ARWIN EGIONAL PECIALISED ETEOROLOGICAL ENTRED R S M C

ISSN 1321 - 4233

January 2013, VOL 32 No 01© PUBLISHED BY THE AUSTRALIAN BUREAU OF METEOROLOGY 2013

DARWIN TROPICAL DIAGNOSTIC STATEMENT

January 2013

ISSUED BY NORTHERN TERRITORY CLIMATE SERVICE CENTRE SUMMARY The beginning of 2013 continued with neutral atmospheric and oceanic conditions across the tropical Pacific Ocean. Sea Surface Temperatures (SSTs) across the equatorial Pacific cool down slightly with all NINO indices falling below zero for the first time since early January 2012. A strong sub-tropical ridge continued to dominate the weather patterns across Australia as it did in the previous month of December 2012. For the first time in two months, an active pulse of the MJO managed to traverse the Maritime Continent. This was the strongest MJO across Australian longitudes since March 2012. The Southern Oscillation Index (SOI) saw a considerable strengthening in January, with values returning to neutral levels after a deep tropical low over Tahiti led to a sharp weakening of the SOI during December. INDICES

Troup’s Southern Oscillation Index (SOI) for January 2013 -1.1 5-month mean (centred upon November) +0.4 Darwin mean MSL pressure for January 2013 1005.8 hPa Pressure anomaly (1933 – 1992 mean) -0.7 hPa Tahiti mean MSL pressure for January 2013 1010.0 hPa Pressure anomaly (1933 – 1992 mean) -0.8 hPa

Time series of Troup’s SOI:

Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 2011 +20 +22 +21 +25 +2 0 +11 +2 +12 +7 +14 +23

2012 +9 +3 +3 -7 -3 -10 -2 -5 +3 +2 +4 -6.0

2013 -1.1 The above table presents monthly values of the SOI from 2011 to date. Fig.1 shows the monthly SOI and its five-month running mean for the past ten years. TROPICAL CYCLONES (TC) [Fig. 2] Five tropical cyclones formed within the Darwin RSMC area in January including tropical cyclone Sonamu, an early season storm in the northern hemisphere. The comparative long-term means for tropical cyclone formation in January are 0.5 tropical storms (0.3 of typhoon intensity) for the north-western Pacific Ocean, 5.6 for the south Pacific and south Indian Oceans combined and 0.1 for the North Indian Ocean including the Bay of Bengal.

