6.S
Conclusions
Government of India is concerned about improving the agricultural economy of the country irrespective of the existing status of infrastructure in a given area and to a certain extent, irrespective of the vagaries of weather too. Itis essential that more inputs would be required for more vulnerable areas if development were to be carried out in a balanced manner across the country. All the existing services must be geared for that purpose. The best form of risk management is planning to ensure that any ensuing risk is manageable. Agroclimatic analyses can help in selection of crops and cropping practices such that while the crop weather requirements match the temporal march of the concerned weather element(s), endemic periods of pests, diseases and hazardous weather are avoided. Such agronomic planning on a micro scale to suit local climate is an essential step in crop-risk management.
Environmental planning would be necessary to avoid or mitigate losses from disasters, by using instruments such as land-use planning and disaster manage- ment. Natural disaster reduction measures are in place in a significant number of the nations surveyed and ongoing research and development to improve and ex- pand these measures are also a feature of many national strategies to minimize ad- verse effects of extreme events on agriculture. Steps are being taken to significantly reduce the vulnerability of people and their communities to natural disasters; this can only be done through mitigation.
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WMO (2004) WMO statement on the states of the global climate in 2003, WMO No. 966
CHAPTER 7
Challenges to coping strategies in Agrometeorology:
The Southwest Pacific
James Salinger
7.1
Introduction
The climate system in the southwest Pacific provides a large source of interannual to multidecadal fluctuations beneath a theme of regional climate warming. These provide challenges especially to coping strategies for agrometeorology in the re- gion. The El Nino-Southern Oscillation (ENSO) provides a large source of sea- sonal to interannual variability across the region promoting seasons of floods and droughts, and warmer and cooler seasons at higher latitudes (Trenberth and Caron 2000).
The Interdecadal Pacific Oscillation (IPO) (Trenberth and Hurrell 1994; De- ser et al. 2004) is an important source of multidecadal climate fluctuations. These cause shifts in climate across the region. With this better understanding of the cli- mate system of the region, these modes place a range of natural variability on the anthropogenic factors that will promote warming in the region during the 21stcen- tury. The latest IPCC projections (IPCC 2001) from the entire range of 35 IPCC scenarios place temperature increases in the range of 1.4 to 5.8°C by the end of the 21" century, with likely increases in heavy rainfall events and drought.Itis the impact of the sources of variability and change on extreme events, such as floods, droughts, tropical cyclones and heatwaves that are significant. These will pose challenges for agriculture and forests to cope with future variability and change in the southwest Pacific.
7.2
EI Nino-Southern Oscillation (ENSO)
El Nino-Southern Oscillation events are a coupled ocean-atmosphere phenome- non. It is a natural feature of the climate system. El Nino involves warming of sur- face waters of the tropical Pacific in the region from the International Date Line to the west coast of South America, with associated changes in oceanic circulation.It is accompanied by large changes in the tropical atmosphere, lowering pressures in the east and raising them in the west, in what is known as the "Southern Oscilla- tion". The total phenomenon is generally referred to as ENSO. El Nino is the warm phase of ENSO and La Nina is the cold phase. Historically, El Nino (EN) events occur about every 3-7 years and alternate with the opposite phases of below aver- age temperatures in the equatorial Pacific (La Nina). A convenient way of measur-
ing ENSO is in terms of the east-west pressure difference, the Southern Oscillation Index, or SOl, which is a scaled form of the difference in mean sea-level pressure between Tahiti and Darwin. A graph of the SOl over the past 30 years is shown in Figure 7.1.
ENSO may be thought in terms of a slopping back and forth of warm surface water across the equatorial Pacific Ocean. The trade winds, blowing from the east towards the west, normally help to draw up cool water in the east and to keep the warmest water in the western Pacific. This encourages low air pressures in the west and high pressures in the east. An El Nino event is when the warm water "spills out" eastwards across the Pacific, the trade winds weaken, pressures rise in the west and fall in the east. Eventually, the warm water retreats to the west again and
"normality" is restored. The movements of water can also swing too far the other way and waters become unusually cool near South America, resulting in what is termed a "La Nina", where the trade winds are unusually strong while pressures are lower than normal over northern Australia. As an El Nino event develops, the compensating shifts of the globe's weather zones and rainfall patterns result in Widespread droughts in some regions, heavy flooding in others, and associated re- gions of warming and cooling. The regions most affected are the tropical and sub- tropical regions of Indonesia, Australia, and the Pacific Islands.
Figures 7.2 and 7.3 show relationships with temperature and precipitation throughout the region. Essentially ENSO events cause increased temperature in the tropical South Pacific from just west of the Date Line to the South American coast, whereas temperatures are decreased in ENSO events over Papua-New Guin- ea south east into the subtropical South Pacific, and New Zealand. The reverse tem- perature anomalies occur during La Nina episodes.
