Synoptic Discussion - June 2014
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.
June marks the beginning 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 the numerous large short-wave troughs and lows traversing the contiguous U.S. in a fast westerly flow indicated that the jet stream was reluctant to migrate north. The troughs and associated surface fronts generated areas of heavy precipitation in the Plains and Midwest, along with severe weather. The circulation pattern inhibited precipitation across much of the West, where drought intensified, but overall the national drought footprint contracted due to the beneficial rains in the Plains. The upper-level circulation pattern still reflected the influence of the West Pacific (WP) mode of variability associated with conditions in the North Pacific. The WP's teleconnections include warmer-than-normal temperatures in the Southwest and northeastern United States. Signs of the influence of other modes of circulation were seen in the temperature and precipitation anomaly patterns, especially the Arctic (AO) and North Atlantic (NAO) modes of variability, as evidenced by colder-than-normal temperatures in the Northwest to Northern Plains and above-normal precipitation in the Plains to Midwest. See below for details.
In the Northern Hemisphere, June is at the beginning 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 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 moving into the contiguous United States (CONUS) more frequently. In June 2014, the North Pacific and North Atlantic subtropical high pressure centers exerted influence over the southern tier states, but the jet stream lingered across the northern half of the U.S., producing a strong westerly flow in the upper-level circulation which sent several weather systems rippling across the CONUS. Occasionally the upper-level trough/ridge pattern would stall and amplify, with upper-level lows and troughs moving slowly across a region.
These upper-level weather systems generated low pressure systems with their associated warm and cold fronts at the surface — warm air flowed ahead of the lows, with cooler air surging southward behind them. There were more record cool high temperature records (676) than record warm high temperature records (391), but warm nighttime temperatures dominated with more record warm low temperatures (1257) than record cold low temperatures (344). When aggregated together, there were more than one and a half times as many record warm daily highs and lows (1648) as record cold daily highs and lows (1020), and this was reflected in the national monthly average temperature which ranked June 2014 as the 33rd warmest June in the 1895-2014 record. With the above-normal temperature anomalies centered over high population areas, the energy needed to cool the nation was up with the June 2014 REDTI (Residential Energy Demand Temperature Index) ranking as the 25th highest June REDTI in the 120-year record.
Severe weather developed along the Front Range and Plains, and points eastward, in association with these weather systems, as the upper-level lows over the Northwest and Northern Plains pulled warm and moist Gulf of Mexico surface air northward. Tornadoes and large hail occurred almost every day throughout the month in this unstable air ahead of the slow-moving upper-level troughs, with the preliminary count of 326 tornadoes ranking above the average count of 243 tornadoes for June. The warm, moist Gulf of Mexico air tended to keep nighttime temperatures warm along and east of the Mississippi River and dampen daytime highs in the Deep South.
The repeated occurrence of the short-wave weather systems traversing the upper-level flow, and the slow-moving nature of some of them, brought heavy precipitation to parts of the country, especially the Central Plains to Midwest, where widespread flooding occurred. Seven states in the Plains to Midwest had the tenth wettest, or wetter, June with Minnesota ranking wettest on record for June. The abundant precipitation helped reduce the area of drought in the Plains and Midwest. A westerly flow in the upper atmosphere inhibited precipitation in the West, where drought intensified and Arizona had the third driest June on record. The net change in drought area was a decrease in the moderate to exceptional national drought footprint compared to the end of May.
When integrated across the month, the circulation produced a pattern of above-normal 500-mb heights (weaker-than-normal long-wave trough) over eastern North America, especially eastern Canada, and below-normal 500-mb heights (weaker-than-normal long-wave ridge) over Alaska and the Pacific Northwest to Northern Plains. The below-normal anomalies in the Northwest to Northern Plains reflected the lingering dominance of troughs at the jet stream level as the circumpolar vortex resisted its seasonal retreat northward.
Above-normal precipitation fell across much of the Central to Northern Plains, Midwest, Mississippi Valley, and eastern Great Lakes. This wet area was ringed on the east and south by areas of below-normal precipitation, especially in southern New England and parts of the Southeast and Southern Plains. On its western side lay a vast area of below-normal precipitation from the Rockies to the West Coast, especially across the Southwest. Frequent upper-level lows and troughs associated with below-normal 500 mb heights brought wetter-than-normal weather to Alaska, giving the state its second wettest June in the 1918-2014 record. The precipitation pattern across Hawaii was mixed.
The storm track, with its frequent upper-level lows and troughs, kept temperatures below normal in Alaska and funneled polar air masses into the Northern Rockies and Northern Plains, resulting in monthly temperature anomalies that were colder than normal in that region. But the warm southerly flow at the surface and above-normal heights in the upper atmosphere contributed to above-normal temperatures east of the Mississippi River, while warmer-than-normal temperatures were associated with above-normal heights in the Southwest.
The upper-level circulation over North America is part of the hemispheric mid-latitude westerly circulation. The circulation during June reflected an interconnected meridional pattern of paired above-normal and below-normal 500-mb height anomalies over the Northern Hemisphere continents and oceans. Compared to earlier months, the pattern of anomalies in June reflects the seasonal shift toward the pole of the jet stream. There were more positive height anomalies over land than negative height anomalies, along with large areas of warmer-than-normal surface temperatures across the Northern Hemisphere associated with the above-normal 500 mb heights.
