particular regional weather extremes

Transcription

1 SUPPLEMENTARY INFORMATION DOI: 1.138/NCLIMATE2271 Amplified mid-latitude planetary waves favour particular regional weather extremes particular regional weather extremes James A Screen and Ian Simmonds SUPPLEMENTARY DISCUSSION 1. Trends in weather variability 34-year linear trends in monthly-mean temperature variability averaged over northern midlatitudes are positive in most months; however, only the July trend is statistically significant (Fig. S1a). Regionally, the most notable trends are decreases in temperature variability over in January and February (Fig. S1b), and increases in temperature variability over, also in January and February (Fig. S1g). Increased temperature variability is identified in and in July (Fig. S1c,d), which are the major regional contributions to the mid-latitude- mean trend a abs T anoms e abs T anoms i abs P anoms m abs P anoms b abs T anoms f abs T anoms j abs P anoms n abs P anoms c abs T anoms g abs T anoms k abs P anoms o abs P anoms d abs T anoms h abs T anoms Supplementary Figure S1: Trends in temperature and precipitation variability. Linear trends ( ) in absolute temperature anomalies averaged and normalised over eight geographical regions: (a), (b), (c), (d), (e), (f), (g) and (h); and in absolute precipitation anomalies over the same eight regions (i-p), respectively. Trends are shown for all months together and each calendar month separately. Error bars and blue shading show the 9% confidence intervals and trends that exceed these. Units are σ 1yr -1. NATURE CLIMATE CHANGE Macmillan Publishers Limited. All rights reserved. l abs P anoms p abs P anoms

2 Trends in monthly-mean precipitation variability averaged over northern mid-latitudes are positive in all months except January; however, none of the monthly trends are statistically significant (Fig. S1i). As for temperature variability, significant trends are found in some regions and months. Precipitation variability has decreased significantly over in November, January and February (Fig. S1j). Four months March, May, September and November show significant increases in precipitation variability over (Fig. S1p). Both temperature and precipitation variability show significant regional changes, but generally the trends are insignificant over northern mid-latitudes as a whole. This general pattern is consistent with a recent study that examined annual-mean temperature variability 1. However, we caution that 34 years is a relatively short time period to assess changes in variability and/or extremes. 2. Physical understanding of monthly-mean planetary-wave amplitude We have calculated planetary-wave amplitudes based on the monthly-mean atmospheric circulation. This is not the same as calculating the monthly-mean amplitude of (sub-) daily planetary waves. The amplitude of the monthly-mean wave and the monthly-mean amplitude of the daily waves are significantly correlated (r =.48; Fig. S2a), but are not equal. They are not equal because the daily waves have varying phase. The amplitude of the monthly-mean wave is also significantly correlated with the variability (standard deviation) of daily wave phase (r = -.46; Fig S2b). When the variability of daily wave phase is low (i.e., wave is relatively stationary) the monthly-mean amplitude is high. Conversely, when the variability of daily wave phase is high (i.e., wave is relatively mobile) the monthly-mean amplitude is low. The monthly-mean planetary-wave amplitude metric combines and captures two physical aspects of the atmospheric circulation: the intensity of meteorological ridges and troughs around mid-latitudes (and hence, via geostrophic balance, the meridional wind); and secondly, the rate of longitudinal (typically west to east) movement of these features. Episodes of atmospheric blocking are a good example of a high-amplitude and slow moving circulation regime, and are often associated with extreme surface weather 2-. a Monthly-mean of daily amplitudes 2-2 b Std. dev. of daily phases Monthly-mean amplitude Monthly-mean amplitude Supplementary Figure S2: Relationship between monthly-mean planetary-wave amplitude and daily-mean planetary-wave properties. Normalised monthly-mean planetary-wave amplitude anomalies plotted against normalized monthly-mean of daily planetary-wave amplitudes (a); and against normalized monthly standard deviation of daily planetary-wave phases (b). Each black cross corresponds to an individual month and wave numbers 3-8. Red crosses denote averages for months and wavenumbers with monthlymean planetary-wave amplitude anomalies equal or greater than 1σ. Black lines show linear fits to the data. Data from the ERA-Interim reanalysis. 214 Macmillan Publishers Limited. All rights reserved.

