Changing Relationship between the Tropical Easterly Jet and the Indian summer Monsoon Rainfall: Role of Indian Ocean Warming

Indian Journal of Geo-Marine Sciences Vol. 44(11), November 2015, pp. 1678-1683 Changing Relationship between the Tropical Easterly Jet and the Indian summer Monsoon Rainfall: Role of Indian Ocean Warming D. Nagarjuna Rao *, Gibies George, C. T. Sabeerali, D.A. Ramu & Suryachandra. A. Rao Indian Institute of Tropical Meteorology, Pashan, Pune-411008, India *[Email: nagarjuna@tropmet.res.in] Received 24 September 2014; revised 22 October 2014 Using long-term observational and reanalysis products, we noticed that the strength of the Tropical Easterly Jet (TEJ) stream shows a weakening trend. Model sensitivity experiments with Atmospheric General Circulation Model (ECHAM5-AGCM) hypothesize that the weakening of the TEJ is mainly a result of the recent Indian Ocean warming and the associated with changes in the convection and circulation. Another notable feature is the weakening of the relationship between the TEJ strength and Indian summer monsoon rainfall (ISMR) in recent decades and sensitivity experiments with AGCM confirm the role of the recent Indian Ocean warming in weakening this relationship. Time series of both TEJ strength and ISMR shows a decreasing trend in recent decades. However, the rate of decrease in ISMR is less compared to the decrease in TEJ strength. This seems to be because of the increases in extreme rainfall events over the Indian landmass in recent decades which can compensate the decrease in total rainfall. It suggests that the local convective rainfall events become more frequent and hence, rainfall due to large scale circulation should decrease to maintain the decreasing trend of total rainfall over Indian landmass. In short, our results indicate that the unequal rate of decrease in the ISMR and the TEJ strength in the context of the recent Indian Ocean warming breaks up the relationship between the ISMR and TEJ. [Keywords: TEJ, ISMR, Indian Ocean Warming, Climate Variability] Introduction The strong upper tropospheric Tropical Easterly Jet (TEJ) stream during the boreal summer season (June through September) is one of the major components of south Asian summer monsoon (ASM) circulation and whose existence was first reported by Koteswaram 8. This narrow belt of strong easterlies are prominent between 100 and 200 hpa vertical levels over ASM region with a wind speed of roughly 35-40 m/s. Establishment of this jet during the early phase of the monsoon is important for the Indian summer monsoon onset. Axis of the TEJ is mainly located between 5 0 S-20 0 N in the north-south direction and its intensity is maximum during the peak monsoon month of July. Over the Atlantic and Pacific Ocean the existence of the TEJ is completely absent. Previous studies 8, 9 have shown that the upper tropospheric meridional thermal gradient that exist between the Asian land mass and Indian Ocean maintain the TEJ. Previous studies have studied the diurnal 10, intraseasonal 4, 23 and interannual variations 5 of the TEJ. The axis of the TEJ oscillates in the northsouth direction between active and break periods of monsoon rainfall 23. During active periods the axis of the TEJ is located near 5 0 N latitude and during break periods it is located around 15 0 N latitude 23. Later, Chen and Van Loon have shown that the interannual variability of the TEJ is mostly controlled by the Walker and regional Hadley circulation variations which indicate that the ENSO has a strong control over the TEJ strength 5. Intensity of the TEJ is weak (strong) during El Nino (La Nina) events 3, 14. It is also shown that the interannual variability of the TEJ and Indian summer monsoon are strongly correlated in such a way that a strong TEJ is associated with excess Indian summer monsoon rainfall (ISMR) and vice versa 5, 4, 14. Using observational datasets, many previous studies have reported that the strength of the TEJ has weakened in recent decades 17,18,1,24.In addition to the intensity of the TEJ, the zonal extent of the TEJ is also weakened in recent decades 18,22.Using reanalysis/observational datasets many theories have been proposed to understand possible reasons for weakening the TEJ 17, 24, 1.Rao et al. have shown that the meridional decrease in the upper tropospheric temperature gradient over ASM region is responsible for weakening the TEJ 17. Sreekala et al. have suggested that the recent reduction in the land-sea contrast during the pre-

