Meridional Tripole Mode of Winter Precipitation over the Arctic and Continental North Africa–Eurasia

Wintertime precipitation is vital to the growth of glaciers in the northern hemisphere. We find a tripole mode of precipitation (PTM), with each pole of the mode extending zonally over the eastern hemisphere roughly between 30°W and 120°E, and the positive/negative/positive structure for its positive phase extending meridionally from the Arctic to the continental North Africa–Eurasia. The large-scale dynamics associated with the PTM is explored. The positive phase of the PTM is associated with the negative while eastward-shifted phase of the North Atlantic Oscillation (NAO) and a zonal band of positive SST anomaly in the tropics, together with a narrowed Hadley cell and weakened Ferrel cell. While being north-eastward tilted and separated from their North Africa-Eurasia counterpart in the climatological mean, the upper-tropospheric westerly jets over the east Pacific and north Atlantic become extending zonally and shifting southward and hence form a circumpolar subtropical jet as a whole by connecting with the westerly jets over the North Africa-Eurasia. The enhanced zonal winds over the north Atlantic promote more synoptic-scale transient eddies which are waveguided by the jet streams. The polar vortex weakens and cold air dips southward from the North Pole. Further diagnosis of the E-vectors suggests that transient eddies have a positive feedback on the weakening of Ferrel cell. Opposite features are associated with the negative phase of the PTM. The reconstructed time series using multiple linear regression on the NAO index and the tropical SST averaged over 20°S– 20°N, can explain 62.4% of the variance of the original the original precipitation time series.

On the Formation of Tropopause Folds and Constituent Gradient Enhancement Near Westerly Jets

The role of differential advection in creating tropopause folds and strong constituent gradients near midlatitude westerly jets is investigated using the University of Wisconsin Non-hydrostatic Modeling System (UWNMS). Dynamical structures are compared with aircraft observations through a fold and subpolar jet (SPJ) during RF04 of the Stratosphere-Troposphere Analyses of Regional Transport (START08) campaign. The observed distribution of water vapor and ozone during RF04 provides evidence of rapid transport in the SPJ, enhancing constituent gradients above relative to below the intrusion. The creation of a tropopause fold by quasi-isentropic differential advection on the upstream side of the trough is described. This fold was created by a southward jet streak in the SPJ, where upper tropospheric air displaced the tropopause eastward in the 6-10 km layer, thereby overlying stratospheric air in the 3-6 km layer. The subsequent superposition of the subtropical and subpolar jets is also shown to result from quasi-isentropic differential advection.The occurrence of low values of ozone, water vapor, and potential vorticity on the equatorward side of the SPJ can be explained by convective transport of low-ozone air from the boundary layer, dehydration in the updraft, and detrainment of inertially-unstable air in the outflow layer. An example of rapid juxtaposition with stratospheric air in the jet core is shown for RF01. The net effect of upstream convective events is suggested as a fundamental cause of the strong constituent gradients observed in midlatitude jets. Idealized diagrams illustrate the role of differential advection in creating tropopause folds and constituent gradient enhancement.

Regime Behaviour in the Upper Stratosphere as a Precursor of Stratosphere- Troposphere Coupling of the Northern Hemisphere

<p>A flow regime index is constructed based on the November-December standard deviation of the Ertel’s potential vorticity (EPV) in the northern upper stratosphere at 1500 K (~40 km). The index reveals two flow regimes in both the stratosphere and the troposphere. In the stratosphere, the two flow regimes involve zonally asymmetric variability that is manifested by a modulation of the Aleutian High and distinct early-to-late winter development of the polar vortex. During the wide-jet regime, an anomalously strengthened, upright polar vortex is found in middle winter, which involves an equatorward shift of the surf zone in the middle to upper stratosphere, a poleward movement of the polar vortex axis, and a sharpening of the polar vortex edge, suggesting a dominant effect of Rossby wave breaking. During the narrow-jet regime, the vortex weakens at least a month earlier in association with enhanced large-scale PV mixing.</p><p>The upper stratospheric flow regimes also have detectable signal in the vicinity of the tropospheric westerly jets in middle winter. The tropospheric responses are also zonally asymmetric. During the wide-jet regime, the largest response is found over the North Pacific with a weakened, poleward shifted westerly jet over north America.  The circulation anomalies during the narrow-jet regime are most strong over the North Atlantic with a weakened, and equatorward shifted westerly jet there. The flow regimes also differ distinctively in their impacts on high-frequency variability downstream of the westerly jets and associated temperature variability. Given the flow regimes in the upper stratosphere leads the tropospheric response by one to two months, improved representation of upper stratospheric variability in climate models may offer more skillful prediction of long-range surface weather forecasts.</p>

