The Guest Who Won’t Leave
Published in the EarthSphere Blog. Feature Image: Cooling Pacific Waters (© Archean Enterprises, LLC; Archeanart)
La Niña conditions, which started two years ago, are set to continue according to the National Oceanic and Atmospheric Administration (NOAA). La Niña conditions in the tropical Pacific Ocean are near record intensity for this time of year. This report raises the likely possibility of a third consecutive fall and winter season with our uninvited guest.
La Niña and her brother El Niño are alternating phases of a natural weather pattern called the “El Niño-Southern Oscillation” (ENSO). La Niña is also referred to as El Viejo — the old man. During the La Niña phase, easterly trade winds strengthen over the Pacific Ocean, leading to an upwelling of cooler water from the deep ocean.
The equatorial trade winds in the Pacific blow from east to west. When the winds are weak, they don’t effectively move the surface waters, and heat from the sun accumulates, warming the ocean’s surface (El Niño). But when ENSO oscillates to the La Niña stage, stronger trade winds push the warmed water into the western Pacific Ocean.
The process starts off the coast of western South America, and as the warm surface waters move westward, colder water from the deep ocean upwells to fill the void. A cold-water wedge is then pushed westward behind the retreating warm water, forming an elongate plume stretching from South America to the western Pacific.
Good News — Bad News
A good aspect of La Niña is cooler water in the tropical Pacific, which exerts a cooling influence on the atmosphere and temporarily dampens global warming. But the bad news is that this temporary cooling does not equate to a reduction in greenhouse gases, and when the warmer El Niño conditions return, global warming will resume with a vengeance.
It is also noteworthy to mention our long-suffering friends in the American SouthWest, who will take no comfort from another year of La Niña. They have watched while a twenty-year megadrought in the American West has drained the region of water, leaving major reservoirs at record low levels and ripening the conditions for massive wildfires.
La Niña tends to push wet weather northward, leaving Southern California and the southwest dry while adding extra moisture to the Pacific Northwest. Our unwanted guest’s lengthy stay exacerbates the ongoing megadrought and drys the region’s forests.
But no one is off the hook, and La Niña conditions reach far eastward, affecting the US Southeast. Residents along the Atlantic and Gulf Coast can look forward to more wind, rain, and destruction.
Pacific to Atlantic — More Hurricanes?
Cooler water in the equatorial Pacific has a knock-on effect. The oceans chill the atmosphere, and because temperature changes drive air circulation, La Niña changes the jet stream’s position pushing it northward. Even though the jet stream rides high above us in the upper atmosphere, its reach extends downward to the surface affecting weather throughout North America. One of the connections La Niña makes is between colder Pacific water and increased tropical storms in the Atlantic Ocean 5,000 to 10,000 miles away.
The connection works through a phenomenon called “wind shear.” Between North Africa and the Caribbean, warm moist air rises from the ocean’s surface. As this warm air leaves the ocean surface, a sea breeze develops to fill the space left by the rising air. The heat from the ocean warms the breeze, and this air also rises. The greater the volume of rising air, the faster the winds blow, and a tropical storm is born.
The storm’s moist, warm air needs somewhere to go, and the only option is up. The higher up it travels, the stronger the circulation cell, and the more intense the storm — this is where wind shear comes into play. Wind shear refers to conditions where mid and upper atmospheric winds travel at different speeds and directions than the surface and lower atmospheric winds. Under these conditions, rising air from a tropical storm reaches a point where upper-level winds blow away the top of the circulation cell. It takes the oomph out of the storm and keeps it from growing stronger.
Simply put, more wind shear equals fewer tropical storms. The antithesis of this is true also. Less high-level atmospheric activity reduces wind shear and promotes more tropical storms. Enter La Niña. She pushes the jet stream northward, leaving calmer mid and upper-level atmospheric conditions over the Atlantic’s hurricane alley. The result is that more tropical storms form, and more of these storms turn into hurricanes.
So it looks like the “Old Man” might spread his grumpiness for another year.
(This article borrows heavily from one of my previous articles: A Cooling Trend Drives Rain, Wind, and Fire.)
Sources:
La Niña heads for rare “triple dip,” dampening global temperatures (Source Axios)