As part of its new emphasis on our Caribbean "back yard," the Carter administration is trying to provide more jobs, investment and development to that economically troubled region through an alliance of rich and poor nations.

Quietly during the past several weeks, U.S. policy planners have taken the initiative in outlining an economic coordinating mechanism tentatively known as "the Caribbean consultative group."

Its purpose: to bring together under one umbrella the isolated, poverty stricken islands that need help and the industrial countries and international lending institutions that have been involved in providing development assistance to the area.

The administration's hope that this group will overcome a present crazy-quilt pattern of outside assistance that results from a tendency among so-called "donor" countries to grant loans and start programs without paying much attention to what other governments are doing.

In the past, that has caused a lot of assistance to be dissipated by duplication of effor or perverted into competitive bidding for influence over recipient countries.

By assembling donors and recipients in a forum where they can examine the region's needs across the board, administration planners argue, the outside assistance can be distributed in ways that are more complementary and efficient.

The consultative group also is supposed to encourage the Caribbean countries - small, scattered and separated by the sea - to explore ways of overcoming their geographic handicaps by working together in regional cooperation efforts.

Administration officals contend that such cooperation is possible in a number of areas - pooling air and sea transportation resources, restructuring educational facilities regionally so they offer complementary rather than competing services, finding ways to industrialize and share the market for the region's common export products like sugar.

The many past efforts to promote such regional cooperation have largely failed. Among the obstacles: cultural aloofness of the Latin countries like Haiti and the Dominican Republic from the English-speaking islands, and jealousies and rivalries among those with an English colonial heritage.

The latest effort to breathe new life into regional cooperation stems from an administration conviction that the United States has a vital stake in Caribbean political and economic stability.

Unemployment - in some sections 30 per cent of the work force - increases the pressures for immigration into the United States illegally. Similarly, U.S. officials point out, continued poverty and backwardness can lead easily to the rise of extremist movements in an area that is often called America's "third border."

For these reasons, such top administration officials as Secretary of State Cyrus R. Vance began talking with Caribbean leaders last spring about a reinvigorated U.S. role. In August, the effort was given a publicity push when U.N. Ambassador Andrew Young made a whirlwind tour.

Following Young's trip, a special Caribbean task force, embracing a broad range of federal agencies, began sounding out other countries about joining the proposed consultative group.

Despite some problems, the response was sufficiently positive to enable Rosalynn Carter to unveil the broad outlines of the plan in a speech before a group of newspaper editors in Puerto Rico last week.

The next step, administration officials say, will be the actual creation of the consultative group. Tentative plans call for that to be done at an early-December meeting.

In its emerging form, the group is expected to include most of the Caribbean islands, except Cuba, and the mainland countries of Guyana, Belize and Surinam, whose basic ties are with the islands. Collectively, that means about 15 countries with a combined population of approximately 15 million.

Initial donor countries are expected to be the United States, whose current aid to the area runs at roughly $100 million a year: Canada, with an annual aid program of approximately $60 million, and Britain, which contributes about $50 million a year to its former Caribbean colonies.

Also in the donor category are two Caribbean-area countries - Venezuela and Trinidad - that have amassed substantial oil revenues in recent years and have started providing financial aid to their neighbors. Last year, Venezuela put about $75 million into this effort, Trinidad about $70 million.

Finally, the group will include the two leading institutions - the World Bank and the Inter-American Development Bank - through which much of the aid to the Caribbean is channeled. Once the group is formed and operating, it will function as a specialized part of the World Bank, with the bank providing the staff to run its affairs.

Some Caribbean countries have resisted the administrative role for the World Bank. That apparently results from the bank's traditional concern with balanced budgets and fiscal austerity - positions that have brought it into conflict with the deficit-spending policies of some Caribbean governments.