Earth could see a 'mega' El Niño this year that would bring some crazy weather

http://www.businessinsider.com/el-nino-2015-potential-effects-and-global-warming-2015-9

As the summer ends, heat is dominating the meteorological landscape, with the warmest month ever recorded and the drought continuing unabated in California.

At the same time, it is clear that an El Niño is building that is expected to culminate in the fall and last until the winter, with the possibility of it becoming a “mega” El Niño.

The hope in California is that the large amounts of precipitation usually associated with extreme El Niño events would lessen the impacts of the state’s multi-year drought by partly refilling reservoirs and groundwater, even as scientists caution that this might not happen to the degree needed to alter the present situation.

What drives the El Niño weather pattern and what do scientists know about El Niño under man-made greenhouse warming?

A tropical Pacific phenomenon with global influence

To be clear, El Niño is a tropical Pacific phenomenon, even though it represents the strongest year-to-year meteorological fluctuation on the planet and disrupts the circulation of the global atmosphere. When sea surface temperature changes – or anomalies – in the eastern equatorial Pacific exceed a certain threshold, it becomes an El Niño.

What are the mechanisms behind El Niño? In normal conditions in the tropical Pacific, the trade winds blow from east to west, driving ocean currents westwards underneath. These currents transport warm water that is heated by low-latitude solar radiation and eventually piles up in the western Pacific. As a result, heat accumulates in the upper ocean.

The warm water evaporates from the ocean surface, and the light, warm and humid air rises, leading to deep convection in the form of towering cumulonimbus clouds and heavy precipitation.

As this air ascends, it reaches upper levels of the troposphere and returns eastwards to eventually sink over the cooler water of the eastern Pacific. This east-west (zonal) circulation is called the Walker Circulation.

What happens to the atmosphere and the ocean during El Niño?

This circulation gets disrupted every few years by El Niño or enhanced by La Niña, the opposite effect. This periodic, naturally occurring phenomenon is called the El-Niño Southern Oscillation (ENSO).

During the typical El Niño, the warm phase of that oscillation, the trade winds weaken, and episodic westerly wind bursts in the western equatorial Pacific generate internal waves into the ocean. These waves trigger the transport of the warm water from the west to the east of the basin.

This induces a reduction of the upwelling (upward motion) of cold water in the east, at the equator and along the coast. It also creates warm sea surface temperature anomalies along the equator from the international dateline in the Pacific to the coast of South America.

As the central part of the Pacific warms up during El Niño, the atmospheric convection that normally occurs over the western warm pool migrates to the central Pacific. That transfer of heat from the ocean to the atmosphere gives rise to extraordinary rainfall in the normally dry eastern equatorial Pacific.

Warm air then flows from the west, feeding this convection and further weakening the east-west-flowing trade winds. This leads to further warming as this feedback loop amplifies the phenomenon and ensures that deep atmospheric convection and rainfall patterns are maintained in the central equatorial Pacific.

El Niño eventually ends when changes in the ocean cause negative feedbacks that reverse the dynamics that create the El Niño effects.