Advances in Atmospheric Sciences. Climate Change in Subtropical Jetstream during

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1 Climate Change in Subtropical Jetstream during -0 Journal: Manuscript ID: AAS--0.R Manuscript Type: Original Article Date Submitted by the Author: n/a Complete List of Authors: B, Abish Joseph, Porathur; Cochin University of Science and Technology, Department of Atmospheric Sciences Johannessen, Ola

2 Page of Climate Change in Subtropical Jetstream during -0 B. Abish *,, P.V. Joseph, and Ola.M. Johannessen, Nansen Environmental Research Centre India, Kochi, India Nansen Environmental and Remote Sensing Center, Bergen, Norway Nansen Scientific Society, Bergen, Norway Department of Atmospheric Sciences, Cochin University of Science and Technology, Kochi, India * Corresponding Author abishb@gmail.com Abstract Study of six decades (-0) of reanalysis data reveals that the subtropical jet stream (STJ) of the southern (northern) hemisphere between longitudes 00 E and 0 E has weakened (strengthened) during both the boreal winter (JF) and summer (JA) seasons. Temperature of the upper troposphere of the midlatitudes had a warming trend in the southern hemisphere and a cooling trend in the northern hemisphere. Correspondingly, the north-south temperature gradient in the upper troposphere had decreasing trend in the southern hemisphere and increasing trend in the northern hemisphere, which affected the strength of the STJ through the thermal wind relation. We devised a method of isotach analysis in intervals of 0. ms - in vertical sections of hemispheric mean winds to study the climate change in STJ core wind speed and also core height and latitude. We found that the upper tropospheric cooling of the Asian midlatitudes has a role in the strengthening of the STJ over Asia. While in the rest of the globe the upper troposphere had a warming trend that weakened the STJ. Available studies showed that the midlatitude cooling of the upper troposphere over Asia is caused by anthropogenic aerosols (particularly sulphate aerosols) and the warming over the rest of the global midlatitude upper troposphere is due to increased green house gases in the atmosphere. Key words: STJ; climate change; upper troposphere; GHG; aerosols

3 Page of Introduction STJ are narrow bands of fast moving westerly winds often observed around 0 hpa levels on either sides of the equator (Krishnamurti ; Peixoto and Oort ; Bluestein ; Archer and Caldeira 0). The intensity of these zonal winds and temperature fields are related through the thermal wind relation and hence the strength and the latitudinal position of the STJ undergo large seasonal variability. In each hemisphere, the STJ is more intense during the winter season and closer to the equator as compared to the summer season (Schulman ; Peixoto and Oort ; Dima and Wallace 0; Galvin 0). The tropical Hadley circulation transfers heat and momentum from low to high latitudes, which has important influence on the STJ (Webster 0). Through this circulation, westerly angular momentum is transferred poleward to the subtropics (Mitas and Clement 0) which contribute to the strong westerly winds associated with STJ. The latitudinal position of the STJ denotes the poleward edge of the Hadley circulation. Recent studies have shown that the Hadley circulation has widened poleward (Hu and Fu 0; Lu et al. 0; Liu et al. ) and thereby shifting the position of the STJ (Fu et al. 0 ; Siedal et al. 0; Hudson ). Between and the poleward movement of the STJ was. latitude in the northern hemisphere (NH) and. latitude in the southern (SH) (Hudson, ). For the reasons not yet understood, such shifts in the core of the STJ and the northward extent of the Hadley cell due to climate change leads to the change in precipitation patterns causing broad societal impacts. Recently Sun () found that the weakening of the East Asian upper level jetstream and the extreme high temperatures over the Jianghuai - Jiangnan region of China is closely related. Abish et al. () studied the climate change and decadal variation of the strength of the Tropical Easterly Jetstream (TEJ) of the June to September season in which they showed that the weakening of

