Jet Stream Dynamics Research Articles

Jet stream dynamics from a potential vorticity gradient perspective: The method and its application to a kilometre‐scale simulation

AbstractThe influence of adiabatic and diabatic processes on the midlatitude circulation is a formidable research question, especially considering their projected changes under global warming. This study presents the prospects, merits, and caveats of a potential vorticity (PV) gradient perspective as a means to disentangle the contributions of adiabatic and diabatic processes affecting the midlatitude circulation. Theoretical considerations reassess the link between the PV gradient and the jet stream. They reveal that the maximum isentropic PV gradient is consistently located on the stratospheric side of the jet, whereas the gradient of is shifted to the tropospheric side but, in general, is better aligned with the jet axis. The stratospheric shift of the PV gradient results from variations in stability across the tropopause, whereas the tropospheric shift of the gradient results from variations in vorticity. Regions of high PV gradient may serve as a proxy for the curvature of the wind field in the case of sufficiently small variations in stability. Otherwise, they depict variations in both wind and thermal stratification along tropopause‐intersecting isentropic surfaces. Lagrangian “PV gradient thinking” is demonstrated in two case studies of jet streak evolution in a simulation with 1.1 km grid spacing performed with the graphics‐processing‐unit‐enabled numerical weather prediction model Consortium for Small‐Scale Modelling featuring on‐line air parcel trajectories. Dry deformation drives the Lagrangian evolution of the PV gradient in the first case, whereas there is a pronounced influence of diabatic modification in the second case. The Lagrangian PV gradient perspective presented offers fresh insight into adiabatic and diabatic processes underlying the midlatitude circulation variability and change.

Probability Distribution of Extreme Events in a Baroclinic Wave Laboratory Experiment

Atmospheric westerly jet streams are driven by temperature differences between low and high latitudes and the rotation of the Earth. Meandering jet streams and propagating Rossby waves are responsible for the variable weather in the mid-latitudes. Moreover, extreme weather events such as heat waves and cold spells are part of the jet stream dynamics. For many years, a simple analog in the form of a simplified laboratory experiment, the differentially heated rotating annulus, has provided insight into the dynamics of the meandering jet stream. In the present study, probability density distributions of extreme events from a long-term laboratory experiment are studied and compared to the atmospheric probability density distributions. Empirical distributions of extreme value monthly block data are derived for the experimental and atmospheric cases. Generalized extreme value distributions are adjusted to the empirical distributions, and the distribution parameters are compared. Good agreement was found, but the distributions of the experimental data showed a shift toward larger extreme values, and some explanations for this shift are suggested. The results indicate that the laboratory model might be a useful tool for investigating changes in extreme event distributions due to climate change. In the laboratory context, the change can be modeled by an increase in total temperature accompanied by a reduction in the radial heat contrast.

Atmospheric jet stream variability reflects vegetation activity in Europe

Jet streams are a key component of the climate system, whose dynamics couple closely to regional climate variability. Yet, the link between jet stream variability and vegetation activity has received little attention. Here, we leverage our understanding of the mid-latitude jet stream dynamics over the Euro-Atlantic sector to probe climate–vegetation interactions across Europe. We link indices related to the meridional location of the jet and the large-scale zonal wind speed with remotely-sensed vegetation greenness anomalies during locally-defined growing seasons. Correlations between greenness anomalies and jet latitude anomalies point to a control of the jet stream's variability on vegetation activity over large parts of Europe. This potential control is mediated by the jet latitude anomalies’ correlations with temperature, soil moisture and downward surface solar radiation. The sign and strength of these correlations depend on location and time of the year. Furthermore, jet stream variability modulates conditions at the onset and end of the growing season. The link between jet latitude anomalies and vegetation greenness is not only specific to the climate zone, but also to the landclass and subperiod within the growing season. It is thus important to use a locally-defined growing season for interpreting the atmospheric controls on regional vegetation phenology. Results consistent with the correlation analysis emerge when focussing on local high or low greenness months only or on zonal wind speed anomalies, confirming the relevance of jet variability for vegetation activity.