3 3. Attributing causality We have shown that weather extremes are often associated with amplified planetary waves. However, this relationship could operate in both directions. It is well known that changes in atmospheric circulation, such as anomalous planetary waves, have a large influence on surface weather. But also, surface temperatures can impact upon the atmospheric circulation. To gain insight into causality, we have examined daily data from the ERA-Interim reanalysis. Supplementary Figure S3 shows time-lagged correlations between daily T L averaged over mid-latitudes and planetary-wave amplitude averaged for wave numbers 3-8. The correlations peak at a lag of 1-2 days, implying that the amplitude anomalies precede the temperature anomalies. We note that because all days are considered here, not just days of extreme temperature, the correlations are modest. A similar time lag is identified when we compare amplitude anomalies before and after days of extreme temperature, relative to those before and after days of near-average temperature. Further insight can be gleaned from the timescale of the interaction, irrespective of the small time lag. Whilst surface temperatures are expected to respond very rapidly to changes in atmospheric circulation, the mid-tropospheric circulation (recall that wave amplitude is defined at hpa) would not be expected to respond to surface temperature anomalies so quickly. Planetary waves are equivalent-barotropic features. The timescale for barotropic adjustment to mid-latitude surface temperature anomalies is estimated to range from tens of days to a few months 6-9. Thus, we would not expect the temperature and planetary-wave amplitude anomalies to co-vary almost simultaneously if the temperature anomalies were driving the amplitude anomalies. Instead, the evidence strongly suggests that the temperature anomalies arise as a result of amplified planetary waves. However, this need not imply the relationship is solely one-way. On longer timescales surface temperature anomalies may feed back on to the circulation anomalies, amplifying the planetary wave disturbance. a.2 b 2. Correlation.1 t statistic Lag/lead (days) Lag/lead (days) Supplementary Figure S3: Relationship between planetary-wave amplitude anomalies and extreme weather at different time lags. Time-lagged correlations between normalised planetary-wave amplitude anomalies averaged for wave numbers 3-8 and normalized midlatitude- (3-6 N) mean absolute temperature anomalies (a). Positive lag times imply that amplitude anomalies precede the temperature anomalies and vice versa. T statistics corresponding to differences in normalized planetary-wave amplitude anomalies averaged for wave numbers 3-8 between time-lagged composites of the 1% largest and smallest absolute temperature anomalies (b). For example at a lag of 1 day, planetary-wave amplitude anomalies 1 day prior to the 1% most extreme days are compared to planetarywave amplitude anomalies 1 day prior to the 1% nearest-average days. In a and b, statistics are shown for daily-mean data (blue) and 31-day running means (black). Data from the ERA-Interim reanalysis. 214 Macmillan Publishers Limited. All rights reserved.

4 4. Amplitude-weather linkages on different timescales In the main text we use solely monthly-mean data. However, it is of interest to consider whether the association between planetary-wave amplitude and extreme weather exists on shorter, or longer, timescales. To assess this, we again use daily data from the ERA-Interim reanalysis. First we correlated daily-mean T L averaged over northern mid-latitudes against planetary-wave amplitude averaged for wave numbers 3-8. We then recalculated the correlation using running means, with averaging windows ranging from 1-4 days. Supplementary Figure S4a shows this correlation as a function of averaging period. The magnitude of the correlation is relatively insensitive to averaging period and correlation is always positive. The strongest correlations, by a fairly small margin, are found for timescales of -14 days. As noted earlier, the correlations are modest because all days are considered, not just those that had temperature extremes. Comparing the composite-mean wave amplitude anomalies during the 1% largest T L anomalies to those during the 1% smallest T L anomalies reveals a very similar sensitivity of the temperature-amplitude relationship to averaging period (Fig. S4b). The amplitude-precipitation relationship shows a different sensitivity to timescale. The correlations diminish for timescales shorter than around 12 days. A similar pattern is seen comparing wave amplitude anomalies during the 1% largest P L anomalies to those during the 1% smallest P L anomalies. There is little difference between planetary-wave amplitude, on average, during short-lived precipitation extremes and similar length periods of nearaverage precipitation. Both the correlations and composite-means suggest that planetary-wave amplitude is a stronger control on longer-timescale precipitation variability and extremes, than it is on short-lived events. We speculate that precipitation variability is closely related to synoptic- or local-scale drivers on short timescales whereas longer-timescale events are more closely tied to the large-scale atmospheric circulation. a Correlation.2.1 b t statistic Averaging period (days) Averaging period (days) Supplementary Figure S4: Relationship between planetary-wave amplitude anomalies and extreme weather on different timescales. Correlations between normalised planetarywave amplitude anomalies averaged for wave numbers 3-8 and normalized mid-latitude- (3-6 N) mean absolute temperature (black) and precipitation (blue) anomalies (a). T statistics corresponding to differences in normalized planetary-wave amplitude anomalies averaged for wave numbers 3-8 between composites of the 1% largest and smallest absolute temperature (black) and precipitation (blue) anomalies (b). In a and b, statistics are shown for running means of daily-mean data, with averaging windows of 1-4 days. Data from the ERA-Interim reanalysis. 214 Macmillan Publishers Limited. All rights reserved.

5 . Wave amplitude during particular regional weather extremes Table 1 in the main material summarizes the results of analyses for particular weather extremes (hot, cold, wet or dry) over seven geographical regions. The full PDFs are presented in Supplementary Figures S (hot and cold extremes) and S6 (wet and dry extremes), and are constructed in a similar manner to Figure 3 in the main material. Frequency (%) Frequency (%) Frequency (%) Frequency (%) a hot t = -.9 (p =.) f = (p =.3) e hot i cold t =.11 (p =.91) f = 1.9 (p =.34) t =.78 (p =.43) f = 1. (p =.9) m cold t = 1. (p =.2) f = 1.1 (p =.86) b hot t = 2.31 (p =.2) f = 1.22 (p =.3) f hot j cold t = -. (p =.96) f = 1.13 (p =.19) t = -.4 (p =.6) f = (p =.26) n cold t =.7 (p =.48) f = -1.3 (p =.7) c hot Amplitude anomaly ( ) Amplitude anomaly ( ) Amplitude anomaly ( ) Amplitude anomaly ( ) Supplementary Figure S: Frequency distributions of planetary-wave amplitude anomalies during months of hot and cold extremes. As Fig. 3, but for months of hot (a-h) and cold extremes (i-p) t = 1.11 (p =.27) f = 1.19 (p =.6) g hot k cold t = 1.48 (p =.14) f = 1.21 (p =.4) t = 3.11 (p =.) f = 1.2 (p =.8) o cold t =.28 (p =.78) f = 1.11 (p =.2) d hot t = (p =.24) f = -1.2 (p =.87) h hot t = (p =.22) f = (p =.26) l cold t = 3.4 (p =.) f = 1.37 (p =.) p cold t =.12 (p =.9) f = 1.14 (p =.) 214 Macmillan Publishers Limited. All rights reserved.

