Synoptic Discussion - February 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.
February is in the heart of the winter season when the cold polar air masses of the circumpolar vortex have the greatest likelihood of expanding south across the United States. 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 very active during February 2013, with a series of strong upper-level weather systems propagating across the country. These generated several winter storms which dumped heavy snow on coastal New England early in the month (raising national snow coverage to about 45 percent by the 11th) and on the Plains states later in the month (raising national snow coverage to about 58 percent by the 22nd). The South was also affected by these winter storms and frontal systems, with heavy rain easing drought conditions in the Southeast (precipitation anomaly maps for weeks 1, 2, 3, 4, 5) and severe weather affecting the Gulf Coast states, causing the preliminary tornado count to rise above normal for February. In addition to bringing precipitation of the liquid and frozen variety, these upper-level weather systems funneled cold Canadian air into the Lower 48 States, especially during the last half of the month (weeks 1, 2, 3, 4, 5). The weather systems east of the Rockies tapped Gulf of Mexico moisture to fuel their precipitation engines, but those moving over the West were moisture-starved, resulting in a generally drier-than-normal month from the Rockies westward.
The movement of the weather systems can be seen in the weekly precipitation anomaly patterns (weeks 1, 2, 3, 4, 5). Hundreds of monthly records were set by daily precipitation (255 preliminary reports) and daily snowfall (172 preliminary reports) reports this month, mostly along the paths of the Southeast rain system and Plains to Midwest snow systems, and (for snow) in the Northeast. The beneficial rain improved drought conditions in the Southeast, where moderate to exceptional drought (D1-D4) contracted from about 43 percent at the end of last month to 27 percent this month and the worst categories (extreme to exceptional drought, D3-D4) disappeared. At least some improvement occurred in most areas — even in the Great Plains, where the February snowstorms nibbled at the edges of the D3-D4 drought area in spite of massive twelve-month precipitation deficits. Five states (Georgia, Alabama, South Carolina, Massachusetts, and Rhode Island) had their tenth wettest, or wetter, February in the 1895-2013 record, with Georgia having its wettest February on record. Two states in the West (California and Oregon) ranked in their top ten driest category, with Montana not far behind at twelfth driest. Overall, the precipitation helped reduce the national (contiguous U.S.) moderate to exceptional drought footprint from 57.7 percent at the end of January to 54.2 percent at the end of February (based on U.S. Drought Monitor statistics). According to the Palmer Drought Index, which goes back to the beginning of the 20th century, 39.6 percent of the contiguous U.S. was in moderate to extreme drought at the end of February, a decrease of about 6 percent compared to last month.
The weekly temperature anomaly maps (weeks 1, 2, 3, 4, 5) show the transition from near- to above-normal temperatures at the beginning of the month to the dominance of colder-than-normal weather during the last half of the month as polar air masses swept into the country. Averaged across the month, temperatures were near to below normal for much of the country, with pockets of above-normal temperatures in the Northern Rockies and adjacent High Plains, New England, and coastal Deep South. Five Southwest states ranked in the cool third of the historical record for February while twelve ranked in the warm third. On a local basis, one and a half times as many record cold highs and lows occurred than record warm highs and lows. About 400 record low temperatures and 500 record cool daily high temperatures were tied or broken. In comparison, over 200 daily high temperature records and about 350 record warm daily low temperatures were tied or broken. (These numbers are preliminary and are expected to increase as more data arrive.) The lack of persistent and widespread extreme monthly temperature anomalies contributed to a national Residential Energy Demand Temperature Index (REDTI) for February 2013 that was near the long-term average.
When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the 49th warmest and 58th driest February 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. For February 2013, in spite of a large spatial extent of long-term drought conditions (seventh largest PDSI component for February) and wet days (13th largest "days with precipitation" component), the national USCEI ranked only 32nd smallest (73rd largest) out of 104 years of record. But the mediocre performance of the first two months of 2013 (46th smallest January-February) was not enough to counter the preponderance of unusual warmth and dryness for much of 2012, with the national USCEI for the last twelve months (March 2012-February 2013) still ranking as the largest on record for March-February.
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 February.
- 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 February, with positive and negative anomalies propagating west to east as the month progressed (5-day index, 15-day index) and cycling out of phase 8 and through phases 1 to 6.
- 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 December-February 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 February 2013 monthly precipitation pattern shows little correlation to the precipitation teleconnection patterns for the MJO. However, the weekly precipitation anomaly patterns correspond in some locations to the teleconnections for the MJO phase during these weeks — February 10-16 has wet anomalies in the Southeast and Upper Midwest, corresponding to the precipitation pattern expected with MJO phase 3 which occurred that week; February 17-23 has wet anomalies in the Southeast and Plains, corresponding to MJO phases 4 and 5 which occurred that week; and February 24-March 2 has wet anomalies from the Southern Plains to Northeast, corresponding to that week's MJO phase 6. The February 2013 monthly temperature pattern shows some similarity to the teleconnections for MJO phases 1 and 2. On a weekly basis (weeks 1, 2, 3, 4, 5), there is some agreement between the temperature anomalies at mid-month and those expected with MJO phase 3, but not for the last half of the month when temperatures were below normal and the MJO teleconnections (phases 4-6) 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 was slightly positive for most of the month, becoming more positive at the end of the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, the temperature teleconnection map for this time of year (January on the maps) shows that a positive PNA is correlated with cooler-than-normal temperatures in the Southeast and warmer-than-normal temperatures along the west coast of North America, especially in western Canada and Alaska. The precipitation teleconnections are generally drier-than-normal weather, especially in the Pacific Northwest, Great Basin, and eastern third of the country. The upper-level circulation anomalies associated with a positive PNA are above-normal heights over western North America and below-normal heights over southeastern North America.
