic Influence and Lapse Rate Changes Dominate the Recent Amplified Saharan Warming

AbstractThe surface air temperature (SAT) over the Sahara Desert has increased at a much faster rate than average tropical land in recent decades. This study examines the relative roles of anthropogenic greenhouse gas (GHG) forcing and sea surface temperature (SST) change in the observed Saharan temperature increase during boreal warm season from 1979 to 2020 using atmospheric general circulation model simulations. It is found that the SST forcing dominates the observed Saharan warming. Further analysis shows that the warming ocean forces the Saharan SAT increase by moving more energy to the Sahara Desert, while the water vapor feedback plays a secondary role. The reason for the stronger Saharan warming than the average tropical land given the same SST forcing is also explored. We found that the largest contributor to the warming contrast is the lapse‐rate feedback, which is attributable to the difference in the vertical warming profile.

Investigation of Mesosphere and Lower Thermosphere Dynamics over Central and Northern Peru Using SIMONe Systems

Abstract One year of Spread spectrum Interferometric Multistatic meteor radar Observing Network (SIMONe) measurements are analyzed and compared for the first time between two low-latitude locations in Peru: Jicamarca (12°S, 77°W) and Piura (5°S, 80°W). Investigation of the mean horizontal winds and tides reveals that mesosphere and lower thermosphere (MLT) planetary-scale dynamics are similar between these two locations, although differences can be seen in some tidal components, e.g., the diurnal tide. On the other hand, 28-day median values of the momentum fluxes obtained with 4-h, 4-km time–altitude bins indicate that the mesoscale dynamics differ significantly between Jicamarca and Piura, places separated by approximately 850 km. From the middle of July until October 2021, a strong acceleration of the background zonal wind by westward-propagating gravity waves (GWs) is observed above ∼90 km at both locations, although with larger amplitudes over Jicamarca. From the middle of January until April 2022, a second strong acceleration of the background zonal wind, again by westward-propagating GWs, is observed, but this time with larger amplitudes over Piura. The latter is further supported by the dominance of negative vertical gradients of the zonal momentum flux above 89 km of altitude. Thus, these results observationally confirm the previous studies based on general circulation model simulations indicating that the directions of the zonal GW drag and the zonal background wind coincide in the low-latitude MLT. The weak correlations between the horizontal wind gradients over Jicamarca and Piura reinforce the fact that the mesoscale dynamics are different at these two locations.

Performance evaluation of ionospheric models over equatorial Indian region

An extensive year-long evaluation of Total Electron Content (TEC) obtained from NCAR Thermosphere-Ionosphere Electrodynamics General Circulation Model (TIE-GCM) simulations along with TEC obtained from International Reference Ionosphere (IRI-2020) is carried out with respect to observations made by a GPS TEC receiver over the dip equatorial location of Thiruvananthapuram (8.5° N, 77° E, 0.5° magnetic), in the Indian region during solar minimum period of 2005–2006. The TIE-GCM model and IRI model were run for the period of 2005 (November December) and till October 2006 for each day, and the hourly values of TEC were compared to GPS TEC observations. Overall the temporal and seasonal characteristics of TEC over Thiruvananthapuram were simulated reasonably well by the TIE-GCM model and IRI model. The skill scores of normalized Root Mean Square Error (nRMSE), Prediction Efficiency (P.E), and Ratio between maximum and minimum values were estimated for both models to evaluate their performance. The mean percentage difference between the measured and the simulated TEC was found to be around −25% for IRI, indicating underestimation of the observations, and between −40% and −50% for TIE-GCM again indicating underestimation. A comprehensive analysis revealed that there is good conformity between the modeled and measured values for the whole observation period, with a correlation coefficient R-value of 0.8 and 0.9 for the TIE-GCM and IRI models respectively. This study brings out the competence of the two models in simulating the temporal and seasonal pattern of variability of ionospheric TEC over the equatorial Indian region.

Application of relative importance metrics for CMIP6 models selection in projecting basin-scale rainfall over Johor River basin, Malaysia

The most recent set of General Circulation Models (GCMs) derived from the Coupled Model Intercomparison Project Phase 6 (CMIP6) was used in this work to analyse the spatiotemporal patterns of future rainfall distribution across the Johor River Basin (JRB) in Malaysia. A group of 23 GCMs were chosen for comparative assessment in simulating basin-scale rainfall based on daily rainfall from the historical period of the Climate Hazards Group InfraRed Precipitation with Station Data (CHIRPS). The methodological novelty of this study lies in the application of relative importance metrics (RIM) to rank and select historical GCM simulations for reproducing rainfall at 109 CHIRPS grid points within the JRB. In order to choose the top GCMs, the rankings given by RIM were aggregated using the compromise programming index (CPI) and Jenks optimised classification (JOC). It was found that ACCESS-ESM1–5 and CMCC-ESM2 were ranked the highest in most of the grid. The final GCM was then bias-corrected using the linear scaling method before being ensemble based on the Bayesian model averaging (BMA) technique. The spatiotemporal assessment of the ensemble model for the different months over the near-future period 2021–2060 and far-future period 2061–2100 was compared with those under Shared Socioeconomic Pathways (SSPs), namely, SSP1-2.6, SSP2-4.5, SSP3–7.0, and SSP5-8.5. Heterogeneous changes in rainfall were projected across the JRB, with both increasing and decreasing trends. In the near-future and far-future scenarios, higher rainfall was projected for December, indicating an elevated risk of flooding during the end of the North East monsoon (NEM). Conversely, August showed a decreasing trend in rainfall, implying an increasing risk of severe drought. The findings of this study provide valuable insights for effective water resource management and climate change adaptation in the region.