4 Page of the TEJ from to 0 was caused mainly by the cooling trend in the temperature of the Tibetan plateau region of the upper troposphere. Increased concentrations of anthropogenic sulphate aerosols have a possible role in this cooling. On the contrary, increase of Green house gases (GHG)) is projected to increase tropospheric temperature by 0. K per decade or 0. K per decade by including the forcing due to sulphate aerosols (Mitchell et al.). On that basis, our study aims to understand the climate change (-0) in the strength of the STJ during two seasons January-February (JF) and July-August (JA) in NH and SH. Using NCEP/NCAR reanalysed dataset of -0 we made a detailed analysis of the STJ of the monsoon hemisphere (MH) between 00 and 0 E longitudes and in the non-monsoon (0 E-0 E) hemisphere (NMH). Section gives details of the data used and section presents the results of this study. Conclusions of the analysis are given in section.. Data used The upper tropospheric monthly mean temperature and wind data obtained from NCEP/NCAR reanalysis at. latitude. longitude is used in this study. NCEP reanalysis data has vertical levels extending from the surface to ~ 0 km (Kalnay et al. ). The network of radiosonde stations in the southern hemisphere covered mostly by ocean is much less than in the northern hemisphere (Duree et al. 0). Over Asia, good data coverage in India started by and in China by with the introduction of wide network of upper air rawindsonde stations. From the Southern Hemisphere, there were considerable pilot balloon data inputs from South America, Australian etc (Kistler et al. 0). Even though reanalysis datasets are said to be affected by the inhomogenities in the observation systems (Bengtsson et al. 0; Bromwich and Fogt 0; Santer et al. ; Simmons et al. 0; Thorne et al 0), the NCEP/NCAR tropospheric winds and temperatures are considered to be free from such changes are best analysed fields (Kalnay et al. ;Kistler et al. 0). We compared the

5 Page of results obtained using NCEP data with a similar analysis using ERA-0 reanalysis data for the years 0-. ERA-0 data, which is based on increased data coverage over the SH particularly over Australasia and around the Antarctic coast from the early 0s (Uppala et al.0). The comparison shows that in both the reanalysis datasets, the warming of the southern hemisphere in the midlatitudes and the cooling over the Asian continent is nearly the same spatially and in magnitude (Abish et al. ). Since, NCEP data provides six decades of continuous data; it was selected in preference to the other dataset for the study of climate change. To study areas of deep convection in the Inter Tropical Convergence Zone (ITCZ), outgoing longwave radiation (OLR) datasets (Liebmann and Smith ) obtained from the National Oceanic and Atmospheric Administration (NOAA) at. latitude. longitude grid resolution has been used and is available only for the period to 0.. Results and Discussions.. Climate Change in STJ in MH (00 and 0 E) (a) Boreal winter season (JF) Fig (a) gives the mean zonal wind at 0 hpa of the period to 0 during the boreal winter season (JF) in the longitudinal belt 00 and 0 E, defined as the monsoon hemisphere (MH). It is seen that STJ is stronger over the winter hemisphere (NH) and its intensity is maximum over Japan while it is weaker over the summer hemisphere (SH). It may be noted that the STJ over the NH exhibits a -wave pattern (Krishnamurti ), while such a -wave pattern is not seen over SH. The change in STJ winds between (00-0) and (-) is illustrated in fig (b). It shows the strengthening of STJ winds by to ms - over Asia, while STJ has weakened over rest of the regions over the NH and throughout the globe over the SH. Statistical analysis using t-test shows that these

