New Zealand Climate: Patterns of Drought and Future Projections Dr Jim Salinger, NIWA, New Zealand

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1 New Zealand Climate: Patterns of Drought and Future Projections Dr Jim Salinger, NIWA, New Zealand Outline New Zealand drought risk Drought index: Potential Evapotranspiration Deficit (PED) Pattern analysis: identification of patterns of drought New Zealand patterns of drought Summary

2 New Zealand drought risk Main areas of drought risk are in east of both islands and inland drier areas of the South Island Main factor in soil moisture deficit patterns is mountain backbone across predominant westerly flow 1997/98 drought resulted in loss of ~$620 M to NZ economy (NZIER) Significant drought can occur in other regions under suitable conditions Regional patterns of drought are linked to favoured persistent climate patterns

3 Potential Evapotranspiration Deficit (PED) Estimate actual evapotranspiration (ET) each month from difference between evapotranspiration (PET) & precipitation (P), using 47 stations, ET =ƒ(pet-p), where ET=PET for (PET P); ET=PET-P for (PET > P) Rainfall in excess of field capacity is lost as runoff + drainage (RO+D) Accumulate PED on a monthly basis, beginning July 1: PED m = PED m-1 + (PET m -ET m ) PED here is the month by month difference between rainfall and long term PET accumulated over the July-June year Accumulated PED is essentially the amount of water that would need to be added to a crop over a year to prevent loss of production due to water shortage Radn,Temp,WindRun,VapPress Potential PET Evapotranspiration Rain SMD Soil Moisture Deficit RO Runoff PED

4 Pattern analysis Principal component analysis (PCA) has been used to identify coherent patterns of spatial variability in elements such as rainfall (Salinger and Mullan, 1999) Principal component analysis was performed on seasonally accumulated PED to identify patterns of drought Rotated principal components, using the Varimax criterion was used to identify stations highly correlated for drought variation For interpretation of the PED patterns, time series for each pattern (RPC scores) were correlated with Trenberth circulation indices and Kidson weather types

5 Pattern analysis Circulation Indices Index Z1 Z2 Z3 Z4 M1 M2 M3 MZ2 MZ3 SOI Definition Auckland Christchurch Christchurch Campbell Is Auckland Invercargill Raoul Is Chatham Is Hobart Chatham Is Hokitika Chatham Is Hobart Hokitika Gisborne Invercargill New Plymouth Chatham Is Tahiti Darwin Wind anomaly (+/- index) West/east (central New Zealand) West/east (south of New Zealand) West/east (New Zealand) West/east (east of North Island) South/north South/north (east of New Zealand) South/north (west of New Zealand) West/northwest/east-southeast Southwest/northeast Northeast/southwest

6 Kidson Weather Types

7 Pattern Analysis Scree Plot PCA used 24 North Island, and 23 South Island stations Five coherent regions for the North Island selected explaining 67% of the variance Separate island analysis gave 3 patterns for the North Island, explaining 72% of the variance, and 4 patterns for the South Island, Scree explaining Plot 70% of the variance Scree Plot New Zealand North Island South Island Eigenvalue 10 5 Eigenvalue Eigenvalue Number of Factors Number of Factors Number of Factors

8 New Zealand Patterns of Drought NZ1 The north of both islands: Northland, Auckland Waikato, Bay of Plenty and Nelson Explains 18% of the variance

9 New Zealand Patterns of Drought High to north west, and southwest flow This pattern associated with strong westerly flow over southern New Zealand, and west to southwest flow over New Zealand Generally predominates with anticyclones to the north west (HNW, SW), with dry west to southwest flow over New Zealand Ridge to south These areas are wet with lows north east of New Zealand, and ridges over the South Island

10 New Zealand Patterns of Drought NZ2 centred on Canterbury, extends from South Canterbury to Wellington and southern Wairarapa, and includes central and coastal Marlborough. Explains 17% of the variance

11 New Zealand Patterns of Drought Southwest flow over New Zealand This pattern associated with strong south westerly flow over southern New Zealand, and southwest flow over New Zealand Generally predominates in Kidson SW type, with dry southwest flow over New Zealand North east flow over New Zealand These areas are wet with north east flow over New Zealand