1

Severe Tropical Storm Sonamu (Auring) Sonamu developed into a tropical depression on the 3rd about 340 km northwest of Zamboanga, Philippines. Sonamu began over the Sulu Sea and crossed the southern end of Palawan before moving into the open waters of the South China Sea on the 4th. By 1500 UTC on the 4th, Sonamu was centred about 800 km east-southeast of Ho Chi Minh City, Vietnam. As Sonamu tracked westward the outer fringes of the storm brought gusty winds, heavy rainfall, and rough coastal conditions to southern Vietnam, from near Ho Chi Minh City southward. Sonamu had maximum sustained winds near 50 knots (92 km/h) at 00 UTC on the 5th and maintained this strength as it continued across the South China Sea. Still over the warm waters of the South China Sea, Sonamu began showing signs of weakening on the 6th as it encountered moderate wind shear and intrusion of dry air. On the 7th, Sonamu’s track began curving to the southwest. By the 8th Sonamu was downgraded to a tropical depression. Tropical depression Sonamu continued a southeast track and landfall 10 Jan on the Malaysian part of Borneo. Severe Tropical Cyclone Narelle A tropical low developed within the monsoon trough over the Timor Sea on 4 January. The system gradually tracked westward and intensified, it was named tropical cyclone Narelle on 8 January by the Perth TCWC. Following a southward track into a region of low wind shear, the tropical cyclone intensified into a severe tropical cyclone on 9 January. The storm attained its peak intensity on 11 January with sustained winds of 100 knots (185 km/h). The system steadily weakened and ultimately fell below tropical cyclone strength on 15 January. Even though Narelle did not make landfall, early in the storm's existence, it brought strong winds, heavy rain, and high winds to many areas in Indonesia. Moderate Tropical Storm Emang Tropical Cyclone Emang developed in the Southern Indian Ocean on Sunday, 13 January, near about 10° S and 81° E. At the time a cluster of thunderstoms began developing around a centre of circulation. On Monday, the 14th, at 0900 UTC Emang's maximum sustained winds were near 35 knots (64.8 kph). Emang was located in a weak steering environment and it is moving slowly along a southwest track under the influence of a building subtropical ridge located to the south of the storm. On the 15th the maximum sustained winds were still near 35 knots (64.8 kph). Forecasters expected atmospheric conditions to improve and that Emang would strengthen. However, on the 16th, satellite imagery showed that the storm had been elongating thus causing the circulation to weaken. Emang did not threaten any land areas. Tropical Cyclone Oswald Tropical cyclone Oswald began as a circulation of low pressure within the monsoon trough on 21 January. At 0600 UTC on the 21st Oswald had maximum sustained winds near 35 knots (64.8 kph). Oswald formed over the Gulf of Carpentaria about 160 km east-northeast of Mornington Island, Australia. As Oswald made landfall along the western coast of the Cape York Peninsula overnight on the 21st. Shortly after making landfall the maximum sustained winds dropped to 25 knots (46.3 kph) and was downgraded to a remnant low pressure area early on the 22nd. Although Oswald was short-lived as a tropical cyclone, the ruminants of the storm moved slowly down Queensland’s northeast coast for the following three days causing floods, tornadoes, power outages, loss of life and billions of dollars in damage. For more information on the affects of former tropical cyclone Oswald see the Bureau of Meteorology’s special

2

climate statement about the event (http://www.bom.gov.au/climate/current/statements/scs44.pdf). Tropical Cyclone Peta On 20 January, a weak low formed over the Kimberley region of Western Australia. Partially overland, convection persisted mainly on the western half of the low as it took in moisture from the Indian Ocean. Tracking west-southwestward in response to a weak mid-level ridge to the southeast, the system gradually intensified. Early on 23 January, surface observations from Cape Lambert and Roebourne indicated that the low had attained gale-force winds, prompting TCWC Perth to upgrade the system to Tropical Cyclone Peta; however, the system may not have met the technical definition of a tropical cyclone as gales did not extend more than halfway around the circulation. Peta then racked south and made landfall near Point Samson, Western Australia, with winds of 75 km/h. Shortly after moving onshore, Peta rapidly weakened and the final advisory was issued during the afternoon of 23 January. SEA SURFACE TEMPERATURE (SST) [Figs. 3a, 3b] Sea surface temperatures across the vast majority of the Darwin RCSM area were above average as a consequence of the relative cloud-free skies across the region. In January, the warmest sea surface temperatures (SST) were off the North West Shelf, the Timor Sea and the Gulf of Carpentaria as well as the area extending to the east of the Bismarck Archipelago and the Solomon Islands. When compared to the long-term average, the SST off Australia’s western coast was up to two degrees above normal. SSTs across some of the eastern Indian Ocean, the Bay of Bengal and the South China Sea were also above average this month. Across the central Pacific Ocean, the January NINO3.4 index was -0.28, the first time it has been negative since April 2012. MEAN SEA LEVEL PRESSURE (MSLP) [Figs. 4a, 4b] In January, the southern hemisphere sub-tropical ridge migrated southwards towards latitudes you will expect it to be at this time of the year. January 2013 finally saw the development of the northern Australian low pressure trough which is indicative of the monsoon season, with a short outburst of the monsoon in mid-January. The strongest MSLP was found over the southeastern Indian Ocean which extended an anomalous stronger than average MSLP across the western Australian continent. Over the eastern seaboard, the remnants of tropical cyclone Oswald led to below average MSLP. Over the northern hemisphere, the sub-tropical ridge intensified over Asia. As observed in the previous month of December, an area of high pressure anomalies was observed over the northwestern Pacific Ocean., while lower than average MSLP was observed over the Indian Ocean. 850hPa FLOW [Figs. 5a, 5b] During the first half in January, a pulse of the Madden Julian Oscillation (MJO) moved slowly across the Maritime Continent contributing to stronger than average westerly winds over the eastern Indian Ocean and the Maritime Continent, just south of the equator. Slightly stronger than average easterly winds were observed between 120° E and the dateline, north of the equator. Across northern Australia, easterly anomalies continued indicative of an inactive North Australian Monsoon. 200hPa FLOW [Figs 6a, 6b] In the northern hemisphere the upper level jet stream sat at around its usual latitude for this time of year. In the southern hemisphere the jet stream was much weaker than normal over central Australia. The upper level jet was, however, stronger than usual off the coast of south Western Australia and over the Great Australian Bight.

