D ARWIN REGIONAL SPECIALISED M ETE OROL OGICAL CENT RE
DECEMBER 2010, VOL 29 No 12
© PUBLISHED BY THE AUSTRALIAN BUREAU OF METEOROLOGY 2010
ISSN 1321 - 4233
DARWIN TROPICAL DIAGNOSTIC STATEMENT
December 2010
ISSUED BY NORTHERN TERRITORY CLIMATE SERVICE CENTRE
SUMMARY
December was characterised by a continuing strong La Nina pattern and an early onset to the
Australian Monsoon. Convection remained enhanced over the Maritime Continent and
suppressed over the western and central equatorial Pacific. The monthly SOI for December
was +27, which is the highest December SOI value on record and the highest value for any
month since November 1973. The 5-month running mean (centred on October) was +21,
continuing to increase slightly from +20 in November. Two tropical cyclones were analysed
in the RSMC analysis area during December.
INDICES
Troup’s Southern Oscillation Index (SOI) for December 2010
5-month mean (centred upon October)
+27
+21
Darwin mean MSL pressure for December 2010
Pressure anomaly (1933 – 1992 mean)
Tahiti mean MSL pressure for December 2010
Pressure anomaly (1933 – 1992 mean)
1004.9 hPa
- 2.5 hPa
1013.7 hPa
+ 2.8 hPa
Time series of Troup’s SOI:
Year Jan
Feb
Mar
Apr
May Jun
2008 + 14
+ 21
+ 12
+4
-4
+5
2009
+9
+ 15
0
+9
-5
2010
-10
-14
-11
+15
+10
Jul
Aug
Sep
Oct
Nov
Dec
+2
+9
+ 14
+ 13
+ 17
+ 13
-2
+2
-5
+4
-15
-7
-7
+2
+21
+19
+25
+18
+16
+27
The above table presents monthly values of the SOI from 2008 to date. Fig.1 shows the
monthly SOI and its five-month running mean for the past ten years.
1
TROPICAL CYCLONES (TC) [Fig. 2]
Two tropical cyclones were analysed in the RSMC area during the month of December. The
first tropical cyclone, Tropical Cyclone Abele, developed over the central Indian Ocean on 1
December and was named while still in the area of responsibility of La Reunion Tropical
Cyclone Warning Centre. The other tropical cyclone, Tropical Cyclone Tasha, formed in the
Coral Sea in the early hours of Christmas Day. The comparative long-term means for tropical
cyclone formation in December are 1.3 tropical storms (0.8 of typhoon intensity) for the
north-western Pacific Ocean, 3.1 for the South Pacific and South Indian Oceans combined
and 0.5 for the North Indian Ocean including the Bay of Bengal.
Tropical Cyclone Abele
A tropical low formed in the central Indian Ocean on the 29 November and drifted south. By
1 December it reached tropical cyclone intensity and rapidly intensified to a category 3
cyclone late on the 2nd. The system was named Tropical Cyclone Abele while in the area of
responsibility of La Reunion Tropical Cyclone Warning Centre. During this time, Abele’s
maximum sustained winds reached 130 km/h and experienced maximum wind gusts of up to
185 km/h. By the time TC Abele moved into the Perth Tropical Cyclone Warning Centre
area of responsibility on the 3 December, it was a category 2 cyclone. Abele weakened to a
tropical low on the 4th as it moved over the cooler waters of the southern Indian Ocean. The
remnant low pressure area continued to weaken as it moved southeast, and dissipated on 6
December. Throughout its life, ex-TC Abele posed no threat to any land mass.
