Synoptic Discussion - October 2015
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.
October 2015 was characterized by a strong El Niño which contributed to an energized upper-level circulation pattern over the contiguous United States (CONUS). Several troughs and ridges migrated through the jet stream flow at northern latitudes. Strong upper-level troughs and cutoff lows competed with the subtropical high pressure systems at lower latitudes. El Niño-enhanced convection triggered tropical cyclones over the eastern equatorial Pacific, and moisture from some of these systems moved across the CONUS. A complex synoptic weather pattern at the beginning of the month funneled tropical moisture into the Carolinas, bringing record flooding rains. Fronts and weather systems moving in the jet stream flow brought areas of above-normal precipitation to parts of the country, especially from the Southwest to Lower Mississippi Valley, and Carolinas to Mid-Atlantic and Tennessee Valley states. High pressure ridging and a northwesterly flow kept the West Coast and Central Plains to Great Lakes drier than normal. Dominant high pressure ridging in the upper levels of the atmosphere kept temperatures warmer than normal and inhibited the occurrence of severe weather. As a result, drought expanded in parts of the Central and Northern Plains to Great Lakes. But tropical moisture-enhanced heavy rains contracted drought in the Southern Plains and Lower Mississippi Valley, and in parts of the Southeast, Northeast, and West. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather and climate of October 2015 were the result largely of atmospheric drivers originating over the Pacific Ocean, with the El Niño playing an important role. See below for details.
In the Northern Hemisphere, October marks the middle of climatological fall (autumn) which is the time of year when solar heating decreases as the sun angle decreases, arctic air masses become much colder, and an expanding circumpolar vortex forces the jet stream to migrate southward. Polar air masses influence the weather over the contiguous U.S. (CONUS) more, and the warm, dry subtropical high pressure belts influence the weather less.
During October 2015, the North Pacific and North Atlantic subtropical high pressure centers continued to have a strong influence on the weather across the CONUS. An active upper-level circulation dominated at higher latitudes. Several short-wave troughs and ridges migrated through the jet stream flow, with troughs frequently plunging south to form cutoff lows which competed with the subtropical highs across the southern tier states. The jet stream circulation was energized by Pacific tropical cyclones spawned under the influence of a strong El Niño.
The oceanic and atmospheric conditions associated with El Niño enhanced tropical cyclone activity in the eastern and central equatorial Pacific Ocean, while increasing vertical wind shear over the North Atlantic Ocean, which inhibited the development of tropical cyclones in the Atlantic and Caribbean. Six tropical cyclones and one tropical depression were active in the central and eastern Pacific during October (Tropical Storm Nora; Hurricanes Oho, Olaf, and Patricia; and Typhoons Choi-wan and Champi). Hurricane Patricia became the strongest hurricane on record in either the Atlantic or eastern North Pacific basins. Tropical moisture from the remnants of Pacific Hurricane Marty (which formed in September) fueled heavy rains across New Mexico and west Texas in October. Tropical moisture from the remnants of Hurricane Patricia moved across Mexico then became entrained in a complex synoptic circulation over the Southern Plains and Lower Mississippi Valley at the end of October — the moisture was caught up in the westerly flow over Mexico along the southern edge of a large upper-level trough, then directed into the Southern Plains in a southwesterly flow. The rains from Patricia's remnants fell on top of heavy rains from previous weather systems that had moved through the area several days earlier, resulting in significant flooding. As it moved to the north, the remnants of Hurricane Oho transitioned into an extratropical low, bringing moisture to the Alaskan panhandle and British Columbia.
No tropical cyclones formed in the Atlantic basin during the month of October, but Joaquin was a major hurricane that was ongoing at the beginning of the month. Moisture from Hurricane Joaquin was drawn into a complex synoptic situation across the Carolinas at the beginning of October — Hurricane Joaquin was nearly stationary over the Bahamas, a slow-moving cutoff low was over the Southeast, and a cold front went stationary along the Southeast coast. This synoptic pattern set up a circulation pattern which funneled onshore winds into the Carolinas, which drew in tropical moisture from Joaquin. As it moved over the stationary front and wrapped around the cutoff low, the moisture from this Atlantic air was squeezed out in the form of constant heavy rain which lasted for several days, resulting in flooding record rainfall.
