On June 30, 2013, the U.S. Southwest experienced some of its hottest days ever since records have been kept. The hottest place of all, Death Valley, tied its June heat record by reaching 129 degrees, just five degrees short of its all-time world heat record. The persistent heat wave also tied or broke many area records for consecutive heat, and it followed a spring which had already had above-average temperatures. Yet heat waves are not unusual in the Southwest. The extreme heat event of June and July 2013 was caused by the same basic factors which cause the Southwest to be hot and dry to begin with, amplified by specific weather conditions.
Much of the American Southwest is occupied by the Sonoran and Mojave Deserts, which are home to Death Valley, the hottest place on Earth. Most of the remainder of the Four Corner states, such as the Colorado Plateau, are either mountainous or arid grassland.
Nearly all parts of the American Southwest, with the exception of a few microclimate pockets, lie in the rain shadow of the Sierra Nevada, the Rocky Mountains and other major mountain ranges. As a result, nearly all of the Southwest is extremely dry, with high evaporation indexes.
However, most parts of the Southwest normally experience occasional precipitation even during the hottest months. In late June and early July of 2013, there was very little precipitation at all. In fact, the Las Vegas Valley recorded no precipitation at all throughout June. This has happened 20 times since records have been kept at McCarran.
The persistent heat of June and July 2013 was directly caused by a ridge of high pressure which was trapped over the Southwest by a deep bend in the jet stream. As long as the bend in the jet stream remained in place, cool air from the north was funnelled to the east, while the high pressure system stayed parked over the west. This is called a blocking pattern.
In turn, the high pressure system created a cap of sinking warm air over the surface air. This effect traps ground level warmth at ground level. It also acts as a barrier to convection, which decreases the chance of rain.
Under unrelenting sunshine and with little chance for the heat to escape, the stagnant air under that high pressure system grew hotter and hotter. Even at night, the temperature in many parts of the Southwest did not go below 90 for days on end.
Heavy use of air conditioning during the heat wave only increased the problem. Although air conditioning ameliorates interior temperatures, it also increases the exterior temperatures in the vicinity of the air conditioning unit. This contributes to the urban heat island effect, especially during heat waves.
This heat wave could not end until the bight in the jet stream moved away from the Southwest. The same bend would go on to produce a severe heat wave in the UK later that July and August.
The jet stream
Deep bends in the jet stream are associated with erratic weather patterns which can remain stagnant for extended periods of time. The trapped weather system may bring prolonged heat, rain or cold, depending on the type of system which is trapped. An extremely deep bend in the jet stream can even temporarily bend back on itself, reversing the normal direction of global winds, which is what caused Hurricane Sandy to turn northwest so late in its life. However, the cause of those bends is not yet understood.
Some research has connected deep bends in the jet stream with low solar activity. However, this cannot explain the June-July 2013 heat wave in the Southwest, which occurred at close to solar maximum.
A new direction of climate study has connected increasing, deeper and more persistent bends in the jet stream with reduced winter snow cover in the far north. Some scientists are studying the effects of land temperature variations caused by decreasing snow cover in Siberia, while others are focusing on the effects of reduced Arctic sea ice. These areas of research are still too new for any degree of certainty.
There is no direct causative link between climate change and any specific weather event. This includes hurricanes, tornadoes and the Southwest heat wave of June-July 2013.
However, climate change is occurring and is a factor in heat anomalies. Qualified scientists have confirmed that the average temperature of the Earth has increased by just over one degree over the last century. Most scientists now agree that by the end of this century, the average temperature of the Earth will increase by another three degrees.
Because severe weather is driven by heat energy, even a slight rise in temperature is likely to make the weather more erratic. Especially, flood and drought cycles tend to become more extreme, both in intensity and in duration. Thus, while the June-July 2013 heat wave was not caused by global warming, its severity may have been affected by it.