Study of the jet stream over India and to its north in winter: Part I

Characteristics of the jet streams over India and to its north in winter were studied with the daily vertical cross sections (1200 GMT) along 75°E from 8oN to 60°N for the period I to 15 February 1967, It was observed that there are three separate jet cores present in this latitl1de belt on most of the days, located on an average at 43°N, 31°N and 23°N. of these three, the most stable and persistent one is the second which is located between Delhi and Srinagar, at 200-mb level with an average maximum speed of 140-150 kt. The one to its south is weaker and quite variable in location as well as altitude. The jet at 31°N, therefore, has been called the primary sl1b-tropical jet over India and its characteristics studied. Based on this study, a. model cross-section has been. prepared for this STJ, The descriptions of the STJ at 23°N and also of PFJ (Polar Front Jet) at 43°N are included.

On the Drivers of Temperature Extremes on the Antarctic Peninsula During Austral Summer

On the basis of surface air temperature (SAT) observations from the Great Wall Station located on the Antarctic Peninsula (AP) and ERA-Interim reanalysis data, the present manuscript investigates the role of atmospheric flow at intraseasonal and synoptic time scales in driving the temperature extremes over the AP during austral summer. Both warm and cold events can persist for multiple days and were maintained mainly by the advection of seasonal air temperature by intraseasonal winds. Synoptic winds can influence the temperature change around the peak time through their advection of seasonal temperature, thus determining the time of peak temperature anomalies. The generation of intraseasonal winds was closely associated with Rossby wave trains propagating along the polar front jet over the Atlantic sector of the Southern Ocean before the warm and cold events. The synoptic height anomalies before the warm events were also manifested as Rossby wave trains propagating along the polar front jet. However, synoptic Rossby wave trains were almost absent when the cold events occurred. Further analysis indicates that the weakened background flow during the cold events may have hindered the eastward travel of synoptic eddies. This study provides an important guidance for subseasonal to seasonal prediction on the AP.

The combined effect of two westerly jet waveguides on heavy haze in the North China Plain in November and December 2015

Severe haze occurred in the North China Plain (NCP) from November to December 2015, with a wide spatial range and long duration. In this paper, the combined effect of the anomalous stationary Rossby waves within two westerly jet waveguides on this haze event in the NCP is investigated based on observational visibility data and NCEP/NCAR reanalysis data. The results show that circulation anomalies in Eurasia caused by the propagation of anomalous stationary Rossby wave energy along two waveguides within the westerly jet originating from the Mediterranean were responsible for haze formation in the NCP. The Rossby waves propagated eastward along the subtropical westerly jet and the polar front jet, causing an anomalous anticyclone over the Sea of Japan and anticyclonic wind shear at 850 hPa over the NCP, which enhanced the anomalous descent in the middle and lower troposphere and subsequently resulted in a stable lower atmosphere. Furthermore, the anomalous stationary Rossby waves propagating along the polar front jet weakened the East Asia trough and Ural ridge and strengthened the anomalous southerly wind at 850 hPa over the coastal areas of eastern China, decelerating the East Asia winter monsoon. The above meteorological conditions modulated haze accumulation in November and December 2015. Meanwhile, continuous rainfall related to ascending motion due to Rossby wave propagation along the waveguide provided by the subtropical westerly jet occurred in a large area of southern China. The associated latent heat release acted as a heat source, intensifying the ascending motion over southern China so that the descending motion over the NCP was strengthened, favoring the maintenance of severe haze. This study elucidates the formation and maintenance mechanism of large-scale haze in the NCP in late fall and boreal winter.