Some of the Pacific tropical storms and powerful hurricanes that moved northward added their energy to the jet stream flow. This energy propagated downstream over North America, contributing to the vigor of upper-level ridges and troughs over the CONUS. These complex interactions in the upper atmosphere were reflected in a stronger index for the PNA (Pacific-North America) Pacific-based teleconnection pattern.
The subtropical high pressure systems and strong El Niño helped keep the jet stream and storm track well to the north. Although cool fronts brought Pacific and Canadian air masses across the CONUS, the subtropical highs and El Niño, in combination with migrating jet stream ridges, kept temperatures warmer than normal across most of the CONUS. Persistent ridging in the jet stream along the west coast of North America gave Alaska a warmer-than-normal month. Temperatures were colder than normal only in parts of the East Coast. By the end of the month, there were 3,247 record warm daily high (1,055) and low (2,192) temperature records, which is almost four times the 845 record cold daily high (668) and low (177) temperature records. Washington had the warmest October in the 1895-2015 record and five other western states ranked second warmest, with the CONUS ranking fourth warmest nationally. The REDTI (Residential Energy Demand Temperature Index) for October 2015 ranked 27th lowest for October, illustrating how the unusually warm temperatures reduced heating demands, especially in the West and Northern Plains.
With subsiding air associated with the subtropical highs and jet stream ridges generally inhibiting convection, severe weather was reduced. Only 40 tornadoes occurred nationwide (according to preliminary reports), which is below the October average of 61. Most of the tornadoes occurred in Texas and the Lower Mississippi Valley with the complex synoptic situation at the end of the month.
An unusual feature of this month's circulation was the frequent development and movement of elongated troughs and cutoff lows along the southern tier states in spite of the presence of the subtropical highs. These systems drew in tropical moisture from the Pacific, Gulf of Mexico, and Atlantic, resulting in a wetter-than-normal month across the Carolinas to parts of the Northeast, the Lower Mississippi Valley and Texas to the Southwest, and the interior West. Wet conditions across the southern tier states is a common characteristic of El Niño, especially as the winter wears on. The subtropical highs and jet stream ridging contributed to drier-than-normal weather across the Far West, parts of the Central Plains to Midwest, and parts of the Southeast. The unusually warm temperatures meant most of the precipitation fell as rain instead of snow, resulting in a smaller-than-normal CONUS snow cover for the month. The persistence of dry weather during the first half of the month (weeks 1, 2, 3, 4, 5) contributed to the development of several large wildfires in parts of the West and Plains. Rainfall later in the month had extinguished most in the West by month's end, although several were still burning in the Southern Plains and Lower Mississippi Valley (maps for October 2, 16, 23, 30).
Nationally, October 2015 ranked as the 20th wettest October in the 1895-2015 record. The heavy tropical rains reduced drought in the Southern Plains to Lower Mississippi Valley and Carolinas. Drought also contracted slightly in parts of the Northeast and West, and above-normal precipitation across parts of Alaska and Puerto Rico reduced drought in those areas. But drier-than-normal weather prompted expansion of drought in parts of the Central and Northern Plains to Great Lakes. Overall, the national moderate-to-exceptional drought footprint shrank to 21.9 percent of the U.S. as a whole.
The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. Several parts of the U.S. experienced temperature and precipitation extremes during October. These included the Northwest region which had the second most extreme October CEI on record due to the most extreme warm minimum and third most extreme warm maximum temperature components, most extreme drought component, and 13th most extreme days with precipitation component (due to frequent upper-level trough and cool front passages). The West region had the second most extreme October CEI on record due to the most extreme warm minimum and 12th most extreme warm maximum temperature components, and ninth most extreme drought component. The Southwest region had the eighth most extreme October CEI on record due to the most extreme warm minimum and 17th most extreme warm maximum temperature components, and seventh most extreme days with precipitation component (due to frequent slow-moving upper-level troughs). When aggregated across the nation, October 2015 had only the 22nd most extreme national October CEI on record.
Troughs and ridges migrated through the jet stream flow at higher latitudes while cufoff lows frequently traversed the lower latitudes of the CONUS. But high pressure ridging tended to favor the western and central CONUS, resulting in an above-normal 500-mb height anomaly pattern over Alaska, western Canada, and much of the western and central CONUS. Migration of strong Pacific hurricanes and typhoons into the North Pacific high latitudes contributed to below-normal 500-mb heights in the central North Pacific. Heights were also below-normal over northeast Canada to Greenland.