NAGARJUNA ET AL.: CHANGING RELATIONSHIP BETWEEN THE TROPICAL EASTERLY JET AND THE INDIAN SUMMER MONSOON RAINFALL: ROLE OF INDIAN OCEAN WARMING 1679 monsoon season might be responsible for the TEJ weakening 24. Using long-term observational datasets, Abish et al. have hypothesized that the recent Indian Ocean warming and associated changes in convection would decrease the upper tropospheric temperature gradient by warming the troposphere over the Indian Ocean and it is responsible for the weakening of TEJ intensity 1. However, an extensive study with Atmospheric General Circulation Models (AGCM) is required to test the robustness of the above hypothesis. In this particular study, we try to understand the role of the Indian Ocean warming in weakening the TEJ using a couple of numerical experiments. Beyond focusing the weakening of the TEJ intensity, the study also investigates how the relationship between the TEJ and ISMR has changed in the recent period. The rest of the paper is organized as follows. Section 2 explains the data, numerical model used and experimental setup. Main result of this paper is presented in section 3. Finally, a conclusion of the study is given in Section 4. Materials and Methods The monthly atmospheric temperature, zonal and meridional wind obtained from the National Centers for Environmental Predictions/National center for Atmospheric Research (NCEP-NCAR) reanalysis for the period 1951-2011 are used in this study 7.We have also used the monthly rainfall data for the meteorological subdivisions provided by the Indian Institute of Tropical Meteorology via www.tropmet.res.in 13. For additional verification, we have used the gridded (1 0 x1 0 ) daily rainfall data provided by the India Meteorological Department 15. In this study, we focus on boreal summer months (June through September). Upper tropospheric temperature (UTT) is calculated as the vertical average of temperature between 600 and 200 hpa levels. To study the interannual variations of TEJ intensity, we have computed the seasonal mean kinetic energy of the upper level flow (200 hpa) by averaging over the region 5 0 S-20 0 N, 30 0 E- 120 0 E. The 11 year running correlations between the ISMR and TEJ intensity have been used to study the relationship between the ISMR and TEJ intensity for the period 1951-2011. The AGCM experiments are performed with the model ECHAM5, from the Max Planck Institute for Meteorology. It is a global atmospheric general circulation model, which we ran at T106 spectral resolution with 19 sigma levels in vertical. More details of the ECHAM5 model can be found in Roeckner et al. 16. Many earlier studies have used this model to study the response of the Indo-Pacific SSTs on the ISMR 19, 20, 21. All these aforementioned studies have shown that this model reasonably simulates the Indo-Pacific SST response and this is important because the primary aim of this particular study is to understand the response of the Indian Ocean SST warming on the relation between the ISMR and TEJ. This model is also reasonable in simulating the intensity of the TEJ during the boreal summer season compared to observations (figure not shown). Two types of AGCM runs are carried out in this study. In the non-warming run (NW run), the monthly trend is removed from the observed global SST for the period 1956-1999, and later half of the trend removed monthly SST is used to force the model and integrated the model from 1979-1998. Indian Ocean warming run (IOW run) is similar to NW run except that the observed monthly SST is used in the Indian Ocean (30 0 S-30 0 N, 40 0 E-120 0 E) and trend removed SSTs are used elsewhere outside the Indian Ocean. Hence, the difference of NW run from the IOW run isolates the influence of the Indian Ocean warming. Since both NW run and IOW runs are long runs (20 years), we have performed only one ensemble member integration in these runs to avoid the huge computational expense. It is found that climatological features of ensemble mean do not change from a single realization as the boundary forcing is the major forcing for long runs. Results and Discussion In this section first we show the change in the TEJ strength using long-term reanalysis products and then try to understand the role of Indian Ocean warming in weakening the TEJ intensity by carrying out SST controlled experiments with an AGCM. To represent the TEJ intensity, we have computed the seasonal mean (June through September) kinetic energy of the upper air flow (200 hpa) by averaging over the region 5 0 S-20 0 N, 30 0 E-120 0 E.