Seasonality in the Response of East Asian Westerly Jet to the Mid‐Holocene Forcing

AbstractThe East Asian westerly jet (EAWJ) features large seasonal variations in its intensity and position and exerts profound influences on local precipitation. In this study, we found the amplified seasonality of the EAWJ in response to the mid‐Holocene forcing. Compared with the preindustrial, the summer EAWJ significantly weakens by 20 ± 4% and shifts northward by 4 ± 2° during the mid‐Holocene, while the winter EAWJ slightly strengthens by less than 5%. Two different mechanisms are proposed to explain the significant summer weakening. From the perspective of stationary waves, the deficient precipitation over the tropical northwest Pacific produces the anomalous cyclonic circulations in the upper troposphere. The easterly winds north of the cyclone act to decelerate the EAWJ. In terms of transient processes, the mid‐Holocene reduced midlatitude meridional temperature gradient suppresses the synoptic‐scale disturbances through changes in the atmospheric baroclinicity. The subsequent decrease in kinetic energy transformation from transient eddies to the mean flow contributes to the weakening of the EAWJ. The reduction of the meridional temperature gradient is related to the subtropical cooling, which is possibly caused by the excessive monsoon rainfall and enhanced convection over East and South Asia. Both mechanisms involve the influence of low‐latitude precipitation on the midlatitude jet stream behavior, which deserves more attention in future studies of the jet stream dynamics.

Synchronous Modes of Terrestrial and Marine Productivity in the North Pacific

The primary productivity of adjacent terrestrial and marine ecosystems can display synchronous responses to climate variability. Previous work has shown that this behavior emerges along the California coast where internal modes of climate variability, such as El Nino Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), alter jet stream dynamics that influences marine ecosystems through upwelling and terrestrial ecosystems through precipitation. This study assesses whether marine-terrestrial synchrony is a widespread phenomenon across the North Pacific by utilizing satellite-derived Solar-Induced Fluorescence (SIF) and chlorophyll-a as proxies for land and sea productivity, respectively. The results show that terrestrial and marine ecosystems are consistently synchronized across 1000's of kms of the North Pacific coastline. This synchrony emerges because both marine and terrestrial ecosystems respond to climate modes with a similar north-south dipole pattern that is mirrored across the coastal interface. The synchrony is disrupted when PDO and ENSO oppose one another because the terrestrial north-south dipole follows the PDO while the marine pattern follows ENSO. The consequence is marine and terrestrial productivity anomalies that are opposite one another along adjacent regions of the coastline. If ENSO and the PDO have shared low-frequency variance, then synchrony would be the dominant state despite local topographic and trophic diversity along the coastlines. This result suggests that climate proxy stacks that include biotically sensitive marine and terrestrial proxies would have a selective sensitivity to modes such as PDO that drive synchrony. Lastly, the coupling of land and sea productivity may have the effect of generating amplified regional responses of the carbon budget to climate variability by simultaneously enhancing the terrestrial and marine carbon sinks.

Jet stream dynamics, hydroclimate, and fire in California from 1600 CE to present

Moisture delivery in California is largely regulated by the strength and position of the North Pacific jet stream (NPJ), winter high-altitude winds that influence regional hydroclimate and forest fire during the following warm season. We use climate model simulations and paleoclimate data to reconstruct winter NPJ characteristics back to 1571 CE to identify the influence of NPJ behavior on moisture and forest fire extremes in California before and during the more recent period of fire suppression. Maximum zonal NPJ velocity is lower and northward shifted and has a larger latitudinal spread during presuppression dry and high-fire extremes. Conversely, maximum zonal NPJ is higher and southward shifted, with narrower latitudinal spread during wet and low-fire extremes. These NPJ, precipitation, and fire associations hold across pre-20th-century socioecological fire regimes, including Native American burning, postcontact disruption and native population decline, and intensification of forest use during the later 19th century. Precipitation extremes and NPJ behavior remain linked in the 20th and 21st centuries, but fire extremes become uncoupled due to fire suppression after 1900. Simulated future conditions in California include more wet-season moisture as rain (and less as snow), a longer fire season, and higher temperatures, leading to drier fire-season conditions independent of 21st-century precipitation changes. Assuming continuation of current fire management practices, thermodynamic warming is expected to override the dynamical influence of the NPJ on climate-fire relationships controlling fire extremes in California. Recent widespread fires in California in association with wet extremes may be early evidence of this change.