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 June. The equatorial Pacific SST anomalies were warmer than normal, suggesting the development of an El Niño, but the collective atmospheric and oceanic conditions continued to 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 June, the typical precipitation pattern would consist of wetter-than-normal conditions in the Southern Plains and Southeast, near-normal to wet conditions in the Southwest, and drier-than-normal conditions across much of the Midwest to Northeast.
- Comparison to Observed: The June 2014 precipitation anomaly pattern is opposite to 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 in phase 2 but quickly went incoherent, emerged in phase 4 at mid-month, then became 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 June 9, 16, 23, and 30).
- 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 May-July teleconnections for precipitation are shown here and for temperature 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. The June 2014 monthly precipitation and temperature anomaly patterns don't match those expected with MJO phase 4, but there seems to be some agreement during week 3 with the temperature anomaly pattern and, east of the Rockies, with the precipitation anomaly pattern.
- 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 negative for the first half of the month then turned positive for the last half, averaging negative 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 shows wetter-than-normal conditions in the northern Great Lakes and drier-than-normal weather along the Northern Rockies to the Southern Plains (although the teleconnections for precipitation are weak). The upper-level circulation anomaly teleconnections are also weak, but for a negative PNA, above-normal heights are evident over eastern Canada.
- Comparison to Observed: The June 2014 upper-level circulation and precipitation anomaly patterns show some agreement with a negative 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 (May-July) is typically associated with dry conditions in the Northeast and Deep South, and wet conditions across the Plains and parts of the Midwest, below-normal temperatures in the Northern Plains and New England, near normal temperatures elsewhere, and upper-level circulation anomalies which are below normal over eastern to central Canada and above normal over Alaska.
- Comparison to Observed: The June 2014 upper-level circulation anomaly pattern is opposite to that expected with a negative AO. The precipitation anomaly pattern agrees in the Plains to Midwest, and the monthly temperature anomaly pattern agrees from the Northern Plains to the Southeast.
- 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 was generally zero to slightly negative, averaging negative for the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative NAO during this time of year (July on the teleconnection maps) is associated with cooler-than-normal temperatures in the Northwest to Northern Plains, warmer-than-normal temperatures in the Southern Plains to Southeast, near normal temperatures over Alaska, wetter-than-normal conditions in the Northern Plains and Midwest, drier than normal in the western Gulf of Mexico coast, near normal precipitation across Alaska (although the teleconnections are weak for precipitation), and negative 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 June 2014 monthly precipitation anomaly pattern agrees over the Northern Plains and part of the Midwest. The temperature and upper-level circulation anomaly patterns generally agree from the Northwest to the Northern Plains.
- 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 June 2014 monthly temperature anomaly pattern is a good match to a negative WP over the CONUS, and the upper-level circulation anomaly pattern is a reasonable match over North America.
- 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 June 2014 saw a continuation of the warmer-than-normal SSTs there, but the magnitude of the warmth decreased. The monthly EP-NP index has been positive since October, pulling the 3-month running mean well into positive territory. The June value was negative, reflecting a weakening of the warm SST anomalies and pulling the 3-month running mean closer to zero.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative EP-NP index during this time of year (July on the maps) is typically associated with warmer-than-normal temperatures from the Northern and Central Rockies to the Great Lakes and beyond into Canada, cooler-than-normal temperatures in part of Alaska, drier-than-normal conditions in the vicinity of Montana and parts of the Midwest (although the precipitation teleconnections are very weak), below-normal upper-level circulation anomalies (weaker upper-level ridge) over the Alaska and western Canada, and above-normal upper-level circulation anomalies (weaker upper-level trough) from the central CONUS to northeastern Canada.
- Comparison to Observed: The June 2014 temperature and upper-level circulation anomaly patterns have some agreement in Alaska but not the CONUS. The precipitation anomaly pattern does not match.
Examination of these circulation indices and their teleconnection patterns, and comparison to observed June 2014 temperature, precipitation, and circulation patterns, suggest that, in the absence of strong forcing from the equatorial Pacific, the weather over the CONUS in June was related to the jet stream and ocean-atmosphere interactions over the North Pacific Ocean, but the North Atlantic and Arctic oceanic drivers were also exerting some influence. 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 incoherent during most of the month, it appeared to have exerted some influence on temperature and precipitation during the third week. The PNA index showed very little correlation to the June weather patterns. The teleconnections for the NAO match June's precipitation, temperature, and upper-level circulation patterns fairly consistently from the Northwest to Midwest, indicating the match is likely more than coincidence. Likewise, the AO teleconnections are fairly consistent in the interior part of the country. The WP showed the strongest correlations, indicating the North Pacific still exerted influence, but EP-NP's influence was waning. The precipitation teleconnections with most of the indices are weak during the summer, but there was some indication that the MJO, PNA, AO, and NAO affected the precipitation patterns in some areas at various times during the month. This month illustrates how the weather and climate anomaly patterns can be the manifestation of influences from several atmospheric drivers (or modes of atmospheric variability).