Tropical Cyclone Tasha
Tropical Cyclone Tasha initially started as an area of low pressure in the northern Coral Sea
on the 20 December. Low wind shear allowed the system to intensify as it gradually drifted
southwest towards the northern Queensland coast. The system, situated over the northwest
flank of the subtropical ridge, intensified and reached tropical cyclone intensity during the
early hours of Christmas Day. The system remained as a category 1 for only a matter of
hours as it soon made landfall near Gordonvale, 20 km south of Cairns, becoming a tropical
low shortly after. At its peak, Tropical Cyclone Tasha sustained winds of up to 70 km/h. As
it moved into the cost, Cyclone Tasha brought widespread rain of the order of 150 to 250 mm
from Cairns to Lucinda. Some parts of Queensland were already suffering from flooding and
the additional rainfall brought in by TC Tasha exacerbated the situation.
SEA SURFACE TEMPERATURE (SST) [Figs. 3a, 3b]
Sea surface temperatures over the eastern and central equatorial Pacific regions remained
cool during December, with cooler than average temperatures extending from near 160° E to
the American continent. Warm anomalies continued to predominate over most the western
tropical Pacific, the Philippine Sea and the Coral Sea. On the other hand, sea surface
temperatures across most of the Maritime Continent returned to more normal values due to
the early onset of the Australian Monsoon. The eastern Indian Ocean remained warmer than
normal, but with positive anomalies contracting along Western Australia. Sub surface
temperatures across the eastern equatorial Pacific remained well below average with some
areas in the central Pacific more than 4°C below the long-term mean.
MEAN SEA LEVEL PRESSURE (MSLP) [Figs. 4a, 4b]
The negative MSLP anomalies that have persisted over the eastern Indian Ocean over the
past few months further strengthened during December. An eastward extension of low
MSLPs from the eastern Indian Ocean into northern Australia correlates with the onset of the
Australian Monsoon in mid-December. The exception was the southern Indian Ocean, south
of about 30o S, where the MSLP anomalies where anomalously high due to the unseasonal
2
strength of the subtropical ridge. Over the North Hemisphere, there was a marked negative
anomaly about 30o N. This is the result of a weaker than normal Siberian high pressure area
and the northern subtropical pressure ridge also being weaker than normal.
850hPa FLOW [Figs. 5a, 5b]
The persistence of an active MJO event in the Maritime Continent region during the last half
of December contributed to a significant westerly wind anomaly across the eastern equatorial
Indian Ocean, reaching more than 10ms-1 in some areas. Easterly flow over the equatorial
Pacific (between 130°E and the dateline) was 5 ms-1 above the long-term mean, consistent
with the La Niña conditions. A weaker than normal sub-tropical ridge in the western North
Pacific, contributed to a cyclonic anomaly near the dateline.
200hPa FLOW [Figs 6a, 6b]
Several significant features highlight the anomalous 200hPa flow during December. Starting
in the Tasman Sea, a large displacement of the sub-tropical jet southwards contributed to a
strong anticyclonic anomaly (reaching 15 – 20ms-1), indicative of a blocking pattern set up.
The second feature, located to the west of Australia is the southward displacement of the subtropical ridge, which spent several weeks 10 – 20 degrees south of its average position,
which is normally near 12oS. Finally, there was almost a complete absence of the sub-tropical
ridge in the Northern Hemisphere east of 150oE, where a very strong (15 – 25ms-1), broad
cyclonic anomaly is evident, centred on the dateline.
OUT-GOING LONG-WAVE RADIATION (OLR) [Figs 7a, 7b]
The La Niña pattern continues to be very strongly evident in the OLR analysis for December.
An area of above average OLR stretches over the western equatorial Pacific as far west as
140° E, where cooler than normal SSTs are suppressing convection. Below average OLR
(consistent with above average cloudiness) dominated the Maritime Continent region, where
the MJO spent a large proportion of December.
CROSS EQUATORIAL INTERACTION [Fig 8]
The development of the Australian Monsoon during December is evident in the crossequatorial wind pattern. Low-level northerly flow (strongest between Indonesia and Borneo)
is overlain by broadscale cross-equatorial southerly flow at around 200hPa. The early shift
south of the ascending branch of the Hadley Circulation, assisting the development of the
Australian Monsoon, is consistent with La Niña events.