3

OUT-GOING LONG-WAVE RADIATION (OLR) [Figs 7a, 7b] As expected, convection throughout January occurred within the tropics in the Darwin RSMC, between 10 N° and 20° S. Enhanced cloud cover and precipitation was observed over the Maritime Continent due to the passage of an MJO and the effects of tropical cyclone Sonamu over the Philippines. Enhance convection was also observed off the northern coast of WA and in western WA due to the passage of tropical cyclones Narelle and Peta. A broad area of suppressed convection was observed over the eastern Indian Ocean to the west of the MJO pulse and over most of Australia due to the lack of monsoonal activity in the north and the passage of any significant fronts in the south. CROSS EQUATORIAL INTERACTION [Fig 8] The ascending branch of the Hadley Cell was situated well into the southern hemisphere this month. Evidence of the Hadley circulation was especially clear over the Indian Ocean. However, there was an area over the western Pacific, near 145° E, where a northerly component of the cross equatorial flow extended into the upper atmosphere. 850hPa WIND COMPONENTS AT DARWIN [Figs. 9a, 9b] The lower level winds at Darwin Airport in January did not resemble climatology. Early in the month the dominant flow for northern Australia was from the north east as a strong high pressure area was situated along the Queensland coast and. After the monsoon onset on the 17th the wind shifted to the west but synoptic scale systems maintained a southerly component to the winds at Darwin for most of this month’s monsoon period. INTRA-SEASONAL VARIATIONS [Figs. 10, 11, 12] After two months of no significant MJO activity along Australian longitudes, an MJO strengthened over the Maritime Continent very early in January with a broad trough active across southern Indonesia. Despite moving south, most of northern Australia (including the Top End) failed to get the typical monsoon rain as the convective activity wrapped around tropical cyclone Narelle, situated to the west of the Top End. Even as tropical cyclone Narelle moved south, the monsoon trough stayed just to the north of Australia, bringing rainfall only to the far north coast. On the third week in January, the monsoon trough finally pushed south over northern Australia, bringing the classical monsoonal conditions. This active phase of the monsoon, however, was very short lived as most of the storm activity soon wrapped around tropical cyclone Oswald near the Cape York Peninsula and tropical cyclone Peta over WA. As such, ‘build-up’ conditions soon returned to northern Australia, conditions that continued throughout the rest of the month. Over the east, the moderate to strong MJO that began to propagate eastwards across the Maritime Continent earlier in the month, stalled over the far western Pacific Ocean and weakened. A quasi-stationary MJO in the later past of January led to enhanced convection over the northern flank of the South Pacific Convergence Zone and the formation of tropical cyclone Garry to the east of the Dateline.

4

Fig. 1 SOUTHERN OSCILLATION INDEX 2003 – 2013 Monthly SOI (bars) and 5-month running mean SOI (Red line).

