WASHINGTON (AP) _ Scientists trying to unravel the mysteries of El Nino are studying floods on the Nile, using information collected by instruments introduced in the days of the pharaohs.

The annual Nile floods were vital to Egyptian agriculture, so the ancient pharaohs introduced the nilometer, a device to measure the height of the river flood.

Now researchers at the Massachusetts Institute of Technology are finding those records useful in estimating the frequency of the El Nino climate phenomenon in the past.

They have discovered a period, just over 1,000 years ago, that had relatively frequent El Ninos, similar to the pattern of the past 20 years, according to a paper in the Feb. 15 edition of Geophysical Research Letters.

Researchers Elfatih A.B. Eltahir and Guiling Wang looked at Nile flood records dating back to A.D. 650 _ long after the pharaohs but still recorded using the nilometer system _ to develop an estimate of past El Ninos.

El Nino, an unusual warming of the water in the eastern tropical Pacific Ocean, can have worldwide impacts on weather. Sea surface temperature records go back only to the late 1800s, so climate researchers have looked for other ways to determine when past El Ninos might have occurred.

When an El Nino is under way, rain is reduced over the sources of the Nile. Studies have found that about 30 percent of the change in water flow on the Nile at Aswan can be attributed to El Nino.

The paper shows frequent El Ninos between about 750 and 1000 with a reduction after that, and then an increase again in the latter years of this century. It did not speculate on the reason for the correlation between the two periods.