The unprecedented swap of Soviet spies for Soviet dissidents is the diplomatic equivalent of the buds of spring-a certain sign that warmer weather is coming.

The Soviet Union and the United States have had to resort to all sorts of unconventional communication during the years they have been pursuing better relations, and this may be the most bizarre signal yet. But its impo rt seems clear: a new strategic arms treaty is coming, and both superpowers want to prepare a friendly reception for it.

Arguably the Soviets have been warming up for this dramatic gesture since September, when they made a deal with Sen. Edward M. Kennedy (D-Mass.) to allow the emigration of a number of Jewish families, including that of Benjamin Levitch, the highest ranking Soviet scientist ever to apply for an exit visa.

The Soviets gave more to Kennedy then than they had ever conceded to the Carter administration during two years of uneven relations. Carter's dealings with Moscow got off to a bad start in his first days as president, when the Soviets reacted sharply to his aggressive human rights campaign.

The Kennedy episode seemed to carry a message: quiet diplomacy, as Henry Kissinger used to call it, would work better than Carter's earlier tactics, such as his personal letter to Soviet dissident leader Andrei D. Sakharov.

The Carter administration has been quieter in recent months, and it became known yesterday that it was negotiating the swap of spies for dissidents for most of that time.

Administration officials made no attempt to conceal their glee that the swap came off. They obviously felt this was a boost for the strategic arms limitation treaty (SALT) as well as a compliment to their own diplomacy.

Some officials held out hope that the Soviets would be pleased by the positive American reaction to the swap and might decide to deal with the human rights issue "head on," as one official put it, by letting many more political prisoners or would-be emigrants leave the Soviet Union.

In fact, the Soviets have repeatedly resorted to this kind of symbolic gesture during the eight years that they have actively pursued detente with the West. They have found that letting people leave the Soviet Union is a relatively easy way to improve their image abroad, at least temporarily. But thaws have always been followed by new freezes in the past.

The timing of the swap, coming as it does on the heels of a series of other positive signals from Moscow, suggests that the Soviets are preparing to engage actively in SALT II politics in the United States.

Just last week, while a delegation of House members was visiting Moscow, the Soviets announced their intention to release four Jews, who-with two of the prisoners released yesterday - took part in the 1970 Leningrad high-jacking incident.

Since late last year the Soviets have permitted a sharp increase in the rate of Jewish emigration. Jews are now leaving the Soviet Union at an annual rate of more than 40,000.

The intense antidissident campaign conducted in 1977-78 by the Soviet political police, the KGB, came to a head in a series of trials last summer, and has since subsided. No dramatic new cases of harsh official action have been taken against dissidents.

In fforeign affairs, too, the Soviets have appeated cautious. Although they continue to support Cuban troops in Africa, there have been no new adventures there in recent months.

If the main message from Moscow has been positive lately, however, there have also been some negative signals. A recent one was the apparent drugging and harassment of Robin Knight, Moscow correspondent of U.S. News & World Report, and his wife while they were on a trip in Tashkent.

The Soviets made no official announcement in Moscow of the trade last night, which is perhaps a sign of uneasiness about the arrangement. In 1976 the Kremlin was badly embarrassed by its willingness to trade another famous political prisoner, Vladimir Bukovsky, for Luis Covalan Lepe, a leader of the Chilean Communist Party jailed by the government of Gen. Augusto Pinochet.

Several West European Communist parties took this swap as a tacit Soviet admission that it held political prisoners who could be compared to those jailed by the capitalist world's most repressive government's.

Whenever they volunteer to engage in horse trading with the West, the Soviets invite more pressure to go even further, as they have discovered repeatedly in the past. Sakharov took this tack in Moscow last night, applauding this swap but immediately listing individuals still in Soviet camps and prisons for political offenses who he felt also should be freed.

In the United States, Jewish groups and others who have taken up the cause of human rights in the Soviet Union will now turn their attention to the most obvious omissions from the list of those swapped yesterday, Anatoly Scharansky and Yuri Orlov, both serving long prison terms.

U.S. sources confirmed that they tried to include at least Scharansky in this trade. It was learned that President Carter proposed trading the Soviet spies caught in New Jersey for Scharansky last spring and summer, but Soviets would not agree.

The Soviet authorities picked five prisoners for yesterday's swap who have all served many years in prisons and labor camps. All of them had been convicted at least twice by Soviet courts.

Andrei Amalrik, a famous former dissident living in the United States suggested last night that this was a signal to those who hoped for the early release of Scharansky and Orlov.

"They are both serving their first terms," Amalrik noted-putting them perhaps in a very different category.

Senior U.S. officials, visiting congressmen, U.S. scientists and others have repeatedly told Soviet officials that Scharansky and Orlov should be freed if Soviet-American scientific exchanges and relations in general are to proceed constructively. At the highest level the Soviets have repeatedly refused to offer any hope for their early freedom.

But the Senate vote on SALT II is months away, and the trade announced yesterday suggests that many surprises are possible before the final votes are cast. CAPTION: Picture, ANATOLY SCHARANSKY . . . not among those released