Changing Polar Low frequency
over the northern North Atlantic
in a changing climate
Matthias Zahn
Institute of Coastal Research
Helmholtz-Zentrum Geesthacht
Matthias.Zahn@hzg.de
September 23, 2013
ACCESS summerschool 23-27 September, Bremen
Polar lows, characteristics
◮
◮
◮
◮
◮
◮
diameter < 1000km
strong winds > 13.9 ms
heavy precipitation
poleward Polar Fronts in
winter
spiral cloud and cloud free
core
comma shape cloud pattern
c
Dundee
Satellite Receiving Station
Development
initial disturbance
◮ lots form along sea ice edge
◮ instable conditions
◮ driven by convective
processes
◮ develop in cold air outbreaks
◮ rapid development
◮ T can drop sharply
◮ short lasting (12 to 36 hrs)
Example: 7 Jan 2009
SST − T500hPa >52K
◮
http://polarlow.met.no/STARS-DAT/
Development
initial disturbance
◮ lots form along sea ice edge
◮ instable conditions
◮ driven by convective
processes
◮ develop in cold air outbreaks
◮ rapid development
◮ T can drop sharply
◮ short lasting (12 to 36 hrs)
Example: 7 Jan 2009
Storm force 10, gusts 62kt
◮
http://polarlow.met.no/STARS-DAT/
Impacts:
◮
◮
◮
◮
◮
◮
oil/gas
rigs
fishery
shipping
aviation
traffic
ocean
circulation
Car hit by polar
c
low, Univisjon
CAFFs, Arctic Flora & Fauna - 2001
Amarube Bridge
accident,
December 28,
1986
protection? short-term and long-term knowledge of
PLs needed
Short term: forecasting
$
%,
,
4
-
=
%
" 4 $
6 2
"
5 $
" + 5 - $-
3 4 6 >
2
>
>
" 4 6
" ?-" 4 $
>
" @ 5
"
$
=*
$
$ 5
$
-
2
*
% **! " !7 8 9:°4
>
*
>
c
Gunnar
Noer, MetNo
Predictability
$
5
2
2
%
%
-
$ - 2
)$ %
$
$ 5
-
*
- 2
:&
$5$
- 2
(
"
,
$
-
c
Gunnar
Noer, MetNo
>
$
Long-term: climatology
Research questions:
◮
◮
◮
how does frequency change?
do areas of occurrence shift?
how do intensities change?
many case studies,
but no long-term climatology
Problem: data
Requirements:
◮
◮
◮
long in time
high in spatial detail
homogeneous
Problem: DO NOT EXIST
Solution: dynamical downscaling
Model grid
◮
◮
global data (re-analysis or GCM) long in time
and homogeneous
downscaled by means of Regional Climate
Model (RCM) COSMO-CLM-2.4.6.
Example of a reproduced PL case
15 Oct 1993, 6:00, The Swan
Example of a reproduced PL case
15 Oct 1993, 6:00, The Swan
Next Problem: detection
Decades of data
◮
◮
◮
eye detection
infeasible for
long-term data
humans decide
subjectively
automated
procedure needed
Solution:
◮
◮
counted cases of 2 yrs, Blechschmidt (2008), GRL
spatial band-pass filter
separates information at different scales
Example: spatial filter
(a) full field
(b) low pass
(c) band pass
(d) high pass
Example of band-pass filtered MSLP
200-600km retained, PLs emerge as distinct minima
Dec 1993 case
2
1
&
A
6
5 ;;
A
+*#$*+%. #0 //
5 ;;
6 &
Jan 1998 case
&
&
A
#3*#*+3. ( //
5 )-
5 )-
6 &
Detection and tracking scheme
1. record all locations at which filt. MSLP minimum ≤ −1hPa
2. combine detected positions to individual tracks, distance to
next (3h) pos ≤ 200km
3. checking further constraints along tracks:
◮
◮
◮
◮
◮
filt. minimum ( ≤ −2hPa once along track)
wind speed (≥ 13.9 ms once along track)
air-sea temperature difference (SST − T500hPa ≥ 43K )
no northward direction of track
limits to allowable adjacent grid boxes
OR: minimum in band-pass filtered MSLP ≤ 6hPa once
Tracks of three (of >3300) polar lows
Reproduced and detected after more than 40 years simulation time
Spatial density of PLs (1949-2005)
80˚
280˚
70
˚
˚
290
60
˚
0˚
350˚
340
˚
0˚
33
0˚
˚
32
40
31
0˚
˚
30˚
20˚
10˚
Number of PLs per area unit
50
0˚
30
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Annual numbers of PLs (1949-2005)
Number of PLs per Polar Low Season (PLS), one PLS from July until June next year
Comparison with observed cases
bias, but qualitative
similarity to observation
data
black: our data (Zahn and v.Storch, 2008)
adjusted to observation data
red: MetNo (pers. comm.)
Wilhelmsen (1985)
Blechschmidt (2008)
Link to large scale flow 1949 - 2005
−
−−−
→
Canonical Correlation Analysis, CCA 1, statistically linking vectors of mean MSLP and
−−−−→
No PL per PLS
Future projections
90˚
280˚
80˚
70
˚
0˚
29
60
˚
0˚
30
˚
50
0˚
350˚
˚
340
0˚
33
0˚
˚
40
30˚
20˚
Model grid
10˚
32
◮
0˚
◮
driven by ECHAM5/MPI-OM
C20: control with GHG 1960-1990
B1,A1B,A2: GHG for 2070-2100 (AR4)
31
◮
Projected cumulative frequency of PLs in
IPCC-scenarios and annual cycle
significant decrease in the
number of PLs per winter
14
Blechschmidt
Wilhelmsen
REA
C20
B1
A1B
A2
average number of polar lows per month
12
10
8
6
4
2
0
Jan
Mar
May
Jul
month
Sep
Number of PLs per month
Nov
Spatial density distribution,
northward shift of genesis region
Spatial density distribution,
difference plots
Polar Lows and projected vertical stability
◮
◮
atmosphere warms stronger
than ocean surface
proxy for frequency of
favourable PL conditions
decreases
Evolution of SST and T500hPa
Polar Lows and projected vertical stability
◮
◮
Area and time-averaged icefree SST − T500hPa
over maritime northern North Atlantic
proxy for frequency of
favourable PL conditions
decreases
large inter model bias, but same
direction of change
Evolution of SST and T500hPa
Linkage of vertical stability changes to
ocean circulation
Spatial response vertical
stability
Regression with ocean
circulation
Multi-model mean response (A1B - C20) in
vertical stability S = T500hPa − SST (= −vdT )
over ice free ocean (Black iso-lines: SST
response)
S regressed on the AMOC response across
CMIP3 model (A1B - C20) space over ice free
ocean (Black iso-lines: regression SST-AMOC)
Frequency changes of Polar Lows
No change in recent past, but high
interannual variability
Significant decrease of annual number
in response to global warming
Decrease linked to more stable mean
conditions
Polar lows, research questions
frequency:
◮
hit much less frequently
areas of occurrence:
◮
◮
hit less frequently in southern regions
hit more often most northerly regions
intensities:
◮
◮
◮
remains to be investigated
RCM data still too coarse
Kolstad (2009): indicates higher intensities
Thank you very much for your attention
http://coast.hzg.de/staff/zahn/
Sensitivity of alorithm, c >0.8
6
K
Perhaps: Link to large scale flow 1949 2005
CCA 2 between mean ~MSLP and No~ PL per PLS
✒