An Objective Identification and Climatology of Upper-Tropospheric Jets near Atlantic Tropical Cyclones

An objective algorithm is developed for identifying jets in 200-hPa flow and applied to reanalysis data within 2000 km of Atlantic tropical cyclones (TCs) during 1979–2015. The resulting set of 16 512 jets is analyzed both qualitatively and quantitatively to describe the climatology of TC–jet configurations and jet behavior near TCs. Jets occur most commonly poleward of TCs within the 500–1000-km annulus, where TC outflow amplifies the background potential vorticity gradient. A rigorous clustering analysis is performed, resulting in statistically distinct clusters of jet traces that correspond to common configurations of large-scale flow near Atlantic TCs. The speed structure of westerly jets poleward of TCs is found to vary with location in the Atlantic basin, but acceleration of jets downstream of their closest approach to the TC due to interaction with the TC’s diabatic outflow is a consistent feature of these structures. In addition to the climatology developed here, this objectively constructed dataset of upper-tropospheric jets opens unique avenues for exploring TC–environment interactions and utilizing jets to quantitatively describe large-scale flow.

Emission, transport, and radiative effects of mineral dust from the Taklimakan and Gobi deserts: comparison of measurements and model results

The Weather Research and Forecasting Model with chemistry (WRF-Chem model) was used to investigate a typical dust storm event that occurred from 18 to 23 March 2010 and swept across almost all of China, Japan, and Korea. The spatial and temporal variations in dust aerosols and the meteorological conditions over East Asia were well reproduced by the WRF-Chem model. The simulation results were used to further investigate the details of processes related to dust emission, long-range transport, and radiative effects of dust aerosols over the Taklimakan Desert (TD) and Gobi Desert (GD). The results indicated that weather conditions, topography, and surface types in dust source regions may influence dust emission, uplift height, and transport at the regional scale. The GD was located in the warm zone in advance of the cold front in this case. Rapidly warming surface temperatures and cold air advection at high levels caused strong instability in the atmosphere, which strengthened the downward momentum transported from the middle and low troposphere and caused strong surface winds. Moreover, the GD is located in a relatively flat, high-altitude region influenced by the confluence of the northern and southern westerly jets. Therefore, the GD dust particles were easily lofted to 4 km and were the primary contributor to the dust concentration over East Asia. In the dust budget analysis, the dust emission flux over the TD was 27.2 ± 4.1 µg m−2 s−1, which was similar to that over the GD (29 ± 3.6 µg m−2 s−1). However, the transport contribution of the TD dust (up to 0.8 ton d−1) to the dust sink was much smaller than that of the GD dust (up to 3.7 ton d−1) because of the complex terrain and the prevailing wind in the TD. Notably, a small amount of the TD dust (PM2.5 dust concentration of approximately 8.7 µg m−3) was lofted to above 5 km and transported over greater distances under the influence of the westerly jets. Moreover, the direct radiative forcing induced by dust was estimated to be −3 and −7 W m−2 at the top of the atmosphere, −8 and −10 W m−2 at the surface, and +5 and +3 W m−2 in the atmosphere over the TD and GD, respectively. This study provides confidence for further understanding the climate effects of the GD dust.

Emission, transport and radiative effects of mineral dust from Taklimakan and Gobi Deserts: comparison of measurements and model results