850 hPa WIND COMPONENTS AND RAINFALL AT DARWIN [Figs. 9a, 9b]
Climatologically, the 850 hPa zonal wind in Darwin has an easterly component for most of
December, with a shift to westerlies late in the month when the monsoon typically arrives.
The early arrival of the Australian Monsoon is evident in the 850 hPa zonal wind plot, which
shows almost continuous westerly winds at Darwin from the middle of the month onwards.
The rainfall during the month was mostly due to the arrival of the monsoon towards the
middle of December. The total monthly rainfall during December was 390.4 mm. The
average Darwin rainfall during December is 252.4 mm and the highest ever recorded
December rainfall was 664.8 mm back in 1974.
3
INTRA-SEASONAL VARIATIONS [Figs. 11, 12, 13]
Throughout most of November the Madden-Julian Oscillation (MJO) was weak and
disorganized. During the second week of December an active MJO pulse moved into the
Maritime Continent region. This pulse, combined with the strong La Niña in the Pacific
Ocean, contributed to the early onset of the Australian Monsoon across northern Australia
towards the middle of the month. The timing of this monsoon, arriving about 2 weeks earlier
than normal, is consistent with previous La Niña events. Although weak, the monsoon trough
and MJO pulse remained active over Australian longitudes throughout the last week of
December, contributing to heavy rainfall events over northern and eastern Australia.
Fig. 1 SOUTHERN OSCILLATION INDEX 2000 – 2010
Monthly SOI (bars) and 5-month running mean SOI (Red line).
4
10°S
01
23
02/50
021270
03/50
20°S
25
241640
Tasha
Abele
04
30°S
Severe tropical cyclone/typhoon
Tropical cyclone/storm
Low Pressure System
DD/ff
wind at 00 UTC (knots)
date
DDHHff
max wind (knots)
time (UTC)
date
90°E
100°E
110°E
120°E
130°E
140°E
Fig. 2 OPERATIONAL TRACKS OF CYCLONES: ABELE AND TASHA FOR DECEMBER 2010.
150°E
40N
°C
30N
31
30
20N
29
28
27
EQ
26
EQ
24
22
20
20S
18
30S
40S
90E
120E
Fig.3(a) SEA SURFACE TEMPERATURE, December 2010.
150E
80E
100E
120E
140E
Fig.4(a) MEAN SEA LEVEL PRESSURE, December 2010.
Isobar interval 2.5 hPa
160E
180E
40N
hPa
7.0
°C
30N
6.0
4.0
3.0
5.0
20N
4.0
3.0
2.0
2.0
1.0
0.5
EQ
- 0.5
1.0
0
EQ
-1.0
-2.0
- 1.0
-3.0
- 2.0
- 3.0
-4.0
20S
-5.0
- 4.0
-6.0
30S
-7.0
No Data
90E
120E
Fig.3(b) SEA SURFACE TEMPERATURE ANOMALY, December 2010.
150E
40S
80E
100E
120E
140E
Fig.4(b) MEAN SEA LEVEL PRESSURE ANOMALY, December 2010.
Contour interval 1 hPa. Heavy line represents zero anomaly.
160E
180E
40N
20N
EQ
20S
40S
80E
100E
120E
140E
160E
180
Fig.5(a) 850 hPa VECTOR WIND ANALYSIS, December 2010.
Arrow length indicates relative magnitude. Isotachs at 5ms-1 intervals are shaded.
40N
20N
EQ
20S
40S
80E
100E
120E
140E
160E
Fig.5(b) 850 hPa WIND ANOMALY, December 2010.
Arrow length indicates relative magnitude. Anomalies > 5 ms-1 are shaded.
180
40N
20N
EQ
20S
40S
80E
100E
120E
140E
160E
180
Fig.6(a) 200 hPa VECTOR WIND ANALYSIS, December 2010.
Arrow length indicates relative magnitude. Isotachs at 10ms-1 intervals are shaded.
40N
20N
EQ
20S
40S
80E
100E
120E
140E
160E
Fig.6(b) 200 hPa WIND ANOMALY, December 2010.
