Synoptic Discussion - March 2013
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.
March is the first month of the Northern Hemisphere's climatological spring (March-May) and usually is a period of transition for the atmospheric circulation — the circumpolar vortex contracts to the north with warm southerly air masses beginning to dominate over cold northerly air masses. During March 2013, however, the polar jet stream (which marks the edge of the circumpolar vortex and the boundary between the cold polar air masses to the north and the warmer sub-tropical air masses to the south) was well entrenched over the eastern United States in association with a strong negative Arctic Oscillation. This pattern reduced the occurrence of severe weather, with the preliminary tornado count below normal for the month, but funneled cold air masses into the country east of the Rockies while keeping the western U.S. generally warmer than normal. Weather systems moving in the very active jet stream flow combined with the frigid air to produce abundant snowfall, resulting in above-average national snow coverage for March. A few weather systems moved through the upper-level ridge over the western U.S., but for the month as a whole, the West averaged drier than normal. A few areas east of the Rockies were wetter than normal, but the precipitation that fell shrank drought areas in the Midwest and Southeast.
The movement of the weather systems can be seen in the weekly precipitation anomaly patterns (weeks 1, 2, 3, 4, 5). The storm track across the central part of the country brought frequent snow cover with over a hundred monthly records set by daily snowfall, mostly along the paths of heavy winter storms from the Central Plains to Midwest, while snow cover over the West was below normal for March. The month began with about 48 percent of the contiguous U.S. snow-covered; coverage gradually decreased to about 21 percent by March 16th, expanded to about 31 percent by the 19th after a snowstorm swept across the Northeast, and reached its greatest daily extent of about 49 percent on the 25th as snow cover expanded across the Rockies to Midwest. This snow cover rapidly melted, with coverage down to about 17 percent by the end of the month. Contiguous U.S. snow cover area, integrated across the month, gave March 2013 a rank of tenth biggest March coverage in the 1967-2013 satellite snow cover record. The trough east/ridge west upper-level circulation pattern, that contributed to extensive eastern snow cover, steered wet weather systems away from the West and funneled cold air masses into the eastern U.S., with the northwest flow blocking the Gulf of Mexico moisture source. Consequently, Louisiana and Wyoming ranked in the top ten driest category for March, with 27 other states falling in the driest third of the historical record. The dryness in the West and parts of the South and East gave March 2013 a rank of fifth driest March in the 1895-2013 record when precipitation is integrated across the country. Drought conditions improved in the Midwest and Southeast, where beneficial precipitation fell, worsened in the Southern Plains (where Texas had the twelfth driest March), and generally held steady elsewhere. Overall, the national (contiguous U.S.) moderate to exceptional drought footprint decreased from 54.2 percent at the end of February to 51.9 percent at the end of March (based on U.S. Drought Monitor statistics). According to the Palmer Drought Index, which goes back to the beginning of the 20th century, 48.7 percent of the contiguous U.S. was in moderate to extreme drought at the end of March, an increase of about 10 percent compared to last month.
The weekly temperature anomaly maps (weeks 1, 2, 3, 4, 5) show the movement of colder-than-normal air masses from the Northern Plains to the Southeast throughout the month, reflecting the dominance of the upper-level trough and negative Arctic Oscillation. This pattern is evident in the monthly temperature anomalies, with 28 states from North Dakota to Florida ranking in the coolest third of the historical record. Eleven of those ranked in the top ten coldest category for March, and five of those states (Florida, Georgia, Alabama, South Carolina, and North Carolina) had an average March 2013 temperature that was colder than their average January 2013 temperature. In contrast, three western states (California, Nevada, and Arizona) ranked in the top ten warmest category for March. On a local basis, nearly two times as many record cold highs and lows occurred than record warm highs and lows. Over 1400 record low temperatures and about 1400 record cool daily high temperatures were tied or broken. In comparison, about 800 daily high temperature records and 800 record warm daily low temperatures were tied or broken. (These numbers are preliminary and are expected to change as more data arrive.) The persistence of much below-normal temperatures over the population centers of the east contributed to a national Residential Energy Demand Temperature Index (REDTI) for March 2013 that was above the long-term average and ranked 31st highest in 119-year record.