6 Page of change in STJ winds are statistically significant at % confidence as shown by the shaded grid cells in fig (C). Temperature analysis indicates that the upper troposphere is colder over the mid-latitude region (see fig d) which results in a large north-south temperature gradient over the subtropical upper troposphere that generates the STJ. These temperature gradients at 00 hpa are larger south of the high-speed centres of STJ in NH. From (-) to (00-0) the upper troposphere has shown a large amplitude cooling over the Asian Continent centred over China - fig (e) - which is related to the strengthening of the STJ of NH (longitudes 00-0 E ) as may be seen in fig (b). Fig e shows three areas of warming between latitudes N and 0 N and three areas of cooling between S and 0 S at the 00 hpa level. The climate change in STJ is strong poleward of these areas as may be seen from fig (b), particularly over the southern hemisphere. These warming and cooling areas are near the locations of the subtropical anticyclones at 00 hpa. The reasons for the temperature changes are not known and therefore require further studies. Over the SH, temperature at 00 hpa (fig e) has shown a warming trend of about C, (00-0) minus (-) in the latitude belt 0 S to 0 S. The equatorial upper troposphere has warmed during the same period only through about C. As a combined effect, the upper tropospheric temperature gradient had a decreasing trend which resulted in the weakening of the STJ of SH. There are three regions of intense deep convection in the ITCZ, namely over the equatorial regions around the maritime continent, Africa and South America as shown by the mean OLR (JF) of -0 in fig (f) which is linked to the three velocity maxima in STJ of NH (Krishnamurti ). Regarding the large area of negative temperature trend at 00hPa over Asia centred over China, Ming et al. () found that the large number of industries over the region emits excessive quantities of mineral aerosols such as sulphates, which scatters incoming solar radiation and causes the cooling.

7 Page of Based on observational data, Kaiser and Qian (0) showed that since 0 the dimming effect of rapidly increasing anthropogenic aerosols could have contributed to the cooling trend over eastern China. Duan (0) reported a strong cooling trend of 0. C per decade in the upper troposphere and lower stratosphere over China based on the records of radiosonde stations during the period 0 to 0. These studies indicate that anthropogenic sulphate aerosols have a dominant role in the cooling trend over China. Figure a gives the vertical section of the zonal wind (u) as a mean for the longitudes 00-0 E and fig (b) gives the change in the mean air temperature (00-0) minus (-) for season JF. The mid-latitude cold anomaly of NH and the warm anomaly of SH extend vertically between 0 hpa and 0 hpa, with maximum anomaly near 0 hpa level. Figs c to f give the vertical section of the mean zonal wind (u) around the core of the STJ for JF season averaged for the MH for decades - and The isotachs marked are at intervals of 0. ms -. By this method the location of the core of STJ in a latitude- height vertical section can be determined precisely. For studying the climate change of STJ the main problem is that the STJ core has differences in its wind speed, latitudinal position and also height (hpa level) and so analysis of time changes of wind at one isobaric level will not help. To study the STJ variability, Strong and Davis (0) used the surface of maximum wind analyses, whereas Archer and Caldeira (0) computed a mass weighted average wind speed between 00 and 0 hpa. Using isotach analysis, we find that in JF season in SH there has been a climate change in the latitude of the STJ core through, from S during (-) to S during (00-0) with a weakening of the STJ. (b) Boreal summer season (JA) Fig (a) gives the mean zonal wind at 0 hpa of the period to 0 which shows a stronger STJ over the SH compared to NH during JA season. The TEJ studied by Abish et al () is seen

8 Page of prominently in fig (a). In fig (b) the climate change in 0 hpa wind from (-) to (00-0) indicates that similar to the JF season in this season also. The STJ intensifies over the NH over Asia and these regions show a statistical significance at % confidence level (see fig. (c)). The mean temperature during the season (fig (d)) shows that the tropical Asian region is warmer compared to the surrounding regions. In the fig (e), the temperature change at 00 hpa between (-) and (00-0) shows that as in JF season, there is upper tropospheric cooling of the mid-latitude region centered over China in NH and warming in mid-latitude SH. Correspondingly STJ had a strengthening trend in NH and a weakening trend in SH (fig b). The box marked in fig (f) between longitudes 00 and 0 E and latitudes S and 0 N contains the area of low OLR or the area of convective heating of the troposphere. This is taken as the broad region of upward motion in the Hadley circulation of JA season that warmed the upper troposphere there. The temperature gradient at 00 hpa between the equatorial box ( S-0 N, 00-0 E) and the midlatitude box (0 S-0 S, 00-0 E) boxes for each JA season is given in fig (a). Fig (b) gives the mean u-wind of the STJ core of SH (00-0 E) for each year of the period to 0 drawing isotachs in a vertical section at intervals of 0. ms - as described earlier. The linear trend in STJ wind speed and the -year moving average are also marked. The main weakening of the STJ was during the third and fourth decades of - 0. Across the decades the weakening was from. ms - in (-) to. ms - in (00-0). The corresponding standard deviations of the STJ wind for these two decades are. ms - and. ms - respectively. Using a t-test the mean STJ core winds of these two decades were found statistically different at a level of significance higher than the % level implying a significant climate change in the STJ. The mean temperature at 00 hpa of the equatorial box ( S-0 N, 00-0 E) and the mid-latitude box (0 S-0 S, 00-0 E) for each JA and their linear trend and year moving average are given