Arrow length indicates relative magnitude. Anomalies > 5 ms-1 are shaded.
180
40N
20N
EQ
20S
40S
80E
100E
120E
140E
160E
180
160E
180
Fig.7(a) OUTGOING LONG WAVE RADIATION, December 2010.
Contour interval 20 watt m-2.
40N
20N
EQ
20S
40S
80E
100E
120E
140E
Fig.7(b) OUTGOING LONG WAVE RADIATION ANOMALY, December 2010.
Contour interval 10 watt m-2.
100
150
200
Southerlies
250
300
400
hPa
500
700
850
Northerlies
1000
70E
80E
90E
100E
110E
120E
130E
140E
150E
160E
170E
Fig.8 EQUATORIAL CROSS SECTION OF MERIDIONAL WIND, December 2010.
Isotachs at 2ms-1 intervals.
W
100
30
90
80
70
10
60
50
0
40
-10
Rainfall (mm)
Wind component (knots)
20
30
20
-20
10
E -30
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
0
Fig.9(a) DARWIN 850 hPa MEAN ZONAL WIND, December 2010.
Black line represents 3-day running mean. Orange line represents the mean seasonal wind.
N
20
15
Wind component (knots)
10
5
0
5
10
15
S 20
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
Fig.9(b) DARWIN 850 hPa MEAN MERIDIONAL WIND, December 2010.
Black line represents 3-day running mean. Orange line represents the mean seasonal wind.
Time/longitude cross section, southern series.
5 day running mean, averaged over 15°S to 5°S
01 JUL
01 AUG
01 SEP
01 OCT
01 NOV
01 DEC
65
85 105 125 145 165
Degrees east longitude
Fig.10(a) Outgoing long wave radiation (watt m-2)
185
340
3.0
320
2.5
300
2.0
280
1.5
260
1.0
240
0.5
220
-0.5
200
-1.0
180
-1.5
160
-2.0
140
-2.5
120
-3.0
65
85 105 125 145 165
Degrees east longitude
Fig. 10(b) Mean sea-level pressure anomaly (hPa)
185
Time/longitude cross section, southern series.
5 day running mean, averaged over 5°S to 5°N
01 JUL
01 AUG
01 SEP
01 OCT
01 NOV
01 DEC
65
85 105 125 145 165
Degrees east longitude
Fig.11(a) Outgoing long wave radiation (watt m-2)
185
340
3.0
320
2.5
300
2.0
280
1.5
260
1.0
240
0.5
220
-0.5
200
-1.0
180
-1.5
160
-2.0
140
-2.5
120
-3.0
65
85 105 125 145 165
Degrees east longitude
Fig. 11(b) Mean sea-level pressure anomaly (hPa)
185
Time/longitude cross section, southern series.
5 day running mean, averaged over 5°N to 15°N
01 JUL
01 AUG
01 SEP
01 OCT
01 NOV
01 DEC
65
85 105 125 145 165
Degrees east longitude
Fig.12(a) Outgoing long wave radiation (watt m-2)
185
340
3.0
320
2.5
300
2.0
280
1.5
260
1.0
240
0.5
220
-0.5
200
-1.0
180
-1.5
160
-2.0
140
-2.5
120
-3.0
65
85 105 125 145 165
Degrees east longitude
Fig. 12(b) Mean sea-level pressure anomaly (hPa)
185
Explanatory Notes
1.
Darwin Tropical Diagnostic Statement is a near real-time monthly diagnostic summary of the major tropical circulations within the Darwin Regional
Specialised Meteorological Centre (RSMC) area of analysis responsibility, which covers 40ºN-40ºS, 70ºE-180º. Caution does need to be exercised when
quoting from this publication as not all information within it has been confirmed.
2.
Features discussed generally include:
. El Niño - Southern Oscillation (ENSO) aspects
. Tropical cyclone (TC) occurrence
. Sea surface temperature (SST)
. Mean sea level pressure (MSLP).
