Investigation of jet indices in warm and cold seasons in the northern and southern hemispheres (1948-2023)

Narrow, rapidly flowing currents of air located near the tropopause are known as jet streams. These jets, often found nearly girdling the globe while exhibiting large meridional meanders, are among the most ubiquitous structural characteristics of Earth’s atmosphere and are known to play a substantial role in the production of sensible weather in the mid-latitudes. Jet streams are classified into two different types subtropical jet and polar-front jet streams. Subtropical jets are driven by angular momentum transport from the tropics and are centered at the poleward boundaries of the Hadley cell and polar-front jets are driven by baroclinic eddies and are associated with weather and climate events, such as precipitation and cold wave processes.

To study whether jet streams have been changing in the past decades, we used the historical data of NCEP/NCAR from the National Centers for Environmental Protection and the National Center for Atmospheric Research, covering 1948 to 2023.We have used the daily means of u, v components, temperature and geopotential height, at 6 levels from 400 to 100 hPa and daily means temperature at 4 levels from 1000 to 700 hPa.

In this paper, we firstly calculated the wind speed index, pressure index and latitude index that were defined by Archer and Caldeira (2008) to characterize the strength, the pressure level, and the latitudinal position of a jet stream, respectively. Then after that the variability of the characteristics of jet streams of both hemispheres in warm and cold seasons were investigated.

In addition, the temperature trend in the upper and lower troposphere was determined by calculating the average seasonal temperatures over latitudinal zones 0°-15°, 15°-30°, 30°-45°, 45°-60°, 60°-75°, 75°-90° in northern and southern hemisphere at upper levels of 400, 300, 250, 200, 150, 100 hPa and lower levels of 1000, 925, 850, 700 hPa.

We found that, in general, the jet streams have moved poleward in both hemispheres so that the shift to the pole of subtropical jets in the northern hemisphere was 0.061 (0.019) °/decade in DJF (JJA) and in the southern hemisphere was 0.307 (0.223) °/decade in DJF (JJA) for 1948-2023.

Also of the southern hemisphere the subtropical jet in winter and summer is weakening but the polar front jet is strengthening. In the northern hemisphere, in both warm and cold seasons the trend of wind speed is increasing in the poleward side of the jet axis, and is decreasing in the southern side of its axis, while in the trend of the maximum wind speed is upward.

One of the important factors of these changes can be related to the upward temperature trend in the tropical and subtropical regions of both hemispheres.

In this statistical period, the smallness of the slope of the zonal mean of seasonal temperature trend line of the lower levels of the tropical region compared to the slope of the average temperature trend of the upper levels of the subtropical region has caused that the strengthening of the Hadley cell convection which is less than the weakening of its meridional circulation range, this can be one of the factors that lowered the trend of the jet streams speed in both hemispheres.

Also, the results showed that the mass-flux weighted pressure in the DJF season is minimum around the equator and in the JJA season around the orbit of 12°N from the west of Mauritania to the southeast of China is minimum.