Implications of Warm Pool Bias in CMIP6 Models on the Northern Hemisphere Wintertime Subtropical Jet and Precipitation

Although the multi-model average compares well with observations, individually most of the latest climate models do not simulate a realistic size of the Indo-Pacific Warm Pool in the present-day climate. This study explores the implications of this warm pool size bias in climate models in Northern Hemisphere winter. The warm pool size bias in phase 6 of the Coupled Model Intercomparison Project models is related to the subtropical jet and precipitation distribution, both in the present-day climate and in response to climate change, through extratropical Rossby wave trains and tropical circulation pathways. Based on these relationships, emergent constraints are developed to observationally constrain the future subtropical jet response over Asia and the Atlantic Ocean and precipitation response over North and Central America, which can help to reduce uncertainty in future projections of these features. Thus, accurate model simulation of the warm pool in the present-day climate is important for future projections of the subtropical jet and precipitation.

Regional characteristics of variability in the Northern Hemisphere wintertime polar front jet and subtropical jet in observations and CMIP6 models

Variability in the position and strength of the subtropical jet (STJ) and polar front jet (PFJ) streams has important implications for global and regional climate. Previous studies have related the position and strength of the STJ to tropical thermodynamic processes, whereas the position and strength of the PFJ are more associated with mid-latitude eddies. These conclusions have largely resulted from studies using idealized models.
In this study, ERA-Interim reanalysis and CMIP6 global climate models are used to examine month-to-month and interannual variability of the wintertime Northern Hemisphere (NH) STJ and PFJ. This study particularly focuses on the regional characteristics of the jet variability, extending previous studies on zonal-mean jet streams. Consistent with idealized modeling studies, a close relationship is found between tropical outgoing longwave radiation (OLR) and the STJ, and between mid-latitude surface temperature gradients and the PFJ. Variations of both jets are also linked to well-known teleconnection patterns.
Variations in tropical convection over the Pacific Ocean are associated with variations of the NH STJ at most longitudes, with different phases of the El Niño-Southern Oscillation (ENSO) associated with the shift and strengthening of the STJ in different regions. CMIP6 models generally capture these relationships, but the models’ tropical convection is often displaced westward when compared to observations, reflecting a climatological bias in OLR in the western tropical Pacific Ocean in many models. The displaced tropical convection in models excites different paths of Rossby wave propagation, resulting in different ENSO teleconnections on the STJ over North America and Europe.

Mapping diurnal aerosol properties in East Asia from Deep Space Climate Observatory

Diurnal variabilities of aerosol properties are important to quantify its impact on public health and climate at high temporal resolution. In this study, we firstly map the hourly aerosol optical depth (AOD) in East Asia using satellite observations from the Deep Space Climate Observatory (DSCOVR). Secondly, we compare our result with the ground AOD observations from AERONET. Both datasets show increasing trends during a day and a higher increase in eastern China. We also show a good correlation between AOD and bottom-up emissions. Our results shed light on future studies to investigate the diurnal variabilities of aerosol from geostationary pollutant observation satellite.

Radiative forcing and precipitation effect of changing ozone

Ozone change plays an important role in Earth’s energy budget and ozone is a significant gas to contribute to radiative forcing (RF) of climate. Tropospheric and stratospheric ozone trends are different and radiative forcing (RF) depends on the altitude of ozone perturbations, so it is important to separate them and show their different effects. For tropospheric ozone perturbation, longwave forcing dominates and longwave and shortwave forcing enhance each other; for stratospheric ozone perturbation, longwave and shortwave forcing counteract each other and contribute nearly equally. The precipitation effect of ozone change is also examined using the theory that global-mean fast precipitation response (FPR) is associated with atmospheric radiative forcing (RFatm) which is the fraction of RF that is directly felt by atmosphere and global-mean slow precipitation response (SPR) is associated with surface temperature change which takes a few decades to respond. In reality, from 1750 to 2011, tropospheric ozone RF leads to negative global-mean FPR and stratospheric ozone RF leads to mainly positive global-mean FPR but smaller than that due to tropospheric ozone. Both tropospheric and stratospheric ozone RF lead to positive global-mean SPR and SPR seems to take up more fraction of total precipitation change.