March 2015 Synoptic Discussion

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.

Summary

March 2015 was characterized by an upper-level circulation pattern that generally consisted of a long-wave ridge over the western contiguous United States (CONUS) and a long-wave trough over the East. Short-wave troughs and low pressure centers moving in this flow brought areas of rain and snow to parts of the country, but the dominant long-wave circulation pattern kept much of the CONUS drier than normal with a reduced occurrence of severe weather. Several of the short-wave troughs generated winter storm systems which tapped Gulf of Mexico moisture to lay down an extensive snow cover east of the Rockies early in the month, but a pattern change to warmer and drier weather rapidly shrank the national snow cover area. The ridge produced near-record warm monthly temperatures in the West and the trough generated much below-normal monthly temperatures in the Northeast. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather and climate of March 2015 were the result of influences from multiple atmospheric drivers originating over the North Pacific, North Atlantic, and Equatorial Pacific. See below for details.

Synoptic Discussion

Animation of daily upper-level circulation for the month. In the Northern Hemisphere, March is the beginning of climatological spring, which is the transition period between winter and summer. The sun angle gets higher in the sky and solar heating increases, which contracts the circumpolar vortex and causes the jet stream to retreat northward. March 2015 began cold and snowy with the long-wave jet stream pattern consisting of an upper-level ridge over the western CONUS and a trough in the East. But, as the month wore on, the jet stream retreated, temperatures warmed (weeks 1, 2, 3, 4), and the snow cover melted away. The long-wave ridge/trough pattern persisted throughout the retreat, with short-wave troughs and low pressure centers migrating through the upper-level flow and distorting its shape. In between the ridge and trough, a northwesterly flow funneled cool and dry Canadian air masses into the CONUS east of the Rockies. This circulation pattern kept the Northeast mostly cold and snow covered. The western ridge was reinforced by the North Pacific High and kept temperatures warmer than normal over the West, with extensive areas experiencing a record to near-record warm March. The retreating jet stream allowed the western ridge to intrude into the Northern Great Plains, where it brought unseasonable warmth. By the end of the month, there were 5,534 record warm daily high (3,105) and low (2,429) temperature records, which is twice as many (2,550) record cold daily high (1,365) and low (1,185) temperature records. The monthly average temperature ranked March 2015 as the 12th warmest March in the 1895-2015 record, showing how the warmth dominated the national statistics. The REDTI (Residential Energy Demand Temperature Index) for March 2015 ranked 50th lowest for March, reflecting the influence of the cold anomalies in the heavily-populated Northeast and indicating that extra energy was needed to heat homes there.

Animation of daily surface fronts and pressure systems for the month. The short-wave troughs and lows moving in the upper-level flow brought areas of precipitation to parts of the Northwest, Southwest, and northern tier states, but the long-wave ridge pattern kept total precipitation amounts well below normal. The combination of drier- and warmer-than-normal weather intensified and expanded drought and abnormally dry conditions in the West, Central to Northern Plains, and western Great Lakes, with several large wildfires developing in the Central to Southern Plains at mid-month. The North Atlantic semi-permanent high pressure system, also known as the Bermuda High, occasionally approached the southeast coast of the CONUS. With the upper-level trough to the north and Bermuda High to the south, cold fronts and surface lows were directed into a storm track from eastern Texas to the Central Appalachians. This region experienced above-normal precipitation for the month, which helped keep temperatures cool, as well as outbreaks of severe weather. The circulation pattern generally inhibited the development of tornadoes, with a preliminary count of only 13 tornadoes developing across the nation compared to a March average of 80. The above-normal precipitation improved drought conditions from Texas and the Lower Mississippi Valley to the Ohio Valley, but the Bermuda High contributed to drier- and warmer-than-normal conditions, and expanding drought, in the Southeast. The dryness in the West, Plains, western Great Lakes, and Southeast resulted in an expansion of the overall national drought footprint, with 36.8 percent of the CONUS experiencing moderate to exceptional drought at the end of March, compared to 31.9 percent at the end of February. The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. The extremes in temperature created by the ridge/trough circulation pattern contributed to extreme climate as measured by the CEI. The March CEI for the U.S. ranked as the 19th highest for the month, due largely to unusually warm maximum and minimum temperatures. Unusual warmth and dryness drove the CEI ranks in the western regions. The West region had the most extreme March CEI on record due largely to the most extreme Palmer drought component, third most extreme warm maximum temperature component, and fourth most extreme warm minimum temperature component. The Northwest region had the eighth most extreme March CEI on record due mostly to the most extreme warm maximum temperature component and fourth most extreme warm minimum temperature component. The West North Central region had the tenth most extreme March CEI on record due largely to the second most extreme warm maximum temperature component and eighth most extreme warm minimum temperature component.