12 New Zealand Patterns of Drought NZ3 a bipolar pattern, with high positive station loadings in the east of the North Island, and negative loadings for stations in the west of the South Island This means when it is dry in Gisborne and Hawke s Bay, it is wet in the west of the South Island Explains 9% of the variance

13 New Zealand Patterns of Drought Westerly flow over New Zealand Ridge over south producing easterly flow over the North Island This pattern associated with strong westerly flow over New Zealand For the east of the North Island, generally occurs in Kidson type W with westerly conditions over New Zealand The east of the North Island is wet with Kidson type R (ridge, producing easterly conditions over the North Island

14 New Zealand Patterns of Drought NZ4 denotes dryness in southern New Zealand, covering central and eastern Otago, and inland Southland. Explains 9% of the variance Best relationships with easterly flow anomalies over the South Island

15 New Zealand Patterns of Drought NZ5 strongest in the west of the North Island, and includes Wairarapa. Explains 14% of the variance

16 New Zealand Patterns of Drought Anticyclonic easterly flow Westerly troughs This pattern associated with easterly flow over the North Island Dry conditions occur in Kidson type HSE with anticyclonic easterly conditions over the North Island The west of the North Island is wet with Kidson type T (trough in westerlies).

17 Drought Statistics New Zealand PCA 1 unrotated Score Year Using unrotated PCs, most widespread high PEDs occurred in 1972/73, 1977/78, 1945/46 and 1982/83 over the 70 year period Years with lowest PEDs were 1979/80,1944/45, 1952/53 and 1985/86

18 Drought Statistics Low and high PED years correlate for the period correlate well with Mullan et al (2005) Drought seasons were associated with the persistence of anticyclonic patterns Areal extent of drought Low PED seasons generally occurred with more frequent troughs From B Mullan

19 Downscaled Rainfall Change in DJF Rainfall to 2080s, 75% scaling Larger rainfall decreases in east for Hadley model

20 Downscaled PET Change in DJF Potential Evapotranspiration to 2080s, 75% scaling Larger PET increases for CSIRO model

21 Future PED scenarios Start with observed daily rain & monthly PET on 0.05 grid (ANUspline), Apply GCM-scenario monthly offsets for PET and rain Produce future daily grid-point rainfall time series by multiplying historical time series, assuming interannual variability unchanged and sequences of wet/dry days unchanged Feed these daily rainfall and monthly PET projections into daily soil moisture balance to calculate and accumulate future PED

22 Future PED scenarios: change in return period Low-medium scenario Medium-high scenario Napier Lincoln Severe droughts occur at least 2x more often to at least 4x more often by 2080s in eastern drought-prone regions

23 Future PED scenarios: Napier & Lincoln Drought severity increases with time & scaling; worse for Hadley than CSIRO Worst historical droughts: 799 PED at Napier 1997/98 & 851 PED at Lincoln 1988/89

24 Change in drought timing Monthly contribution to July-June PED accumulation Largest PED accumulation occurs in Dec/Jan By 2080s, under Hadley 75% scaling, get about 1-month advance in drying tendency

25 Change in drought variability PED Probability Distribution Function PDF broadens, implying more variability in year to year PED

26 Summary Five coherent regions for PED deficit anomalies occur in NZ: those in eastern North Island are coupled with opposite anomalies in west of the South Island Patterns are produced by a persistence in circulation or increased frequency of a synoptic type interacting with NZ orography More frequent anticyclones were a common thread for high PED: wetter seasons occurred with more troughs persisting No distinct trends Drought over extensive areas evidence of seasonal persistence of atmospheric circulation and synoptic weather types

27 Summary Drought risk is expected to increase this century in already drought-prone areas Severe droughts occur at least 2x more often ( lowmedium ) to at least 4x more often ( medium-high ) by 2080s in eastern drought-prone regions Drying of pasture in Spring is advances up to a month in dry eastern regions ( medium-high ) relative to the present Scenarios spanned only central portion (~25% to 75%) of IPCC SRES range Rainfall time series assumed proportional change in amount of rain each wet day, but not change in number of rain days compared to present