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.
The weather pattern over North America during October 2012 consisted of the seasonal battle between subtropical high pressure (High, or upper-level ridge) to the south and the polar jet stream and associated storm track to the north. As the sun angle decreases during Northern Hemisphere autumn, polar air masses get colder and normally expand the polar jet toward the south. The jet stream was a little more active than seasonal normals this month, propagating numerous cool upper-level troughs into the contiguous U.S., with surface cold fronts and low pressure systems following in their wake. When averaged over the month, this pattern resulted in cooler-than-average temperatures beneath a stronger-than-average upper-level trough stretching from western Canada to the Southeast United States. The storm track directed weather systems into the Pacific Northwest, across the northern Plains and southern Canadian Prairies, then into the Great Lakes and Midwest. This jet stream pattern inhibited the formation of tornadoes and wildfires, both of which were below normal for October, and brought beneficial rainfall which improved drought conditions in the Midwest. It also brought snow to parts of the Northwest, Northern and Central Rockies, and adjoining parts of the Great Plains at mid-month. But, averaged over the month, above-normal upper-level geopotential heights and descending air ("subsidence") associated with the subtropical High reduced precipitation and kept temperatures above-normal over the Southwest. The October dry weather extended into the Central and Southern Plains, where the most intense drought continued its grip. Five tropical systems (two hurricanes and three tropical storms) formed in the Atlantic basin during October, well above the long-term average. The overall circulation pattern normally deflects such coastal systems further out into the Atlantic. But for Hurricane Sandy, which wrought devastation in the Caribbean and western North Atlantic islands before moving up the eastern U.S. seaboard, an unusual upper-level jet stream flow pulled it into the northeastern U.S., where it combined with a cold front to cause locally heavy rain, strong winds, widespread flooding, and — in the central Appalachians — areas of heavy snow.
The movement of several weather systems — wet low pressure over the Southeast at the beginning of the month, an upper-level low with rainfall over the Southwest early in the month, fronts across the northern states later in the month, and the remains of Sandy in the Northeast at the end of the month — can be seen in the weekly above-normal precipitation anomaly patterns (weeks 1, 2, 3, 4, 5). Areas of persistent dryness, especially across parts of the Plains, are also evident. Nine states (mostly in the Southwest and Southern to Central Plains) ranked in the driest third of the historical record, with Texas having the ninth driest October in the 1895-2012 record. Twenty-three northern tier and Midwest states ranked in the wettest third of the historical record, with seven of them in the top ten category and Delaware ranking wettest on record for October. According to the end-of-October (October 30) U.S. Drought Monitor, 60.2% of the contiguous U.S. (50.4% of the U.S. including Alaska, Hawaii, and Puerto Rico) was affected by moderate to exceptional drought overall. These values are lower than those at the end of September, reflecting improvement in the Northeast, Mid-Atlantic, Midwest, and parts of the Northern Plains. According to the Palmer Drought Index, which goes back to the beginning of the 20th century, 49.2% of the contiguous U.S. was in moderate to extreme drought, a decrease of about 3 percent compared to last month. The 2012 Palmer Drought Index percent area values have been exceeded only by the droughts of the 1930s and 1950s.
The movement of the cool fronts can be seen in the weekly temperature anomaly maps (weeks 1, 2, 3, 4, 5). The persistence of warm anomalies in the Northeast, and frequent excursions of cool air masses — especially east of the Rockies — gave 19 states, from Montana to Alabama, monthly temperature ranks in the coolest third of the historical record, and 14 states, split between the Northeast and Southwest, monthly temperature ranks in the warmest third of the historical record. On a local basis, there were more record cold highs and lows than record warm highs and lows, which is a change from the last several months where warm records dominated over cool records. About 1300 record low temperatures and 2400 record cool daily high temperatures were tied or broken. In comparison, a little over 900 daily high temperature records and about 2350 record warm daily low temperatures were tied or broken. (These numbers are preliminary and are expected to increase as more data arrive.) On balance, the warm and cold anomalies, combined with the time of year, contributed to a national Residential Energy Demand Temperature Index (REDTI) for October 2012 that was near average.
When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the 44th coolest and 52nd wettest October in the 118-year record. Averaging extremes tends to cancel them out (as is the case for this month). But when extremes are combined cumulatively, like in the U.S. Climate Extremes Index (USCEI), they may tell a different story. Nationally, the large spatial extent of very dry conditions ranked the PDSI component fourth largest for October 2012 (behind October 1934, 1956, and 1954) and the number of days with extreme precipitation component ranked 14th largest for October 2012, but the other components ranked low or mid-range, giving the U.S. an October USCEI that ranked only 39th largest. Even with Sandy (tropical cyclone component) considered, the national October USCEI still ranked only 34th largest. However, the preponderance of unusual warmth and dryness for much of 2012 has ranked the national USCEI largest for the year-to-date (January-October) and second largest for the last twelve months (November-October).
Subtropical highs, and cold fronts and low pressure systems moving in the storm track flow, are influenced by the broadscale atmospheric circulation. Five such large-scale atmospheric circulation drivers were potentially influential during October:
El Niño Southern Oscillation (ENSO)
- Status: Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific continued in an ENSO-neutral state during October, although equatorial sea surface temperatures were close to weak El Niño 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 can be analyzed to show typical temperature and precipitation patterns associated with the ENSO episodes. For an El Niño, the typical August-October temperature anomaly pattern is patchy below-normal temperatures for most of the country. The typical El Niño August-October precipitation anomaly pattern consists of drier-than-normal conditions in the mid-Atlantic to Northeast, parts of the Great Lakes, and Southern Plains to Lower Mississippi Valley, while wetter-than-normal conditions are dominant in the Southeast, Central Plains, and Northwest. For a La Niña, the typical August-October temperature anomaly pattern consists of warmer-than-normal conditions east of the Rockies and cooler-than-normal in the West, and precipitation anomaly pattern is drier than normal from the Southwest to Central Plains, Tennessee and Ohio valleys, and eastern Great Lakes.