6 Frequency (%) Frequency (%) Frequency (%) Frequency (%) a wet t =. (p =.88) f = 1.18 (p =.8) e wet i dry t =.93 (p =.3) f = 1.9 (p =.3) t =.11 (p =.91) f = 1.3 (p =.72) m dry t = 2.4 (p =.1) f = 1.1 (p =.31) b wet t = 1.7 (p =.29) f = 1.7 (p =.4) f wet j dry t = 1. (p =.29) f = 1. (p =.8) t = 2. (p =.1) f = 1.6 (p =.4) n dry t = -.86 (p =.39) f = -1.1 (p =.91) c wet Amplitude anomaly ( ) Amplitude anomaly ( ) Amplitude anomaly ( ) Amplitude anomaly ( ) Supplementary Figure S6: Frequency distributions of planetary-wave amplitude anomalies during months of wet and dry extremes. As Fig. 3, but for months of wet (a-h) and dry extremes (i-p) t = (p =.26) f = -1.3 (p =.8) g wet k dry t = 2.2 (p =.1) f = 1.2 (p =.1) t =.14 (p =.89) f = -1.2 (p =.89) o dry t = 2.94 (p =.) f = 1.11 (p =.26) d wet t = 1.3 (p =.18) f = 1.19 (p =.6) h wet t =. (p =.88) f = 1.22 (p =.3) l dry t =.2 (p =.8) f = 1.3 (p =.74) p dry t = -. (p =.96) f = -1.7 (p =.48) 214 Macmillan Publishers Limited. All rights reserved.

7 6. Catalogue of extreme weather events Supplementary Tables S1-4 list the months included in the various composites analysed. Extreme T Extreme P Region Months Jan 26, Mar 28, Sep 1994, Oct 1987, Mar 22, Dec 22, Nov 1993, Jun 23, Apr 1997, Oct 22, Dec 1984, May 1999, Feb 1984, Sep 1986, Nov 199, May 22, Mar 1999, Aug 21, Feb 22, Nov 1998, Aug 1984, Apr 1987, Dec 21, Oct 1997, Feb 1979, Nov 2, Jul 1994, May 1991, Feb 212, Sep 1993, Aug 1981, Dec 2, Aug 23, Dec 1989, Jan 1987, Jul 211, Jan 21, Jun 1992, Apr 21, Jul 1993 Nov 198, Mar 22, Feb 1989, Aug 199, Jul 1993, Jun 1992, Dec 1983, May 1993, May 1998, Feb 1991, Feb 1979, Jan 1982, Jan 1996, Sep 198, Dec 199, Apr 28, Oct 1988, Apr 211, Apr 1982, Aug 1981, Jun 1982, Sep 199, May 21, Jan 1981, Apr 1987, Oct 1984, Mar 1992, Dec 1996, Jan 1979, Jul 198, Oct 1979, Nov 26, Sep 1983, Sep 1986, Sep 199, Jul 211, Dec 28, Apr 1992, Apr 22, Feb 1994 Dec 1983, Jun 1988, Mar 212, Oct 22, Mar 22, Aug 1983, Jan 26, Nov 198, Aug 24, Feb 1979, Jul 29, Oct 29, Sep 29, Jan 1982, Jul 1992, Jul 212, May 1983, Apr 26, Feb 1984, May 22, Apr 1987, Jan 1979, Sep 1998, Dec 2, Nov 1999, Feb 1998, May 1987, Nov 199, May 24, Dec 1989, Jul 1993, Jan 1994, Jun 199, Nov 1996, May 1997, Sep 1986, Feb 1989, Sep 1993, Jun 24, Jun 1982 Dec 1989, Sep 1999, Nov 1986, Jun 1999, Jul 1992, Feb 1981, May 1999, Aug 1982, Apr 21, Dec 21, Jan 1994, Sep 23, May 2, Aug 198, Jan 26, Mar 1999, Oct 29, Feb 21, Oct 27, Feb 1993, Aug 1984, Feb 1984, Jul 21, Apr 23, Jul 29, Sep 1986, Jul 1993, Aug 1994, Mar 212, Aug 212, Jun 212, Sep 198, Mar 1981, Dec 21, Jun 1986, Jan 24, Mar 2, Dec 198, Oct 1984, Apr 1987 Jun 23, Mar 1987, May 1991, Oct 21, Dec 21, Aug 23, Feb 199, Jan 198, Jul 26, Apr 211, Jan 1987, Feb 1986, Sep 1996, Oct 23, Jun 1992, Apr 1997, May 198, Sep 1987, Apr 1986, Jul 198, Jul 1994, Aug 1997, Dec 21, May 1984, Aug 1992, Dec 1981, Nov 2, Nov 198, Mar 21, Nov 21, Sep 1999, Nov 1998, Jun 1991, May 1987, Jul 1983, Mar 1989, Dec 2, Oct 1992, Feb 198, Sep 26 Dec 22, Nov 1993, Aug 21, Jun 23, Sep 1994, Jul 21, Oct 1987, Feb 22, Mar 28, Sep 