5

Fig. 2 OPERATIONAL TRACKS OF CYCLONES: Emang, Narelle, Oswald, Peta and Sonamu for January 2013.

15° N

25° S

15° S

5° S

5° N

Severe tropical cyclone/typhoon

Tropical cyclone/storm

Low Pressure SystemDD/ff

DDHHff

wind at 00 UTC (knots)date

max wind (knots)time (UTC)date

35° S105° E95° E85° E75° E 155° E145° E135° E125° E115° E

Emang

15

Narelle 08 09/60

10/75

11/85

12/100

13/75

14/50

112100

Peta 24

230335

23

Oswald 22 210635 21

130635

15

14/35 13

Sonamu

03 050050

07/45 08

06/50 04/35

Fig.3(a) SEA SURFACE TEMPERATURE, January 2013.

Fig.3(b) SEA SURFACE TEMPERATURE ANOMALY, January 2013. Fig.4(b) MEAN SEA LEVEL PRESSURE ANOMALY, January 2013.Contour interval 1 hPa. Heavy line represents zero anomaly.

Fig.4(a) MEAN SEA LEVEL PRESSURE, January 2013.Isobar interval 2.5 hPa

40N

20N

EQ

20S

40S

40N

20N

EQ

20S

40S

80E 100E 120E 140E 160E 180E

80E 100E 120E 140E 160E 180E

No Data

40N

20N

EQ

20S

40S

40N

20N

EQ

20S

40S

- 0.5

4.0

3.02.01.00.5

- 4 .0- 3 .0- 2 .0- 1 .0

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0

-1.0

-2.0

-3.0

-4.0

-5.0

-6.0

-7.0

hPa

°C

20

28262422

101214

18

°C3230

16

80E 100E 120E 140E 160E 180E

80E 100E 120E 140E 160E 180E

1027.5

1025.0

1022.5

1020.0

1017.5

1015.0

1012.5

1010.0

1007.5

1005.0

1002.5

1000.0

997.5

995.0

992.5

hPa

Arrow length indicates relative magnitude. Anomalies > 5 ms-1 are shaded.Fig.5(b) 850 hPa WIND ANOMALY, January 2013.

80E 100E 120E 140E 160E 180

40N

20N

EQ

20S

40S

Arrow length indicates relative magnitude. Isotachs at 5ms-1 intervals are shaded.Fig.5(a) 850 hPa VECTOR WIND ANALYSIS, January 2013.

80E 100E 120E 140E 160E 180

40N

20N

EQ

20S

40S

Arrow length indicates relative magnitude. Anomalies > 5 ms-1 are shaded.Fig.6(b) 200 hPa WIND ANOMALY, January 2013.

80E 100E 120E 140E 160E 180

40N

20N

EQ

20S

40S

Arrow length indicates relative magnitude. Isotachs at 10ms-1 intervals are shaded.Fig.6(a) 200 hPa VECTOR WIND ANALYSIS, January 2013.

80E 100E 120E 140E 160E 180

40N

20N

EQ

20S

40S

Contour interval 10 watt m-2.Fig.7(b) OUTGOING LONG WAVE RADIATION ANOMALY, January 2013.

80E 100E 120E 140E 160E 180

40N

20N

EQ

20S

40S

Contour interval 20 watt m-2.Fig.7(a) OUTGOING LONG WAVE RADIATION, January 2013.

80E 100E 120E 140E 160E 180

40N

20N

EQ

20S

40S

N

S

Win

d co

mpo

nent

(kno

ts)

W

E

Win

d co

mpo

nent

(kno

ts)

Fig.8 EQUATORIAL CROSS SECTION OF MERIDIONAL WIND, January 2013.Isotachs at 2ms-1 intervals.

Fig.9(a) DARWIN 850 hPa MEAN ZONAL WIND, January 2013.Black line represents 3-day running mean. Orange line represents the mean seasonal wind.

