Memo should be writing from the perspective of an HR manager.
Write a memo to the Intern managers explain that the Intern numbers will be cut to half. Explain that this is to help ensure open completion of internships interns are able to be converted to full time workers instead of just leaving the program empty handed. Explain that this might be temporary and we will adjust based on the business climate. Explain that their currently interns will also not be extended. This email should sound empathetic.