1997, Apr 1987, Jan 26, Jan 1987, May 1979, Sep 1993, Apr 212, Mar 1987, Feb 212, May 1999, Dec 1984, Jan 28, Aug 27, Jan 21, Feb 198, Jun 1979, Oct 212, Jul 21, Oct 1981, Apr 2, Nov 211, Nov 1998, Mar 198, Nov 21, Oct 1997, Jun 21, Jan 27, Jul 1994, Mar 22, Jul 26, Oct 1991 Sep 1992, Dec 1984, Apr 1997, Dec 212, Oct 1987, Mar 1999, Jun 1987, Feb 2, May 21, Mar 28, Oct 2, Nov 1993, Aug 26, Oct 1997, Jan 21, Jan 22, Jan 26, May 1993, Aug 1998, Jan 211, Nov 1987, Feb 21, Jul 1999, Mar 1997, Jul 21, Aug 1981, Jul 22, Jul 23, Nov 2, Feb 22, Oct 1981, Feb 1984, Aug 22, Mar 22, Sep 1994, Nov 1998, Apr 211, Jul 21, Jan 28, Sep 1998 Apr 21, Jun 1983, Apr 198, Dec 2, Oct 199, Sep 211, Mar 28, Sep 1997, Dec 212, Mar 1999, Mar 199, Feb 22, May 22, Aug 1981, Jul 1994, Jun 21, Jan 22, Nov 1981, Nov 29, Mar 21, Jan 21, Aug 23, Jul 1987, May 199, Feb 27, Feb 2, Oct 22, Sep 2, Nov 22, Jul 2, May 2, May 29, Jan 211, Aug 1998, Aug 198, Jun 28, Jun 1998, Nov 1987, Dec 199, Aug 22 Jul 1993, Dec 21, Apr 1983, May 1984, Apr 1989, Dec 1996, Jun 1988, Mar 21, Nov 1993, Aug 1998, Aug 22, Feb 1998, Jan 1996, Sep 1996, Oct 1987, Oct 23, Jul 1994, Mar 1994, Oct 1997, Jun 1992, Aug 198, Jan 1981, Oct 1998, Mar 199, Sep 29, Jan 1979, Nov 1983, Sep 1994, Mar 1996, Nov 199, Aug 26, Feb 1979, May 27, Nov 2, Jan 21, Dec 1981, Feb 199, Feb 22, Sep 1987, Jan 199 Aug 1991, Jun 1982, Jul 1983, Jan 198, Oct 22, Jan 199, May 1998, Dec 1983, Mar 1983, Mar 27, Dec 1996, Apr 23, Feb 1999, Jul 21, Feb 1989, Nov 26, Sep 1986, Oct 199, Jul 1997, Aug 1983, May 21, May 1996, Sep 1993, Apr 1996, Nov 1983, Jun 1987, May 2, Nov 198, Apr 1987, Aug 1989, Sep 1991, Apr 1981, Jun 1997, Sep 1987, Dec 1989, Oct 198, Feb 1998, Jul 2, Oct 1987, May 199 Jul 1993, Dec 1982, Sep 1986, Nov 198, May 199, Jun 1988, Jan 1981, Jan 2, Oct 29, Apr 211, Oct 1984, Jul 1992, Jun 1998, Aug 27, Mar 1998, Jun 2, Aug 198, Apr 1999, Apr 1986, Mar 1979, Feb 199, Sep 24, Oct 2, Mar 21, Apr 1989, Dec 199, Oct 1998, Mar 1997, Feb 21, May 24, Aug 198, Sep 1979, Aug 21, Nov 1991, Mar 24, Apr 21, Feb 1987, Aug 199, Jan 1986, May 1981 Apr 1983, Jan 1979, Jul 1996, Sep 1999, Jun 26, Dec 1988, Mar 26, Jun 1988, Aug 199, Mar 1994, Mar 1983, Dec 199, Aug 211, Feb 1981, Jul 1989, Sep 1996, Apr 21, Oct 2, Sep 21, Nov 21, May 1984, Oct 1997, Feb 1988, Nov 212, Jan 24, Jun 199, May 1989, Aug 199, Dec 1996, Jun 1982, Jan 21, Mar 1999, Nov 199, May 1997, Feb 1987, Aug 29, Nov 1983, Oct 1994, Dec 1993, Sep 27 Jun 1992, Feb 199, Mar 21, Apr 1998, May 1984, Jul 1988, Mar 1994, Jul 1983, Oct 199, Nov 1981, Aug 26, Dec 1981, Apr 1983, Jul 27, Jan 1996, Feb 1986, Dec 1996, Oct 198, Apr 27, Jan 1997, Aug 22, Aug 199, Dec 199, May 1983, May 28, Nov 211, Oct 23, Feb 22, May 198, Jun 1997, Sep 21, Sep 1994, Dec 1999, Dec 1993, Feb 1979, Sep 1996, Nov 2, Aug 1992, May 21, Jun 1988 May 1991, Nov 1993, Oct 1987, Nov 21, Jul 1994, Feb 1984, Mar 1986, Jun 23, Mar 1996, Oct 1997, Aug 1997, Jan 1987, Feb 198, Apr 22, Sep 1987, Jun 1992, Sep 29, Dec 1981, Jan 24, Dec 21, Jul 1997, Dec 21, Jul 21, Aug 26, Dec 26, Sep 1996, Mar 198, Apr 