Extratropical Highlights –October 2015
1. Northern Hemisphere
The mean 500-hPa circulation during
October featured above-average heights over Alaska, western North America, and
Scandinavia, and below-average heights over the high latitudes of the North Pacific,
eastern Canada, the eastern North Atlantic, and central Russia (Fig. E9). Over
the Pacific/ North America sector, the anomaly pattern projected onto the
strong positive phase of the Pacific/ North American (PNA) teleconnection
pattern (Table E1,
At 200-hPa, a significant El Niño
response continued in the streamfunction (Fig.
T22) and wind (Fig. T21) fields. This response featured anticyclonic streamfunction
anomalies over the subtropical North and South Pacific, straddling the region
of enhanced convection (Fig. T25).
The main land-surface temperature signals
during October included well above-average temperatures across Alaska and the
western half of North America, (Fig. E1). The main precipitation signals included above-average
totals in the south-central U.S. and southeastern Europe, and below-average
totals in northwestern Europe and Scandinavia (Fig. E3).
a. North Pacific/ North America
At 500-hPa, the circulation during October
featured above-average heights over the subtropical central North Pacific Ocean
and western North America, and below-average heights over the high latitudes of
the North Pacific and eastern Canada (Fig.
E9). At 200-hPa, the circulation featured
anticyclonic streamfunction anomalies across the subtropical central North
Pacific, in association with El Niño (Fig.
Over the Pacific/ North America
sector, the anomaly pattern projected onto the strong positive phase of the
Pacific/ North American (PNA) teleconnection pattern (Table E1, Fig. E7). Over North America, the anomaly
pattern reflected an amplified ridge in the West and trough in the East. This overall
circulation pattern contributed to exceptionally warm surface temperatures across
Alaska and western North America, with departures in many areas exceeding the
upper 90th percentile of occurrences (Fig. E1).
Precipitation was above average
across the southwestern, south-central, and mid-Atlantic regions of the U.S.,
and below average in the central U.S. (Fig.
E3). According to the U.S. Drought Monitor, severe
or extreme drought persisted in Washington, Oregon, Idaho and western Montana. Exceptional
drought continued across central California and western Nevada.
b. North Atlantic
The 500-hPa circulation during
October featured above-average heights over Scandinavia and below-average
heights over the eastern North Atlantic (Fig.
E9). The associated split-flow pattern contributed to increased storminess
and above-average precipitation across southern Europe, and to well
below-average precipitation in northwestern Europe and Scandinavia (Figs. E3, E4).
Across the Atlantic hurricane Main
Development Region (MDR, which spans the Caribbean Sea and tropical Atlantic
Ocean between 9°N-21.5°N), an amplified Tropical Upper-Tropospheric Trough
(TUTT) was again present during October in association with El Niño (Fig. T22).
This amplified TUTTcontributed to increased vertical
wind shear, anomalous upper-level convergence (Fig. T23) and sinking motion (Fig. T30) across large portions of the
MDR. Similar conditions have suppressed hurricane activity throughout
the peak months (Aug.-Oct.) of the Atlantic hurricane season.
In contrast, conditions in the
central and eastern North Pacific hurricane basins remained exceptionally
conducive to very active hurricane seasons. These conducive conditions have
included an amplified upper-level ridge in association with El Niño (Fig. T22), weak vertical wind shear,
anomalous upper-level divergence (Fig. T23), strong low-level
convergence, anomalous rising motion (Fig. T29), and an amplified ITCZ (Fig. T25).
2. Southern Hemisphere
The mean 500-hPa circulation during
October featured above-average heights over the southern portions of the three
continents, and below-average heights over the high latitudes of both the
central South Pacific and the eastern North Atlantic (Fig. E15). At 200-hPa, a significant El
Niño response was evident in the streamfunction field throughout the global
tropics and subtropics. This response featured a zonal wave-1 pattern of
streamfunction anomalies in both hemispheres (Fig. T22), with anticyclonic anomalies over the subtropical North and South
Pacific straddling the region of enhanced convection (Fig. T25).
In the SH, this pattern was
associated with 1) a strengthening and eastward extension of the South Pacific
jet stream to well east of the date line, and 2) an eastward shift of that
jet’s exit region to the eastern South Pacific (Fig. T21). This jet stream
pattern represents major dynamical and kinematic changes in the mid- and
upper-level circulation during El Niño, and it also represents a fundamental
manner in which El Niño’s circulation impacts are communicated downstream.
The main surface temperature signals
during October included well above-average temperatures across southern
Australia and southern Africa, with departures in many locations exceeding the
upper 90th percentile of occurrences (Fig. E1). In contrast, surface
temperatures were well below average over southern South America. Precipitation
signals during October included exceptionally dry conditions in southeastern
and northeastern Australia, and in southern Africa.
The South African rainy season
lasts from October to April. Many areas during October 2015 recorded rainfall
totals that were in the lowest 10th percentile of occurrences (Fig. E3).
Rainfall for the region as a whole during October was the lowest in the
1979-present record (Fig. E4).
The Antarctic ozone hole typically develops during August
and reaches peak aerial extent in September and early October (Fig. S8). The
ozone hole then typically weakens rapidly during October and November. In
contrast, the ozone hole during October 2015 decreased only slightly (Fig. S6), and
still spanned approximately 21 million square kilometers at the end of the
month. This size is largest in the 2005-2014 record (Fig. S8, top), and it is nearly twice
that of the 2005-2014 average.
This record ozone hole was associated with the largest SH
polar vortex observed in the 2005-2014 record (Figs. S8, middle, Fig. S1) and with a well above-average coverage
of polar stratospheric cloud (Fig. S8, bottom). These conditions were accompanied by well below-average
stratospheric temperatures for the past several months (Fig. S4, right). This record-size ozone
hole follows a near-average size in 2014, and below-average sizes during both
2012 and 2013.