Friday, November 28, 2008
Wednesday, November 26, 2008
Bay depression may intensify, to cross TN coast
The Hindu Business Line, Nov 25, 2008
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.
Tuesday, November 18, 2008
Recent intensification of tropical climate variability in the Indian Ocean
Nature Geoscience Published online: 16 November 2008 doi:10.1038/ngeo357
Nerilie J. Abram1,2, Michael K. Gagan1, Julia E. Cole3, Wahyoe S. Hantoro4 & Manfred Mudelsee5
The interplay of the El Niño Southern Oscillation, Asian monsoon and Indian Ocean Dipole (IOD)1, 2, 3 drives climatic extremes in and around the Indian Ocean. Historical4, 5 and proxy6, 7, 8, 9 records reveal changes in the behaviour of the El Niño Southern Oscillation and the Asian monsoon over recent decades10, 11, 12. However, reliable instrumental records of the IOD cover only the past 50 years1, 3, and there is no consensus on long-term variability of the IOD or its possible response to greenhouse gas forcing13. Here we use a suite of coral oxygen-isotope records to reconstruct a basin-wide index of IOD behaviour since AD 1846. Our record reveals an increase in the frequency and strength of IOD events during the twentieth century, which is associated with enhanced seasonal upwelling in the eastern Indian Ocean. Although the El Niño Southern Oscillation has historically influenced the variability of both the IOD and the Asian monsoon3, 8, 10, we find that the recent intensification of the IOD coincides with the development of direct, positive IOD–monsoon feedbacks. We suggest that projected greenhouse warming may lead to a redistribution of rainfall across the Indian Ocean and a growing interdependence between the IOD and Asian monsoon precipitation variability.
Monday, November 17, 2008
Coral study points to more harsh droughts
BY NYSSA SKILTON
17/11/2008 8:52:00 AM, The Canberra Times
New coral records have revealed Australia is likely to experience more frequent and intense droughts.
Scientists studying the tropical weather patterns stored in corals have discovered climate variability in the Indian Ocean has intensified during the 20th century. This suggests Australia and the region can expect less rain while eastern Africa gets wetter.
An international research team, led by the Australian National University, analysed corals from tropical waters north-east of Australia to build a picture of climate change going back to 1846.
Their findings are published in the journal Nature Geoscience, available online today.
To date, reliable records of the Indian Ocean Dipole a climate phenomenon similar to El Nino go back about 50 years.
Palaeoclimatologist Mike Gagan, of the ANU's Research School of Earth Sciences, said the researchers' techniques allowed them to analyse sea-surface temperature and salinity stretching back hundreds, even thousands, of years.
Indian Ocean Dipole events occur when the ocean temperature and winds along the equatorial Indian Ocean reverse from their normal state. These changes bring drought to western Indonesia and southern Australia and heavy rains to eastern Africa and southern India.
Dr Gagan said the frequency of dipole events was increasing. ''In the last 160 years, there's been about 21 dipole events that we can see, and there's only been about five very strong events,'' he said.
''But it turns out, four out of the five very strong events have occurred since 1960, and three of the very strong events have occurred since 1994.''
Dr Gagan said this meant farmers across southern Australia would not be able to rely on spring rains as they might have in the past.
''We have one dipole event every four years now on average. Who knows, there may be one every two years in 30 or 40 years,'' he said.
The researchers analysed giant porite corals, some aged 400 years, to glean information about past conditions. They drilled cores from the centre of the coral and studied the chemistry preserved in the skeleton of the coral.
''In trees, you get annual growth bands. In coral, you get annual changes in coral density, and you can count these annual density bands to keep track of the time,'' Dr Gagan said. ''We analyse it virtually month by month.''
He said the Indian Ocean Dipole was like ''the canary in the mine of climate change''.
But there was hope. ''If we can even level off CO2 and get it to be stable, even if it's a higher level, we might not like the climate that we have, but at least it's going to be more predictable and easier to adapt to,'' Dr Gagan said.
Droughts to become more frequent, severe: researchers
ABC News, Mon Nov 17, 2008
Canberra scientists say they have proven that the world's climate is changing faster than ever before.
The international research team drilled core samples from living corals off the Indonesian coast and found an increased frequency in the weather phenomenon known as the "Indian Ocean Dipole".
Like the El Nino weather effect, the Indian Ocean Dipole has a dramatic impact on the Australian climate, and can cause severe droughts.
Australian National University researcher Dr Mike Gagan says his coral samples show the dipole is occurring more regularly and that is changing Australian weather patterns.
"There's going to be not only more propensity for drought, there's going to be more variability," he said.
"If you get a dipole event superimposed onto an El Nino event, what may be a moderate El Nino event turns out to be a very strong drought."
Dr Gagan says the Dipole used to occur every 20 years, but is now happening about every four years.
He says Australia is in for more severe droughts in coming years.
"Now that we have a 160-year record we can see a clear trend towards more frequent dipole events," he said.
"When you look at what the climate models are telling us should happen as you warm the planet, a stronger Indian Ocean Dipole is something that climate models predict."