1986, Jun 21, Apr 1989, Aug 22, Jan 1981, Jun 1981, Jun 1979, Apr 27, Jan 1996, Apr 1984, Mar 199, Mar 28, Dec 1984 Feb 1998, Sep 1987, Jun 22, May 27, Oct 1987, Nov 1994, Aug 1981, Jul 1993, Dec 1991, Sep 1992, Apr 22, Jul 199, Oct 1998, Apr 1987, Mar 21, Mar 22, Jun 1981, Jan 211, May 2, Dec 1981, Apr 1997, Feb 21, Aug 1991, Jan 28, Nov 1993, Oct 21, Mar 21, Nov 21, Aug 1992, Jul 1979, Jan 1992, Mar 1993, Mar 1996, Jan 1984, Oct 1979, May 1991, Jan 1988, Aug 2, Dec 1996, Dec 1986 Apr 1983, Dec 21, Aug 1998, Mar 27, Jan 22, Dec 198, Nov 212, Jun 29, Sep 211, Jul 1994, Oct 24, Aug 1981, Jul 26, May 1998, May 23, Sep 1994, Aug 198, Feb 24, Feb 199, Oct 1997, May 24, Mar 211, Jan 28, Jun 1996, Mar 29, Jul 1991, Oct 22, Oct 1991, Feb 2, Nov 199, Nov 23, Sep 1992, Sep 29, Oct 198, Nov 26, Apr 22, Mar 26, Oct 1994, Jan 21, Jul 2 Supplementary Table S1: Months of extreme temperature and precipitation. The 4 months of largest T' (upper rows) and largest P (lower rows) area-averaged over eight geographical regions (shown in Fig. 2). 214 Macmillan Publishers Limited. All rights reserved.

8 Near-average T Near-average P Region Months Apr 2, Jun 1997, May 1989, Jul 2, Apr 1988, Jul 1991, Jun 1996, Sep 1989, Jun 1984, Nov 27, May 1982, Jan 29, Oct 1996, Aug 1988, Jul 1982, Sep 1981, Aug 1989, May 1994, Apr 1993, Feb 1996, Oct 1989, Feb 1997, Nov 1991, Jan 1999, Oct 2, Aug 211, Jul 28, Nov 1989, May 26, May 27, Mar 1983, Aug 1996, Sep 23, Oct 24, Jul 199, Jan 199, Oct 1983, Dec 23, Apr 21, Jan 1988 Jul 198, Jun 199, Apr 212, Dec 1982, Aug 29, Sep 198, Jun 1994, Jan 211, Oct 2, Aug 1988, May 1989, Jul 1991, Sep 28, May 1988, Aug 1984, Oct 1989, Dec 21, May 28, Dec 1994, Dec 23, Aug 1996, Jun 1983, May 1986, Jul 1984, Mar 198, Mar 21, Dec 1988, Jul 2, Oct 2, Oct 211, Jan 22, Dec 2, Jan 199, Jun 1997, Jun 27, Oct 1983, Jul 199, Jan 29, Mar 199, Oct 199 Sep 1982, Apr 1988, Jun 1979, May 1993, Dec 1998, Feb 26, Dec 1981, Sep 1991, Aug 26, Sep 1989, Sep 1988, Feb 1996, Aug 1999, Mar 1983, Feb 29, Jun 21, Nov 1988, Apr 1994, Dec 1988, Oct 1996, Jul 1982, Jul 1991, Nov 27, May 1989, Aug 199, Jan 199, Jun 1989, Mar 199, Feb 1997, Oct 199, Apr 24, Jun 199, Jan 28, Jan 2, Oct 21, Sep 28, Aug 212, Apr 1993, Jun 28, Apr 23 Jul 1982, Aug 1998, May 1993, Oct 1998, Nov 1983, Nov 1998, Oct 1983, Oct 1979, Jun 27, Nov 2, Nov 199, May 1987, Jan 1999, Mar 211, Apr 24, Mar 1994, Jan 1996, Jun 199, Jan 1997, Sep 1989, Oct 1996, Jan 21, Jun 1993, Aug 211, Feb 26, Mar 29, Mar 1988, Aug 1999, Apr 198, May 1986, Jan 1987, Jun 23, Aug 23, Nov 1984, Dec 1997, Apr 2, Apr 1998, Dec 1979, Jan 2, Jun 1996 Nov 1987, Nov 1991, Jul 2, May 1982, Dec 1983, Feb 1984, Nov 1997, Feb 24, Jan 199, Aug 28, Mar 29, Sep 23, Jan 211, Feb 29, Apr 2, Oct 1999, Mar 211, Jul 28, Nov 199, Mar 2, Jul 198, Feb 1987, Oct 1983, Jul 24, Apr 1992, Nov 1979, Dec 1997, Apr 22, Dec 1999, Jan 1986, Aug 1982, Jan 1999, Dec 23, Apr 21, Mar 1983, Jul 21, Aug 21, May 27, Sep 1981, Jun 1994 Jul 28, Apr 2, May 1998, Jun 1984, May 1982, May 26, Oct 1998, May 29, Dec 198, Mar 1988, Jun 198, Jul 1991, Feb 1997, Aug 