1680 INDIAN J. MAR. SCI., VOL. 44, NO. 11 NOVEMBER 2015 (Fig. 1(a)) shows the interannual variations of the TEJ intensity for the period 1951-2011. It is clear that the TEJ intensity shows a decreasing trend over last six decades. We have also plotted the spatial trend of boreal summer (JJAS) UTT for the period 1951-2011 (Fig. 2) and it is evident that the boreal summer UTT over the Indian Ocean shows an increasing trend compared to northern latitudes where it shows a decreasing trend in most places. Fig. 1 (a) Interannual variations of TEJ intensity for the period 1951-2011 (bars) and its trend line (black dotted line). Green line: interannual variations of Indian summer monsoon rainfall anomalies (ISMR) for the period 1951-2011 and its trend line (green line). 1(b) Interannual variations of the extreme rainfall events (rainfall >100 mm/day) over the Indian landmass for the period 1951-2008. The linear trend for the period 1951-2008 is represented by the dotted line. All the trend lines are significant at 90% confidence level. This upward UTT trend over the Indian Ocean region is associated with increased SSTs (Fig. 2). These results are consistent with previous studies 17, 18, 1, 24. Using reanalysis datasets, Abish et al. have suggested that the recent Indian Ocean warming and the associated changes in the convection weaken the UTT gradient and thereby, weaken the TEJ intensity 1. However, SST forced experiments with an AGCM are required to confirm this theory. Differences between IOW run and NW run are shown in (Fig. 3) to highlight the impact of Indian Ocean warming on TEJ. It is quite clear that in response to the pronounced Indian Ocean warming, an anomalous low level moisture convergence has formed over the central equatorial Indian Ocean (Fig. 3(a)). Because of this anomalous moisture convergence, the regional Hadley circulation over ASM region has modified by producing an anomalous upward motion over the Indian Ocean and an anomalous downward motion over the Indian landmass (Fig. 3(b)). Anomalous upward (downward) limb of the regional Hadley circulation has increased (decreased) the UTT over the Indian Ocean (Asian landmass) region by enhancing (reducing) the latent heat release and thereby, weakened the meridional UTT gradient (Fig. 3(c)). It has been shown that due to low Coriolis force over the TEJ domain, a small change in the meridional thermal gradient and associated changes in the thermal wind can make large variations in the intensity of the TEJ 12. Fig. 2 Long-term trend of boreal summer (JJAS) upper tropospheric temperature in 0 C (600-200 hpa) for the period 1951-2011. The contours represent the SST trend in 0 C/year for the same period. Fig. 3 (a) Difference in the boreal summer (JJAS) divergent component of moisture transport in kg m-1 s-1 between the Indian Ocean warming run (IOW) and Non warming run (NW run). 3(b), 3(c), and 3(d) is same as 3(a) but for the regional Hadley circulation, upper tropospheric temperature in 0 C (600-200 hpa) and 200 hpa zonal wind in m/sec respectively.

NAGARJUNA ET AL.: CHANGING RELATIONSHIP BETWEEN THE TROPICAL EASTERLY JET AND THE INDIAN SUMMER MONSOON RAINFALL: ROLE OF INDIAN OCEAN WARMING 1681 The difference of the zonal wind at 200 hpa between the IOW run and NW run is shown in (Fig. 3(d)). It is evident that the upper tropospheric zonal wind has shown a westerly anomaly in the IOW run compared to NW run (Fig. 3(d)), which means that the reduction in the upper tropospheric meridional thermal gradient has weakened the strength of TEJ considerably. These AGCM experiments confirm the role of Indian Ocean warming in weakening the strength of TEJ as suggested by Abish et al 1. Earlier studies have shown a strong association between the ISMR and TEJ intensity, because the strength of both ISMR and TEJ is maintained by the meridional upper tropospheric thermal gradient 5, 4, 14. To study this relationship for the period 1951-2011, we have computed the 11 year running correlation between the TEJ intensity and ISMR using reanalysis products and observational datasets and it has shown a decreasing trend for last three decades (Fig. 4). This decreasing trend suggests that the relationship between the ISMR and TEJ has weakened in recent decades. means that the model truthfully simulate the relation between the ISMR and TEJ for the period 1979-1998. However, the simulated running correlations in the model should be viewed by considering the limitations of the existing general circulation models (GCMs). It is found that the running correlation drops significantly once we introduce the Indian Ocean warming in the model (Fig. 5) which indicates that the relation between the ISMR and TEJ is weak in the IOW run compared to NW run. The idea we put forward is that the drop in correlation between the TEJ and ISMR is due to Indian Ocean