The response of the polar jet to climate warming and rapid Arctic change is a leading uncertainty in changing general circulation and critical to the future of mid-latitude surface weather. Previous studies suggest that CMIP5-6 model projections fall into two groups reflecting different characteristics of climate change, especially in the North Atlantic. Here we present distinct warming patterns emerging by the late 21st century from the first two generations of the Community Earth System Model Large Ensemble (CESM-LE) and use daily diagnostics to assess associated changes in mid-latitude circulation. We show that the subsequent versions of CESM represent categorically different storylines of North Atlantic climate change. The first version of CESM-LE (CESM1-LE, hereafter LENS1) exhibits severe Arctic amplification (AA) along with minor reductions in jet waviness. In contrast, CESM2-LE (hereafter LENS2) presents subdued AA, a more pronounced North Atlantic warming hole, and a late-century climate dominated by upper-tropospheric tropical warming (UTW). Uniquely, in LENS2 during winter, the North Atlantic sector projects less warming in the Arctic than mid-latitude mid-troposphere. The projected North Atlantic jet is reinforced and poleward-shifted, with reduced sinuosity, blocking, and synoptic variability. The surface weather response includes greater precipitation over Northern Europe, more intense drying in the eastern Mediterranean, and a slower decline in cold extremes by late century compared to LENS1.

Reconstructing hydroclimate over the Common Era is essential for understanding the dominant mechanisms of precipitation change and improving climate model projections, which currently suggest Northeast Mexico will become drier in the future. Tree-ring reconstructions have suggested regional rainfall is primarily controlled by Pacific sea surface temperatures (SST). However, tree ring records tend to reflect winter-spring rainfall, and thus may not accurately record total annual precipitation. Using the first multiproxy speleothem record spanning the last millennium, combined with results from an atmospheric general circulation model, we demonstrate mean annual rainfall in Northeast Mexico is highly sensitive to Atlantic SST variability. Our findings suggest precipitation in Northeast Mexico may increase in the future in response to the relative warming of Tropical North Atlantic SSTs.

Similar to many areas of the globe, South America is under a monsoon regime. The Amazon rain-forest is one of the regions critical for this regime, known as the South American Monsoon System (SAMS). Besides being crucial to the Earth’s carbon budget, the Amazon is also important for SAMS development. During SAMS onset, the trade winds intensify and a northwesterly low-level jet, known as the South American Low Level Jet (SALLJ), is formed east of the Andes. Moisture originating in the Atlantic Ocean is recycled and reinvigorated over the Amazon and carried by the SALLJ to continental and subtropical South America. In this project, we analyze the projected changes in precipitation and extreme events of rainfall over South America by the end of the 21st century using the CESM Large Ensemble Project (LENS). LENS is able to reproduce the important elements of the SAMS. We find that wet season rainfall is projected to increase over the east coast and central South America. The SALLJ is projected to become stronger, especially during late wet season, and carry more moisture to subtropical South America. As a result, moisture convergence between the SALLJ and the South Atlantic Subtropical High increases, creating the conditions to rainfall over Central and Southeastern South America. Meanwhile, Amazon is projected to become dryer during both dry and wet seasons. As a result, rain-forest productivity is reduced during late dry season in Southern Amazon. As vegetation struggles to save water, evapotranspiration, and consequently surface latent heat flux are reduced. These factors have been suggested as crucial to SAMS initiation and their reduction is responsible for a delay in SAMS onset by the end of the century. Extreme wet events are projected to become more frequent, especially over Northeastern Brazil, Southern Brazil and Uruguay. As dry season rainfall is reduced in all of the regions analyzed, drought events are projected to become more frequent and longer over both the wettest (Northern Amazon) and the driest (Northeastern Brazil) regions analyzed.

The teleconnection between the Quasi-Biennial Oscillation (QBO) and the Arctic polar vortex is investigated using Coupled Model Intercomparison Project phase 6 (CMIP6) models. We use 14 CMIP6 models, reanalysis, three experiments with prescribed QBOs, one of which has no free polar stratospheric variability, and branched runs in which a QBO is imposed in runs previously devoid of a QBO. Each CMIP6 model underestimates the Holton-Tan effect (HTE), the weakening of the polar vortex with QBO easterlies in the lower stratosphere. To establish why, 850 Kelvin potential vorticity (PV) maps are used to study zonal asymmetries in the teleconnection. The QBO initiates the HTE by promoting equatorward (poleward) intrusion of high (low) PV over mid-latitude Asia (60°E-120°E). The presence of the PV intrusion in a model response is highly correlated with polar cap warming and the HTE. Models with stronger 10 hPa QBO amplitudes generally include the PV intrusion.

We develop a hierarchy of simplified ocean models for coupled ocean, atmosphere, and sea ice climate simulations using the Community Earth System Model version 1 (CESM1). The hierarchy has four members: a slab ocean model, a mixed-layer model with entrainment and detrainment, an Ekman mixed-layer model, and an ocean general circulation model (OGCM). Flux corrections of heat and salt are applied to the simplified models ensuring that all hierarchy members have the same climatology. We diagnose the needed flux corrections from auxiliary simulations in which we restore the temperature and salinity to the daily climatology obtained from a target CESM1 simulation. The resulting 3-dimensional corrections contain the interannual variability fluxes that maintain the correct vertical gradients of temperature and salinity in the tropics. We find that the inclusion of mixed-layer entrainment and Ekman flow produces sea surface temperature and surface air temperature fields whose means and variances are progressively more similar to those produced by the target CESM1 simulation. We illustrate the application of the hierarchy to the problem of understanding the response of the climate system to the loss of Arctic sea ice. We find that the shifts in the positions of the mid-latitude westerly jet and of the Inter-tropical Convergence Zone (ITCZ) in response to sea-ice loss depend critically on upper ocean processes. Specifically, heat uptake associated with the mixed-layer entrainment influences the shift in the westerly jet and ITCZ. Moreover, the shift of ITCZ is sensitive to the form of Ekman flow parameterization.