Note: This Synoptic Discussion describes recent weather events and climate anomalies in relation to the phenomena that cause the weather. These phenomena include the jet stream, fronts and low pressure systems that bring precipitation, high pressure systems that bring dry weather, and the mechanisms which control these features — such as El Niño, La Niña, and other oceanic and atmospheric drivers (PNA, NAO, AO, and others). The report may contain more technical language than other components of the State of the Climate series.
July is in the middle of climatological summer in the Northern Hemisphere, which is the season of maximum solar heating and warmest temperatures. This season is normally characterized by a contraction of the circumpolar vortex and dominance of the subtropical high pressure centers, but July 2014 was characterized by an active jet stream with high amplitude meridional flow. Cooler-than-normal temperatures dominated the central and eastern U.S. beneath a strong upper-level long-wave trough, while warmer- and drier-than-normal weather prevailed across much of the West due to an intense upper-level ridge. Monsoon showers dumped moisture over the southwestern U.S., while weather systems moving through the eastern trough brought above-normal rainfall to parts of the Southern Plains and East Coast. The upper-level circulation pattern inhibited precipitation over the Northern and Central Plains to Midwest, reduced the occurrence of severe weather, and contributed to large wildfires across the West. Drought contracted where it rained, and expanded where it didn't, resulting in a national drought footprint about the same as a month ago. The upper-level circulation pattern suggested the combined influence of the East Pacific-North Pacific (EP-NP) and North Atlantic (NAO) modes of variability associated with conditions in the North Pacific and North Atlantic oceans. The warmer-than-normal temperatures in the West can be traced to the NAO's teleconnections while the cooler-than-normal temperatures east of the Rockies can be related to the EP-NP's teleconnections. See below for details.
In the Northern Hemisphere, July is in the middle of climatological summer which is the time of year when solar heating forces the jet stream and circumpolar vortex to contract poleward. Polar air masses can still influence the weather, but they are usually rare and not as cold as in previous months. The warm, dry subtropical high pressure belt normally dominates the weather as it shifts northward, with warm southerly air masses holding sway across the contiguous United States (CONUS). In July 2014, the North Pacific subtropical high pressure center exerted influence over the western states and occasionally the southern tier states, but a strong upper-level trough dominated the eastern CONUS. A vigorous upper-level circulation sent several weather systems rippling across the northern and eastern CONUS throughout the month.
These upper-level weather systems generated low pressure systems with their associated cold fronts at the surface — warm air flowed ahead of the lows, with cooler air surging southward behind them. The western ridge produced over 2500 (2596) record warm daily high (720) and low (1876) temperature records, but the eastern trough generated more than twice as many (5508) record cold daily high (3333) and low (2175) temperature records. This was reflected in the national monthly average temperature which ranked July 2014 as the 48th coolest July in the 1895-2014 record. The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. The cold air masses were so persistent and extreme that they gave the Central region the second most extreme CEI component rank in the 1910-2014 record for the cold maximum and cold minimum temperature components. The East North Central region had the fourth most extreme cold maximum temperature component on record. On the other hand, the western ridge gave the Northwest region the second most extreme warm minimum temperature component and third most extreme warm maximum temperature component, and the West region the second most extreme warm minimum temperature component and eighth most extreme warm maximum temperature component. With below-normal temperature anomalies centered over the high population areas of the central and eastern CONUS, the energy needed to cool the nation, as a whole, was down with the July 2014 REDTI (Residential Energy Demand Temperature Index) ranking as the 33rd lowest July REDTI in the 120-year record.
The western ridge and eastern trough set up a predominantly northwest flow in the upper atmosphere over the central CONUS, which inhibited the development of severe weather. The preliminary count of 85 tornadoes ranked below the average count of 134 tornadoes for July.