1989, Jun 1997, Apr 21, Oct 24, Mar 1983, Jun 2, Aug 211, Feb 1991, Sep 1981, Mar 1984, Aug 199, Feb 1987, Feb 1993, Jan 29, Jul 2, Jan 199, Oct 1983, Jul 199, Mar 211, Aug 2, Jan 212, Sep 1998, Jan 211, Jun 211, Jan 1991, Apr 1988, Feb 1988 Jun 1984, May 1994, Nov 199, Aug 1982, Sep 1987, Jun 1993, Aug 1986, Apr 2, Jul 1982, Jul 1992, Jun 198, Jul 199, Nov 1999, Nov 1989, Jun 2, Apr 21, Jan 1988, Apr 1988, Jun 21, Jun 1997, May 1984, Nov 1991, Jul 198, Nov 198, Mar 1984, Feb 21, Aug 1997, May 29, May 1989, Mar 1994, Apr 28, Mar 2, Aug 24, Feb 199, Jan 29, Dec 28, Oct 23, Apr 1999, May 1996, Feb 1981 Feb 1991, Aug 1992, May 1993, Jul 212, Jan 23, Sep 1984, Feb 1997, Jan 29, Oct 1991, Aug 1983, Jul 198, Jul 1982, Aug 1996, Feb 1996, May 1989, Dec 1999, May 1997, Jan 28, Nov 1999, Oct 198, Jul 2, Nov 1991, Oct 1989, Feb 1987, Dec 1993, Oct 27, Nov 198, Jan 1996, Jan 2, Dec 26, Jun 2, Mar 1996, May 1991, May 198, Nov 28, Jun 23, Oct 23, Feb 23, Jan 1982, Jan 1988 May 198, Jun 211, Nov 1987, Nov 28, Aug 1982, Mar 1982, Sep 2, Jul 1986, Sep 1991, Mar 1999, Jan 1982, Oct 1983, Oct 28, Sep 1988, Dec 1994, Jun 1986, Aug 2, Feb 26, Nov 23, May 29, Oct 1989, Dec 23, Mar 1984, Oct 1986, Apr 26, Feb 1992, Apr 199, Feb 1981, Sep 1982, Feb 1987, Apr 1979, Aug 21, Nov 198, Jan 1994, Jul 1981, Dec 1987, May 1982, Oct 211, Jan 23, Jun 24 Apr 21, Sep 2, Nov 211, Apr 1994, Feb 211, Jan 212, Mar 198, Jun 1994, Nov 28, Oct 1989, Sep 1996, Jun 28, Aug 1997, Apr 2, Jul 1991, Mar 1999, Jan 1988, Dec 1993, Sep 1983, Nov 22, Aug 198, Mar 1993, Jan 24, Sep 1988, Jul 1989, Jan 22, Oct 21, Jul 1998, Mar 2, May 2, Nov 21, May 28, Dec 1994, Nov 1992, Aug 1988, Dec 1982, Oct 1994, Jun 21, Jun 21, Oct 28 Feb 1984, Aug 199, May 198, Sep 1988, May 1986, Feb 2, Feb 1992, Nov 21, May 1997, Mar 1982, Oct 1981, Aug 1989, Apr 27, Nov 1993, Jan 1991, Aug 24, Oct 1993, Dec 23, Jan 198, Mar 1992, Mar 1993, Oct 198, Apr 29, Jun 21, Apr 23, May 1989, Oct 198, Apr 199, Aug 1986, Jan 2, Jun 1999, May 2, Dec 212, Mar 26, Nov 1984, Jul 211, Nov 28, May 1979, Aug 211, Jul 2 Feb 199, May 198, Jan 27, Nov 1979, Feb 198, Jun 1987, Nov 1982, Jul 1982, May 29, May 22, Oct 198, Apr 22, Dec 211, Sep 1982, Oct 1983, Nov 1987, Feb 1986, May 212, Aug 2, Feb 1982, Aug 23, Mar 1997, Jun 211, Jul 21, Dec 1987, Oct 1999, Dec 1994, Jan 199, Mar 1982, Nov 23, Mar 2, Jan 1983, Aug 1981, Aug 26, Mar 1987, Nov 24, Dec 1982, Oct 1992, Sep 1993, Jul 199 Jan 1998, Jan 1999, Apr 1979, Sep 28, May 1999, Jun 211, Nov 23, Jul 23, Oct 212, Sep 1991, Sep 212, Dec 1997, May 212, Sep 2, Dec 199, Nov 2, Jun 22, Jan 1982, Jan 211, Dec 1992, Nov 28, Nov 198, Jul 212, Aug 1987, Dec 23, Jan 198, Dec 1983, Feb 26, Oct 24, Mar 1999, Nov 212, Sep 24, Oct 1988, Feb 21, Dec 212, Apr 1987, Aug 21, Nov 199, Feb 1981, Sep 1988 Sep 24, Feb 1991, Nov 1987, Jun 1982, Aug 199, Aug 1994, Jul 211, Feb 2, Sep 26, Dec 1979, Jun 199, Aug 1991, Dec 1988, Jan 1994, Sep 198, Apr 1979, Mar 1999, Mar 1993, Jan 23, Apr 2, Aug 1981, Jan 211, Feb 1981, Jul 1991, Aug 21, Jul 1986, Aug 1982, Jun 1986, Nov 1997, Jun 28, Jan 1982, Apr 1983, Oct 