The short answer: whether tropical Pacific sea surface temperatures run high or low
El Niño and La Niña are told apart by sea surface temperatures in the tropical Pacific. The KMA Climate Information Portal defines El Niño as a phenomenon in which sea surface temperatures in the tropical eastern and central Pacific stay higher than usual for several months or more, recurring every two to five years; La Niña is its opposite. During La Niña, sea surface temperatures in the central and eastern Pacific fall below their usual levels and the trade winds blow more strongly than normal. A warmer-than-normal ocean means El Niño, a cooler one La Niña.
In Korea, the difference shows up in rainfall and temperature. According to the KMA Climate Information Portal, November and December, when El Niño is at its strongest, tend to bring more precipitation and higher temperatures to the Korean Peninsula; during La Niña those same two months tend to be drier and colder. NOAA describes the two as the warm and cool phases of ENSO, a natural climate pattern of the tropical Pacific. The easiest way to follow all this is to look first at the state of the ocean and then at the weather tendencies that follow.
NOAA Climate.gov's thresholds: 0.5°C in the Niño-3.4 region and five consecutive periods
NOAA puts numbers to the criteria in its ENSO guide on Climate.gov. El Niño conditions mean that the average sea surface temperature in the equatorial Pacific's Niño-3.4 region — the band spanning 5°N–5°S and 120°W–170°W — was at least 0.5°C above normal in the previous month. La Niña conditions mean the average there was at least 0.5°C below normal. Either way, the anomaly must also have lasted, or be expected to last, for five consecutive three-month running-average periods.
NOAA adds that the pattern swings back and forth irregularly every two to seven years, changing ocean temperatures and disrupting the usual wind and rainfall patterns of the tropics. The KMA Climate Information Portal, for its part, says El Niño generally begins in spring or summer, develops through winter, then gradually weakens and dies out the following spring or summer. Taken together, the two explanations suggest that a determination draws on both spatial and temporal criteria.
What the JMA FAQ explains: trade winds, upwelling and the 1988/89 La Niña
The Japan Meteorological Agency (気象庁) gives both a region and a duration in its El Niño FAQ. It defines El Niño as a phenomenon in which sea surface temperatures across the equatorial Pacific — from around the date line to the coast of South America — rise above normal and stay that way for about a year. When temperatures in the same waters remain below normal, the phenomenon is called La Niña. The same source says each occurs at intervals of several years, and notes that the term ENSO is often used when the Southern Oscillation and the two phenomena are viewed as a single linked variation of atmosphere and ocean.
The JMA also traces the movements of wind and water. During El Niño the easterly winds are weaker than usual, so warm water that had piled up in the west spreads eastward and the upwelling of cold water in the east weakens. During La Niña the easterlies blow more strongly than usual, warm water piles up more thickly in the west, and the upwelling of cold water in the east grows stronger than usual.
The FAQ also offers a past case. The 1988/89 La Niña began in the spring of 1988, peaked in December 1988 and ended in the spring of 1989. The JMA considers El Niño and La Niña possible drivers of abnormal weather around the world, including in Japan.
Comparing the definitions: regions and durations at the KMA, NOAA and JMA
The three agencies point to similar stretches of ocean but describe them differently. The KMA Climate Information Portal cites the tropical eastern and central Pacific; the JMA cites the equatorial Pacific from around the date line to the coast of South America; NOAA uses the Niño-3.4 region as its benchmark.
The wording on duration differs too: several months or more for the KMA, about a year for the JMA, and five consecutive three-month running-average periods for NOAA. On frequency, the KMA says every two to five years, NOAA every two to seven years and the JMA at intervals of several years. The 0.5°C numerical threshold comes from NOAA's explanation.
All three sources are publicly available, so readers can go straight to the originals: the KMA Climate Information Portal's climate prediction guide, NOAA's Climate.gov ENSO page and the JMA's El Niño FAQ. NOAA's Climate Prediction Center, together with the International Research Institute for Climate and Society (IRI), issues the official ENSO status update on the second Thursday of each month. When definitions seem to clash, it is arguably best to check first whose criteria are in play.
Korea's impacts as compiled by the KMA Climate Information Portal: November 1982, 1997 and 2015
The KMA Climate Information Portal breaks Korea's tendencies down by period. From mid-July to mid-August, precipitation tends to increase during El Niño and decrease during La Niña, mainly in the southern part of the Korean Peninsula. In September during La Niña, an anticyclonic flow settles to the southeast of the peninsula and a southerly airstream appears, bringing more precipitation and higher temperatures. In November and December, when El Niño is at its strongest, precipitation tends to increase and temperatures tend to rise; in those same months during La Niña, precipitation tends to decrease and temperatures tend to fall.
The portal's section on effects over the Korean Peninsula names specific years: "The years in which November precipitation reached 100 mm or more — more than twice the normal amount — were 1982, 1997 and 2015, all years in which El Niño developed strongly." NOAA, however, notes that the effects of El Niño and La Niña are weaker in the Northern Hemisphere summer than in winter. Taken together, the two explanations suggest that the strength of the signal varies with the season.
In summary
El Niño and La Niña are separated by whether tropical Pacific sea surface temperatures run above or below normal. The KMA Climate Information Portal describes the two as opposite states. NOAA offers a measurable test: a 0.5°C anomaly in the Niño-3.4 region sustained over five consecutive three-month running-average periods. The JMA gives a region and a duration of about a year, along with the shifts in the easterly winds and in upwelling.
The effects on Korea come as month-by-month tendencies. In November and December, when El Niño is at its strongest, more precipitation and higher temperatures are reported; in the same months during La Niña, less precipitation and lower temperatures. One could say that choosing explanations that spell out the numbers, the months and the regions together leaves less room for misunderstanding.