North America monthly upper-level circulation pattern and anomalies. When integrated across the month, the atmospheric circulation indicated a pattern of above-normal 500-mb heights (stronger-than-normal long-wave ridge) over the western CONUS and all along the North American west coast to Alaska, above-normal heights extending across the central to southeastern CONUS (weaker-than-normal long-wave trough in the Southeast), and below-normal 500-mb heights (stronger-than-normal trough) over eastern Canada.

Map of monthly precipitation anomalies. Most of the West, Central to Northern Plains, Upper Midwest, Northeast, and Southeast were drier than normal during March 2015. Precipitation was above normal across southern and eastern Texas, the Lower Mississippi Valley, and Ohio Valley. March was drier than normal across much of the Hawaiian Islands and southern to interior Alaska.

Map of monthly temperature anomalies. March 2015 temperatures averaged warmer than normal across Alaska and the western, central, and southeastern CONUS. Temperatures were colder than normal from Texas to the Northeast.

Global monthly upper-level circulation pattern and anomalies.

Atmospheric Drivers

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: During March 2015, weak El Niño conditions were observed as the above-average SSTs across the western and central equatorial Pacific continued to be coupled to the tropical atmosphere.
  • Teleconnections (influence on weather): To the extent teleconnections are known, the typical temperature and precipitation patterns associated with El Niño during March include above-normal precipitation in the Southwest, Gulf of Mexico coast to Mid-Atlantic coast, and Central Plains; below-normal precipitation from the Ohio Valley to Great Lakes and parts of the Northern Rockies; below-normal temperatures across the southern half of the CONUS; and above-normal temperatures across parts of the Pacific Northwest and North Central States.
  • Comparison to Observed: The March 2015 temperature and precipitation anomaly patterns do not agree with those 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 rapidly passed through phases 5, 6, 7, 8, 1, and 2. The MJO indices appeared to be influenced by other modes of coherent subseasonal tropical variability, including the background El Niño state, tropical cyclone activity, and Kelvin and Rossby waves (MJO updates for March 2, 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 February-April 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. The March 2015 monthly temperature and precipitation anomaly patterns match some of the phases and don't match others. The weekly temperature anomaly patterns (weeks 1, 2, 3, 4) are a reasonable match for the MJO phases that existed during each respective week. The weekly precipitation patterns (weeks 1, 2, 3, 4) are a reasonable match for the first two and last weeks, but not for the third week.
• 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 then slightly positive during the month, averaging negative for the month as a whole. The 3-month-averaged index was approaching zero after being positive for the last several months.
  • Teleconnections (influence on weather): To the extent teleconnections are known, for a negative PNA, the temperature teleconnection map for this time of year (April on the teleconnection maps) shows cooler-than-normal temperatures in Alaska, Canada, and the Pacific Northwest of the U.S., and warmer-than-normal temperatures for the Southern and Central Plains to Atlantic Coast of the CONUS. The precipitation teleconnection map shows drier-than-normal weather from Utah to Kansas and wetter-than-normal anomalies over the western Great Lakes, but the precipitation teleconnections are weak. The upper-level circulation anomaly teleconnections show above-normal heights over the southeastern CONUS, and below-normal heights across Canada and the Pacific Northwest.
  • Comparison to Observed: The March 2015 temperature and upper-level circulation anomaly patterns show some agreement in the Southeast with that expected with a negative PNA, but disagreement elsewhere. The precipitation anomaly pattern agrees near Kansas, but the teleconnections are weak.
• 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 mostly positive, with the index averaging positive for the month. The 3-month-averaged index was positive.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a positive AO this time of year (February-April) is typically associated with dry conditions in the Southeast, Southwest, and Central Plains; wet conditions in the Mid-Mississippi to Ohio Valleys; above-normal temperatures east of the Rocky Mountains, near normal temperatures in the West; and upper-level circulation anomalies which are below normal across the Arctic into northern North America, and above normal over the eastern CONUS, North Atlantic and North Pacific.