199, May 1988, Apr 1998, Apr 198, Oct 1986, Oct 1994, Oct 198, Sep 1981 Apr 21, Feb 1992, Jan 198, Apr 1984, Jan 1989, Aug 2, Aug 1996, Dec 29, Oct 23, Aug 1982, Dec 1997, May 211, May 1994, Feb 1994, Jul 28, Jul 2, Sep 199, Jul 1991, Sep 1981, Jun 1997, Sep 2, May 212, Aug 24, Jan 2, Jan 1993, Nov 2, Dec 1993, Nov 1997, Mar 1994, Apr 1999, Nov 199, Nov 1998, Mar 29, Jun 24, Oct 1986, Aug 28, Mar 1999, Oct 199, Nov 1982, Feb 28 May 1987, Jun 1987, Feb 1986, Sep 1997, Mar 199, Jul 1984, Sep 1979, Jun 198, Feb 26, Feb 198, Feb 1987, Jan 21, May 198, Mar 1986, Oct 1989, Nov 1999, Jan 2, Jun 211, Jul 2, Jun 1994, May 1992, Apr 1999, Nov 2, Jul 1981, Oct 27, Oct 21, Oct 1984, Nov 1996, Apr 199, Jun 1991, Aug 24, Jan 1986, Feb 1984, Sep 24, Sep 1991, Nov 1986, Jan 1994, Sep 1984, Jul 198, Jul 1989 Supplementary Table S2: Months of near-average temperature and precipitation. The 4 months of smallest T (upper rows) and smallest P' (lower rows) area-averaged over eight geographical regions (shown in Fig. 2). 214 Macmillan Publishers Limited. All rights reserved.

9 Hot extremes Cold extremes Region Months May 21, Feb 22, Jul 212, Apr 212, May 212, Mar 199, Dec 1999, Jan 22, Jun 212, Aug 21, Dec 1979, Jan 27, Jan 1983, Jul 1999, Mar 28, Jun 26, Jul 21, May 27, Sep 1994, Apr 2, Sep 2, Nov 21, Feb 1998, Feb 1999, Jan 1992, Oct 211, Jun 21, Jan 1989, Apr 198, Aug 212, Jun 27, Sep 212, Nov 198, Aug 2, Sep 24, Sep 1998, Aug 23, Nov 21, Mar 1981, Feb 24 Jun 1992, May 1993, Feb 1991, Oct 1988, Aug 1981, Jan 1981, Jun 24, Sep 199, Jul 198, Mar 1992, Oct 1979, Sep 199, May 1997, Apr 1987, Oct 1993, Oct 1987, Aug 1994, Sep 1979, Apr 1992, Dec 22, Mar 1986, Apr 1989, May 1992, Oct 1986, Jun 1982, Jul 24, Jan 1986, Oct 198, Mar 1984, Aug 24, Nov 1987, Feb 1984, Aug 1986, Mar 1994, Dec 1991, Apr 198, Oct 23, Dec 1989, Dec 1997, Jul 199 Mar 212, Jun 1988, Aug 1983, Jan 26, Jul 212, Sep 1998, Apr 26, Nov 1999, Apr 1987, May 1987, Sep 29, Feb 1984, Nov 29, Jun 1987, Feb 1998, Nov 1981, Aug 1984, Nov 21, Sep 199, Aug 23, Oct 2, Sep 1994, Jan 212, Feb 1987, Oct 27, Feb 1991, Jan 199, Apr 21, Oct 21, May 198, Jul 1989, Apr 198, Mar 2, Feb 1999, May 1991, Jun 212, Jan 1987, Dec 22, Mar 1986, Mar 21 Sep 1999, Feb 1981, Jun 1999, May 1999, Jan 26, Apr 21, Sep 23, Aug 1984, Feb 1984, Aug 212, Dec 21, Mar 1999, Oct 27, Oct 199, Mar 2, Feb 21, May 1979, Mar 21, May 1998, Apr 26, Jul 1999, Jul 212, Dec 21, Nov 26, Nov 29, Jul 28, Apr 1987, Jun 26, May 26, Oct 21, Jul 1994, Nov 211, Mar 212, Mar 1981, Mar 1979, Apr 1986, Dec 1996, Jul 26, Dec 212, Jun 1989 Jun 23, Apr 211, Aug 23, Oct 21, Feb 199, Jul 26, Dec 2, Aug 1992, Jul 1994, Sep 1999, Apr 27, Oct 26, Mar 1989, Nov 2, Sep 1987, Jan 28, Nov 1994, Sep 26, Mar 1991, Aug 1994, Jan 1988, Sep 1982, Mar 1994, Sep 211, Mar 212, May 2, Nov 1992, Feb 28, Oct 199, Jul 1983, Jul 23, Jun 1982, Nov 1984, Jun 27, Feb 1989, Jan 1994, Apr 29, Dec 198, May 1992, Sep 1991 Aug 21, Jul 21, Mar 28, Feb 22, Oct 212, Apr 212, Nov 21, Aug 27, Oct 1991, Sep 1994, Jun 21, Jan 27, Apr 2, Oct 1981, Mar 22, Jul 211, May 212, May 1996, Feb 