9 Page of in figs c and d. The equatorial box has shown only a weak warming trend while the mid-latitude box had a strong warming trend. The linear correlation coefficient between the temperature gradient of each JA and the corresponding STJ core wind speed for the period -0 is 0. and that between their year moving averages 0. which are both high and statistically significant. This reveals that the climate change in STJ core wind is mainly controlled by the climate change in upper tropospheric temperature gradient. Computations done as in Abish et al () has shown that thermal wind change in the layer 0 hpa to 0 hpa is the main factor in the climate change in STJ core wind speed... Climate Change in STJ of NMH (0 E and 0 E) The vertical section of the mean zonal wind during (-0) is given in figures (a, b, c) shows the intensity of STJ over SH. Figures (d, e, f) gives the difference in temperature of decades (00-0) minus (-), which shows the subtropical cooling between latitudes S and 0 S, the midlatitude cooling of NH and the mid-latitude warming of SH. The equatorial upper troposphere ( S- 0 N) has only a weak warming. The mid-latitude upper troposphere warming of the SH is as prominent as for the MH discussed in section.. In the NH the mid-latitude upper troposphere of the JF season has shown a warming trend between latitudes 0 N and N. In the JA season this latitude zone had only a weak warming. Studies show that the warming over these oceanic regions is due to increase in green house gases that are less affected by cooling due to sulphate aerosols. The subtropical zones N-0 N and S-0 S has shown cooling in both JF and JA seasons. Thus while the north south temperature gradients in mid-latitude upper troposphere had a strong decreasing trend in the SH, there was a very little change in the corresponding areas of the NH.

10 Page of Conclusions A prominent feature noticed in this study is the large warming trend of the SH mid-latitude upper troposphere at all longitudes 0 to 0 in both JF and JA seasons. Over this vast ocean covered area available research suggests that the cause of the observed warming trend is the increase in GHG. Another prominent feature is the mid-latitude upper tropospheric cooling trend in the NH between longitudes 00 and 0 E. This cooling trend is due to the increased emission of anthropogenic sulphate aerosols that offset the warming trend due to GHG. We designed an isotach analysis method to study the climate change in the core of the STJ. The difference (climate change) in wind strength, latitude and height of the STJ core winds during JF and JA seasons during -00 are given in table- and the causative mechanisms are described schematically in fig. According to Hadley cell theory, one of the effects of the global warming and its increasing convective heating of the equatorial upper troposphere (UT) is the upward and poleward expansion of the Hadley circulation. The schematic is for the monsoon half of the globe (continental region) between the longitudes 00 and 0 E. For the non-monsoon half which is mainly ocean covered, mid-latitude UT has a warming trend in the SH; whereas in the NH mid-latitude, UT shows a mild warming trend. The main findings of this analysis are: (a) (b) (c) STJ has weakened over SH and strengthened over NH in MH. The changes in the NMH are much smaller except the STJ weakening in the JA season in SH There is large poleward shift of STJ in the JF season of MH and JF and JA seasons of NMH. In general there is very little change in the height of STJ core except in JF season of both MH and NMH where STJ core moved downwards from hpa to hpa.

