A document by Kezhou Lu. Click on the document to view its contents.

A single column model with parameterized large-scale dynamics is used to better understand the response of steady-state tropical precipitation to relative sea surface temperature under various representations of radiation, convection, and circulation. The large-scale dynamics are parametrized via the weak temperature gradient (WTG), damped gravity wave (DGW), and spectral weak temperature gradient (Spectral WTG) method in NCAR’s Single Column Atmosphere Model (SCAM6). Radiative cooling is either specified or interactive, and the convective parameterization is run using two different values of a parameter that controls the degree of convective inhibition. Results are interpreted in the context of the Global Atmospheric System Studies (GASS) Intercomparison (Daleu et al. 2016). Using the settings given in Daleu et al. (2016), SCAM6 under the WTG and DGW methods produces erratic results, suggestive of numerical instability. However, when key parameters are changed to weaken the strength with which the circulation acts to eliminate tropospheric temperature variations, SCAM6 performs comparably to single column models in the GASS Intercomparison. The Spectral WTG method is less sensitive to changes in convection and radiation than are the other two methods, performing at least qualitatively similarly across all configurations considered. Under all three methods, circulation strength, represented in 1D by grid-scale vertical velocity, is decreased when barriers to convection are reduced. This effect is most extreme under specified radiative cooling, and is shown to come from increased static stability in the column’s reference radiative-convective equilibrium profile. This argument can be extended to interactive radiation cases as well, though perhaps less conclusively.

Canonical understanding based on general circulation models (GCMs) is that the atmospheric circulation response to midlatitude sea-surface temperature (SST) anomalies is weak compared to the larger influence of tropical SST anomalies. However, the horizontal resolution of modern GCMs, ranging from roughly 300 km to 25 km, is too coarse to fully resolve mesoscale atmospheric processes such as weather fronts. Here, we investigate the large-scale atmospheric circulation response to idealized Gulf Stream SST anomalies in Community Atmosphere Model (CAM6) simulations with 14-km regional grid refinement over the North Atlantic, and compare it to the response in simulations with 28-km regional refinement and uniform 111-km resolution. The highest resolution simulations show a large positive response of the wintertime North Atlantic Oscillation (NAO) to positive SST anomalies in the Gulf Stream, a 0.8-standard-deviation anomaly in the seasonal-mean NAO for 2°C SST anomalies. The lower-resolution simulations show a weaker response with a different spatial structure. The enhanced large-scale circulation response results from an increase in resolved vertical motions with resolution and an associated increase in the influence of SST anomalies on transient-eddy heat and momentum fluxes in the free troposphere. In response to positive SST anomalies, these processes lead to a stronger North Atlantic jet that varies less in latitude, as is characteristic of positive NAO anomalies. Our results suggest that the atmosphere responds differently to midlatitude SST anomalies in higher-resolution models and that regional refinement in key regions offers a potential pathway to improve multi-year regional climate predictions based on midlatitude SSTs.

By injecting SO2 into the stratosphere at four latitudes (30°, 15° N/S), it might be possible not only to reduce global mean surface temperature but also to minimize changes in the equator-to-pole and inter-hemispheric gradients of temperature, further reducing some of the impacts arising from climate change relative to equatorial injection. This can happen only if the aerosols are transported to higher latitudes by the stratospheric circulation, ensuring that a greater part of the solar radiation is reflected back to space at higher latitudes, compensating for the reduced sunlight. However, the stratospheric heating produced by these aerosols modifies the circulation and strengthens the stratospheric polar vortex which acts as a barrier to the transport of air toward the poles. We show how the heating results in a feedback where increasing injection rates lead to stronger high-latitudinal transport barriers. This implies a potential limitation in the high-latitude aerosol burden and subsequent cooling.