  • Comparison to Observed: The March 2015 monthly precipitation anomaly pattern is a very good match for the teleconnections with a positive AO. The upper-level circulation anomaly pattern agrees over eastern Canada and the Southeast U.S., but not elsewhere. The temperature anomaly pattern does not match.
• 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 positive throughout the month, averaging positive for the month. The 3-month-averaged index was positive.
  • 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 drier-than-normal conditions in the Southeast, Central Plains, and parts of the West (although the teleconnections are weak for precipitation); warmer-than-normal temperatures everywhere in the CONUS except the Gulf of Mexico coast to Mid-Atlantic; positive upper-level circulation anomalies across the North Atlantic and most of the CONUS, and negative upper-level circulation anomalies over the Canadian Archipelago and Greenland.
  • Comparison to Observed: The March 2015 precipitation anomaly pattern agrees with that for a positive NAO where teleconnections exist. The monthly temperature and upper-level circulation anomaly patterns agree over the CONUS, but the temperature pattern does not agree over Alaska and the upper-level circulation anomalies are shifted over eastern Canada.
• 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 positive for the month, while the three-month average WP index remained in negative territory.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a positive WP during this time of year (April on the maps) is typically associated with below-normal temperatures in the Southwest, above-normal temperatures from the Ohio Valley to Northern Plains, wetter-than-normal conditions in the Northwest, and above-normal circulation anomalies over the western CONUS and below-normal circulation anomalies over the Northern Plains (although the precipitation and circulation teleconnections are weak).
  • Comparison to Observed: The March 2015 monthly temperature, precipitation, and upper-level circulation anomaly patterns show little resemblance to those expected for a positive WP 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 magnitude of the warmth of the SSTs in the northeastern North Pacific changed little this month, with the March SST pattern still showing above-normal SSTs along the North American coast and cooler-than-normal SSTs in the central to western North Pacific. The monthly EP-NP index was positive during March, keeping the 3-month running mean well in positive territory.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a positive EP-NP index during this time of year (April on the maps) is typically associated with colder-than-normal temperatures east of the Rockies, warmer-than-normal temperatures across the extreme West Coast and in Alaska, wetter-than-normal conditions in Wyoming and parts of the Ohio Valley (although the precipitation teleconnections are very weak), above-normal upper-level circulation anomalies (stronger upper-level ridge) over Alaska and western Canada and into the western CONUS, and below-normal upper-level circulation anomalies (stronger upper-level trough) over the northeastern CONUS and eastern Canada.
  • Comparison to Observed: The March 2015 upper-level circulation anomaly pattern over North America and the North Pacific agrees with a positive EP-NP pattern, although the observed above-normal 500-mb heights over the Central and Southeast CONUS correspond to zero teleconnection on the EP-NP maps. The March temperature anomaly pattern matches, but the cold anomalies appear over a narrower band east of the Rockies than indicated on the teleconnection map. The precipitation anomaly pattern shows some agreement but few teleconnections exist.

Examination of these circulation indices and their teleconnection patterns, and comparison to observed March 2015 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in March reflected influences from several atmospheric drivers. The EP-NP, PNA, and NAO teleconnection indices indicate that the North Pacific region drivers influenced the overall circulation pattern, but the North Atlantic region driver also exerted an influence. The EP-NP, NAO, and MJO teleconnection indices reflected a shared influence on CONUS temperature by the North Pacific, North Atlantic, and Equatorial Pacific region drivers. The AO, NAO, and MJO teleconnection indices had the best overall correlation with the observed precipitation pattern.

This month illustrates how the weather and climate anomaly patterns can reflect the combined influence of several atmospheric drivers (or modes of atmospheric variability).

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