Jewish emigration from the Soviet Union, which rose dramatically with the advent of detente in the early 1970s and reached its record level last year, now appears to be on a sharp decline, presumably due to a cooler international climate.

Obsevers here believe that the indications of a more restrictive Soviet emigration policy were linked to worsened Soviet-American relations. The reasons also may be bureaucratic and linked to increase visa demands for the upcoming Moscow Summer Olympics.

Western diploma tic sources said 6,139 Jews were granter permission to leave the Soviet Union in the first two months of 1980 compared with 8,166 for the same period last year. The decline had long been predicted by foreign sources here familiar with Soviet bureaucratic problems in processing visas for Olympic visitors and Soviet resentment at the loss of possible U.S. trade concessions in the aftermath of Moscow's invasion of Afghanistan last December.

These sources said the flow of Jewish emigres from the major Ukranian cities of Kiev, Odessa and Kharkov has virtually ceased in recent weeks, accounting for the overall decline. They said permission to emigrate apparently has continue to be granted for Soviet Jews from Moscow, Leningrand, Riga, Minsk and other cities, at about the same level.

Jewish emigration from the Soviet Union topped 50,000 last year, a record, and was the highest number recorded since 1973, when 35,000 Jews were granted visas. Emigration of Jews from the Soviet Union, which has closed borders, has been tied in the past to apparent Soviet hopes for improved trade credits from Washington. The Jackson-Vanik amendment to the 1972 trade act tied Jewish emigration to Moscow's most favored-nation trade status.

The sources said there is no apparent reason for the marked reductions in Soviet approvals for emigration by Ukrainain Jews. Jewish activists here have said in recent weeks that regional emigration offices in the Ukraine and elsewhere have reduced working hours sharply. There is throught to be a growing backlog of unprocessed exit applications from Soviet Jews because of rising applications.

Many Moscow Jews have predicted with foreboding that emigration will be cut in connection with the Olympics. Some of these sources have said that Jews in the Byelorussion capital of Minsk have been told to apply now if they ever wish to leave.

The sense of depression has increased in the aftermath of U.S. retaliation against Moscow for the Afghan intervention. Jews here have applauded the actions of President Carter. At the same time they ruefully surmise, in the words of one activist, that "this removes any incentive" for the government to continue allowing Jews to leave at 1979 levels.

While some diplomatic sources predict that emigration in 1980 will average 2,800 to 3,000 a month in line with previous years, Soviet policy on the Jewish exodus is clouded by unknowns. The soviets have never conceded any connection between emigration and foreign pressure. It is widely believer here, however, that U.S. granting of most-favored-nation trade benefits to China, Moscow's archrival, ahead of the Soviet Union, can only have an adverse impact on emigration.

Jewish activists and Western sources alike have said that since last fall, Soviet officials increasingly have rejected applications from Jews seeking to reunite abroad with anyone other than immediate relatives -- parents, spouses, siblings, children. In previous years, more distant relations, such as aunts, uncles, and cousins, had been considered in many cases as sufficiently close to grant exit visas under reunification provisions of various international agreements signed by the Kremlin.

There are an estimated 2.5 million to 3 million Jews in the Soviet Union. Together wiith Volga Germans and Armenians, they are about the only Soviet citizens with any hope of being permitted to emigrat e.