The British–Baikal Corridor: A Teleconnection Pattern along the Summertime Polar Front Jet over Eurasia

The British–Baikal Corridor (BBC) pattern, a new teleconnection along the summertime upper-tropospheric polar front jet (PFJ), is investigated based on observational and reanalysis datasets. The BBC pattern consists of four geographically fixed centers over the west of the British Isles, the Baltic Sea, western Siberia, and Lake Baikal, respectively. It features a zonally oriented and meridionally confined wavelike structure with a zonal wavenumber 5, and it influences the climate along its route significantly. The BBC pattern forms from the trapped effect of the PFJ waveguide that is characterized by a strong meridional gradient of stratification. As a preferred dynamical mode inherent in the PFJ, it is maintained through the baroclinic energy conversion from the basic flow and the feedback forcing of high-frequency transient eddies. Meanwhile, its geographical location is determined by the barotropic energy conversion, which is sensitive to the configuration of the basic flow. The interannual variability of the BBC pattern is dominated by atmospheric internal dynamics considering its loose relation with immediate atmospheric external forcing. Further analyses suggest that the BBC pattern is excited by the active multiscale interactions among the climatological mean flow, the low-frequency flow, and the synoptic-scale transient eddies in the exit region of the North Atlantic jet, which may also determine the preferential upstream forcing region and anchor the BBC pattern geographically. Budget analyses on vorticity, temperature, and water vapor are performed to interpret the physical nature of the BBC pattern. The possible linkage to the North Atlantic Oscillation is also discussed.

Improvement in Prediction of the Arctic Oscillation with a Realistic Ocean Initial Condition in a CGCM

This study verifies the impact of improved ocean initial conditions on the Arctic Oscillation (AO) forecast skill by assessing the one-month lead predictability of boreal winter AO using the Pusan National University (PNU) coupled general circulation model (CGCM). Hindcast experiments were performed on two versions of the model, one does not use assimilated ocean initial data (V1.0) and one does (V1.1), and the results were comparatively analyzed. The forecast skill of V1.1 was superior to that of V1.0 in terms of the correlation coefficient between the predicted and observed AO indices. In the regression analysis, V1.1 showed more realistic spatial similarities than V1.0 did in predicted sea surface temperature and atmospheric circulation fields. The authors suggest the relative importance of the contribution of the ocean initial condition to the AO forecast skill was because the ocean data assimilation increased the predictability of the AO, to some extent, through the improved interaction between tropical forcing induced by realistic sea surface temperature (SST) and atmospheric circulation. In V1.1, as in the observation, the cold equatorial Pacific SST anomalies generated the weakened tropical convection and Hadley circulation over the Pacific, resulting in a decelerated subtropical jet and accelerated polar front jet in the extratropics. The intensified polar front jet implies a stronger stratospheric polar vortex relevant to the positive AO phase; hence, surface manifestations of the reflected positive AO phase were then induced through the downward propagation of the stratospheric polar vortex. The results suggest that properly assimilated initial ocean conditions might contribute to improve the predictability of global oscillations, such as the AO, through large-scale tropical ocean–atmosphere interaction.

The Impact of the East Asian Subtropical Jet and Polar Front Jet on the Frequency of Spring Persistent Rainfall over Southern China in 1997–2011

Spring persistent rainfall (SPR) over southern China has great impact on its society and economics. A remarkable feature of the SPR is high frequency. However, SPR frequency obviously decreases over the period of 1997–2011. In this study, the possible causes have been investigated from the perspective of the individual and concurrent effects of the East Asian subtropical jet (EASJ) and East Asian polar front jet (EAPJ). A close relationship is detected between SPR frequency and EASJ intensity (but not EAPJ intensity). Associated with strong EASJ, abundant water vapor is transported to southern China by the southwesterly flow, which may trigger the SPR. Additionally, frequencies of both strong EASJ and weak EAPJ events are positively correlated with SPR frequency. Further investigation of the concurrent effect indicates a significant positive correlation between the frequencies of SPR and the strong EASJ–weak EAPJ configuration. Associated with this configuration, southwesterly flow strengthens in the lower troposphere, while northerly wind weakens in the upper troposphere. This provides a dynamic and moist condition, as enhanced ascending motion and intensified convergence of abundant water vapor over southern China, which favors the SPR. All analyses suggest that the EASJ may play a dominant role in the SPR occurrence and that the EAPJ may play a modulation role. Finally, a possible mechanism maintaining the strong EASJ–weak EAPJ configuration is proposed. Significant cooling over the northeastern Tibetan Plateau may induce a cyclone anomaly in the upper troposphere, which could result in an accelerating EASJ and a decelerating EAPJ.