The weather research and forecasting model with chemistry (WRF-Chem) was used to investigate a typical dust storm event that occurred from 18th to 23rd March 2010 and swept across almost all of China, Japan, and Korea. WRF-Chem captured the spatial and temporal variations in dust aerosols and the meteorological conditions over East Asia well, and the results were used to further investigate details of processes related to dust emission, long-range transport, and radiative effects of dust aerosols over the Taklimakan desert (TD) and Gobi desert (GD). Results showed that the differences of weather conditions and topography and surface types in dust source regions may lead to the differences of dust emission, uplift height and transport. The typical dust event over East Asia was classified into two main stages. In the first stage (18th–20th March), the GD was located in the warm zone in advance of a cold front. The enhanced convection increased momentum transfer in the middle and lower troposphere because of the instability in the atmosphere. Moreover, the GD is located in relatively flat, high altitude regions influenced by the confluence of the northern and southern westerly jets. Therefore, the GD dust transport was the primary contributor to the dust concentration over East Asia. The strength of the dust emission decreased greatly during the second stage (21st–23rd March). The TD dust emission contributed to the dust concentration over East Asia. Cold air was lifted over the Pamir Plateau and intruded into the Tarim basin causing a strong uplifting motion. The average TD dust emission flux was 27.2 ± 4.1 μg m−2 s−1. However, the transport contribution of the TD dust (1.1 ton day−1) to the dust sink was smaller than that of the GD dust (1.4 ton day−1) because of the complex terrain and the prevailing wind in the TD. It is noted that the TD is not the main source region in China but a small amount of the TD dust was lofted to more than 5 km and transported over greater distances under the influence of the westerly jets. Moreover, the radiative forcing induced by dust particles is estimated as −3 W m−2 and −7 W m−2 at the top of the atmosphere, −8 W m−2 and −10 W m−2 at the surface, and +5 W m−2 and +3 W m−2 in the atmosphere over the TD and GD, respectively. The study provided confidence for further understanding the climate effect of the TD and GD dust.

Equilibration of a Baroclinic Planetary Atmosphere toward the Limit of Vanishing Bottom Friction

This paper discusses whether and how a baroclinic atmosphere can equilibrate with very small bottom friction in a dry primitive equation general circulation model. The model is forced by a Newtonian relaxation of temperature to a prescribed temperature profile, and it is damped by a linear friction near the lower boundary. When friction is decreased by four orders of magnitude, kinetic energy dissipation by friction gradually becomes negligible, while “energy recycling” becomes dominant. In this limit kinetic energy is converted back into potential energy at the largest scales, thus closing the energy cycle without significant frictional dissipation. The momentum fluxes are of opposite sign in the upper and lower atmosphere: in the upper atmosphere, eddies converge momentum into the westerly jets; however, in the lower atmosphere, the eddies diverge momentum out of the westerly jets. The secondary circulation driven by the meridional eddy momentum fluxes thus acts to increase the baroclinicity of the westerly jet. This regime may be relevant for the Jovian atmosphere, where the frictional time scale may be much larger than the radiative damping time scale.

A Barotropic Model of the Angular Momentum–Conserving Potential Vorticity Staircase in Spherical Geometry

An idealized analytical model of the barotropic potential vorticity (PV) staircase is constructed, constrained by global conservation of absolute angular momentum, perfect homogenization of PV in mixing zones between (prograde) westerly jets, and the requirement of barotropic stability. An imposed functional relationship is also assumed between jet speed and latitudinal separation using a multiple of the “dynamical Rossby wave” Rhines scale inferred from the strength of westerly jets. The relative simplicity of the barotropic system provides a simple relation between absolute angular momentum and PV (or absolute vorticity). A family of solutions comprising an arbitrary number of jets is constructed and is used to illustrate the restriction of jet spacing and strength imposed by the constraints of global conservation of angular momentum and barotropic stability. Asymptotic analysis of the theoretical solution indicates a limiting ratio of jet spacing to the dynamical Rhines scale equal to the square root of 6, meaning that westerly jets are spaced farther apart than predicted by the dynamical Rhines scale. It is inferred that an alternative “geometrical” Rhines scale for jet spacing can be obtained from conservation of absolute angular momentum on the sphere if the strength of zonal jets is known from other considerations. Numerical simulations of the full (nonaxisymmetric) equations reveal a pattern of zonal jet evolution that is consistent with our construction of ideal PV staircases in spherical geometry (which can be considered as limiting cases), as well as with the asymptotic analysis of a geometrical Rhines scale. The evolution of the PV staircase originating from an upscale cascade of energy in the barotropic model is therefore seen to depend on conservation of energy (for the strength of jets) and conservation of absolute angular momentum (for the spacing and number of jets). Further analysis of the numerical results confirms a “Taylor identity” relating the flux of eddy potential vorticity to mean-flow acceleration. Eddy fluxes are responsible for the occasional transitions between mode number as well as for maintaining the sharp westerly jets against small-scale dissipation. Suggestions are made for extending the theoretical model to PV staircases that are asymmetric between hemispheres or with latitudinal variation of amplitude, as modeled in the shallow-water system.