Over 180 000 people have been evacuated from their homes after damage to the Lake Oroville Dam (America's highest dam) after damage was discovered to the main spillway of the dam on Tuesday 7 February 2017 (a spillway is an overflow channel that releases water from a dam to stop it overfilling). The flow of water was initially switched to a second, emergency spillway, but damage to this channel was discovered on Sunday 12 February, prompting the evacuations.
Damage to the auxiliary spillway at Lake Oroville Dam. Kelly Grow/California Department of Water Resources.
The damage is particularly worrying as it comes after a winter of exceptional rains as California has been battered by a succession of Pacific storms, bringing widespread flooding and triggering a number of landslip and subsidence events. This is due to exceptionally high temperatures over the Pacific Ocean off the coast of the state this year due to a La Niña weather system over the southern Pacific. This meant that the dam was already filled to capacity and regularly using the spillways to remove water when the damage was discovered, with another major series of storms due to start ariving later this week.
The La Niña weather system is the opposite of the El Niño weather system, in which unusually cold surface temperatures spread across the equatorial Pacific from the upwelling zone on the South American coast. This traps warm water from the western Pacific, preventing it from spreading east and warming the central Pacific. This leads to lower evaporation over the (cooler) east Pacific, leading to low rainfall on the west coast of South America, and higher evaporation over the (warmer) west Pacific, leading to higher rainfall over East and Southeast Asia and northern Australia.
The effects of a La Niña weather system in December-February. NOAA.
This also leads to a breakdown in surface circulation in the North Pacific, which generally rotates clockwise, so that the same body of water stays off the coast of California, where it is constantly warmed by the Sun, leading to high levels of evaporation and onshore winds that bring high rainfall and flood events to the state.
The effect of the La Niña weather system on the weather of North America. NOAA.
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