- Observed: The October and August-October 2012 precipitation patterns match the La Niña teleconnections from the Southwest to Central Plains, but not elsewhere, and don't match El Niño's. Although it should be noted that, before Sandy struck, the Northeast precipitation pattern for October was below normal and reasonably matched the El Niño teleconnections. The October and August-October 2012 temperature patterns show little similarity to the El Niño or La Niña teleconnections.
- The Pacific/North American (PNA) pattern
- Status: The PNA index was negative for most of the month except for a blip into positive territory at mid-month.
- Teleconnections (influence on weather): To the extent teleconnections are known, the temperature teleconnection map for this time of year (October on the maps) shows that a negative PNA is correlated with warmer-than-normal temperatures in the Southeast and cooler-than-normal temperatures along the west coast of North America, especially in western Canada and Alaska. The precipitation patterns are weakly correlated this time of year, but show some hint of wetter-than-normal conditions from the Ohio Valley to Great Lakes with a negative PNA.
- Observed: October 2012 was wetter than normal in the Ohio Valley to Great Lakes and cooler than normal from the southeastern U.S. into southeast Alaska. Otherwise, the U.S. temperature and precipitation anomaly maps bore little resemblance to that expected with a negative PNA.
- The Arctic Oscillation (AO) pattern
- Status: The AO index was negative throughout the month, turning strongly negative during the last half of the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative AO this time of year (August-October) is typically associated with cooler-than-normal temperatures across the Northern Plains, warmer-than-normal temperatures across the Southern Plains, wet conditions in the Central Plains, and spotty drier-than-normal conditions from the Southern Plains to Southeast and Lower Mississippi Valley. The August-October 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 northeastern 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 August-October 2012 temperature pattern matches the negative AO where there is a correlation, but the precipitation patterns show little similarity. However, the October 2012 temperature pattern more closely resembles what would be expected during the winter or spring. The August-October 2012 upper-level circulation had an above-normal pattern over the northern Atlantic, Greenland, and adjacent Arctic, with above-normal pattern across western North America and a trough over the eastern United States. This has some resemblance to a negative AO in the east but not in the west.
- The North Atlantic Oscillation (NAO) pattern
- Status: The NAO index was near neutral to slightly negative during the first half of the month and turned strongly negative during the last half.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative NAO during this time of year (October on the teleconnection maps) is typically associated with drier-than-normal conditions in the Ohio Valley, while temperatures show little correlation.
- Observed: The weak correlations this time of year make it difficult to relate the October and August-October 2012 precipitation and temperature anomaly patterns to the NAO. The temperature patterns more closely resemble what would be expected from a negative NAO in the winter or spring.
- The East Pacific-North Pacific (EP-NP) pattern relates sea surface temperature (SST) and upper-level circulation patterns over the eastern and northern Pacific to temperature, precipitation, and circulation anomalies downstream over North America.
- Status: The SST pattern over the northeast Pacific has experienced a warming trend over the last five months with SST anomalies becoming less cool (June, July, August) to warmer than normal in places (September, October). This has resulted in the EP-NP index (3-month running mean) transitioning from negative to positive for the last few months.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive EP-NP index during this time of year (October on the teleconnection maps) is typically associated with cooler-than-normal temperatures east of the Rockies, warmer-than-normal temperatures along the West Coast and Alaska, drier-than-normal conditions over the Northwest (although the correlations are weak for precipitation), 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 Canada and Alaska. A negative EP-NP index is typically associated with the opposite pattern — 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.
- Observed: The October and August-October 2012 temperature patterns readily reflect the positive EP-NP teleconnection. The precipitation correlations are weak and are reflected somewhat in the precipitation anomaly maps for August-October and the first half of October (drier-than-normal Pacific Northwest) but not in the full-month October precipitation map (due to wet conditions developing in the Pacific Northwest during the last half of the month). The pattern of above-normal upper-level heights (stronger ridge) over western North America, and trough over the east, in the August-October 2012 upper-level circulation matches the pattern associated with a positive EP-NP. This upper-level circulation pattern (stronger-than-normal ridge in the west and stronger-than-normal trough in the east) is distorted (negatively tilted) but still evident on the October 2012 map.
Examination of these circulation indices and their teleconnection patterns, and comparison to observed October and August-October 2012 temperature, precipitation, and circulation patterns, suggests that ENSO, PNA, and NAO had little influence on the observed weather patterns, but the teleconnections are weak this time of year for the PNA, AO, and NAO. The AO and EP-NP may have exerted some influence on the weather this past month and season. The temperature patterns over the last three months, especially the frequent movement of cold fronts across the U.S. east of the Rockies, and upper-level circulation patterns reasonably reflect a positive EP-NP. The temperature patterns could reflect the negative AO and NAO teleconnections — if it were winter. As noted above, some of the indices have weak teleconnections in the fall, so it appears that the EP-NP pattern dominated. When the atmospheric circulation drivers are neutral or in a state of transition, their influence becomes difficult to trace and can be overwhelmed by other competing forces, including random fluctuations in the atmosphere. But even when the atmospheric drivers are strong, their influence can be masked by short-term fluctuations like the highly unusual occurrence of Sandy.