Synoptic Discussion - April 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.
April is in the middle of climatological spring, which is the season of transition from the atmospheric forcings of cold, dark winter to a new set of forcings behind the hot radiance of summer. This change was reflected in the long-wave circulation pattern for April 2014 over North America, which reflected a weakening of a mode of atmospheric variability (East Pacific-North Pacific) which has influenced the weather of the last several months. The circulation pattern consisted of a retreating upper-level trough over central Canada which extended into the north central U.S., with short-wave troughs traversing the contiguous U.S. in a fast westerly flow. This long-wave pattern still reflected the influence of an atmospheric mode of variability (West Pacific) associated with the North Pacific driver, whose teleconnections include colder-than-normal temperatures in the north central U.S. and warmer-than-normal temperatures in the Southwest, although signs of the influence of other modes of circulation were evident in the temperature, precipitation, and upper-level circulation anomaly patterns. The short-wave troughs generated large areas of precipitation east of the Rockies, and some produced severe weather when slowed by blocking patterns in the atmospheric circulation. Strong winds kicked up by frequent cold frontal passages contributed to large wildfires and, in the Southern Plains, numerous dust storms. The circulation pattern inhibited beneficial precipitation across much of the West and Plains, where drought expanded or intensified. See below for details.
In the Northern Hemisphere, April is in the middle of climatological spring which is the time of year when solar heating forces the jet stream and circumpolar vortex to contract poleward. Cold polar air masses can still influence the weather, but the warm, dry subtropical high pressure belt begins to exert more influence as it shifts northward, with warm southerly air masses moving into the contiguous United States (CONUS) more frequently. In April 2014, a strong westerly flow in the upper-level circulation sent several weather systems rippling across the CONUS, which played havoc with a cold long-wave trough that tried to maintain its grasp over eastern and central North America.
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 cold air surging southward behind them. There were more than one and a half times as many record cold daily highs and lows (2799) as record warm daily highs and lows (1583), but the parade of competing cold and warm air masses resulted in a monthly temperature pattern where monthly extremes were washed out. Snow cover generally decreased as the month wore on and temperatures in general warmed, but the air was still below freezing in the Plains and northern states behind cold fronts at mid-month and near the end of the month to result in temporary expansions of the snow cover area.
Severe weather developed in the Central to Southern Plains and eastward in association with these weather systems, with major outbreaks occurring at the beginning, middle, and end of the month. A large, slow-moving upper-level trough near the end of the month triggered severe outbreaks of tornadoes along and ahead of its surface low and fronts, resulting in significant destruction and dozens of fatalities. The end-of-month outbreak brought the preliminary count of 217 tornadoes well above the average count of 155 tornadoes for April.
The short-wave weather systems moving in the upper-level flow brought precipitation to many parts of the country, but the long-wave trough over central and eastern North America imparted a slight northerly component to the overall circulation which inhibited precipitation in the western and central CONUS. The short-wave troughs intensified as they moved into the long-wave trough and tapped Gulf of Mexico moisture to lay down bands of heavy precipitation in the Southeast, along the Ohio Valley, and in the Upper Mississippi Valley to Great Lakes, giving five states their tenth wettest, or wetter, April. The rain from these weather systems shrank the drought and abnormal dryness areas in the Midwest, Southeast, and Pacific Northwest, but drought expanded in the West and Plains where the month was drier than normal, resulting in the national drought area remaining relatively unchanged compared to a month ago. The frequent frontal passages in the Southern Plains kicked up numerous dust storms. The wind, low humidities, and prolonged dryness contributed to the development of large wildfires in the Plains and Southern Appalachian regions throughout the month.
When integrated across the month, the circulation produced a pattern of below-normal 500-mb heights (stronger-than-normal long-wave trough) over central Canada extending into the north central CONUS. Above-normal 500-mb heights extended over the southwestern CONUS and along the east coast of North America, reflecting an upstream and downstream response to the long-wave trough in the westerlies as well as hints of intensifying and expanding North Atlantic and North Pacific subtropical highs.
