Flooding is common in St. Cloud MN

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At the beginning of the year there was a very sharp rise in water temperature on the coast of Peru and Ecuador. Image: NOAA

In the past few weeks, there have been more frequent reports of floods in Peru. Heavy rains caused one of the worst flood disasters in the last 20 years. Although it is currently the rainy season in Peru, the current rainfall of this magnitude is absolutely unusual for the regions. Repeatedly violent thunderstorms brought amounts of rain of sometimes more than 200 mm within one day. This is double to triple the amount of precipitation that is usual in Germany for a whole month. Often these rains have been linked to the El Nino phenomenon. El Nino is a circulation and water temperature anomaly in the central Pacific that often occurs around Christmas time. This is why El Nino has its name, which means "boy" or "Christ child" in German.

Usually the east trade winds drive warm surface water from the Peruvian coast to the west. As a result, cooler deep water in the area of ​​the northwest coast of South America swells to the surface. This water comes from the cold Humboldt Current. With the El Nino phenomenon, the east trade winds weaken significantly, which means that less cold water swells to the surface and the water temperature rises significantly off the coast. More information about the creation of El Nino can be found in the topic of the day from 08/09/2015).

After an El Nino subsides, this anomaly usually reverses into its opposite. Then the east trade winds are stronger, the Humboldt Current brings significantly colder deep water to the surface along the coast. This anomaly is called La Nina, "the girl". But how does El Nino cause rain on the Peruvian coast? Usually the cold coastal water is a barrier to the thunderstorms that form over the Pacific. The coastal areas of Peru are usually relatively dry. This barrier is absent at warmer water surface temperatures during the El Nino phases. On the contrary: The thunderstorms are supplied with additional moisture and energy by the warm water and can move to the mainland, where they then accumulate on the Andes. During the El Nino phases, heavy rains often occur there. However, the soil in the otherwise dry areas cannot absorb as much water. Flash floods and mudslides are the result, which spread from the Andes far downstream towards the coast.

The rains in Peru are currently not caused by the classic El Nino phenomenon. According to the classic definition, the conditions in the central Pacific are currently neutral. That means, there are neither El Nino nor La Nina conditions. However, El Nino showed an unusual development. After the years 2014 to 2016, in which the strongest El Nino event since 1997/98 occurred, at the end of last year the La Nina conditions were only briefly weak before the conditions changed back to "neutral". However, at the beginning of the year there was a very sharp rise in the water temperature on the coast of Peru and Ecuador, which resulted in heavy rainfall there due to the processes described above (see figure of the water surface temperature). The Peruvian Meteorological Service has therefore issued a so-called "Coast El Nino" warning. However, since a weak La Nina circulation pattern still prevails over a large area, the thunderstorms have it easier to pull ashore, which means that the amounts of rain are more devastating than during normal El Nino phases.

In the next few days, the northern province expects more heavy rainfall, before easing begins on Sunday. But as long as the warm water is off the coast, strong thunderstorms and heavy rains must still be expected.

Dipl.-Met. Christian Herold
German Weather Service
Prediction and advice center
Offenbach, March 29th, 2017

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