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Climate Diagnostics Bulletin
Climate Diagnostics Bulletin - Home Climate Diagnostics Bulletin - Tropics Climate Diagnostics Bulletin - Forecast

 

  Extratropical Highlights

  Table of Indices  (Table 3)

  Global Surface Temperature  E1

  Temperature Anomalies (Land Only)  E2

  Global Precipitation  E3

  Regional Precip Estimates (a)  E4

  Regional Precip Estimates (b)  E5

  U.S. Precipitation  E6

  Northern Hemisphere

  Southern Hemisphere

  Stratosphere

  Appendix 2: Additional Figures

Extratropical Highlights

OCTOBER 2015

1

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, Fig. E7).

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. T22).

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.

 

 

 


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