The dry and stable air masses delivered by the upper-level northwest flow also resulted in drier-than-normal weather for much of the Northwest, Northern Rockies, Northern and Central Plains, and Midwest. Short-wave weather systems traversing the upper-level flow tapped into Gulf of Mexico moisture as they reached the Southern Plains, dropping above-normal precipitation which helped contract drought areas. As the weather systems curved northward on the other side of the long-wave trough, they generated areas of above-normal rainfall along the East Coast. Hurricane Arthur contributed moisture to the region, especially the Northeast, as it merged with a cold front along the coast. Meanwhile, hot and dry weather, along with low humidities, dominated the West, especially parts of the Northwest, beneath the upper-level ridge. This contributed to the expansion of drought areas and the development of large wildfires across the West, especially in the Northwest. Monsoon showers brought above-normal precipitation to parts of the Southwest, helping to contract drought, especially in New Mexico. The net change in drought area was a slight expansion in the moderate to exceptional national drought footprint compared to the end of June.
When integrated across the month, the circulation produced a pattern of above-normal 500-mb heights (stronger-than-normal long-wave ridge) over western North America and below-normal 500-mb heights (stronger-than-normal long-wave trough) over eastern North America centered over the Great Lakes.
Above-normal precipitation fell across much of the Southwest and Northeast and parts of the Southern Plains and East Coast. Precipitation was below normal across the rest of the West and much of the Plains, with patchy dryness from the Midwest to the Southeast. The precipitation pattern was mixed across Alaska, but with wet conditions more evident. The month was generally wetter than normal across Hawaii.
The dominant long-wave circulation pattern of western ridge/eastern trough funneled polar air masses into the CONUS east of the Rockies, resulting in monthly temperature anomalies that were colder than normal in that region, while warmer-than-normal temperatures dominated the West. Alaska averaged mostly colder than normal.
The upper-level circulation over North America is part of the hemispheric mid-latitude westerly circulation. The Northern Hemisphere westerlies during July were characterized by a high amplitude meridional pattern over Eurasia as well as over North America. As with North America, unusually warm and dry weather in Eurasia was associated with the ridges, and unusually cold weather occurred in association with the troughs.
Subtropical highs, and fronts and low pressure systems moving in the mid-latitude storm track flow, are influenced by the broadscale atmospheric circulation. The circulation of the atmosphere can be analyzed and categorized into specific patterns. The tropics, especially the equatorial Pacific Ocean, provides abundant heat energy which largely drives the world's atmospheric and oceanic circulation. The following describes several of these modes or patterns of the atmospheric circulation, their drivers, the temperature and precipitation patterns (or teleconnections) associated with them, and their index values this month:
El Niño Southern Oscillation (ENSO)
- Description: Oceanic and atmospheric conditions in the tropical Pacific Ocean can influence weather across the globe. ENSO is characterized by two extreme modes: El Niño (warmer-than-normal sea surface temperature [SST] anomalies in the tropical Pacific) and La Niña (cooler-than-normal SST anomalies), with the absence of either of these modes termed "ENSO-neutral" conditions.
- Status: Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific continued in an ENSO-neutral state during July. The equatorial Pacific SST anomalies have become near average in the central and east-central Pacific, while remaining warmer than average in the western and eastern Pacific, but the collective atmospheric and oceanic conditions still reflect ENSO-neutral conditions.
- Teleconnections (influence on weather): To the extent teleconnections are known, while in a neutral state, ENSO normally is not a player in the month's weather. Historical data has been analyzed by NOAA to show typical temperature and precipitation patterns associated with El Niño and La Niña ENSO episodes. Teleconnections are not available for ENSO-neutral conditions at that NOAA web site. If an El Niño were happening during July, the typical precipitation pattern would consist of wetter-than-normal conditions in the Northern Rockies and Southeast and parts of the Central Plains, and drier-than-normal conditions in New England, the Southern Plains, and parts of the Midwest.
- Comparison to Observed: The July 2014 precipitation anomaly pattern does not match that expected with an El Niño.
Madden-Julian Oscillation (MJO)
- Description: The MJO is a tropical disturbance or "wave" that propagates eastward around the global tropics with a cycle on the order of 30-60 days. It is characterized by regions of enhanced and suppressed tropical rainfall. One of its indices is a phase diagram which illustrates the phase (1-8) and amplitude of the MJO on a daily basis. The MJO is categorized into eight "phases" depending on the pattern of the location and intensity of the regions of enhanced and suppressed tropical rainfall. The MJO can enter periods of little or no activity, when it becomes neutral or incoherent and has little influence on the weather. Overall, the MJO tends to be most active during ENSO-neutral years, and is often absent during moderate-to-strong El Niño and La Niña episodes.