Arctic-midlatitude weather linkages in North America

There is intense public interest in whether major Arctic changes can and will impact midlatitude weather such as cold air outbreaks on the central and east side of continents. Although there is progress in linkage research for eastern Asia, a clear gap is conformation for North America. We show two stationary temperature/geopotential height patterns where warmer Arctic temperatures have reinforced existing tropospheric jet stream wave amplitudes over North America: a Greenland/Baffin Block pattern during December 2010 and an Alaska Ridge pattern during December 2017. Even with continuing Arctic warming over the past decade, other recent eastern US winter months were less susceptible for an Arctic linkage: the jet stream was represented by either zonal flow, progressive weather systems, or unfavorable phasing of the long wave pattern. The present analysis lays the scientific controversy over the validity of linkages to the inherent intermittency of jet stream dynamics, which provides only an occasional bridge between Arctic thermodynamic forcing and extended midlatitude weather events.

Tropopause folds in ERA‐Interim: Global climatology and relation to extreme weather events

AbstractTropopause folds are intimately linked to upper level frontogenesis and jet stream dynamics. They play an important role for stratosphere‐troposphere exchange, the dynamical coupling of upper and lower tropospheric levels, and for generating severe weather events. This study presents a global climatology of tropopause folds using ERA‐Interim reanalysis data from 1979 to 2012 and a refined version of a previously developed 3‐D labeling and fold identification algorithm. This algorithm objectively separates stratospheric and tropospheric air in complex situations, e.g., in regions with strong low‐level inversions, and in extratropical cyclones where diabatically generated potential vorticity anomalies typically occur. Three classes of tropopause folds are defined (shallow, medium, and deep), and their geographical distribution, vertical extent, and seasonal cycle are investigated. Most shallow folds occur along the subtropical jet stream, in agreement with previous studies. Hot spots of medium and deep tropopause folds are found west of Australia and along the coast of Antarctica in the Southern Hemisphere and around the east coast of North America in the Northern Hemisphere. Seasonal cycles show maxima in winter for all fold classes. Medium and deep folds are frequently associated with surface wind gust and precipitation extremes, as quantified for folds over the southern Indian Ocean. Wind gust extremes occur mainly in an elongated band upstream and equatorward of folds, whereas precipitation extremes occur mainly east and poleward of folds. Overall, in the considered region, about 20% of medium folds and 33% of deep folds are associated with surface wind or precipitation extremes in the vicinity of the fold.

Bracketing Climatological Mid-tropospheric Temperatures in the Northern Hemisphere: An Observational Study 1979-2013

We examine mid-tropospheric temperatures in the Northern Hemisphere using the 500 mb pressure surface from reanalysis data as a representative level. This standard analysis level is significant meteorologically (i.e. for frontal identification and jet stream dynamics) and climatologically (e.g. changes in long term front and jet structures would be expected to extend throughout the troposphere as would tropospheric warming). We find that 500 mb temperatures are bracketed between about –42°C and –3°C with very few excursions beyond these brackets suggesting a limiting physical process or processes. In this paper we update the data for the –42°C limit which we have proposed in previous papers, document the –3°C limit for the first time, and briefly discuss the possible physical mechanisms responsible for this observed temperature bracketing concluding that the limits on both maximum and minimum temperatures are due to convective processes. This self-regulation of tropospheric temperatures constrains changes in jet stream and baroclinic storm dynamics and therefore constrains changes in climate variability.

