Convective forcing of the North American Monsoon anticyclone at intraseasonal and interannual time scales.

In the upper troposphere and lower stratosphere (UTLS), large-scale anticyclones associated with monsoons play major roles in tropospheric and stratospheric transport and mixing. To understand the forcing of the North American monsoon anticyclone (NAMA), this study examines the connection between precipitation over the tropics and subtropics of the North American longitude sector and the variability of the troposphere and lower stratosphere. Using ERA5 reanalysis and outgoing longwave radiation (OLR) data from 1979âAS2019, we assess the relationship at the intraseasonal time scale using pentad-mean time series. We show that OLR anomalies are correlated with circulation anomalies northwest and northeast of the region of precipitation. Decreased OLR (increased precipitation) corresponds to increased geopotential heights and anticyclonic circulation anomalies in the 300 - 100 hPa layer and an opposite response in the lower tropospheric 850 - 600 hPa layer. The results are consistent with the established theory of the Rossby wave response to latent heating. The increase in height, which is strongest near 150 hPa, indicates that increased precipitation is associated with a strengthened NAMA. UTLS temperatures also have significant correlations with OLR, with cold (warm) anomalies occurring above (below) the core of the anticyclonic anomaly consistent with large-scale balance. The vertical structure of geopotential and temperature anomalies is compared to simulations using an idealized general circulation model, which shows that such a vertical structure is a consistent response to diabatic heating. Correlations at the interannual time scale resemble those at the intraseasonal time scale, demonstrating that precipitation is related to the NAMA at both time scales.

A 22-Year Evaluation of Convection Reaching the Stratosphere Over the United States.

Stratosphere-reaching moist convection can significantly alter the dynamics, chemistry, and climate of the Earth system. This study seeks to add to the emerging understanding of the frequency, depth, and stratospheric impact of such events using 22 years (1996-2017) of ground-based radar observations in the contiguous United States. While most prior studies identify such storms using the temperature lapse-rate tropopause (LRT) as a troposphere-stratosphere boundary, this study is the first to identify convection that reaches into stratospheric air below the LRT (tropopause depressions, excluding folds) as well. It is found that tropopause depression (TD) overshooting and LRT overshooting occur at similar frequency over the United States, with TD overshooting being more episodic in nature than LRT overshooting. TD overshooting is also found more often throughout the cooler months of the year, while LRT overshooting dominates all overshooting in the summer months. Stratospheric residence of overshoot material, as estimated using trajectory calculations driven by large-scale winds, suggests that the vast majority of TD overshoot material does not remain in the stratosphere within 5 days downstream and rarely impacts altitudes more than 1 km above the LRT. Conversely, the majority of LRT overshoot material remains in the stratosphere downstream and routinely impacts altitudes >1 and >2 km above the tropopause.

Unsteady Vortex Behavior in the Asian Monsoon Anticyclone.

The Asian monsoon anticyclone (AMA), which is primarily driven by the latent heat released by monsoon precipitation, is one of the dominant features of the Northern Hemisphere summer circulation in the upper troposphere and lower stratosphere. Due to variations in the diabatic heating, interactions with Rossby waves propagating along the subtropical jet, and internal dynamics within the anticyclone, the circulation of the AMA is unsteady. Here we use the ERA-Interim dataset and trajectories computed with ERA-Interim winds to show that the AMA contains two or three distinct synoptic-scale subvortices 69% of the time, while a single circulation center is present only 23% of the time. More than three simultaneous subvortices are uncommon. Observed behaviors of the subvortices include 1) splitting of a single vortex into two vortices; 2) merger of two vortices into a single vortex; 3) vortex shedding in the eastward direction; 4) vortex shedding in the westward direction; and 5) formation, movement, and dissipation of a vortex. The evolution of the subvortices is closely tied to stirring and transport.

Stratospheric ozone over the United States in summer linked to observations of convection and temperature via chlorine and bromine catalysis.

We present observations defining (i) the frequency and depth of convective penetration of water into the stratosphere over the United States in summer using the Next-Generation Radar system; (ii) the altitude-dependent distribution of inorganic chlorine established in the same coordinate system as the radar observations; (iii) the high resolution temperature structure in the stratosphere over the United States in summer that resolves spatial and structural variability, including the impact of gravity waves; and (iv) the resulting amplification in the catalytic loss rates of ozone for the dominant halogen, hydrogen, and nitrogen catalytic cycles. The weather radar observations of ∼2,000 storms, on average, each summer that reach the altitude of rapidly increasing available inorganic chlorine, coupled with observed temperatures, portend a risk of initiating rapid heterogeneous catalytic conversion of inorganic chlorine to free radical form on ubiquitous sulfate-water aerosols; this, in turn, engages the element of risk associated with ozone loss in the stratosphere over the central United States in summer based upon the same reaction network that reduces stratospheric ozone over the Arctic. The summertime development of the upper-level anticyclonic flow over the United States, driven by the North American Monsoon, provides a means of retaining convectively injected water, thereby extending the time for catalytic ozone loss over the Great Plains. Trusted decadal forecasts of UV dosage over the United States in summer require understanding the response of this dynamical and photochemical system to increased forcing of the climate by increasing levels of CO2 and CH4.