11 Page of Acknowledgements This work is part of Project No., EC-FP Project "INDOMARECLIM". The Research Council of Norway through the Project India-Clim led by Ola M.Johannessen also funds this work.. References Abish, B., P.V. Joseph and O.M. Johannessen, : Weakening Trend of the Tropical Easterly Jetstream of the Boreal Summer Monsoon season - 0, Journal of Climate. doi:./jcli- D Archer, C. L., and K. Caldeira 0: Historical trends in the jet streams, Geophys. Res. Lett.,, L00, doi:./0gl0 Bengtsson, L., S. Hagemann, and K.L. Hodges, 0: Can climate trends be calculated from reanalysis data? J. Geophys. Res.,, D, doi:./0jd00. Bluestein, H. B., : Synoptic-Dynamic Meteorology in Midlatitudes, vol., Observations and Theory of Weather Systems, pp., Oxford Univ. Press, New York Bromwich, D.H., and R.L. Fogt, 0: Strong trends in the skill of the ERA-0 and NCEP-NCAR reanalyses in the high and middle latitudes of the Southern Hemisphere, -0. J. Clim.,, 0-. Dima, I.M., and Wallace, J.M., 0: On the seasonality of the Hadley Cell, J. Atmos. Sci., 0, -. Duan, A.,0: Cooling trend in the upper troposphere and lower stratosphere over China, Geophys. Res. Lett.,, L0, doi:./0gl0 Durre, Imke, Russell S. Vose, David B. Wuertz, 0: Overview of the Integrated Global Radiosonde Archive. J. Climate :. doi: Fu, Q., C. M. Johanson, J. M. Wallace, and T. Reichler 0: Enhanced mid-latitude tropospheric warming in satellite measurements, Science,,. Galvin, 0: The weather and climate of the tropics, Weather,, -. Hu, Y and Fu, Q.,0: Observed poleward expansion of the Hadley circulation since, Atmos. Chem. Phys.,, -. Hudson, R.D., : Measurements of the movement of the jet streams at mid-latitudes, in the Northern and Southern Hemispheres, to, Atmos. Chem. Phys.,, -0, Kaiser, D. P. and Qian, Y., 0: Decreasing trends in sunshine duration over China for : indication of increased haze pollution?, Geophys. Res. Lett.,,, doi:./0gl00. Kalnay, E., M. Kanamitsu, R. Kistler, W. Collins, et al. : The NCEP/NCAR Reanalysis 0-year Project, Bull. Am. Meteorol. Soc.,,

12 Page of Kistler, R., et al., 0: The NCEP NCAR Year Reanalysis: Monthly Means CD ROM and Documentation. Bull. Amer. Meteor. Soc.,,. Krishnamurti, : The subtropical jetstream of winter, Journal of Meteorology,, Liebmann, B., and C. A. Smith, : Description of a complete (interpolated) outgoing longwave radiation dataset. Bull.Amer. Meteor. Soc.,,. Liu, J., Song, M., Hu, Y. and Ren, X., : Changes in the strength and width of the Hadley Circulation since, Clim. Past,, -. Lu, J., G. A. Vecchi, and T. Reichler 0: Expansion of the Hadley cell under global warming, Geophys. Res. Lett.,, L00, doi:./0gl0 Mitas, C. M., and A. Clement 0: Has the Hadley cell been strengthening in recent decades?, Geophys. Res. Lett.,, L00, doi:./0gl0 Mitchell, J. F. B., T. C. Johns, J. M. Gregory, and S. F. B. Tett, : Climate response to increasing levels of greenhouse gases and sulphate aerosols, Nature,,. Peixoto J.P. and Oort A.H.,: Physics of Climate, American Institute of Physics New York, pages Schulman, : On the summer Hadley cell, Quart. J. R. Met. Soc,, -. Santer, B.D., et al., : Uncertainties in observationally based estimates of temperature change in the free atmosphere. J. Geophys. Res.,, 0-. Seidel, D. J., Q. Fu, W. J. Randel, and T. J. Reichler 0: Widening of the tropical belt in a changing climate, Nat. Geosci.,,. Simmons, A.J., et al., 0: Comparison of trends and low-frequency variability in CRU, ERA-0 and NCEP/NCAR analyses of surface air temperature. J. Geophys. Res.,, D Strong, Courtenay, Robert E. Davis, 0: Variability in the Position and Strength of Winter Jet Stream Cores Related to Northern Hemisphere Teleconnections. J. Climate,,. Sun, J., : Record-breaking SST over mid-north Atlantic and extreme high temperature over the Jianghuai Jiangnan region of China in. Chin. Sci. Bull.,, 0. Thorne P.W., et al., 0: Revisiting radiosonde upper air temperatures from to 0, J. Geophys. Res., 0, D, doi:./0jd00. Uppala SM, et al. 0: The ERA-0 re-analysis. Q.J.R. Meteorol. Soc., : 0. doi:./qj.0. Webster, P. J., 0: The Elementary Hadley Circulation. The Hadley Circulation: Past, Present and Future. Editors: H. Diaz and R.Bradley. Cambridge University Press. pp. -0.