- Status: The MJO started the month weakly incoherent, then rapidly transitioned through phases 4 through 7 during the month, finally ending the month weak and incoherent again. The MJO indices appeared to be influenced by other modes of coherent subseasonal tropical variability, including Kelvin and Rossby waves and the developing background state related to the transition to El Niño (as indicated by the MJO discussions for July 7, 14, 21, 28, and August 4).
- Teleconnections (influence on weather): The MJO's temperature and precipitation teleconnections to U.S. weather depend on time of year and MJO phase. To the extent teleconnections are known, the June-August teleconnections for temperature are shown here and for precipitation are shown here.
- Comparison to Observed: The MJO is transitory and can change phases (modes) within a month, so it is more closely related to weekly weather patterns than monthly. But the MJO transitioned rapidly this month through several phases, making weekly matches difficult to identify. The July 2014 monthly precipitation anomaly pattern shares some agreement with the teleconnections associated with MJO phases 5 and 6 in the Northern Rockies to Plains and phases 4 and 5 in the Southeast. The July 2014 monthly temperature anomaly pattern, and weekly temperature anomaly patterns for weeks 3, 4, and 5, are similar to the teleconnections for MJO phases 6 and 7.
- The Pacific/North American (PNA) pattern
- Description: The PNA teleconnection pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. PNA-related blocking of the jet stream flow in the Pacific can affect weather downstream over North America, especially the West and especially in the winter half of the year.
- Status: The daily PNA index was positive for the first half of the month then turned negative during the last half, averaging positive for the month as a whole.
- Teleconnections (influence on weather): There are virtually no significant temperature teleconnections during the summer. To the extent teleconnections are known, the precipitation teleconnection map for this time of year (July on the teleconnection maps) shows wetter-than-normal conditions from the Northern Rockies to Southern Plains and drier-than-normal weather in the northern Great Lakes (although the teleconnections for precipitation are weak). The upper-level circulation anomaly teleconnections are also weak, but for a positive PNA, below-normal heights are evident over eastern Canada.
- Comparison to Observed: The July 2014 upper-level circulation and precipitation anomaly patterns show little agreement with a positive PNA.
- The Arctic Oscillation (AO) pattern
- Description: The AO teleconnection pattern relates upper-level circulation over the Arctic to circulation features over the Northern Hemisphere mid-latitudes and is most active during the cold season.
- Status: The daily AO index was generally zero to slightly negative, averaging negative for the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative AO this time of year (June-August) is typically associated with dry conditions in the Northeast and along the Gulf Coast, and wet conditions across parts of the Plains, Midwest, and Southeast, below-normal temperatures in the Northern Plains, New England, and parts of the West, near normal temperatures elsewhere, and upper-level circulation anomalies which are below normal over eastern to central Canada and into the Northern Plains.
- Comparison to Observed: The July 2014 upper-level circulation, precipitation, and temperature anomaly patterns do not match those expected with a negative AO.
- The North Atlantic Oscillation (NAO) pattern
- Description: The NAO teleconnection pattern relates upper-level circulation over the North Atlantic Ocean to circulation features over the Northern Hemisphere mid-latitudes.
- Status: The daily NAO index fluctuated around zero during July, averaging near zero (slightly positive) for the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive NAO during this time of year (July on the teleconnection maps) is associated with warmer-than-normal temperatures in the Northwest to Northern Plains, cooler-than-normal temperatures in the Southern Plains to Southeast, little correlation elsewhere across the CONUS and over Alaska, drier-than-normal conditions in the Northern Plains and Midwest, wetter than normal in the western Gulf of Mexico coast, little correlation elsewhere across the CONUS and Alaska (although the teleconnections are weak for precipitation), and positive upper-level circulation anomalies across the Northern and Central Rockies, Northern Plains, and eastward to New England and the Canadian Maritimes.
