The Tropical Easterly Jet, or TEJ, emerges each year with the South Asian Summer Monsoon. It owes its existence primarily to a vast temperature contrast in the upper troposphere between the heated, elevated Tibetan Plateau region and the cooler Indian Ocean. This contrast sets the jet’s speed through what physicists call thermal wind balance: the stronger the north-south temperature difference, the faster the westward flow aloft. But the new analysis reveals that this textbook picture is only the first approximation. The diverging, rotating outflow at the tops of deep convective storms can push the jet faster or slower than the thermal wind alone would predict, producing swings of about 3 metres per second in the wind over the Arabian Sea during the monsoon.

To reach these conclusions, the team exploited a dataset in which profiles of horizontal wind measured by Aeolus are perfectly co-located with profiles of cirrus and deep convective clouds. Aeolus carries ALADIN, a 355-nanometre spaceborne Doppler wind lidar that measures wind by detecting the Doppler shift of laser light scattered back from molecules and particles in the air. Because the instrument points 35 degrees off-nadir and nearly perpendicular to the satellite track, it essentially measures the east-west, or zonal, component of the wind, with the difference from the true zonal wind amounting to only about one percent. That geometry happens to be ideal for the TEJ, which is almost purely zonal.

The study covers 2020, with a focus on the monsoon months from June to October. Aeolus overflies the Arabian Sea at 06:00 local time and the Bay of Bengal at 18:00 local time, and thanks to its sun-synchronous orbit, the track over the Arabian Sea on three days out of every seven sits roughly 1150 kilometres west of the track that crossed the Bay of Bengal twelve hours earlier. An air parcel carried westward at about 25 metres per second by the jet therefore arrives almost exactly where the satellite will look half a day later. This orbital quirk turns Aeolus into a natural experiment for tracking storm outflows and their cloud cargo across the region.

The researchers first verified the thermal wind framework. Comparing the wind difference observed by Aeolus between 7 and 15 kilometres of altitude with the thermal wind computed from ERA5 reanalysis temperatures across the Arabian Sea, they found a median discrepancy of only about 2 metres per second, confirming that the jet broadly follows the temperature gradient. But the day-to-day fluctuations around this balance told a richer story. On days when the observed jet was stronger than the thermal wind prediction, the previous evening had seen abundant deep convection over north-east India and the Bay of Bengal, with cloud cover reaching about 40 percent, and weak convection over the south-eastern Arabian Sea. On days when the jet was weaker than expected, the pattern flipped: convection dominated the south-eastern Arabian Sea while the Bay of Bengal quieted down.

The mechanism, the authors propose, lies in the anticyclonic circulation that crowns every deep convective storm. Air diverging outward at the cloud top is deflected clockwise by the Coriolis force, producing a westward wind perturbation to the south-west of the storm and an eastward perturbation to the north-east, a process previously documented over West Africa and the Sahel. When convection peaks over the Bay of Bengal, its outflow strengthens the westward jet to the south-west, over the Arabian Sea, twelve hours later. When convection instead peaks over the south-eastern Arabian Sea, its outflow generates an eastward perturbation that weakens the jet. Averaged over the whole Arabian Sea, the convective contribution reached minus 1.7 metres per second on strengthened days and plus 1.3 metres per second on weakened days, a total swing of about 3 metres per second that was statistically significant at the 90 percent confidence level and up to 5 metres per second in the jet’s centre. Because the wind anomaly lags the convection by twelve hours, the storms are the likely drivers, not the other way around.

Having established what moves the jet, the team turned to what the jet moves: cirrus clouds. These optically thin ice clouds form either in situ from water vapour in cold upper-tropospheric air or as material detrained from the tops of deep convective storms, and they can drift more than 1000 kilometres before dissipating. The analysis showed a striking sensitivity to wind speed. On days when the westward wind between 14 and 17 kilometres over the Arabian Sea exceeded the June-to-October median of 23.6 metres per second, the average cirrus cloud cover at 06:00 local time was 14 percent. On slower-wind days it was only 6 percent, less than half as much. Cirrus fractions above 8 percent were almost never observed when westward winds were slower than 10 metres per second, and their occurrence peaked near 23 metres per second.

Crucially, the researchers worked to separate the wind’s role from two rival explanations: temperature and convective intensity. Temperature, it turned out, was not the limiting factor. Comparing ERA5 temperatures over the Arabian Sea across quartiles of cirrus cloud fraction, they found no significant dependency; large cirrus fractions were not systematically associated with colder regional mean temperatures, partly because cirrus themselves can warm the upper troposphere locally by up to 4 kelvin and because small-scale cold anomalies from gravity waves escape regional averages. Convective intensity, by contrast, mattered: cirrus fractions over the Arabian Sea rose steadily with the deep convective cloud fractions observed over the Bay of Bengal twelve hours earlier. Yet fast-wind days also coincided with stronger convection over northern India and the Bay of Bengal, raising the question of which factor truly controls the cirrus.

To disentangle the two, the team constructed a diagnostic that measures the cirrus fraction’s share relative to the preceding convection, for a given convective input. The result was clear: for the same amount of deep convection over India or the Bay of Bengal, faster westward winds produced larger cirrus fractions over the Arabian Sea twelve hours later, with the effect most pronounced as winds increased from 20 to 25 metres per second before plateauing. The picture that emerges is of cirrus clouds detrained from afternoon storms over the Bay of Bengal riding the jet westward overnight, arriving over the Arabian Sea in time for the satellite’s morning pass. Wind and convection work hand in hand: the storms supply the ice and vapour, and the jet delivers them.

The findings carry a sobering implication for a warming world. Climate models project that the TEJ will weaken over the Indian Ocean, with one CMIP-6 based study estimating an 18 percent decline in central jet intensity by the end of the twenty-first century, while convection over the Bay of Bengal is also expected to decrease. If both the water supply and the delivery mechanism fade together, the authors predict that cirrus cloud cover over the Arabian Sea may decline in the coming decades. Because cirrus clouds trap longwave radiation while the convective anvils they accompany reflect sunlight, shifts in their coverage could feed back on the region’s energy balance. The study’s authors caution that Aeolus’s fixed observation times leave the phase of the advected water, whether ice crystals or vapour, unresolved, and they plan to explore these relationships further in a global circulation model under both current and warming climates.

Subject of Research: Relationships between deep convective clouds, cirrus clouds and the Tropical Easterly Jet over the Indian region observed with the Aeolus spaceborne Doppler wind lidar

Article Title: Relationships between deep convective clouds, cirrus clouds, and the tropical easterly jet over the Indian region observed with Aeolus spaceborne Doppler wind lidar

Article References: Titus, Z., Bonazzola, M., Chepfer, H., Feofilov, A., & Roussel, M.-L. (2026). Relationships between deep convective clouds, cirrus clouds, and the tropical easterly jet over the Indian region observed with Aeolus spaceborne Doppler wind lidar. Atmospheric Chemistry and Physics, 26(20), 15017-15036. https://doi.org/10.5194/acp-26-15017-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-15017-2026

Keywords: Aeolus, Doppler wind lidar, Tropical Easterly Jet, cirrus clouds, deep convection, South Asian Summer Monsoon, Arabian Sea, Bay of Bengal, thermal wind balance, upper troposphere, cloud advection, climate change