Future Summer Drying in the U.S. Corn Belt and the Role of Midlatitude Storm Tracks

During the summer, the Midwest United States, which covers the main US corn belt, has a net loss of surface water as evapotranspiration exceeds precipitation. The net moisture gain into the atmosphere is transported out of the region to northern high latitudes through transient eddy moisture fluxes. How this process may change in the future is not entirely clear despite the fact that the corn belt region is responsible for a large portion of the global supply of corn and soybeans. We find that increased CO2 and the associated warming increases evapotranspiration. while precipitation reduces in the region leading to further reduction in precipitation minus evaporation (P-E) in the future. At the same time, the poleward transient moisture flux increases leading to enhanced atmospheric moistures export from the corn belt region. However, storm track intensity is generally weakened in the summer due to reduced north-south temperature gradient associated with amplified warming in the midlatitudes. The intensified transient eddy moisture transport as storm track weakens can be reconciled by the stronger mean moisture gradient in the future. This is found to be caused by the climatological low-level jet transporting more moisture into the Great Plains region due to the thermodynamic mechanism under warmer conditions. Our results, for the first time, show that in the future, the US Midwest corn belt will experience more hydrological stress due to intensified transient eddy moisture export leading to drier soils in the region.

Roles of vertical distributions of atmospheric transient eddy dynamical forcing and diabatic heating in midlatitude unstable air–sea interaction

Atmospheric transient eddy dynamical forcing (TEDF)-driven midlatitude unstable air–sea interaction has recently been recognized as a crucial positive feedback for the maintenance of the extratropical decadal variabilities. Our recent theoretical work (Chen et al., Clim Dyn 10.1007/s00382-020-05405-0, 2020) has characterized such an interaction through building an analytical midlatitude barotropic atmospheric model coupled to a simplified upper oceanic model. This study extends the analytical model to including a two-layer quasi-geostrophic baroclinic atmospheric model and then identifies the roles of vertical distributions of atmospheric TEDF and diabatic heating in midlatitude unstable air–sea interaction. It is found that midlatitude air–sea coupling with more realistic vertical profiles of atmospheric TEDF and diabatic heating destabilizes oceanic Rossby wave modes over the entire range of zonal wavelengths, in which the most unstable coupled mode features an equivalent barotropic atmospheric low (high) pressure over a cold (warm) oceanic surface. Spatial structure and period of the most unstable mode are more consistent with the observation than those from in previous model. Although either TEDF or diabatic heating alone can lead to a destabilized coupled mode, the former makes a dominant contribution to the instability. The increase of low-layer TEDF stimulates the instability more effectively if the TEDF in upper layer is larger than in lower layer, while the TEDF in either high or low layers can individually cause the instability. The surface heating always destabilizes the air–sea interaction, while the mid-level heating always decays the coupled mode. The results of this study further confirm the TEDF-driven positive feedback mechanism in midlatitude air–sea interaction proposed by recent observational and numerical experiment studies.

A nonlinear multi-scale theory of atmospheric blocking: Structure and evolution of blocking linked to meridional and vertical structures of storm tracks

This paper examines the impact of the meridional and vertical structures of a preexisting upstream storm track (PUST) organized by preexisting synoptic-scale eddies on eddy-driven blocking in a nonlinear multi-scale interaction model. In this model, the blocking is assumed, based on observations, to be comprised of barotropic and first baroclinic modes, whereas the PUST consists of barotropic, first baroclinic and second baroclinic modes. It is found that the nonlinearity (dispersion) of blocking is intensified (weakened) with increasing amplitude of the first baroclinic mode of the blocking itself. The blocking tends to be long-lived in this case. The lifetime and strength of blocking are significantly influenced by the amplitude of the first baroclinic mode of blocking for given basic westerly winds (BWWs), whereas its spatial pattern and evolution are also affected by the meridional and vertical structures of the PUST.It is shown that the blocking mainly results from the transient eddy forcing induced by the barotropic and first baroclinic modes of PUST, whereas its second baroclinic mode contributes little to the transient eddy forcing. When the PUST shifts northward, eddy-driven blocking shows an asymmetric dipole structure with a strong anticyclone/weak cyclone in a uniform BWW, which induces northward-intensified westerly jet and storm track anomalies mainly on the north side of blocking. However, when the PUST has no meridional shift and is mainly located in the upper troposphere, a north-south anti-symmetric dipole blocking and an intensified split jet with maximum amplitude in the upper troposphere form easily for vertically varying BWWs without meridional shear.

Interdecadal changes in synoptic transient eddy activity over the Northeast Pacific and their role in tropospheric Arctic amplification

Arctic amplification refers to the greater surface warming of the Arctic than of other regions during recent decades. A similar phenomenon occurs in the troposphere and is termed “tropospheric Arctic amplification” (TAA). The poleward eddy heat flux and eddy moisture flux are critical to Arctic warming. In this study, we investigate the synoptic transient eddy activity over the North Pacific associated with TAA and its relationship with the subtropical jet stream, and propose the following mechanism. A poleward shift of the subtropical jet axis results in anomalies of the meridional gradient of zonal wind over the North Pacific, which drive a meridional dipole pattern of synoptic transient wave intensity over the North Pacific, referred to as the North Pacific Synoptic Transient wave intensity Dipole (NPSTD). The NPSTD index underwent an interdecadal shift in the late 1990s accompanying that of the subtropical jet stream. During the positive phase of the NPSTD index, synoptic eddy heat flux transports more heat to the Arctic Circle, and the eddy heat flux diverges, increasing Arctic temperature. This mechanism highlights the need to consider synoptic transient eddy activity over the North Pacific as the link between the mean state of the North Pacific subtropical upper jet and TAA.