Black line represents 3-day running mean. Orange line represents the mean seasonal wind.Fig.9(b) DARWIN 850 hPa MEAN MERIDIONAL WIND, January 2013.

70E 80E 90E 100E 110E 120E 130E 140E 150E 160E 170E1000

850

700

500

400

300

250

200

150

100

hPa

350

450

550

600

650

750

800

950

900

ms -1

Northerlies

Southerlies

Rain

fall

(mm

)

20

15

10

5

0

5

10

15

201 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31

-30

-20

-10

0

10

20

30

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 310102030405060708090100

3.0

2.5

2.0

1.5

1.0

0.5

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

Time/longitude cross section, southern series.5 day running mean, averaged over 15°S to 5°S

65 85 105 125 145 165 185 65 85 105 125 145 165 185

01 SEP

01 AUG

01 NOV

01 JAN

01 OCT

01 DEC

340

320

300

280

260

240

220

200

180

160

140

120

Degrees east longitudeFig.10(a) Outgoing long wave radiation (watt m-2)

Degrees east longitudeFig. 10(b) Mean sea-level pressure anomaly (hPa)

3.0

2.5

2.0

1.5

1.0

0.5

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

Time/longitude cross section, equatorial series.5 day running mean, averaged over 5°S to 5°N

65 85 105 125 145 165 185 65 85 105 125 145 165 185

01 SEP

01 AUG

01 NOV

01 JAN

01 OCT

01 DEC

340

320

300

280

260

240

220

200

180

160

140

120

Degrees east longitudeFig.11(a) Outgoing long wave radiation (watt m-2)

Degrees east longitudeFig. 11(b) Mean sea-level pressure anomaly (hPa)

3.0

2.5

2.0

1.5

1.0

0.5

-0.5

-1.0

-1.5

-2.0

-2.5

-3.0

Time/longitude cross section, northern series.5 day running mean, averaged over 5°N to 15°N

65 85 105 125 145 165 185 65 85 105 125 145 165 185

01 AUG

01 JAN

01 OCT

01 DEC

01 SEP

01 NOV

340

320

300

280

260

240

220

200

180

160

140

120

Degrees east longitudeFig.12(a) Outgoing long wave radiation (watt m-2)

Degrees east longitudeFig. 12(b) Mean sea-level pressure anomaly (hPa)

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 - Cyclonic storm ISO - Intra-seasonal oscillation JMA - Japan Meteorological Agency JTWC - Joint Typhoon Warning Center, Pearl Harbour MJO - Madden-Julian Oscillation MSLP - Mean Sea Level Pressure MT - Monsoon trough NET - Near-equatorial trough OLR - Out-going long-wave radiation PAGASA - Philippine Atmospheric, Geophysical and Astronomical

Services PNG - Papua New Guinea RSMC - Darwin Regional Specialised Meteorological Centre (see

note 1)

SCS - South China Sea SOI - Southern Oscillation Index SPCZ - South Pacific convergence zone SST - Sea Surface Temperature STC - Severe tropical cyclone STR - Subtropical ridge TC - Tropical cyclone (see note 3(ii)) TD - Tropical depression TXLAPS - Tropical region Extended Limited Area Prediction Scheme TS - Tropical storm (generally used for TC in northern Hemisphere sector) TUTT - tropical upper tropospheric trough VSCS - Very severe cyclonic storm

5. Subscription rates All costs in $AUSTRALIAN:

Annual subs. Postage Subs (incl postage) 95.50 (86.80 ex GST) 14.40 (Australia) 109.90 36.00 (Asia/Pacific) 131.50 51.60 (Rest of the world) 147.10 6. For further details contact: The Regional Director, Bureau of Meteorology, PO Box 40050, Casuarina, Northern Territory 0811 AUSTRALIA Telephone: (International: 61) (08) 8920 3813 Fax: (International: 61) (08) 8920 3832 E-mail: climate.nt@bom.gov.au