Climatology of Upper Tropospheric–Lower Stratospheric (UTLS) Jets and Tropopauses in MERRA

Abstract A global climatology (1979–2012) from the Modern-Era Retrospective Analysis for Research and Applications (MERRA) shows distributions and seasonal evolution of upper tropospheric jets and their relationships to the stratospheric subvortex and multiple tropopauses. The overall climatological patterns of upper tropospheric jets confirm those seen in previous studies, indicating accurate representation of jet stream dynamics in MERRA. The analysis shows a Northern Hemisphere (NH) upper tropospheric jet stretching nearly zonally from the mid-Atlantic across Africa and Asia. In winter–spring, this jet splits over the eastern Pacific, merges again over eastern North America, and then shifts poleward over the North Atlantic. The jets associated with tropical circulations are also captured, with upper tropospheric westerlies demarking cyclonic flow downstream from the Australian and Asian monsoon anticyclones and associated easterly jets. Multiple tropopauses associated with the thermal tropopause “break” commonly extend poleward from the subtropical upper tropospheric jet. In Southern Hemisphere (SH) summer, the tropopause break, along with a poleward-stretching secondary tropopause, often occurs across the tropical westerly jet downstream of the Australian monsoon region. SH high-latitude multiple tropopauses, nearly ubiquitous in June–July, are associated with the unique polar winter thermal structure. High-latitude multiple tropopauses in NH fall–winter are, however, sometimes associated with poleward-shifted upper tropospheric jets. The SH subvortex jet extends down near the level of the subtropical jet core in winter and spring. Most SH subvortex jets merge with an upper tropospheric jet between May and December; although much less persistent than in the SH, merged NH subvortex jets are common between November and April.

Corrigendum to "Jet stream wind power as a renewable energy resource: little power, big impacts" published in Earth Syst. Dynam., 2, 201–212, 2011

Fig. 4. Sensitivity of extracted kinetic energy from jet streams Pex and total atmospheric dissipation Dn to the additional drag Cex imposed by wind turbines. Fig. 5. Sensitivity of jet stream dynamics to the intensity of kinetic energy extraction Cex from jet streams with vjet = 25 m s −1 in terms of (a) the mean uand v-wind velocities at 200 hPa and (b) the dissipation rate within those atmospheric regions at which the wind velocity is > 25 m s−1.

Surface Temperature Gradients as Diagnostic Indicators of Midlatitude Circulation Dynamics

Abstract Zonal and meridional surface temperature gradients are considered to be determinants of large-scale atmospheric circulation patterns. However, there has been limited investigation of these gradients as diagnostic aids. Here, the twentieth-century variability in the Northern Hemisphere equator-to-pole temperature gradient (EPG) and the ocean–land temperature contrast (OLC) is explored. A secular trend in decreasing EPG and OLC is noted. Decadal and interannual (ENSO-related) variations in the joint distribution of EPG and OLC are identified, hinting at multistable climate states that may be indigenous to the climate or due to changing boundary forcings. The NH circulation patterns for cases in the tails of the joint distribution of EPG and OLC are also seen to be different. Given this context, this paper extends past efforts to develop insights into jet stream dynamics using the Lorenz-1984 model, which is forced directly and only by EPG and OLC. The joint probability distribution of jet stream and eddy energy, conditional on EPG and OLC scenarios, is investigated. The scenarios correspond to (i) warmer versus colder climate conditions and (ii) polarized ENSO phases. The latter scenario involves the use of a heuristic ENSO model to drive the Lorenz-1984 model via a modulation of the EPG or the OLC. As with GCMs, the low-order model reveals that the response to El Niño forcing is not similar to an anthropogenic warming signature. The potential uses of EPG and OLC as macro-level indicators of climate change and variability and for comparing results across GCMs and observations are indicated.

