La Nina is a weather pattern that develops when the waters of the Pacific Ocean near the equator become cooler than normal. This is the direct opposite of El Nino, which occurs when the equatorial Pacific becomes warmer than average. Both conditions can have impacts on the weather in the southwestern United States. Here is a look at La Nina impact on weather in the southwest.
The cooler water near the equator allows the formation of a high-pressure center in the atmosphere that prevents the jet stream from dipping as far south as it normally does during the winter months. Because most frontal systems that produce rain tend to follow the jet stream from west to east, this farther north placement prevents rainfall from occurring in the states of Arizona, New Mexico, and in the region known as the four corners.
Because the southwest is known for its low rainfall amounts already, this shift in the jet stream creates a drought condition. This can be especially hazardous following an El Nino because the additional rainfall created by El Nino causes local vegetation to flourish. The ensuing drought can then dry everything to a point where the slightest spark can set off a wildfire that can cover thousands of acres in a very short time.
The lack of rainfall in the mountains on the western edge of this region, as well as the lack of snowfall in winter, reduces the amount of groundwater flowing into rivers and streams in the area. It can also cause a fall in the level of water in the aquifer that provides well water to the region. This can create a shortage of drinking water and water available for the irrigation of crops.
The region has a period during the summer months that is known as its monsoon season. During this time, it rains almost daily. However, the rains are usually only seen in very localized areas and fall so quickly that the water quickly runs off. Storms in winter are quite different. These tend to cover a larger area and last for days or weeks.
La Nina reaches its peak effect in winter. This means that the winter storms that produce the vast majority of the rain and snow in the region travel a more northerly course and miss the southwest almost entirely. This reduces the moisture in a region that is already considered to be deficient to fall well below sustainable levels.
The effects of this drought are likely to be felt for years, even after the drought ends and normal rain and snow patterns are restored. Water that normally runs down from the mountains in the spring and feeds rivers such as the Rio Grande is not there to flow down.
This shortage is widespread and affects not only humans but animal and plant life that are native to the region. The drop in the water table below the surface of the ground can take several years to be brought back to previous levels. The constant drain on the ground water supply by people in the area can prolong this effect.