Wetter-than-normal monthly precipitation anomalies dominated across the Carolinas to parts of the Northeast, the Lower Mississippi Valley and Texas to the Southwest, and the interior West. October was drier than normal across the Far West, parts of the Central Plains to Midwest, and parts of the Southeast. The precipitation anomaly pattern was mixed in Alaska, Hawaii, and Puerto Rico.
North America was part of a long-wave pattern that stretched across the Northern Hemisphere. Above-normal 500-mb heights occurred across the CONUS and western Canada, then stretched from Alaska and along the Arctic coast of Eurasia to Scandinavia. Below-normal 500-mb heights occurred in between the above-normal heights over northeast Canada and Greenland, and pockets of below-normal 500-mb heights occurred along the periphery of the above-normal heights over the north central North Pacific, interior western Asia, and near Japan. The above-normal 500-mb heights were associated with upper-level ridging, above-normal surface temperatures, below-normal snow cover (over the CONUS and southwest Canada), and (over Scandinavia and north central Russia) below-normal precipitation. The below-normal 500-mb heights were associated with upper-level troughing, near- to below-normal surface temperatures, and (over interior western Asia) above-normal precipitation and snow cover. With large portions of the continents having warmer-than-normal temperatures, and large portions of the equatorial Pacific Ocean (due to El Niño) and eastern North Pacific Ocean having warmer-than-normal sea surface temperatures, the October 2015 global temperature was well above normal.
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 October 2015, El Niño conditions were observed as above-average SSTs continued across the western and central equatorial Pacific and continued to be coupled to the tropical atmosphere. Collectively, the atmospheric and oceanic features reflected strong El Niño.
- Teleconnections (influence on weather): To the extent teleconnections are known, the typical temperature and precipitation patterns associated with El Niño during October include above-normal precipitation in parts of the Central and Southern Plains, Northwest, West, Tennessee Valley, and Southeast; below-normal precipitation across the Mid-Atlantic region, Great Lakes, and Northeast, although there is a trend toward wetter conditions in the Northeast; and near- to below-normal temperatures across much of the CONUS except the Northwest to Northern Plains, although there is a trend toward warmer-than-normal temperatures for much of the CONUS.
- Comparison to Observed: The October 2015 precipitation anomaly pattern is a reasonable match with that expected historically with an El Niño, but with some differences. The temperature anomaly pattern does not match the historical teleconnections, but does fit the trend.
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 index stayed inside the center circle for most of October, indicating an incoherent signal, but emerged into the Indian Ocean phase (phase 2) near the end of the month. Other modes of coherent subseasonal tropical variability, including the background El Niño state, tropical cyclone activity, and Kelvin waves, influenced the tropical Pacific and global climate (MJO updates for October 5, 12, 19, 26, and November 2).
- 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 September-November 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 October 2015 monthly temperature and weekly (weeks 4 and 5) temperature anomaly patterns generally do not match those expected with MJO phase 2. The monthly precipitation anomaly pattern matches that expected with MJO phase 2 in the Mid-Atlantic to Great Lakes and Central Plains, but not in the Southern Plains and Far West. The weekly (weeks 4 and 5) precipitation anomaly patterns do not match that expected with MJO phase 2.
- 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 mostly positive, averaging positive for the month as a whole. The 3-month-averaged index was also positive.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive PNA for this time of year (October on the teleconnection maps) is correlated with above-normal temperatures over Alaska, western Canada, and the extreme western CONUS; below-normal temperature anomalies over the Southern Plains and Southeast; above-normal upper-level circulation anomalies over western North America; and below-normal circulation anomalies over the Southern Plains to East Coast and the north central North Pacific. The precipitation teleconnection map shows little correlation to the PNA except for a hint of dry anomalies over the Midwest.
- Comparison to Observed: The October 2015 temperature and upper-level circulation anomaly patterns are a good match for the teleconnections for a positive PNA across western North America, but not so much over the Southeast. The October 2015 precipitation anomaly pattern agrees in the Midwest, but few correlations exist.
- 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 oscillated between positive and negative throughout the month, averaging negative for the month. The 3-month-averaged index was negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative AO this time of year (September-November) is typically associated with dry conditions in the Southern Plains, Southeast, Midwest, and coastal Northwest; wet conditions in parts of the Great Lakes, New England, and Southern Florida; below-normal temperatures across parts of the Northern Plains; and upper-level circulation anomalies which are below normal in the North Central Pacific, southeastern Canada, north central to eastern CONUS, and across the North Atlantic, and above normal over the Arctic Ocean and Greenland. For temperature and precipitation, most of the country has little correlation to the AO this time of year.
