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.



Summary


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.


Synoptic Discussion


Animation of daily upper-level circulation for the month
Animation of daily upper-level circulation for the month.
Animation of daily surface fronts and pressure systems for the month
Animation of daily surface fronts and pressure systems for the month.

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.

North America monthly upper-level circulation pattern and anomalies
North America monthly upper-level circulation pattern and anomalies.

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.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

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.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

October 2015 averaged warmer than normal across Alaska and most of the CONUS, especially in the West. Temperatures were cooler than normal across parts of the East.

Northern Hemisphere monthly upper-level circulation pattern and anomalies
Northern Hemisphere monthly upper-level circulation pattern and anomalies.
Global Linkages: The upper-level circulation anomaly pattern over 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.


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:


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.


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for October 2015, published online November 2015, retrieved on August 15, 2020 from https://www.ncdc.noaa.gov/sotc/synoptic/201510.

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