When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the 43rd coolest and fifth driest March in the 119-year record. Averaging extremes tends to cancel them out. But when extremes are combined cumulatively, like in the U.S. Climate Extremes Index (USCEI), they may tell a different story. The national USCEI for March 2013 ranked only 37th largest (68th smallest) in spite of a large spatial extent of long-term drought conditions (fourth largest PDSI component for March) because, although widespread below-normal temperatures occurred (20th largest cool max and 24th largest cool min components), many other years have been much colder. The below-normal temperatures for March, combined with the cool start to the year, have been enough to knock the USCEI for the last 12 months out of first place, with April 2012-March 2013 ranking only 13th largest on record for April-March.
Subtropical highs, and cold fronts and low pressure systems moving in the storm track flow, are influenced by the broadscale atmospheric circulation. The following describes several such large-scale atmospheric circulation drivers and their potential influence 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 March.
- 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 can be analyzed to show typical temperature and precipitation patterns associated with El Niño and La Niña ENSO episodes. Teleconnections are not available for ENSO-neutral conditions.
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 time-longitude graph showing the areas affected by these regions and their propagation and another is a phase diagram. 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. 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 continued to be active during March, with positive and negative anomalies propagating west to east as the month progressed (5-day index, 15-day index) and cycling out of phase 6 and through phases 7 to 3.
- 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 January-March teleconnections for precipitation are shown here and for temperature are shown here.
- 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 2013 monthly precipitation pattern shows little correlation to the precipitation teleconnection patterns for the MJO this time of year. However, the weekly precipitation anomaly patterns correspond in some locations to the teleconnections for the MJO phase during these weeks — above-normal precipitation in the Midwest is frequently associated with MJO phases 6 and 7 (precipitation here was above normal during the first half of the month [weeks 1, 2, and 3] and the MJO was in phases 6-7 for March 1-16). The March 2013 monthly temperature pattern shows some similarity to the teleconnections for MJO phases 1-3. On a weekly basis (weeks 1, 2, 3, 4, 5), there is some agreement between the temperature anomalies during the last half of the month and those expected with MJO phases 1-3 in the east but not in the west, nor for the first half of the month when temperatures were below normal and the MJO teleconnections (phases 6-7) called for warm anomalies.
- 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 PNA index began March positive, turned negative briefly at mid-month, then fluctuated around zero (neutral) for the last third of the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, the temperature teleconnection map for this time of year (April on the maps) shows that a positive PNA is correlated with cooler-than-normal temperatures in the Southeast and warmer-than-normal temperatures in the Pacific Northwest and especially in western Canada and Alaska. A negative PNA is correlated with the inverse pattern. The precipitation teleconnections are weak, but show some hint of wetness over and around Colorado and dryness in the Upper Mississippi Valley for a positive PNA (the inverse pattern for a negative PNA). The upper-level circulation anomalies associated with a positive PNA are above-normal heights over much of Canada, especially western Canada, and below-normal heights over southeastern North America (the inverse pattern for a negative PNA).
- Observed: Alaska was cooler than normal during March (consistent with a negative PNA), but the southeastern U.S. was also cooler than normal which is consistent with a positive PNA. The March precipitation anomaly pattern is consistent with a negative PNA over Colorado and Minnesota. The March 2013 upper-level circulation pattern had strong negative anomalies across the eastern third of the U.S. and positive anomalies across the western third of the country; except for an area of overlap in the Southeast with a positive PNA pattern, this circulation anomaly pattern does not match any PNA pattern.