- Comparison to Observed: The July 2014 monthly precipitation anomaly pattern agrees over the Northern Plains to the Midwest. The temperature and upper-level circulation anomaly patterns generally agree from the Northwest to the Northern Plains, but not elsewhere (especially the eastern CONUS).
- The West Pacific (WP) pattern
- Description: The WP teleconnection pattern is a primary mode of low-frequency variability over the North Pacific and reflects zonal and meridional variations in the location and intensity of the (East Asian) jet stream in the western Pacific.
- Status: The monthly WP index was negative for the month, with the three-month average WP index negative as well.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative WP during this time of year (July on the maps) is typically associated with above-normal temperatures in the Southwest and Midwest to Northeast, above-normal circulation anomalies over the Southwest, Midwest to Northeast, and eastern Canada, and below-normal circulation anomalies over western Canada. There are virtually no significant precipitation teleconnections during the summer.
- Comparison to Observed: The July 2014 monthly temperature anomaly pattern matches that expected with a negative WP over the Southwest, but is opposite that expected east of the Rockies, and the upper-level circulation anomaly pattern does not match what is expected over the CONUS.
- The East Pacific-North Pacific (EP-NP) pattern
- Description: The EP-NP teleconnection pattern relates SST and upper-level circulation patterns (geopotential height anomalies) over the eastern and northern Pacific to temperature, precipitation, and circulation anomalies downstream over North America. Its influence during the winter is not as strong as during the other three seasons.
- Status: The SST pattern over the northeastern North Pacific during July 2014 saw a continuation of the warmer-than-normal SSTs there, with the magnitude of the warmth increasing over much of the area. The monthly EP-NP index was positive during July, but it has fluctuated around zero in recent months, pulling the 3-month running mean to near zero.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive EP-NP index during this time of year (July on the maps) is typically associated with cooler-than-normal temperatures from the Central Rockies to the Great Lakes and beyond into Canada, warmer-than-normal temperatures in part of Alaska, wetter-than-normal conditions in the vicinity of Montana and parts of the Midwest (although the precipitation teleconnections are very weak), above-normal upper-level circulation anomalies (stronger upper-level ridge) over Alaska and western Canada, and below-normal upper-level circulation anomalies (stronger upper-level trough) from the central CONUS to northeastern Canada.
- Comparison to Observed: The July 2014 temperature and upper-level circulation anomaly patterns have some agreement over the CONUS, especially with the below-normal upper-level circulation anomalies centered over the Great Lakes and the below-normal temperatures from the Northern and Central Plains to the Great Lakes. The above-normal circulation anomalies expected with a positive EP-NP over Alaska and western Canada appear to be shifted to the northwestern CONUS through north central Canada. The precipitation anomaly pattern does not match.
Examination of these circulation indices and their teleconnection patterns, and comparison to observed July 2014 temperature, precipitation, and circulation patterns, suggest that the weather over the CONUS in July was related to the normal chaotic nature of summer convection, but there were hints that the jet stream and ocean-atmosphere interactions over the North Pacific Ocean and North Atlantic Ocean played a role, and the equatorial Pacific may have exerted some limited influence via the MJO mechanism. ENSO was neutral, but equatorial Pacific oceanic conditions were slowly evolving toward an El Niño state, which was affecting the MJO. Even though the MJO was weak or incoherent during most of the month, it appeared to have exerted some influence on temperature and precipitation. The PNA, AO, and WP indices showed very little correlation to the July weather patterns. The teleconnections for the EP-NP match July's upper-level circulation and temperature patterns in the eastern CONUS, while the teleconnections for the NAO match July's upper-level circulation and temperature patterns in the western CONUS and precipitation patterns in parts of the central CONUS, but the value of the NAO index indicated it was very weak. The precipitation teleconnections for all of the indices are very weak this time of the year, indicating that random atmospheric processes (i.e., convection from solar heating) played an important role for July precipitation. This month illustrates how the weather and climate anomaly patterns can be the manifestation of normal (random) atmospheric variability, but also reflect influences from several atmospheric drivers (or modes of atmospheric variability).