Precipitation anomalies can be much more dramatic (Figure 7.3). ENSO events can bring drought and decreased precipitation anomalies over the Phillipines, In-
3,----,----,.---.----.---,---....,
. . . M • • • • • • • • •
EI Nino .
ããã;-ããã..fãããtããããããtããã..ããã
~75 1980 1985 1990
Year
1995 2000 2005
Fig.7.1. The Southern Oscillation Index (SOl) for the last 30 years. Negative excursions indicate El Nino events, and positive excursions indicate La Nina events. The irregular nature of ENSO events is evident in the time sequence
Chapter 7: Challenges to coping strategies inAgrometeorology: The Southwest Pacific 101
Fig.7.2. Correlations between the May - April Southern Oscillation Index and surface tempera- ture 1958-2004, after Trenberth and Caron (2000)
Fig.7.3. Correlations between the May - April Southern Oscillation Index and precipitation 1958-2003, after Trenberth and Caron (2000).
donesia, northern and eastern Australia, the subtropical Southwest Pacific and the north east of New Zealand. Increased precipitation occurs in the equatorial Pacific from Kiribati (west of the Date Line) through to the Galapagos Islands. The reverse anomalies occur in La Nina episodes.
Tropical cyclones develop in the South Pacific over the wet season, usually from November through to April. Peak cyclone occurrence is usually during January, February and March based on historical tropical cyclone data analysis. Those countries with the highest risk include Vanuatu, New Caledonia, Fiji, Tonga and Niue. Taken over the whole of the South Pacific, on average nine tropical cyclones can occur during the November to April season, but this can range from as few as three in 1994/95, to as many as 17 in 1997/98, during the last very strong El Nino.
The mean frequency of tropical cyclones for the 1970 - 2000 period for El Nino epi- sodes is 11.5,and La Nina events 8.6 per season. The tropical cyclone track densities vary depending on the ENSO state (Figure 7.4). During El Nino episodes a higher frequency of tropical cyclone tracks occur near Vanuatu and Fiji, and their occur- rence spreads further east to 1600 Wto affect the Cook Islands and most of French Polynesia. In contrast, during La Nina events the maximum occurrence is largely confined to the Coral Sea area of the Southwest Pacific centering on 1600 E ,200S and affecting New Caledonia in particular.
12o"w
Fig.7.4. Tropical cyclone densities for El Nino (upper) and La Nina (lower) seasons in the South- west Pacific. Contour interval is 0.25, starting at 1.0
Chapter 7: Challenges to coping strategies in Agrometeorology: The Southwest Pacific 103 7.3
Decadal Variability
Decadal-to-interdecadal variability of the atmospheric circulation is most prom- inent in the North Pacific, where fluctuations in the strength of the wintertime Aleutian Low (AL) pressure system co-vary with North Pacific sea surface temper- atures (SST) in the Pacific Decadal Oscillation (PDO). These are linked to decadal variations in atmospheric circulation, SST and ocean circulation throughout the whole Pacific Basin in the IPO. This is an 'ENSO-like' feature of the climate system that operates on time scales of several decades. The main centre of action in SST is in the north Pacific centred near the Date Line at 400 N .Three phases of the IPO have been identified during the 20thcentury: a positive phase (1922-1946), a nega- tive phase (1947-1976) and the most recent positive phase (1977-1998). The IPO has been shown to be a significant source of decadal climate variation through- out the South Pacific (Salinger et al. 2001), New Zealand and Australia (Power et al. 1999)
Folland et al. (2002) showed that the IPO significantly affects the movement of the South Pacific Convergence Zone (SPCZ) in a way independent of ENSO.
The South Pacific Convergence Zone (SPCZ) is one of the most significant fea- tures of the subtropical Southern Hemisphere climate (Kiladis et al. 1989; Vincent and Dias 1999; Vincent 1994).Itis characterized by a quasi-permanent band of low-level convergence, enhanced cloudiness and precipitation originating from the west Pacific warm pool in the Indonesian region and trending south-east towards French Polynesia. The SPCZ when it is active during the November - April South Pacific wet season produces a good proportion of the precipitation. In the negative phase of the IPO the SPCZ is displaced southwest producing a drying in climates to the north east, and increases in precipitation to the southwest. The east and north of New Zealand becomes wetter, whilst the west and south drier. In Australia rain- fall tends to increase in eastern Australia. The positive phase reverses the climate anomalies: the SPCZ is displaced north east increasing precipitation to the north- east and making the subtropical southwest Pacific drier. The north and east of New Zealand becomes drier, whilst the south and west wetter. Similarly eastern Austra- lia tends to be drier.