Above-normal precipitation fell in the Upper Mississippi Valley to Great Lakes, in the Ohio Valley, Southeast, and Mid-Atlantic Coast where upper-level weather systems and surface lows were able to tap into Gulf of Mexico moisture. Pacific weather systems moving in the upper-level flow brought a few areas of above-normal precipitation to the West and Central Plains, but the overall pattern of western ridging and a component of northwesterly flow brought widespread below-normal precipitation to these areas. In between the bands of heavy precipitation in the east, April was drier than normal across the southern Great Lakes and along the Appalachian chain to New England. The strong westerly flow over the Pacific kept weather systems away from Alaska, which was mostly drier than normal.
With the roller coaster pattern of warm fronts and cold fronts traversing the CONUS, monthly temperatures averaged near normal in many areas. A colder-than-normal pool was located in the north central states, in association with the upper-level long-wave trough, and in New England and the Lower Mississippi Valley. Warmer-than-normal temperatures were found in the West, Ohio Valley, and parts of the coastal Southeast. Temperatures in Alaska averaged warmer than normal in the west and north, and cooler than normal in the southeast.
The upper-level circulation over North America in April is an example of an interconnected and amplified meridional pattern. The atmospheric circulation is interconnected around the world, and this interconnectedness was manifested this month by a pattern of paired above-normal and below-normal 500-mb height anomalies over North America, the North Atlantic, and Eurasia. Similar to last month, a dipole of below-normal heights can be traced from the high latitudes over north central Asia, extending across the Arctic into lower latitudes over central Canada. Above-normal heights dominated Europe and East Asia. The above-normal heights were reflected at the surface over Eurasia as widespread above-normal temperatures and below-normal snow cover, with areas of below-normal precipitation.
Subtropical highs, and cold fronts and low pressure systems moving in the 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 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 April, but SST anomalies were warming and the atmospheric and oceanic conditions collectively indicated a continued evolution toward El Niño.
- 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.
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 3, transitioned to phases 4 and 5 as the month progressed, then became weak and incoherent, finally ending the month weakly in phase 8. 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 April 7, 14, 21, and 28).
- 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 March-May 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. But the April 2014 monthly precipitation anomaly pattern, as well as the precipitation pattern for weeks 1 and 2, appear to match that expected with MJO phase 3 in the Ohio Valley to Southeast (wetter than normal); drier-than-normal conditions in the Southwest and Central Appalachians, associated with MJO phases 4 and 5, agree on the precipitation anomaly patterns for the month as well as week 2; and wetter-than-normal conditions in parts of the West (Oregon-Idaho-Nevada), associated with MJO phase 3, appear to match the precipitation anomaly pattern there for week 1. The monthly temperature anomaly pattern, as well as the pattern for week 2, appear to match the teleconnections for MJO phases 4 and 5 in the West and, for week 2, in the Central Plains (warmer than normal).
- 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 near zero for the first half of the month then positive for the last half, averaging near zero (neutral) for the month as a whole.
- Teleconnections (influence on weather): Teleconnections are not available for a neutral PNA at that NOAA web site.
- Comparison to Observed: The April 2014 temperature, precipitation, and upper-level circulation anomaly patterns cannot be compared to any teleconnections for a neutral PNA, since such teleconnections are not available. But the PNA was weakly positive during part of the month, and this month's anomaly patterns do not agree with those expected 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 began the month neutral (near zero), turned strongly positive for much of April, and ended the month neutral, averaging positive for the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive AO this time of year (March-May) is typically associated with wet conditions in coastal Washington and eastern Texas, dryness across parts of the West, Central Plains, Gulf of Mexico coast, and East Coast, above-normal temperatures from the Rockies to the Appalachians, and upper-level circulation anomalies which are below normal over northern Canada and above normal over the eastern to south central CONUS.
- Comparison to Observed: The April 2014 upper-level circulation anomaly pattern is similar to that expected with a positive AO over Canada and the eastern CONUS, but appears shifted to the south; it is generally consistent over the Arctic and Eurasia; but it does not agree over the North Pacific or North Atlantic. The precipitation anomaly pattern agrees in coastal Washington, other parts of the West, and New England. The monthly temperature anomaly pattern shows little resemblance to that associated with a positive 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 began April near zero, turned positive for much of the month, then ended negative, and averaged slightly positive for the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive NAO during this time of year (April on the teleconnection maps) is associated with warmer-than-normal temperatures across most of the CONUS, near normal temperatures over Alaska, drier-than-normal conditions across the Southeast and in parts of the West and Central Plains, near normal precipitation across Alaska, and positive upper-level circulation anomalies across the CONUS.
- Comparison to Observed: The April 2014 monthly temperature anomaly pattern agrees in the Southwest but not elsewhere. The precipitation anomaly pattern agrees over the Central Plains and West where teleconnections exist, but not in the Southeast. The upper-level circulation anomaly pattern matches over the Southwest and East Coast, but not over the interior CONUS where the teleconnections are strongest.
- 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 (April on the maps) is typically associated with above-normal temperatures in the Southwest, below-normal temperatures in the Northern Plains to Midwest, wetter-than-normal weather in coastal Washington (although the precipitation teleconnections in general are very weak), above-normal circulation anomalies over the Southwest, and below-normal circulation anomalies over the Northern Plains into central Canada.
- Comparison to Observed: The April 2014 monthly temperature, precipitation, and upper-level circulation anomaly patterns are a good match to a negative WP in the CONUS where teleconnections exist. The upper-level circulation anomaly pattern is also a good match over the North Pacific (below-normal heights) and East Asia (above-normal heights).
- 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 April 2014 saw a continuation of the warmer-than-normal SSTs there, but the magnitude of the warmth continued to decrease. The monthly EP-NP index has been positive since October, pulling the 3-month running mean well into positive territory. The April value was near zero, reflecting a weakening of the index in the short-term. The positive phase of the EP-NP pattern is associated with above-average surface temperatures over the northeastern Pacific (thus, the negative phase is associated with below-normal SSTs).
- Teleconnections (influence on weather): Teleconnections are not available on the NOAA web site for a neutral (near zero) EP-NP. But, to the extent these teleconnections are known, a positive EP-NP index during this time of year (April on the maps) is typically associated with cooler-than-normal temperatures across the U.S. east of the Rockies, warmer-than-normal temperatures along the immediate West Coast and in Alaska, wetter-than-normal conditions in the vicinity of Wyoming and parts of the Ohio Valley (although the precipitation teleconnections are weak), below-normal upper-level circulation anomalies (stronger upper-level trough) over the eastern half of Canada and the northeastern CONUS, and above-normal upper-level circulation anomalies (stronger upper-level ridge) over western Canada and Alaska and extending weakly into the western CONUS.
- Comparison to Observed: The April 2014 temperature anomaly pattern agrees from the Northern Plains to New England, along the West Coast, and over western Alaska, but not elsewhere. The precipitation anomaly pattern is a reasonable match in the Ohio Valley. The upper-level circulation anomaly pattern shows some agreement over central Canada and the southwestern CONUS, as well as the North Pacific (below-normal heights), but shows little correlation over western Canada and Alaska.
Examination of these circulation indices and their teleconnection patterns, and comparison to observed April 2014 temperature, precipitation, and circulation patterns, suggest that the weather over the CONUS in April was most closely related to the jet stream and ocean-atmosphere interactions over the North Pacific Ocean. ENSO was neutral, and thus not a player. Equatorial Pacific oceanic conditions were slowly evolving toward an El Niño state, which was affecting the MJO, but the MJO still may have exerted some influence on temperature and precipitation. The teleconnections for the NAO match April's temperature and circulation patterns in the Southwest and East, and precipitation patterns in parts of the Central Plains and West, but not elsewhere, so the match could have been coincidence. Likewise, the AO teleconnections match for precipitation in parts of the country, but don't match for temperature, so the match could have been coincidence, but the upper-level circulation pattern suggests the AO did exert some influence. The PNA index shows essentially no significant correlation to the April weather patterns. It is the North Pacific indices which have the best correlations — EP-NP and WP with the April temperature and circulation anomaly patterns. But the decreasing Northeast Pacific SST anomalies, and limited agreement with April temperature, precipitation, and upper-level circulation anomaly patterns, suggest that the EP-NP's influence is weakening. The undercutting of the western North America ridge by moist short-wave troughs in recent months may also be contributing to the weakened EP-NP influence. The WP shows some agreement with the precipitation anomaly pattern where teleconnections exist, but so do the MJO, AO, and NAO. It should be noted that precipitation relationships with these indices are generally weak. This month illustrates how the anomaly patterns can be strongly represented by an index (WP) measuring one atmospheric driver (the North Pacific Ocean) for circulation and temperature but also have elements of several other drivers (or modes of atmospheric variability), and how precipitation can be a reflection of several of the drivers.