Reposted by The Cliff Mass Weather Blog
This winter was dominated by La Nina. a period of below average sea surface temperatures in the tropical Pacific. The impact of La Nina on the west coast has been profound, with above-average snow cover in the northwest and dry conditions in the southwestern United States
Typical conditions during a La Nina winter
But there is important news at the front of La Nina. La Nina is collapsing now and should be history in a few months.
To show this, let’s first consider the sea surface temperature anomaly (difference from normal) for the central tropical Pacific, an area known as the Nino 3.4 region (see map).
As shown below, sea surface temperatures in this area were colder than normal (blue colors), peaking at 1.8 ° C below normal in late October. Whenever the temperature is 0.5 ° C or colder than normal, we consider it the La Nina period.
But look carefully: the cold temperature anomaly has weakened significantly to around 0.6 ° C. La Nina is radically weakened.
In fact, if it gets much lower (less than 0.5 ° C below normal) we would call it a neutral or normal period.
Another way to see the situation is by looking at sea surface temperatures beneath the waves – in the top few hundred meters of the ocean. This is shown by vertical east-west cross sections along the equator over the top 300 meters of the Pacific for February 7th through late March. Orange / red means above average sea temperatures and blue means below average.
In early February there was considerable cold water in the upper parts of the Pacific, with warm water being restricted to the western part of the ocean. But by the end of March, almost all of the cold water was gone, and warm water had flowed below the surface about three-quarters of the way across the Pacific Ocean to the east. When this warm water mixes with the surface, La Nina quickly becomes history.
Almost all current forecasts of the atmospheric-ocean model (see below) assume that the surface temperatures of the Central Pacific Sea in the Nino 3.4 area will warm up. resulting in a transition to neutral or La Nada conditions. I should note that such spring forecasts are not reliable, a phenomenon known as the “spring forecast barrier”. But by July we should have a very good idea of what the next winter will bring.
These rapid changes in La Nina conditions are good news for California, which has seen two dry winters in a row, as neutral conditions should put the Golden State at increased risk for humid atmospheric river conditions. And such neutral years are often associated with the most active weather in the northwest, which is estimated by meteorologists.