199, May 1979, Jun 1998, Apr 1983, Dec 198, Aug 28, Apr 28, Aug 26, Mar 21, May 27, Feb 199, Jun 212, Dec 1999, Oct 1999, Feb 1999, Sep 21, Nov 1996, Jul 22, Jun 26, Jan 1994, Jul 212, Dec 1981 Apr 1997, Aug 1998, May 21, Mar 28, Jan 22, Oct 1997, Mar 1997, Feb 22, Apr 211, Aug 2, Dec 1989, May 1999, Mar 22, Sep 1988, May 24, Apr 212, Apr 27, Oct 1999, Sep 27, Jul 1996, Feb 1999, Jun 1994, Jul 1999, Aug 1987, Jul 27, Jun 21, Jul 28, Aug 1999, Feb 24, Nov 198, Nov 1994, Nov 1988, Jan 1992, Jun 212, May 1997, Feb 27, Nov 28, Nov 199, Jan 27, Feb 1979 Oct 199, Mar 28, Mar 199, Jun 21, Jan 22, May 22, Dec 199, Sep 2, Feb 22, May 29, Sep 27, Jul 1999, Aug 2, Jul 2, Oct 199, Nov 199, Aug 27, Jun 2, Jan 1989, Feb 1989, Jan 199, Aug 1994, Apr 1997, Dec 199, Feb 1993, Oct 211, Aug 211, Apr 1998, Nov 24, Mar 22, May 2, Aug 26, Jan 27, Feb 27, Mar 1989, Dec 27, Nov 199, Sep 1983, Jul 1994, Jan 1991 Nov 1993, Oct 1987, Oct 22, Dec 1984, Apr 1979, Jul 1993, Jun 1993, Aug 29, Mar 198, Sep 1996, Jun 1983, Mar 1996, Sep 1993, Dec 22, Nov 198, May 1983, Jun 1992, Jun 198, Dec 212, Mar 198, Aug 1987, Jul 1992, Jan 1979, May 22, Apr 23, Jul 1987, Feb 212, Apr 198, May 21, Jan 1996, Jul 29, Sep 1997, Oct 198, Feb 2, Dec 29, Apr 22, Jan 198, Aug 1993, Aug 198, Apr 1996 Nov 198, Mar 22, Feb 1989, Aug 199, Jan 1982, Dec 199, Feb 1979, Sep 198, May 21, Dec 1983, Apr 28, May 1999, Jan 1979, Oct 1984, Nov 26, Dec 28, Apr 1982, Apr 211, Jul 1999, Jun 212, Aug 198, Jan 1996, Feb 1994, Mar 29, Jul 211, Sep 1986, Jan 1993, Jul 1993, Sep 2, Mar 26, Jun 1991, May 22, Jul 1983, Oct 29, Aug 198, Apr 22, Sep 1983, Dec 29, Feb 199, Nov 1994 Oct 22, Dec 1983, Mar 22, Aug 24, Jul 29, Oct 29, Jul 1992, Feb 1979, Nov 198, Jan 1982, May 1983, Sep 1993, Jan 1979, May 22, Dec 2, Jun 24, Apr 1997, Nov 1996, Sep 1984, Dec 1989, Jun 1982, Apr 199, Mar 1996, Feb 1989, Aug 1992, Aug 29, Oct 26, Apr 1982, Dec 28, Sep 198, Nov 199, Jul 1993, Jan 1994, Apr 1983, May 1997, Nov 1991, Sep 26, Feb 1994, Nov 1986, Jun 1992 Dec 1989, Jul 1992, Aug 1982, Nov 1986, Jan 1994, Oct 29, Feb 1993, Apr 23, Dec 198, Sep 1986, Jul 21, Oct 1993, Feb 199, Jun 24, Apr 1992, May 1997, Apr 1982, Aug 1997, May 199, Mar 1997, Mar 28, Jan 1991, Apr 27, Jun 1986, Sep 1988, Nov 22, Sep 199, Mar 1989, Jun 198, May 1994, Mar 1992, Jun 1988, Oct 1988, Apr 1997, Feb 1979, May 22, Sep 29, Mar 23, Jul 1996, Nov 199 May 1991, Jan 198, Dec 21, Jan 1987, Mar 1987, Oct 23, Dec 21, Sep 1996, Jun 1991, May 198, Feb 1986, Jul 198, Apr 1986, May 1987, May 1984, Aug 1979, Nov 1998, Oct 21, Apr 1991, Apr 198, Sep 199, Jan 21, Mar 1996, Mar 26, Aug 2, Sep 27, Nov 1993, Jul 1981, Mar 1984, May 21, Jun 1987, Jun 1984, Sep 21, Dec 1998, Nov 21, Nov 198, Dec 1981, Feb 212, Jul 1979, Jun 198 Nov 1993, Dec 22, Oct 1987, Jan 26, Sep 1997, Jun 23, Apr 1987, Dec 1984, Sep 1993, Jan 28, Mar 1987, Feb 212, Nov 211, Mar 198, May 1992, May 1999, Jan 198, Jul 198, Aug 198, Sep 1987, Jul 26, Apr 1981, Mar 198, Oct 198, Jul 1994, Nov 1999, Jun 1982, Aug 29, Feb 1994, Jan 1987, Jun 1994, Jan 1996, May 2, Mar 1996, May 22, May 1981, Mar 23, