Synoptic Discussion - June 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.
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June marks the beginning of the Northern Hemisphere's climatological summer (June-August). With the sun's angle at its maximum inclination in the Northern Hemisphere and, thus, solar heating at its greatest intensity, the circumpolar vortex is normally weak and contracted far to the north with warm southerly air masses associated with the subtropical high pressure belt influencing the weather over the United States. During June 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) continued in an active phase, with many troughs and ridges migrating across the country in the upper-level circulation.
Cold fronts and warm fronts moving with these upper-level systems brought migrating spells of cooler-than-normal and warmer-than-normal weather to parts of the country (weeks 1, 2, 3, 4). Some of these troughs moved very slowly and, in the east, tapped Gulf of Mexico and Atlantic moisture, resulting in heavy rains and causing extensive flooding, especially in the Upper Midwest and along the East Coast, where Delaware and New Jersey had their wettest June on record and New York and North Carolina ranked second wettest. These upper-level troughs and their complex circulations also triggered tornado outbreaks and widespread wind damage from the Plains to the East Coast, although the preliminary national monthly count of 145 tornadoes was below the June average of 243.
Although cool troughs and their associated fronts traversed the West during the month, high pressure ridging dominated — bringing hot and dry weather to the southwestern third of the country, where June ranked in the top ten warmest category for six states and top ten driest category for four states, including Utah which had its driest June on record. Dozens of large wildfires flared up, especially in the Southwest but also in Alaska (where the statewide average temperature ranked third warmest for June in the 1918-2013 record). The total number of fires in the country was below the June average, but the acreage burned was above average. About 2800 record warm daily highs and lows were recorded across the nation, almost four times as many record cool daily highs and lows. When integrated across the month, the Southwest averaged warmer than normal and the north central parts of the country averaged cooler than normal with a mixture of anomalies elsewhere.
June marks the beginning of the Atlantic hurricane season. Two tropical storms formed this month in the North Atlantic basin, one of which (Tropical Storm Andrea) merged with a cold front and contributed to heavy rain along the East Coast during the first week of June. Based on a 30-year (1981-2010) average, a tropical storm forms in June in the basin about once every other year.
The rain-producing systems shrank drought in the East and Midwest, but hot and dry weather expanded drought in the West and Alaska. Overall, the national (all of the U.S.) drought footprint expanded from 37.2 percent in moderate to exceptional drought last month to 39.3 percent at the end of June, but remained about the same at 44.1 percent for the contiguous U.S., according to U.S. Drought Monitor statistics. When the temperature and precipitation anomalies are integrated across the country and across the month, June 2013 ranked as the 15th warmest and 13th wettest June for the nation.
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 June, but SSTs were cooler-than-average over the eastern equatorial Pacific.
- 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 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. 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 was weak and incoherent during the first third of June, then transitioned through phases 4 to 8 as the month progressed, ending the month in phase 1 (as indicated by the MJO discussions for June 10, 17, 24, and July 1). The propagation of enhanced precipitation anomalies eastward across the equatorial Pacific is associated with this transition.
- 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 April-June 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 June 2013 monthly precipitation pattern shows little correlation to the precipitation teleconnection patterns for MJO phases 4-1 this time of year. However, the weekly precipitation anomaly patterns correspond in some locations to the teleconnections for the MJO phase during these weeks — dry weather in the western and central parts of the U.S. during week 2 can be associated with phase 4, dryness in the West and Northeast and precipitation in the central U.S. during week 3 can be associated with phases 5-6, and above-normal precipitation along the West Coast during week 4 could be associated with phase 1. The June 2013 monthly temperature pattern shows some similarity to the teleconnections for MJO phases 4-7 and 1 (warmer than normal in the Southwest). This is especially the case for week 2 (associated with phase 4) and week 3 (associated with phases 5 and 6).
- 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 negative for the first half of June, then turned positive for the second half, averaging negative for the month as a whole.
- Teleconnections (influence on weather): To the extent teleconnections are known, the temperature teleconnection map for this time of year (July on the maps) shows little correlation with the PNA. The precipitation and upper-level circulation anomaly teleconnections are likewise weak.
- Observed: The lack of notable teleconnections suggests that the PNA has little influence on the weather during the summer months.
