Analyze monthly mean SSTs in 2° ´ 2° grids.
2. Calculate monthly SST anomalies averaged for the
area 4°N–4°S and 90°–150°W. This is essentially
the region known as “Niño 3” (although the latter
extends to ±5° latitude).
3. Find periods during which 5-month running means
of the monthly SST anomalies in the above-mentioned
area are +0.5°C or more for at least six consecutive
months.