The Great red spot used to be bigger. Over the course of twentieth century, it lost half of its size and between 1996 to 2006, its area diminished by 15%. If the shrink rate is constant, then the spot will vanish within 20 years. However, computer simulation showed that Jovian weather is stable enough to sustain smaller red spots as Hubble space telescope showed two smaller red spots besides the great red spot indicating that small red spots will persist even if the Great red spot vanishes. The reason for this shrinkage is unknown but scientist suggested it has something to do with Jovian equivalent of global warming and climate change.
Unlike Earth, Jupiter does not have solid landmass so energy lost due to friction is out of question. This explains the long lasting nature of Jovian storms. The Great Red Spot is indeed long-lived, even by Jupiter standards. Researchers are able to pinpoint that Jovian storms that are located in cloud bands with the same direction of rotation tend to be longer lasting. So, the shrinkage of this spot might indicate that the spot is drifting from the clouds bands.
The Great Red Spot is confined by an eastward jet to its north and a westward jet to its south, confining the storm to a constant latitude. However, it has undergone considerable changes in longitude over time, and recent evidence suggests that its rate of westward longitudinal motion is increasing. The bands have also undergone little change in latitude over the time during which they have been observed. So, researchers suggested that this dynamic change of the cloud bands and drifting of the storm might be the reason for the change in the shape of The Great spot.
Although the reason has not been proved yet, researchers are trying to trace the evolution of the Great Red Spot, analyzing its size, shape, color, drift rate and storm’s internal wind speeds from the information available from spacecraft and hope they will provide an explanation in future.
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- Historical and Contemporary Trends in the Size, Drift, and Color of Jupiter's Great Red Spot by Simon et.al., The Astronomical Journal, Volume 155, Number 4, 2018 (link)