ALONG with the blast of cold air that howls out of Canada with the approach of winter, there's another wind blowing as well - a warming trend in Canadian-US relations.
It is evident in the words and actions of Canada's new leadership. No sooner was it installed two months ago than Brian Mulroney's government began sounding a message of friendship with Washington. Prime Minister Mulroney quickly acted on campaign promises of improved ties with the United States. And the new external affairs minister, Joe Clark, after a meeting with US Secretary of State George P. Shultz, suggested the two nations were embarking on ''a new chapter in relations.'' All this must be music to Washington's ears. During the two decades that Pierre Elliott Trudeau dominated Canadian politics, there were plenty of discordant notes.
To back up this new tone, Prime Minister Mulroney is moving on a number of fronts to remove irritants. US companies are finding it easier to do business in Canada. Expanded trade between the two neighbors is in the offing as Canada lowers import barriers. And Canadian defense spending is likely to increase moderately - allaying US criticism that Canada has not been shouldering its load in North Atlantic defense commitments.
None of this should suggest that Canada under Prime Minister Mulroney is simply bowing to US pressure or is becoming a doormat for Washington. Far from it. There are plenty of irritants remaining, many of which are up to Washington to ameliorate if the relationship is to warm still more. So far, for example, little progress has been made on the acid-rain issue - an issue that angers Canadians who hold that emissions from US industrial plants cause serious damage to Canada's ecology. In his meeting with External Affairs Minister Clark, Secretary Shultz observed: ''You gave me a hard time on a lot of subjects, like acid rain.''
Washington should move more quickly on this issue now that Canada has shown the way on other issues. What worries Canadians, including some in the Mulroney government, is that Washington in the past has simply ignored Canadian pleadings on the subject - and ''ridden roughshod'' over the pleadings, as the Winnipeg Sun put it recently. It is easy for Washington to do so. After all, despite its enormous geographic size, Canada lies in the shadow of its mighty neighbor to the south. And many Americans tend to take Canada for granted. It would be unfortunate, with the goodwill emanating from Ottawa these days, for Washington to continue this path. Prime Minister Mulroney has given the US the opportunity to correct some of the slights, omissions, and mistakes of the past.