And analysts at Bank of America Merrill Lynch published a note last week concluding that days during which Trump tweets relatively frequently tend to see negative stock returns of 9 basis points on average. (Photo: Shutterstock)

(Bloomberg) –Two of the largest Wall Street banks are trying to measure the market impact of Donald Trump's tweets.

Analysts at JPMorgan Chase & Co. have created an index to quantify what they say are the growing effects on U.S. bond yields. Citigroup Inc.'s foreign exchange team, meanwhile, report that these micro-blogging missives are also becoming "increasingly relevant" to foreign-exchange moves.

JPMorgan's "Volfefe Index," named after Trump's mysterious covfefe tweet from May 2017, suggests that the president's electronic musings are having a statistically significant impact on Treasury yields. The number of market-moving Trump tweets has ballooned in the past month, with those including words such as "China," "billion," "products," "Democrats" and "great" most likely to affect prices, the analysts found.