. Lower and upper level wind
. Up-motion and convection
. Intra-seasonal variability
3.
Data sources:
(i) SOI = 10 x(∆PTAH - ∆PDAR) /σ
where ∆PTAH =Tahiti (91938) monthly pressure anomaly
(monthly mean minus 1933-1992 mean, averaging 3-hourly observations)
∆PDAR = Darwin (94120) monthly pressure anomaly (monthly mean
minus 1933-1992 mean, averaging 0900, 1500LT observations)
σ = monthly deviation of the difference.
(ii) Operational tropical cyclone tracks based upon Darwin RSMC manual operational analyses. A tropical cyclone or cyclonic storm is defined as having
mean wind ≥ 17 ms-1 (34 kn) or a named system. Standard practice is to accept intensity and position as promulgated by the responsible warning agency,
whenever possible. This may cause apparent discontinuities in intensity or track when cyclones cross warning area boundaries. Limited post analysis may
sometimes be performed when warranted. A severe TC (equivalent to typhoon or hurricane) or very severe cyclonic storm is defined as having mean wind
≥32 m s-1 (63 Kn).
(iii) Tropical cyclone climatology for the northwest Pacific and the south Indian and Pacific Oceans is based on 2008 Annual Tropical Cyclone Report, by
Cooper, G.A. and R.J. Falvey, (2009), US Naval Pacific Meteorology and Oceanography Center/ Joint Typhoon Warning Center, Pearl Harbour, Hawaii,
USA, (available at https://metoc.npmoc.navy.mil/jtwc/atcr/2008atcr/2008atcr.pdf). North Indian Ocean records are taken from WMO Technical Document No.
430, Tropical Cyclone Report No.TCP-28 (Mandal, 1991).
(iv) SST analysis based on Darwin RSMC automated operational analyses (RSMC subset of the Australian National Meteorological and Oceanographic
Centre (NMOC) global analysis: blended in situ and satellite data, 1ºC resolution). The 1ºx 1º global SST climatology from the US National Centers for
Environmental Prediction (Reynolds and Smith 1995). A high resolution global sea surface temperature climatology, J. Clim., 8, 1571-1583 is used for the
calculation of anomalies and as the default field for the analysis first guess.
(v) Mean MSLP, upper wind data, anomalies and velocity potential data from the Bureau of Meteorology’s Global Assimilation and Prediction System (GASP
- refer Bourke et al 1990. The BMRC global assimilation and prediction system. ECMWF Seminar proceedings: Ten years of medium-range weather
forecasting, Sep 89) and NCEP2 22 year climatology, 1979-2000.
Equatorial cross section of meridional wind field was derived from the Bureau of Meteorology’s operational Tropical region Extended Limited Area
Prediction System (TXLAPS_PT375n) model. (Refer- Analysis and Prediction Operations Bulletin No 59. Bur. Met., Australia.)
A full web version available at: http://www.bom.gov.au/nmoc/bulletins/nmc_bulletin.shtml).
(vi) The mean seasonal cycles for the Darwin 850 hPa wind components were constructed by averaging daily values over 57 years (1950 to 2006).
(vii) OLR time longitude plots and maps derived from the US National Oceanic and Atmospheric Administration.
4. Some commonly-used acronyms:
CS
ISO
JMA
JTWC
MJO
MSLP
MT
NET
OLR
PAGASA
PNG
RSMC
-
Cyclonic storm
Intra-seasonal oscillation
Japan Meteorological Agency
Joint Typhoon Warning Center, Pearl Harbour
Madden-Julian Oscillation
Mean Sea Level Pressure
Monsoon trough
Near-equatorial trough
Out-going long-wave radiation
Philippine Atmospheric, Geophysical and Astronomical
Services
- Papua New Guinea
- Darwin Regional Specialised Meteorological Centre (see
note 1)
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South Pacific convergence zone
Sea Surface Temperature
Severe tropical cyclone
Subtropical ridge
Tropical cyclone (see note 3(ii))
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Hemisphere sector)
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