North Atlantic sea-surface temperatures (SSTs) exhibit variability on seasonal to decadal timescales, providing a potential source of predictability for the atmospheric circulation and regional climate on these timescales. Recent work has shown that initialized climate models have skill in predicting the decadal evolution of North Atlantic SSTs [1], but this will only help to predict regional climate in the surrounding continents if models can correctly simulate the atmospheric response to these SST anomalies. There is growing evidence that models systematically underestimate the atmospheric response to extratropical SST anomalies [2], and that this may be rectified by increasing the atmospheric resolution to resolve mesoscale processes over ocean frontal zones [3]. Here, we investigate the large-scale atmospheric circulation response to idealized Gulf Stream SST anomalies in two configurations of the Community Atmospheric Model (CAM6), one with 1-degree resolution globally and one with regional grid refinement of 1/8-degree over the North Atlantic. The variable resolution configuration, which resolves mesoscale atmospheric processes, shows a large negative response of the wintertime North Atlantic Oscillation (NAO) to a strengthening of the SST gradient across the Gulf Stream (a 2-standard-deviation NAO anomaly for SST anomalies that vary between ±2°C). The response is substantially weaker and has a different spatial structure in the lower resolution simulations. The large-scale atmospheric circulation response in the variable resolution simulations results from mesoscale processes that enhance convection over the Gulf Stream and lead to latent-heating and divergence anomalies in the upper troposphere. These results suggest that the atmospheric circulation response to extratropical SST anomalies may be fundamentally different at higher resolution. Regional refinement in key regions offers a potential pathway towards improving simulation of the atmospheric response to extratropical SST anomalies and thus improving multi-year regional climate predictions. [1] Yeager, S.G., et al., 2018, https://doi.org/10.1175/BAMS-D-17-0098.1. [2] Simpson, I.R., et al., 2018, https://doi.org/10.1175/JCLI-D-18-0168.1. [3] Czaja, A., et al., 2019, https://doi.org/10.1007/s40641-019-00148-5.

Many recent studies have confirmed that variability in the stratosphere is a significant source of surface sub-seasonal prediction skill during northern hemisphere winter. It may be beneficial, therefore, to think about times in which there might be windows-of-opportunity for skilful sub-seasonal predictions based on the initial or predicted state of the stratosphere. In this study, we propose a simple, minimal model that can be used to understand the impact of the stratosphere on tropospheric predictability. Our model purposefully excludes state dependent predictability in either the stratosphere or troposphere or in the coupling between the two. Model parameters are set up to broadly represent current sub-seasonal prediction systems by comparison with four dynamical models from the sub-seasonal to seasonal prediction project database. The model can reproduce the increases in correlation skill in sub-sets of forecasts for weak and strong stratospheric states over neutral states despite the lack of dependence of coupling or predictability on the stratospheric state. We demonstrate why different forecast skill diagnostics can give a very different impression of the relative skill in the three sub-sets. Forecasts with large stratospheric signals and low amounts of noise are demonstrated to also be windows-of-opportunity for skilful tropospheric forecasts, but we show that these windows can be obscured by the presence of unrelated tropospheric signals.

We examine the response of the Community Earth System Model versions 1 and 2 (CESM1 and CESM2) to abrupt quadrupling of atmospheric CO$_2$ concentrations (4xCO2) and to 1% annually increasing CO2 concentrations (1%CO2). Different estimates of equilibrium climate sensitivity (ECS) for CESM1 and CESM2 are presented. All estimates show that the sensitivity of CESM2 has increased by 1.5K or more over that of CESM1. At the same time the transient climate response (TCR) of CESM1 and CESM2 derived from 1%CO2 experiments has not changed significantly - 2.1K in CESM1 and 2.0K in CESM2. Increased initial forcing as well as stronger shortwave radiation feedbacks are responsible for the increase in ECS seen in CESM2. A decomposition of regional radiation feedbacks and their contribution to global feedbacks shows that the Southern Ocean plays a key role in the overall behavior of 4xCO2 experiments, accounting for about 50% of the total shortwave feedback in both CESM1 and CESM2. The Southern Ocean is also responsible for around half of the increase in shortwave feedback between CESM1 and CESM2, with a comparable contribution arising over tropical ocean. Experiments using a thermodynamic slab-ocean model (SOM) yield estimates of ECS that are in remarkable agreement with those from fully-coupled earth system model (ESM) experiments for the same level of CO2 increase. Finally, we show that the similarity of TCR in CESM1 and CESM2 masks significant regional differences in warming that occur in the 1%CO2 experiments for each model.

The 2021 Pacific Northwest heatwave featured record-smashing high temperatures, raising questions about whether extremes are changing faster than the mean, and challenging our ability to estimate the probability of the event. Here, we identify and draw on the strong relationship between the climatological higher-order statistics of temperature (skewness and kurtosis) and the magnitude of extreme events to quantify the likelihood of comparable events using a large climate model ensemble (CESM2-LE). In general, CESM2 can simulate temperature anomalies as extreme as those observed in 2021, but they are rare: temperature anomalies that exceed 4.5σ occur with an approximate frequency of one in a hundred thousand years. The historical data does not indicate that the upper tail of temperature is warming faster than the mean; however, future projections for locations with similar climatological moments to the Pacific Northwest do show significant positive trends in the probability of the most extreme events.