warming. This confirms that Indian Ocean warming is one of the major reasons that are responsible for weakening the relationship between the ISMR and TEJ intensity in recent decades. Fig. 4 Eleven year running correlation between the Indian summer monsoon rainfall in mm/day (ISMR) and TEJ strength in m2 /sec2 for the period 1951-2011. The correlation values are plotted at the center of the each 11 year period. The blue line represents 90% confidence level value. To test the role of the Indian Ocean warming in weakening the relationship between the TEJ intensity and ISMR, we have compared the 11 year running correlation between the ISMR and TEJ intensity simulated in the IOW run and NW run (Fig. 5). In this study, the model integration has been performed for the period 1979-1988 and details are discussed in section 2. In both observations and model, the 11 year running correlation between the TEJ and ISMR is insignificant for the period 1979-1998, which Fig. 5 Eleven year running correlation between the Indian summer monsoon rainfall in mm/day (ISMR) and TEJ strength in m2 /sec2 for the period 1979-1998. Blue line: 11 year running correlation between the ISMR and TEJ intensity computed in the Non warming run (NW run). Red line: 11 year running correlation between the ISMR and TEJ intensity computed in the Indian Ocean warming run (IOW run). Although the numerical experiments confirm the role of the Indian Ocean warming in weakening the relationship between the ISMR and TEJ intensity, it would be interesting to know how this relationship has decreased in the context of the recent Indian Ocean warming. From the interannual variations of the ISMR and TEJ intensity, it is clear that both ISMR and TEJ intensity has shown a decreasing trend for the period 1951-2011 (Fig. 1(a)). However, the rate of decrease in ISMR is less compared to the rate of decrease in TEJ intensity (Fig. 1(a)). An increasing trend in the extreme rainfall events (rainfall >/100 mm/day) over the ISMR region have also been shown over last six decades (Fig. 1(b)) consistent with previous

1682 INDIAN J. MAR. SCI., VOL. 44, NO. 11 NOVEMBER 2015 studies 6, 2. This increasing trend in extreme rainfall events in the warming environment can compensate the total rainfall decrease and might be one reason for the reduced rate of ISMR decrease. Mani et al. have shown that higher frequency of extreme rainfall events and increased potential instabilities resulted in decreased predictability of weather 11. It is found that extreme rainfall events arise due to the convective instabilities of the atmosphere and an increasing trend in the Convective Available Potential Energy (CAPE) and a decreasing trend in Convective Inhibition Energy (CINE) are reported in recent decades 11. Finally we conclude that the unequal rate of decrease in the ISMR and TEJ intensity in the context of recent Indian Ocean warming can lead to a weakening of the relationship between the ISMR and TEJ intensity as shown in (Fig. 4). Conclusion In this study, by using observational datasets and reanalysis products we have shown a weakening of the strength of the TEJ in recent decades consistent with previous studies. By using a couple of AGCM experiments with different SST forcing, we have confirmed the role of the Indian Ocean warming in weakening the TEJ intensity. Analysis shows that due to the recent Indian Ocean warming an anomalous low level moisture convergence produce over the Indian Ocean which in turn modulates the regional Hadley circulation by creating an anomalous upward motion over the Indian Ocean and an anomalous downward motion over the Asian landmass. As a result, the upper tropospheric temperature over the Indian Ocean region warm more compared to the troposphere over the Asian landmass by increasing or reducing the latent heat release and thereby, weaken the north-south temperature gradient. This reduced upper tropospheric thermal gradient due to Indian Ocean warming is mainly responsible for the weakening TEJ intensity in recent decades. In addition, we have also shown that the relationship between the ISMR and TEJ intensity has weakened in recent decades. We have used two SST forced AGCM experiments with ECHAM5 viz., the Indian Ocean warming (IOW) run and non warming (NW) run to confirm the role of the Indian ocean warming in weakening the relationship between the ISMR and TEJ intensity. These runs show a weakening relationship between the ISMR and TEJ in the Indian Ocean warming (IOW) run compared to the non-warming (NW) run. The unequal rate of decrease in the ISMR and TEJ intensity explains how the relation between the ISMR and TEJ intensity weakens in the context of the recent Indian warming. It is clear that both the ISMR and TEJ intensity show a decreasing trend over last six decades. 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