- Observed: Alaska was warmer than normal in the south (consistent with a positive PNA) but cooler than normal in the north. The February temperature anomaly pattern for the contiguous U.S. was consistent in some areas but not in others, especially the southern half of the country. The February precipitation anomaly pattern is consistent in the Northwest to Great Basin and Tennessee Valley to Northeast, but not otherwise. The February 2013 upper-level circulation pattern had strong negative anomalies from the Southwest to Northeast and positive anomalies over the Pacific and just touching the North American coast. There is some agreement with the teleconnections over parts of the eastern U.S., but generally the pattern is not consistent 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 AO index began the month slightly positive, then turned strongly negative, especially during the last half of the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative AO this time of year (December-February) is typically associated with cooler-than-normal temperatures from the Northern High Plains to the East Coast and Florida, dry conditions from the Southern Plains to Ohio Valley, and patchy wetter-than-normal conditions in Northern California and New England. A positive AO is related to warmer-than-average conditions east of the Rockies, drier-than-normal conditions in the coastal Southeast and Northeast, and wetter-than-normal conditions in the Tennessee Valley and coastal Northwest. The December-February 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 half of the U.S. and over the North Pacific Ocean, and above-normal 500-mb heights (which translates to stronger ridge or weaker trough, depending on the circulation) over the Arctic. A positive AO correlates to the opposite conditions.
- Observed: The February monthly temperature anomaly pattern is consistent with a negative AO for much of the eastern half of the country, but not elsewhere. However, on a weekly basis, the teleconnection with a positive AO at the beginning of the month agrees with the temperature anomalies for January 27-February 2, and the temperature anomalies for February 17-23 and February 24-March 2 are consistent with a negative AO, except the Southwest is also colder than normal. There is no agreement with the February precipitation pattern. The February 2013 upper-level circulation is consistent over the eastern U.S. and from Hudson Bay to Greenland, but not elsewhere.
- 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 positive at the beginning of the month and turned negative for the last part of the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive NAO during this time of year (January on the teleconnection maps) is typically associated with drier-than-normal conditions in Northern California and the Great Basin and warmer-than-normal temperatures for much of the country east of the Rockies. The upper-level circulation anomalies for a positive NAO are above-normal 500-mb geopotential heights over the eastern two-thirds of the contiguous United States. A negative NAO is associated with the opposite temperature, precipitation, and circulation anomalies.
- Observed: The February weekly temperature anomaly patterns generally track the change from a positive to negative NAO, with the monthly temperature anomaly patterns matching those expected with a negative NAO over much of the country east of the Rockies. The precipitation anomalies match where teleconnections exist. The February 2013 upper-level circulation pattern over the U.S. and Hudson Bay to Greenland is consistent with a negative NAO.
- 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 averaged cooler than normal during February 2013, but the cool anomaly pool shrank and the warm anomaly pool in the central North Pacific expanded. The EP-NP index (3-month running mean) has been positive for the last several months but turned negative for February.
- Teleconnections (influence on weather): Teleconnection maps are not available for the winter (January), but they are for the bookend seasons autumn and spring. To the extent these teleconnections are known, a negative EP-NP index during these seasons is typically associated with warmer-than-normal temperatures east of the Rockies, cooler-than-normal temperatures along the West Coast and Alaska, above-normal upper-level circulation anomalies (weaker upper-level trough) over eastern North America, and below-normal upper-level circulation anomalies (weaker upper-level ridge) over western Canada and Alaska (the correlations are weak for precipitation).
- Observed: The February temperature and upper-level circulation anomaly patterns are not consistent with those expected for a negative EP-NP.
- The Tropical/Northern Hemisphere (TNH) pattern
- Description: The TNH teleconnection pattern reflects large-scale changes in both the location and eastward extent of the Pacific jet stream, and also in the strength and position of the climatological mean Hudson Bay Low, and is dominant in the winter months. The pattern significantly modulates the flow of marine air into North America, as well as the southward transport of cold Canadian air into the north-central United States.
- Status: The TNH index was positive during February.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive TNH is typically associated with cooler-than-normal temperatures for much of the country, drier-than-normal conditions in the West, and slightly wetter-than-normal conditions in the Tennessee Valley. The upper-level circulation anomalies for a positive TNH are below-normal 500-mb geopotential heights over the eastern two-thirds of Canada and the north central U.S., and above-normal heights over the northeastern Pacific and Gulf of Mexico into the western North Atlantic.
- Observed: The February precipitation anomaly pattern matches that expected with a positive TNH in the West and the temperature anomaly pattern matches for much of the country except the Northern High Plains and Northern Rockies. The February upper-level circulation pattern over the North Pacific matches the teleconnections for a positive TNH, but the below-normal upper-level circulation pattern over eastern Canada seems shifted south over the eastern United States.
Examination of these circulation indices and their teleconnection patterns, and comparison to observed February 2013 and December 2012-February 2013 temperature, precipitation, and circulation patterns, suggests that no single atmospheric driver dominated the weather during February, but the weather was influenced in part by several of the drivers. ENSO was neutral and, thus, not a player. The influence of the EP-NP driver is weak during the winter. The NAO transitioned from positive to negative, which could have masked its influence. In spite of its transition, the upper-level circulation pattern seemed to match the NAO teleconnection closest but was modified by the AO and TNH. The NAO, AO, and TNH drivers apparently conspired to dominate the temperature pattern. The TNH and PNA drivers appeared to account for the dryness in the West. The AO and PNA drivers are normally associated with below-normal precipitation over the eastern half of the U.S., but weather systems, likely driven by the MJO, brought a mixed pattern of above-normal precipitation east of the Rockies. 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.