- 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 AO index oscillated in negative territory throughout the month, reaching a March record negative value during the last half of the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative AO this time of year (January-March) is typically associated with cooler-than-normal temperatures across the contiguous U.S. from the Rockies to the east coast and dry conditions in the Lower Mississippi and Tennessee valleys, although the precipitation correlations are weak. The January-March averaged upper-level circulation anomalies for a negative AO are below-normal 500-millibar (mb) geopotential heights (which translates to stronger trough or weaker ridge, depending on the circulation) over the eastern two-thirds of the U.S., and above-normal 500-mb heights (which translates to stronger ridge or weaker trough, depending on the circulation) over the Arctic.
- Observed: The March monthly temperature anomaly pattern is strongly consistent with a negative AO. There is some agreement with the March precipitation pattern where correlations exist. The March 2013 upper-level circulation is consistent over the eastern half of the U.S., the North Atlantic, and Arctic basin, but not over the western U.S. or northeast Pacific. Both the March 2013 and January-March 2013 upper-level circulation patterns have positive height anomalies over the northeastern Pacific, which is more consistent with a positive AO than 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 NAO index was negative for most of the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative NAO during this time of year (April on the teleconnection maps) is typically associated with cooler-than-normal temperatures for much of the country except the South, wetter-than-normal conditions in the Southeast, Central Plains, and parts of the West (although the correlations are weak for precipitation), and negative upper-level circulation anomalies across most of the country, especially over the Central Rockies.
- Observed: The March 2013 temperature anomaly patterns are opposite those expected with a negative NAO for much of the country (except the central U.S.), as are the March 2013 precipitation anomaly patterns for those areas where correlations exist. The March 2013 upper-level circulation anomalies are opposite those expected with a negative NAO in the western U.S. but consistent in the east.
- The East Pacific-North Pacific (EP-NP) pattern
- Description: The EP-NP teleconnection pattern relates SST and upper-level circulation patterns 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 northeast Pacific during March 2013 consisted of a dipole of cooler-than-normal SSTs along the North American coast (which was shrinking) and warmer-than-normal SSTs in the central North Pacific. The EP-NP index (3-month running mean) flipped from negative in February to positive in March, returning to the positive realm it was in for the last half of 2012.
- Teleconnections (influence on weather): 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 east of the Rockies, warmer-than-normal temperatures along the West Coast and Alaska, below-normal upper-level circulation anomalies (stronger upper-level trough) over eastern North America, and above-normal upper-level circulation anomalies (stronger upper-level ridge) over western North America (the correlations are weak for precipitation).
- Observed: The March temperature anomalies are a good match for the eastern U.S., but not Alaska. The upper-level circulation anomaly patterns are consistent with those expected for a positive EP-NP over western North America, while in the east they seemed shifted south (a positive EP-NP is related to negative upper-level anomalies over eastern Canada and neutral over the eastern U.S., while March 2013 had positive anomalies over eastern Canada and negative over the eastern United States).
Examination of these circulation indices and their teleconnection patterns, and comparison to observed March 2013 and January-March 2013 temperature, precipitation, and circulation patterns, suggests that the AO was responsible for the temperature pattern during March, but the precipitation pattern was influenced in part by a couple other drivers. The MJO was in phase with the AO near the end of the month, and the PNA and EP-NP may have also influenced the temperature pattern in the eastern United States. Precipitation early in the month could have been influenced by the MJO, and the PNA and AO might have had some influence in some areas, but in general, precipitation is weakly correlated with these latter two patterns this time of year. ENSO, which can have significant impacts on precipitation, was neutral and, thus, not a player. The upper-level circulation pattern strongly matched that associated with a negative AO in the eastern U.S., but it seemed to match the EP-NP over the western U.S. and PNA over the North Pacific. This month illustrates how competing atmospheric drivers can result in a complex weather pattern and how, when the atmospheric circulation drivers are neutral or in a state of transition, their influence can become difficult to trace and can be overwhelmed by other competing forces, including random fluctuations in the atmosphere.