(1)
(7)
Summary
Discussion
➢ The Autumn season of 2000 saw record rainfall and flooding in western Europe. It was the
wettest Autumn in England and Wales since records began in 1766, several regions from France
to Norway received double their average rainfall and there were severe floods and mud-slides in
the southern Alps (2).
➢ Seasonal forecasts have shown skill in predicting the large-scale climate anomalies associated
with the last El Niño in 1997, but they were less successful in predicting the extreme Autumn
2000 European weather. The results here may provide a basis for understanding the current
lack of predictability and improving forecast skill.
The UK Record-Breaking Wet Autumn of 2000:
Patterns of variability associated with Autumn precipitation in the UK
➢ The extreme weather was linked to changes in the Atlantic jet-stream (3), which were part of an
extensive pattern of anomalies stretching from mid-Atlantic across Eurasia (centre: left-panel).
➢ Previous studies have shown the potential for Amazonian deforestation (either due to direct
land-use changes or induced by climate change) to influence Atlantic and European climate in
winter. Our results suggest that changes in Amazonian precipitation, associated with natural
variability or deforestation, could influence European climate in Autumn.
➢ Wet UK Autumns in the preceding 42 years were associated with a remarkably similar pattern,
closely resembling the Scandinavia pattern, which has previously been identified as a preferred
pattern of northern hemisphere variability (4; centre: middle-panel).
➢ There was evidence in Autumn 2000 of anomalous descent and dryness in the tropics, over the
Atlantic and South America. Precipitation variability there shows a weak (negative) correlation
with the index of the Scandinavia pattern (5). Could anomalies in this region trigger the pattern?
Michael Blackburn(1) and Brian J. Hoskins(2)
➢ Further numerical experimentation is planned using a baroclinic global model, to investigate the
wavelength of the response generated by the barotropic model, and the possible influence on
the South American region of the anomalous Indonesian precipitation observed in Autumn 2000.
(1) Centre for Global Atmospheric Modelling;
(2) Department of Meteorology,
➢ The England-Wales precipitation time-series contains no significant trend in total Autumn
precipitation, but there is the suggestion of increasing interannual variability, which warrants
further investigation. Any such trend would increase the risk of future extreme events.
➢ Numerical experiments with a barotropic model confirm this hypothesis, with anomalous tropical
vorticity forcing generating a wavetrain extending over Europe (6; centre: right-panel).
(2)
University of Reading, UK
(6)
European Precipitation in Autumn 2000
England-Wales Autumn precipitation anomaly
European Precipitation (percentage of normal)
Sep - Nov 2000
250
Observations - SON 2000
Regression on England-Wales precip
Barotropic model response
300hPa Geopotential Height
300hPa Geopotential Height
Streamfunction anomaly
ECMWF analyses
SON 1958-1999
Convergence forcing (45W,5N), (-fD)
Anomaly from ERA-15
(bold: 99% confidence level)
SON climate 300hPa basic state
• A global barotropic model has been used to investigate
possible triggering of the Scandinavia pattern by
anomalous tropical convergence over the Atlantic and
South America.
Autumn rainfall relative to average (mm)
200
150
100
50
180
180
EQ
EQ
150W
EQ
150W
150E
150E
-100
30N
-150
30N
30N
120W
120E
120W
• The response is robust to modest displacement of the
forcing region and other modelling choices.
120E
-200
60N
-250
1750
1800
1850
1900
1950
60N
60N
2000
• The wavelength of the response is longer than observed,
possibly due to omission of vertical structure in the
barotropic model.
Year
Daily precipitation for England and Wales (Sep − Dec, 2000)
90W
90E
90N
90W
90E
90N
90N
40
source: Alexander & Jones, 2001
35
30
precipitation (mm)
Example of the barotropic model’s streamfunction
response to equatorial divergence forcing over
Indonesia, using a super-rotation basic-state.
• Idealised convergence centred on (45°W, 5°N) leads to a
wavetrain propagating north-east over the Atlantic and
Eurasia (centre-panel: right), similar to the Autumn 2000
anomalies and to the regressed field associated with wet
UK Autumns.
0
-50
Barotropic Model Experiments
60E
60W
25
60W
• The barotropic model simulates the global response of the vorticity field to specified vorticity forcing, associated with either
divergence, orography or transient weather systems (Sardeshmukh and Hoskins, 1988, J. Atmos. Sci., 45, 1228-1251).
60E
• The record England-Wales precipitation in Autumn 2000 amounted to
503mm, or 186% of the long-term average of 270mm.
20
15
30W
• The persistent wet weather in the UK began in mid-September and
lasted until mid-December.
10
0
Sep
Oct
Met i00
Office
i15
Nov
• Most of western Europe was wet, with double the average in several
regions. In contrast, large parts of central and eastern Europe were
exceptionally dry (and warm).
Dec
Hadley Centre for Climate Prediction and Research
hadva 15/06/2001 1433
Max 23.8162
20.1091
0.0390048
( courtesy
of L. V. Alexander ) Min 0.0960518
Mean 7.14422
6.86672
30W
30E
30E
0
0
-100
5
-120
-60
-80
-20
-40
• In the tropics the model represents the upper tropospheric flow in the layer of divergent convective outflow. In the extra-tropics
it represents the deep equivalent-barotropic atmospheric structure.
20
0
(metres)
60
40
100
80
-35
120
-40
-25
-30
-15
-20
-5
-10
5
0
15
10
25
20
35
-1.2e+07
30
40
-1.4e+07
-8e+06
-1e+07
-4e+06
-6e+06
0
-2e+06
4e+06
2e+06
8e+06
6e+06
• The Autumn climatological flow at 300hPa has been held fixed in the model (by computing the climatological vorticity forcing
necessary to maintain it against self-advection) and a specified convergence anomaly has been added. The model is integrated
forward in time to obtain a steady state response. Linear damping is included to crudely represent non-adiabatic processes.
