The nation's merchandise trade deficit was $9.9 billion in November, reaching $115.4 billion for the first 11 months of the year and heading toward new heights for 1984, the Commerce Department reported yesterday.

But Deputy Commerce Secretary Clarence J. Brown predicted the deficit is likely to fall somewhat short of the predicted $130 billion total for the year because the growth in imports has begun to slow.

"Unless December figures are aberrational," he said, "the 1984 trade deficit is likely to be closer to $125 billion than the higher mid-year projection of $130 billion."

Even so, the record 1984 trade deficit will be almost twice last year's record of $69.4 billion and will mark the first time the United States' trade deficit has topped $100 billion.

The soaring trade deficit is slowing down the country's economic growth as the increased demand is being met by foreign producers, Commerce Department Chief Economist Robert Ortner said. The third-quarter gross national product would have been a robust 5.5 percent instead of the more anemic 1.6 percent if it had not been for the trade deficit.

"Demand went up, but so did imports," Ortner said.

The November figures were slightly higher than October's $9.2 billion deficit. On the bright side, the November trade deficit ran below the monthly average of $10.5 billion and was far less than the the peak month of July, when the deficit soared to a monthly record of $14.06 billion.

Much of the trade deficit has been caused by a flood of imports, drawn by a strong dollar that made them less expensive to American consumers and by the U.S. economic recovery, which has been the most vigorous in the world.

Although imports for the first 11 months of the year increased 27.8 percent over last year, the slowdown in their growth indicates that the recovery is spreading to other countries. In October, there was a substantial, 16.7 percent decline in imports from the month before, while the November increase in imports was a slight 2.7 percent.

Brown said fourth-quarter totals should fall "well below" the $36.6 billion level for the third quarter.

Brown placed major blame for the continued trade deficit on the strong dollar, which raises the cost of American-made products in foreign markets and makes imports less expensive in the United States. With the dollar remaining high, he said, "the trade deficit is likely to increase further in the early part of 1985."

Imports in November totaled $28.3 billion, a 2.7 percent increase over October and 17.1 percent higher than November 1983. But the November import deficit stood $300 million below the monthly average of $28.6 billion for the first 11 months of 1984.

There were large increases in imports of manufactured products, including new cars, electrical machinery and clothing.

The import deficit was held down by a negligible, 0.2 percent increase in oil imports. Heating oil distributers continued to keep stocks low in anticipation of prices falling even further. The average price per barrel of petroleum products has dropped almost $1.50 from last year.

Exports remained essentially flat after two straight months of increases. The November export figures, however, were $1.3 billion higher than the same month last year, a slightly greater increase than the average for the first 11 months of 1984.

Gains in exports of farm products -- especially corn, soybeans, tobacco and animal feeds -- and petroleum products overcame decreases in overseas sales of American manufactured goods. Products such as electrical machinery, chemicals, parts for data-processing equipment and telecommunications equipment suffered export declines.

Agricultural sales jumped 15.2 percent, or $3.19 billion last month.

Once again, the United States registered its largest trade deficit with Japan, $2.7 billion for the month. It was, however, the smallest monthly trade deficit with Japan since April, as the 11-month total climbed to a record $34 billion.

Japan accounts for almost one-third of the entire U.S. merchandise trade defict, which is expected to be a major item on the agenda when President Reagan meets Japanese Prime Minister Yasuhiro Nakasone in California on Wednesday.

Western Europe recorded a $1.9 billion November trade surplus with the United States, followed by Canada, with a $1.8 billion surplus, and the members of the Organization of Petroleum Exporting Countries, with a $1.1 billion surplus. The United States, however, maintained a $121.6 million trade surplus with Saudi Arabia.