The Lorenz Energy Cycle: Trends and the Impact of Modes of Climate Variability

<p>The atmospheric circulation is driven by heat transport from the tropics to the polar regions, implying energy conversions between available potential and kinetic energy through various mechanisms. The processes of energy transformations can be quantitatively investigated in the global climate system through the Lorenz energy cycle formalism. In this study, we examine these variations and the impacts of modes of climate variability on the Lorenz energy cycle by using reanalysis data from the Japanese Meteorological Agency (JRA-55). We show that the atmospheric circulation is overall becoming more energetic and efficient. For instance, we find a statistically significant trend in the eddy available potential energy, especially in the transient eddy available potential energy in the Southern Hemisphere. We find significant trends in the conversion rates between zonal available potential and kinetic energy, consistent with an expansion of the Hadley cell, and in the conversion rates between eddy available potential and kinetic energy, suggesting an increase in mid-latitudinal baroclinic instability. We also show that planetary-scale waves dominate the stationary eddy energy, while synoptic-scale waves dominate the transient eddy energy with a significant increasing trend. Our results suggest that interannual variability of the Lorenz energy cycle is determined by modes of climate variability. We find that significant global and hemispheric energy fluctuations are caused by the El Nino-Southern Oscillation, the Arctic Oscillation, the Southern Annular Mode, and the meridional temperature gradient over the Southern Hemisphere.</p>

What controls probability distribution of local wave activity in the midlatitudes?

<p>We examine probability distributions of <em>local wave activity</em> (LWA), a measure of the jet stream's meander, and factors that control them.  The observed column-mean LWA distributions exhibit significant seasonal, interhemispheric, and regional variations but are always positively skewed in the extratropics, and their tail often involves disruptions of the jet stream.  A previously derived 1D traffic flow model driven by observed spectra of transient eddy forcing qualitatively reproduces the shape of the observed LWA distribution.  It is shown that the skewed distribution emerges from nonlinearity in the zonal advection of LWA even though the eddy forcing is symmetrically distributed.  A slower jet and stronger transient and stationary eddy forcings, when introduced independently, all broaden the LWA distribution and increase the probability of spontaneous jet disruption.  Quasigeostrophic two-layer model also simulates skewed LWA distributions in the upper layer.  However, in the two-layer model both transient eddy forcing and the jet speed increase with an increasing shear (meridional temperature gradient), and their opposing influence leaves the frequency of jet disruptions insensitive to the vertical shear.  When the model's nonlinearity in the zonal flux of potential vorticity is artificially suppressed, it hinders wave-flow interaction and virtually eliminates reversal of the upper-layer zonal wind.  The study underscores the importance of nonlinearity in the zonal transmission of Rossby waves to the frequency of jet disruptions and associated weather anomalies. </p>

Roles of vertical distributions of atmospheric transient eddy dynamical forcing and diabatic heating in midlatitude unstable air-sea interaction

Atmospheric transient eddy dynamical forcing (TEDF)-driven midlatitude unstable air-sea interaction has recently been recognized as a crucial positive feedback for the maintenance of the extratropical decadal variabilities. Our previous theoretical work by Chen et al. (2020) characterizes such an interaction with building an analytical midlatitude barotropic atmospheric model coupled to a simplified upper oceanic model. This study firstly extends the analytical model to a two-layer quasi-geostrophic baroclinic atmospheric model coupled to a simplified upper oceanic model and then identifies the roles of vertical distributions of atmospheric TEDF and diabatic heating in midlatitude unstable air-sea interaction. It is found that the midlatitude air-sea coupling through atmospheric TEDF and diabatic heating with more realistic vertical profile destabilizes the oceanic Rossby wave mode over the entire range of zonal wavelengths, and the most unstable mode exhibits an equivalent barotropic structure with geopotential lows (highs) over cold (warm) water. The spatial configuration structure and period of the most unstable coupled mode are more consistent with the observation than those from the previous model. Although either TEDF or diabatic heating alone can lead to unstable air-sea interaction, the former is dominant to the instability. TEDF in both higher and lower layers can cause unstable coupled mode individually, while the lower-layer forcing stimulates instability more effectively. Surface diabatic heating always destabilizes the coupled mode, while the mid-level heating always decays the coupled mode. Moreover, the influences of oceanic adjustment processes, air-sea coupling strength and background zonal wind on the unstable coupled mode are also discussed. The results of this study further prove the TEDF-driven positive feedback mechanism in midlatitude air-sea interaction proposed by recent observational and numerical experiment studies.