1.2e+07
1e+07
1.4e+07
(Interval 2∗106 m2s-1)
(metres for 1 Std.Dev. precipitation)
pressure 500
Pressure
Dummy7
0
500 hPa
millibars
(3)
The Atlantic Jet-stream in Autumn 2000
(4)
(5)
Previous wet Autumns: the Scandinavia Pattern
Links with Tropical South America
Scandinavia pattern for October
700hPa Geopotential height (CPC)
Velocity Potential Anomaly 200hPa ECA - ERA-15
September 2000
Isotachs of the Autumn mean 500hPa wind
ECMWF analyses (SON)
135W
30N
60N
90N
45E
• In Autumn 2000 the Atlantic jet-stream was
displaced east, with an accentuated jet-exit
region south of the UK.
• The displaced jet acted to “steer” intense weather
systems repeatedly into western Europe.
• The weather systems tended to slow in the jetexit, leading to prolonged precipitation events.
18
90
• A regression of geopotential height with England-Wales
precipitation, for Autumns 1958-1999, results in a pattern of
statistically significant anomalies remarkably similar to that in
Autumn 2000 (centre-panel: middle). The regression pattern
appears in other fields, such as sea level pressure.
-1E6
70
2E6
30
-2E6
10
3E6
1E6
2E6
-10
0
1E6
-30
0
1E6
-50
• The seasonal England-Wales precipitation data are slightly
skewed relative to a Gaussian distribution but a square root
transform, which minimises skewness, and detrending make
no discernable difference to the regressions.
-70
0
0
-90
-180
-150
-120
-90
-60
-30
0
30
60
90
120
0
45W
Climatology (coloured)
(ms-1)
6
3
Autumn 2000 (contours)
12
9
18
15
24
21
• The displaced Atlantic jet-stream was part of an
extensive pattern of large-scale anomalies,
stretching from the mid-Atlantic across Eurasia.
The pattern was dominated by low pressure over
the UK and a strong ridge downstream, over
Scandinavia (centre-panel: left).
• The regression pattern closely resembles the Scandinavia
pattern, originally identified as a preferred pattern of northern
hemisphere variability in a rotated principal component
analysis by Barnston and Livezey (1987, Mon. Wea. Rev.,
115, 1083-1126).
• A monthly index of the Scandinavia pattern, computed by
NOAA’s Climate Prediction Center (CPC), correlates with
England-Wales precipitation for Autumn, with a correlation
coefficient of 0.60.
Regressions using NCEP-NCAR reanalysis data were performed at
the NOAA-CIRES Climate Diagnostics Centre, Boulder, Colorado,
using their Web site at http://www.cdc.noaa.gov/.
180
90
0
• A composite of wet UK Autumns (>50mm anomaly) for 19581999 reveals a very similar pattern.
3E6
50
-1E6
-3E6
0
2E6
• First, both ECMWF and NCEP analyses showed a
region of anomalous upper level convergence over
this region in Autumn 2000 (left). Satellite derived
monthly OLR anomalies were broadly consistent
with the divergence anomalies.
250
Incidence
of England-Wales
precipitation
England-Wales
Autumn Precipitation
and the1950-2000
Scandinavia
pattern
in Autumn
Scandinavia
Index (CPC)
10
-10
200
150
100
2E6
-50
1E6
-70
1E6
-90
-180
-150
-120
-90
-60
-30
0
30
60
90
120
150
180
November 2000
90
50
1E6
70
50
0
0
0
1E6
0
30
-4E6
-10
-100
-30
-50
-150
Linear regression
1950-1999
r = 0.60
-200
-1.5
-1.0
-0.5
0.0
0.5
Scandinavia Index
1.0
1.5
0
-1E6
2E6
0
-70
0
-1E6
1E6
-90
-180
2.0
-150
-120
-90
-60
-30
0
30
60
90
120
150
• Second, there is a weak historical (negative)
correlation between tropical South American
precipitation and the Scandinavia pattern in
Autumn, using a new precipitation dataset from
1960-1990 compiled by Webber and Willmott. The
correlations are only marginally significant at the
95% level (right).
• There are stronger (positive) correlations of South
American precipitation over Amazonia with the
Southern Oscillation Index, so it may not be
possible using observational data to separate the
impact of South American and Pacific anomalies on
the Scandinavia pattern and European weather.
0
0
0
10
-50
-250
-2.0
0
4E6
0
-30
1950s
1960s
1970s
1980s
1990s
2000
Correlation between
South American Precipitation
and the Scandinavia Index
Willmott/Webber SAmerican precip (cai)
in cAutumn
(SON, 1960-90)
SON
cai ScandI
n/d
Mean -0.0199881
Max 0.563747
Min -0.580569
10N
EQ
10S
30
England-Wales Precipitation Anomaly (mm)
90W
0
• There is dynamical forcing of a thermally indirect
vertical circulation in a jet-exit, with ascent on the
poleward side and descent on the equatorward
side. In Autumn 2000 the ascent region was
accentuated and displaced close to the UK.
12
150
October 2000
70
21
15
• Two pieces of data suggest that the Scandinavia
pattern in Autumn 2000, and possibly other years,
may have been triggered by anomalous weather in
the tropics, over the Atlantic and South America.
50
180
20S
30S
40S
50S
60S
90W
80W
-0.45
-0.5
70W
-0.35
-0.4
60W
-0.25
-0.3
-0.2
50W
0
0.25
0.2
40W
0.35
0.3
30W
0.45
0.4
0.5