Finnish President Alexander Stubb has developed an unconventional personal rapport with U.S. President Donald Trump after the two played golf earlier this year, according to people familiar with the matter. The relationship has evolved into an informal back-channel that European heads of state and Ukrainian officials now use to pass messages directly to the White House, filling a communications gap created by strained trans-Atlantic ties. Stubb and Trump exchange text messages and phone calls at irregular hours, sources said. European officials describe the conversations as "frank," noting that Trump, who is often portrayed as aloof toward multilateral diplomacy, appears willing to listen when approached through the Finnish leader. Trump’s Nordic outreach has also included Norway. The business daily Dagens Næringsliv reported that during a call last month with Norwegian Finance Minister Jens Stoltenberg on trade tariffs, Trump pivoted to inquire about the Nobel Peace Prize—an award he has repeatedly said he deserves for recent cease-fire negotiations. The White House and Norway’s finance ministry declined to comment.