- 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 was neutral to slightly positive throughout June, and averaged positive for the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive AO this time of year (April-June) is typically associated with warmer-than-normal temperatures in the central U.S., dry conditions from the central Rockies to Midwest and Tennessee Valley to New England, and patchy wet conditions in parts of the Southeast, although the wet correlations are weak. The April-June averaged upper-level circulation anomalies for a positive AO are a band of above-normal 500-millibar (mb) geopotential heights (which translates to weaker trough or stronger ridge, depending on the circulation) over the central to eastern U.S. and mid-latitude North Atlantic and North Pacific, and below-normal 500-mb heights (which translates to weaker ridge or stronger trough, depending on the circulation) over the Arctic, Alaska, and northern Canada.
- Observed: The June 2013 monthly temperature anomaly pattern shows some agreement with the positive AO teleconnection in the Southwest to Southern Plains, but not in the Central Plains to Great Lakes. The April-June 2013 temperature anomaly pattern is a good match for a negative AO. There is some agreement with the June 2013 precipitation pattern in the central Rockies and Plains, but not in the East. The April-June 2013 upper-level circulation pattern generally matches the teleconnections for a positive AO over the Pacific and Atlantic, but the negative height anomalies observed in 2013 seem shifted off of northern Canada and further into the Arctic. The June 2013 upper-level circulation shows considerable deviation from that expected with a positive 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 fluctuated between positive and negative values for the first half of June, then turned positive for the last half, averaging positive for the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive NAO during this time of year (July on the teleconnection maps) is typically associated with warmer-than-normal temperatures over the northwestern third of the country, cooler-than normal conditions across the Southern Plains and Southeast, drier-than-normal conditions in the Northern Plains and Midwest, patchy wet conditions along parts of the Gulf Coast (although the correlations are weak for precipitation), and positive upper-level circulation anomalies in the northwestern quarter of the country centered over Montana, with negative anomalies touching the Gulf of Mexico coast.
- Observed: The June and April-June 2013 temperature anomaly patterns show little agreement with that expected for a positive NAO, except for some hint of agreement in the Southeast. The June and April-June precipitation anomaly pattern has little resemblance to that expected with a positive NAO. The June 2013 upper-level circulation anomaly pattern is somewhat consistent with a positive NAO over western North America, the North Atlantic, and the Arctic, but not the Pacific or Gulf of Mexico.
- 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 June 2013 was dominated by a large pool of warmer-than-normal SSTs in the central North Pacific, while the cooler-than-normal SSTs along the western U.S. coast all but disappeared to be replaced by gradually warming SSTs. The EP-NP index (both the monthly index and 3-month running mean) was positive.
- Teleconnections (influence on weather): To the extent these teleconnections are known, a positive EP-NP index during this time of year (July on the maps) is typically associated with cooler-than-normal temperatures in the north central U.S., warmer-than-normal temperatures in parts of Alaska, below-normal upper-level circulation anomalies (stronger upper-level trough) over eastern Canada into the north central U.S., and above-normal upper-level circulation anomalies (stronger upper-level ridge) over western North America, including Alaska (the correlations are weak for precipitation).
- Observed: The temperature anomalies for June (contiguous U.S. and Alaska) and April-June (contiguous U.S.) are a good match for a positive EP-NP. The June 2013 upper-level circulation anomaly patterns over western Canada and Alaska (positive anomalies) are consistent with those expected for a positive EP-NP; while an upper-level trough is evident over eastern North America, the trough was not strong enough to produce negative height anomalies which are expected from the teleconnections.
Examination of these circulation indices and their teleconnection patterns, and comparison to observed June 2013 and April-June 2013 temperature, precipitation, and circulation patterns, suggests that the MJO and AO drivers each exerted some influence during June (the influence for temperature is stronger than for precipitation this time of year) but the EP-NP driver was most dominant. ENSO was neutral, and thus not a player; the PNA changed signs during the month, thus making its signal difficult to decipher; and the NAO showed no agreement to the observed patterns. The MJO signal could be seen during the second and third weeks in both the temperature and precipitation patterns. The EP-NP driver seemed to be the most influential for June temperature in the north central U.S. while the MJO driver seemed influential for June temperature in the West. This month illustrates how dominant signals can be extracted from the data, even though competing atmospheric drivers may result in a complex weather pattern.