Wednesday, December 24, 2008

Pacific indicators point to la nina: bureau
December 23, 2008, Weekly Times
A CONTINUED cooling of the equatorial Pacific in the past two weeks raises the possibility of indicators reaching levels of a la nina, often associated with above-average rainfall in Australia, the Bureau of Meteorlogy says.

If a la nina develops, summer crops such as sorghum and cotton in northeast Australia might experience several rainfall events in the coming months, potentially boosting production. Already, these cropping areas in northern New South Wales and southern Queensland received above-average November rainfall. "Given current conditions and recent trends, the development of a la nina during the southern summer can't be ruled out. However, the majority of climate models forecast neutral conditions, with a cooler than normal Pacific, during the first quarter of 2009," the bureau said in a regular review of Pacific conditions. The bureau also cited persistently stronger than normal trade winds for some months in the western Pacific and cloudiness suppressed along much of the central and eastern equatorial Pacific.

Its Southern Oscillation Index has remained strongly positive since late August. The value of the SOI in the 30 days ended December 21 was +13, falling a little from +17 in November. Sustained positive values of the SOI are associated with a la nina. Historically, it is unusual for la nina thresholds to be reached during the southern summer, though this did occur as recently as the summer of 1999/2000, it said.

The bureau also reiterated the Indian Ocean Dipole is neutral and is expected to remain neutral through summer. The bureau previously identified a positive mode for the IOD as a key influence for a drought in 2007 that wrecked winter crops including wheat in southeast Australia. A positive mode for the IOD is an effect that often hinders the formation of the northwest cloudbands that are an important source of winter/spring rainfall in southeast Australia.


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Wednesday, November 26, 2008

{News} 081125! Bay depression may intensify, to cross TN coast

Bay depression may intensify, to cross TN coast

The Hindu Business Line, Nov 25, 2008
Vinson Kurian

Thiruvananthapuram, Nov. 25 The well-marked low-pressure area over north Sri Lanka and adjoining southwest Bay of Bengal has concentrated into a depression and lay centred 200 km southeast of Pamban and about 300 km south-southeast of Nagapattinam on Tuesday afternoon.

India Meteorological Department (IMD) expected the system to intensify further and move slowly in a northwesterly direction and cross Tamil Nadu coast by Wednesday night. International models seemed to delay the landfall until Thursday along the Pattukottai-Vedaranyam belt.

Heavy to very heavy falls and isolated extremely heavy falls are likely over coastal Tamil Nadu during the next two days. Isolated heavy to very heavy falls are also likely over interior Tamil Nadu and Kerala during the same period.

Squally winds speed reaching 45-55 km/hr gusting to 65 km/hr are likely along and off the Tamil Nadu and Puducherry coasts. Sea condition is rough to very rough along and off Tamil Nadu coast. Fishermen are advised not to venture out.

Meanwhile, the US Joint Typhoon Warning Centre (JTWC) upgraded as good the potential of the system off Sri Lanka and extreme south Indian peninsula to develop into a tropical cyclone.

The threat of vertical wind shear (that kills building storms) was assessed as moderate to low on Tuesday. A limiting feature could be the proximity to land – especially since the system is shown to enter Tamil Nadu in the neighbourhood. But conversely, extended stay over seawaters and ensuing moisture feed could help crank up system strength.

A welcome result could be that the heavy rains would have cooled down the seawaters, denying purchase for an incoming easterly wave to grow to cyclonic strength.
Earlier forecasts by the Fleet Numerical Meteorology and Oceanography Centre of the US had indicated that a powerful system could be in the making as the easterly wave made inroads into the Bay waters.

Dr Swadhin Behera, Sub-Leader, Climate Variations Research Programme at the Tokyo-based Frontier Research Centre for Global Change (FRCGC), Japan Agency for Marine-Earth Science and Technology, informed Business Line that the northeasterlies are usually strong in northeast and parts of peninsular India during winter monsoon following a positive Indian Ocean Dipole (IOD) event.

The FRCGC team discovered the IOD event, which refers to sea-surface temperature differential between southwest and east Indian Ocean. A positive IOD features warmer seawaters to the southwest, which aids precipitation over India.

“So, those regions are expected to receive above normal rainfall. Eastern parts of Sri Lanka would also receive heavy rain following the positive IOD,” Dr Behera said.

“We have not done a detail analysis, but winter rains in northwest India tend to be weaker during a La Nina that is persisting in the equatorial Pacific. However, the surface temperature is usually colder than normal in such cases.

“So, relative to the passing of western disturbances, we may expect occasional heavy snow/severe cold waves in Jammu and Kashmir and the northern hills this season,” Dr Behera added.


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