13 Figures (a) (b) iew ev rr Fo On (c) ly Page of Fig. (a) Mean wind at 0 hpa (b) change in zonal wind during boreal winter (JF). Vectors denote the direction and color indicate its magnitude. (c) Shaded are regions of statistical significance at % level (continued)

14 Page of (d) (e) (f) Fig. (d) Mean temperature at 00 hpa (e) Horizontal section of the temperature change at 00 hpa (f) Monthly mean OLR showing deep convection over the south of the Equator.

15 Page of (a) (b) Fig.(a) Vertical section of climatology in u-wind and (b) temperature change during the JF season (continued).

16 Page of (c) (d) Fig. Isotachs of U- wind over SH at 0. ms - resolution for - (d) 00-0 and (e),(f) that over NH (continued).

17 Page of (e) (f) Fig. Isotachs of U- wind over SH at 0. ms - resolution for - (d) 00-0 and (e),(f) that over NH.

18 Page of (a) iew ev rr (b) Fo On (c) ly Fig same as in fig., but in JA season (continued.)

19 Page of (d) (e) (f) Fig (d) and (e)same as in fig., but in JA season (f) Monthly mean OLR indicating deep convection associated with summer monsoon and adjoining regions.

20 Page of Temperature gradient (JA) b U-Wind (JA) a 0 SH- 00hPa JA 0 0 Years S - 0N ; 0E-0E 0 S - S ; 0E-0E Temperature (JA) SH-STJ-JA Years Fig (a) Temperature gradient at 00 hpa during JA over SH (b) corresponding wind speed during the same season (c) -year moving average of temperature over the tropics and (d) the mid-latitudes during JA c d Temperature (JA) hPa JA 0 Years 00hPa JA 0 S - 0N ; 0E-0E Years 0 0 S - S ; 0E-0E

21 Page of (a) (b) (c) Fig.. (a,b,c) shows the vertical section of climatology in u-wind (continued)

22 Page of (d) (e) (f) Fig.. (d,e,f) Temperature change during the JA and JF seasons.

23 Page of SH Midlatitudes Warming trend of UT possibly by increase of GHG Decreasing trend of STJ core speed Poleward trend of STJ Core Decreasing trend of UT temperature gradient Equator Increasing trend of convective heating of UT in global warming Poleward widening of Hadley Circulation Observed slight downward shift of STJ Core Fig. A schematic diagram showing the mechanism of climate change in STJ Increasing trend of UT temperature gradient NH Midlatitudes Cooling Trend of UT by increase of anthropogenic aerosols Increasing trend of STJ core speed Poleward trend of STJ Core

24 Page of Table : Wind Strength in ms -, latitude and height (hpa) of the core of STJ in the MH and NMH of JA and JF seasons during the decades - and 00-0 Year, Season, Hemisphere -, JA, NH 00-0,JA, NH -, JF, NH 00-0,JF, NH -, JA, SH 00-0,JA, SH -, JF, SH 00-0,JF, SH Sector 0-0 Sector 0-0 Strength Latitude Height Strength Latitude Height