A Numerical Investigation on the Formation of the Stalling of the Cross‐Polar Jet in Midnight Thermospheric Winds

AbstractThermospheric winds usually flow from noon to the midnight inside the polar cap and spill further equatorward around midnight, due to the combined effects of solar radiation‐driven pressure gradient and ion drag. Recent observations from the Scanning Doppler Imager at Poker Flat indicated a distinct feature that the cross‐polar jet stalls over short horizontal distances around the magnetic midnight. However, the mechanisms responsible for the occurrence of stalling have not been well understood. In this study, we investigated the physical mechanisms of the stalling of the cross‐polar jet based on the Thermosphere Ionosphere Electrodynamics General Circulation Model simulations. The model reproduced the general stalling features at midnight over a short horizontal distance. The occurrence of stalling is mainly associated with Joule heating, which enhances the neutral temperature and then turns winds poleward by the pressure gradient force, and ion‐neutral coupling, which drags neutral winds westward in the pre magnetic local time midnight sector and contributes to the poleward turning of the meridional winds by the poleward Coriolis force. Meanwhile, the stalling region has a large latitudinal range and depends on the interplanetary magnetic field (IMF). Under southward IMF Bz, thermospheric winds are likely to stall at lower latitudes associated with strong ionospheric convection. The stalling of the cross‐polar jet in the midnight thermospheric winds has UT/longitude dependence, which could be associated with the orientation of the alignment of geomagnetic and geographic poles with respect to ionospheric convection.

The Role of Diffusivity Changes on the Pattern of Warming in Energy Balance Models

Abstract Atmospheric macroturbulence transports energy down the equator-to-pole gradient. This is represented by diffusion in energy balance models (EBMs), and EBMs have proven valuable for understanding and quantifying the pattern of surface temperature change. They typically assume climate-state-independent diffusivity, chosen to well represent the current climate, and find that this is sufficient to emulate warming response in general circulation models (GCMs). Meanwhile, model diagnoses of GCM simulations have shown that the diffusivity changes with climate. There is also ongoing development for diffusivity theories based on atmospheric dynamics. Here, we examine the role that changes in diffusivity play in the large-scale equator-to-pole contrast in surface warming in EBMs, building on previous analytic EBM theories for polar-amplified warming. New analytic theories for two formulations of climate-state-dependent diffusivity capture the results of numerical EBM solutions. For reasonable choices of parameter values, the success of the new analytic theories reveals why the change of diffusivity is limited in response to radiative forcing and does not eliminate polar-amplified warming.

PyESDv1.0.1: an open-source Python framework for empirical-statistical downscaling of climate information

Abstract. The nature and severity of climate change impacts vary significantly from region to region. Consequently, high-resolution climate information is needed for meaningful impact assessments and the design of mitigation strategies. This demand has led to an increase in the application of empirical-statistical downscaling (ESD) models to general circulation model (GCM) simulations of future climate. In contrast to dynamical downscaling, the perfect prognosis ESD (PP-ESD) approach has several benefits, including low computation costs, the prevention of the propagation of GCM-specific errors, and high compatibility with different GCMs. Despite their advantages, the use of ESD models and the resulting data products is hampered by (1) the lack of accessible and user-friendly downscaling software packages that implement the entire downscaling cycle, (2) difficulties reproducing existing data products and assessing their credibility, and (3) difficulties reconciling different ESD-based predictions for the same region. We address these issues with a new open-source Python PP-ESD modeling framework called pyESD. pyESD implements the entire downscaling cycle, i.e., routines for data preparation, predictor selection and construction, model selection and training, evaluation, utility tools for relevant statistical tests, visualization, and more. The package includes a collection of well-established machine learning algorithms and allows the user to choose a variety of estimators, cross-validation schemes, objective function measures, and hyperparameter optimization in relatively few lines of code. The package is well-documented, highly modular, and flexible. It allows quick and reproducible downscaling of any climate information, such as precipitation, temperature, wind speed, or even short-term glacier length and mass changes. We demonstrate the use and effectiveness of the new PP-ESD framework by generating weather-station-based downscaling products for precipitation and temperature in complex mountainous terrain in southwestern Germany. The application example covers all important steps of the downscaling cycle and different levels of experimental complexity. All scripts and datasets used in the case study are publicly available to (1) ensure the reproducibility and replicability of the modeled results and (2) simplify learning to use the software package.

A Mechanism for the Generation of a Warm SST Anomaly in the Western Equatorial Pacific: A Pathway Perspective

AbstractProcesses leading to the onset and development of an El Niño event in the tropical Pacific remain elusive. Observed data and Ocean General Circulation Model (OGCM) simulations are used to reveal a well‐defined pattern of sea surface temperature (SST) perturbations along the mean North Equatorial Countercurrent (NECC) pathways in association with the onset and evolution of some El Niño events. The OGCM‐based sensitivity experiments are conducted to illustrate how a warm SST anomaly (SSTA) on the equator can result from a thermal forcing that is prescribed north of 10°N, similar to observed SST anomalies in December 1988. Within approximately one year, the imposed SST anomaly north of 10°N tends to be transported to the dateline region on the equator by the mean ocean circulation in the western Pacific (the low‐latitude western boundary current and the NECC). In due course, an upper‐layer ocean warming is generated off the equator at 6–10°N and then on the equator, which acts to induce a westerly wind anomaly response; a simple statistical atmospheric wind stress model is then used to depict an expected westerly wind response. These resultant SST and surface wind perturbations can couple together over the western tropical Pacific, forming air‐sea interactions and setting up a stage for El Niño onset. As such, this pathway mechanism can reasonably well explain the first appearance of a warm SST anomaly on the equator in the dateline region and the corresponding development of westerly wind anomalies over the western Pacific in association with El Niño onset.