- Comparison to Observed: The October 2015 monthly precipitation, temperature, and upper-level circulation anomaly patterns show little to no agreement with those associated with 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 daily NAO index oscillated between neutral and positive, averaging positive for the month. The 3-month-averaged index was negative, reflecting the influence of the previous two months.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive NAO during this time of year (October on the teleconnection maps) is associated with above-normal upper-level circulation anomalies over the eastern CONUS. There are no temperature teleconnections, and the precipitation teleconnections are very weak, with the Ohio Valley wetter than normal.
- Comparison to Observed: The October 2015 temperature, precipitation, and upper-level circulation anomaly patterns either show little agreement or have few or no teleconnections with a positive NAO.
- 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: Both the monthly WP index and three-month average WP index were negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative WP during this time of year (October on the maps) is typically associated with below-normal temperatures in the Central to Northern Plains; above-normal precipitation in the Northwest, Upper Midwest, and Deep South; below-normal circulation anomalies over central Canada and the north central CONUS, and above-normal circulation anomalies over the eastern North Pacific just off the West Coast.
- Comparison to Observed: The October 2015 monthly temperature, precipitation, and upper-level circulation anomaly patterns show little agreement with a negative WP.
- 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 October SST pattern showed a continuation of widespread above-normal SSTs in the North Pacific from the International Dateline to North America and pockets of cooler-than-normal SSTs west of the Dateline. The Pacific SSTs warmed along the North American coast but cooled significantly just east of the International Dateline, resulting in a cooler-than-normal pocket of SSTs in the midst of the warm SSTs that dominate the North Pacific east of the Dateline. The monthly EP-NP index was slightly positive during October, while the 3-month running mean was slightly negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive EP-NP index during this time of year (October on the maps) is typically associated with cooler-than-normal temperatures across central to eastern Canada and across the CONUS east of the Rockies (except for the Southeast), and warmer-than-normal temperatures across Alaska and the extreme West Coast of the CONUS; above-normal upper-level circulation anomalies (stronger upper-level ridge) over Alaska and western Canada, and below-normal upper-level circulation anomalies (stronger upper-level trough) over the CONUS east of the Rockies, eastern Canada, and the central North Pacific. The precipitation teleconnections are too weak to be mapped in any detail, although there is a hint of drier-than-normal anomalies in the Northwest.
- Comparison to Observed: The October 2015 temperature and upper-level circulation anomaly patterns agree with the teleconnections for a positive EP-NP along the west coast of North America, but not over the central or eastern regions. The precipitation anomaly pattern is not a good match in the Northwest, where a hint of teleconnections exist.
Examination of the available circulation indices and their teleconnection patterns, and comparison to observed October 2015 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in October reflected influences largely from Pacific atmospheric drivers. The AO and NAO showed little correlation with the October anomaly patterns, indicating that the Arctic and North Atlantic atmospheric drivers had little influence on October's weather over the CONUS. The WP showed little correlation, and the EP-NP's teleconnection matched October's anomaly patterns along the west coast of North America but not elsewhere, which indicates that these specific Pacific drivers were overwhelmed by other drivers. The PNA matched over parts of North America and the CONUS, but not others. The precipitation teleconnections for El Niño were a reasonable match with the October anomalies in most areas, but the temperature teleconnections not so much. The MJO precipitation teleconnections matched in some parts of the CONUS, but not others, and there was no agreement for temperature.
The El Niño created atmospheric and oceanic conditions that enhanced the development of tropical storms and hurricanes in the eastern and central tropical Pacific Ocean. Moisture and energy from the remnants of some of these tropical cyclones directly impacted the CONUS. Others moved further north in the North Pacific, transferring energy into the mid-latitude circulation and mixing the waters at the sea surface. This likely affected the strength and location of the East Asian jet stream, which was reflected in the PNA index. MJO contributed energy near the end of the month which may have influenced the precipitation pattern over the eastern CONUS. But the lack of agreement (except for PNA) between the teleconnections and the temperature anomaly pattern suggests other factors in play, or possibly the random variations of an energized jet stream flow.
This month illustrates how the weather and climate anomaly patterns can reflect the influence of one atmospheric driver (or mode of atmospheric variability), but also how random variability can influence the month's weather.