7.4
Regional Warming
Climate model results show that globally average surface temperature is projected to increase by 1.4 to 5.8°C over the period 1990 - 2100 (IPCC 2001). Since the Third Assessment Report (TAR) (IPCC 2001), future climate change projections have been updated regionally (Ruosteenoja et al. 2003). Lal (2004) have applied these to the South Pacific. Models all project increases in temperature: for the 2080s in- creases of between 1.0 and 3.1°C are indicated with the 2100 surface air tempera- ture to be at least 2.5°C more than in 1990. The models only simulate a marginal increase or decrease in annual precipitation (10 percent), with a drying in the sub- tropical South Pacific whilst the equatorial areas become wetter. During summer
more precipitation is projected while an increase in daily rainfall intensity causing more heavier rainfall events is likely.
For Australia and New Zealand, scenarios based on more-detailed projections have been developed by CSIRO (2001) and NIWA (Wratt et al. 2004), respectively.
Within 800 km of the Australian coast, a mean warming of 0.4 to 6.7°C is likely by the year 2080, relative to 1990. A tendency for decreased rainfall is likely over most of Australia, except Tasmania and New South Wales. A tendency for less run-off is also likely. In New Zealand, a warming of 0.5-3SC is likely by 2080s. The mid- range projection for the 2080s is a 60% increase in annual mean westerly winds (Wratt et al. 2004). Consequently, precipitation is likely to be biased towards in- creases in the south and west, and decreases in the north and east.
Global warming from anthropogenic forcing is likely to increase extreme events.
Extreme temperatures above 30 and 35°C are likely throughout the region. In Aus- tralia the number of days over 35°C increasses sihnificantly by 2020 with a 10 to 80% decrease in days below O°C (Suppiah et al. 2007). In New Zealand there is likely to be a 50-100% decrease in frosts in the lower North Island, and a 50% de- crease in the South Island, and a 10-100% increase in the number of days over 30°C (Mullan et al. 2001).
Under 3 x CO2conditions, there is a 56% increase in the number of simulated tropical cyclones over north-eastern Australia with peak winds greater than 30 ms' (Walsh et al. 2004). Maximum tropical cyclone wind intensities could increase 5 to 10 percent by around 2050 (Walsh 2004) with peak precipitation rates likely to in- crease by 25 percent as a result of increases in wind intensities.
For Australia, increases in extreme daily rainfall are likely where average rain- fall increases, or decreases slightly. For example, the intensity of the l-in-20 year daily-rainfall event increases by up to 10% in parts of South Australia by the year 2030 (McInnes et al. 2002), 5 to 70% by the year 2050 in Victoria (Whetton et al.
2002), up to 25% in northern Queensland by 2050 (Walsh et al. 2001) and up to 30% by the year 2040 in south-east Queensland (Abbs 2004). In New Zealand the frequency of high intensity rainfall is likely to increase, especially in western ar- eas.
In the South Pacific, projected impacts include extended periods of drought.
Projected changes in rainfall and evaporation have been applied to water balance models, indicating that reduced soil moisture and runoff is very likely over most of Australia and eastern New Zealand. Two climate models simulate up to 20%
more droughts (defined as soil moisture in lowest 10% from 1974-2003) over most of Australia by 2030 and up to 80% more droughts by 2070 in south-western Aus- tralia (Mpelasoka et al. 2005). By the 2080s in New Zealand, severe droughts (the current one-in-twenty year soil moisture deficit) are likely to occur at least twice to four times as often in the east of both islands, and parts of Bay of Plenty and Northland (Mullan et al. 2005). The drying of pastures in eastern New Zealand in spring is very likely to be advanced by a month, with an expansion of droughts into spring and autumn.
Finally, an increase in fire danger in Australia is likely to be associated with a reduced interval between fires, increased fire-line intensity, a decrease in fire ex- tinguishments and faster fire spread (Cary 2002;Tapper 2000;Williams et al. 2001).
In south-east Australia, the frequency of very high and extreme fire danger days
Chapter 7: Challenges to (oping strategies in Agrometeorology: The Southwest Padfi( 105 is likely to rise 4-25% by 2020 and 15-70% by 2050 (Hennessy et al. 2006). By the 2080s, 10-50% (6-18) more days with very high and extreme fire danger are likely in eastern areas of New Zealand, the Bay of Plenty, Wellington and Nelson regions (Pearce et al. 2005), with increases of 1-5 days in some western areas. Fire season length is likely to be extended, starting earlier in August and finishing in May in many parts of New Zealand, compared with the current October to April season.
7.5
Challenges to Agriculture and Forestry 7.5.1