Characterizing the North Pacific Jet Stream for Understanding Historical Variability in Western United States Winter Precipitation

We examined historical winter-season relationships among precipitation across the western United States, the northern Pacific jet stream (NPJ) across the northern Pacific Ocean basin, and the El Niño-Southern Oscillation (ENSO) phenomenon to determine if the NPJ significantly adds to the existing ENSO-based understanding of inter-annual variability in precipitation. The results indicate that the NPJ is significantly related to winter precipitation across a much larger area of the western United States than the ENSO signal. For areas where the ENSO signal is significantly related to precipitation, there generally exists a stronger or more consistent NPJ-precipitation relationship. The position of the NPJ is the most important characteristic of the jet in its relationship with precipitation across the western United States. However, winter precipitation across the northwest, especially for regions of higher elevation, is more significantly related to the strength of the NPJ. The results indicate that there is value in considering seasonal jet stream characteristics as independent variables, along with climate teleconnections, when considering seasonal precipitation variability in regions where large-scale jet stream dynamics are influential.

A 40‐year climatological study of relative vorticity distribution over the Mediterranean

AbstractA climatological study of the spatial and temporal distribution of mean relative geostrophic vorticity over the Mediterranean region is examined for a period of 40 years (1958–1997). Two major cyclonic vorticity maxima are found in the Mediterranean at low levels, one covers the Gulf of Genoa and south Italy and the second is located over the southeastern Aegean Sea with different diurnal and seasonal characteristics, controlling cyclone formation. At upper levels, a large scale cyclonic vorticity zone covers the whole Mediterranean region with notably increased values at 300 hPa, being associated with jet stream dynamics, especially in the eastern Mediterranean. The seasonal changes of the horizontal vorticity distribution substantially reduce with height while the diurnal changes become unimportant at upper levels. Furthermore, the investigation of the vertical vorticity structure separately for the main cyclonic centres in the Mediterranean implies the different mechanisms that are responsible for cyclone formation in the western and eastern basin. Copyright © 2001 Royal Meteorological Society

Simulations of the 14 July 1987 squall line using a fully compressible model

Abstract Numerical simulations of the severe squall line of 14 July 1987 are discussed within the context of semi‐Lagrangian and semi‐implicit integrations. The fully compressible non‐hydrostatic Euler set of equations constitutes the basic dynamical framework of the numerical model. With elementary precipitation physics and with a generalized treatment of lateral boundary conditions, nested integrations simulate the observed structure of the squall line. The numerical solution and observations show a well organized precipitation system including a mesoscale fast‐propagating prefrontal squall line and a slow‐propagating system moving with the synoptic wave. The prefrontal squall line is seen to be a manifestation of the organization of an inertia‐gravity wave and has characteristics of a wave‐CISK mechanism. Owing to the interaction between the upper jet stream dynamics and moisture, the prefrontal perturbation is initiated locally and subsequently irradiates away from its point source. The high computational efficiency and the accuracy of the model emphasize its potential and demonstrate its value as an interesting tool for mesoscale modelling.

Circulations of the West Pacific Jet Stream

Abstract The jet stream of the west Pacific is a very steady feature of the winter circulation, with almost continuousintensification over East China and Japan and weakening in the central Pacific. In this study the National Meteorological Center spectral forecast model with normal mode initialization is used as a tool to provide diagnostic elements in a region of nonstandard data sources. Cross sections transverse to the jet stream in its intensification zone show a deep single-celled direct solenoidal circulation around a single frontal zone to be producing large increases of kinetic energy. Farther east, in the central Pacific, a strong indirect solenoidal circulation produces a strong decrease of kinetic energy. The irrotational wind components at jet stream levels are suggestive of an inertial cycle as part of the jet stream dynamics. A simple model of a modified inertial cycle is integrated in which the geostrophicwind is allowed to vary as a function of the ageostrophic flow transverse to the jet a...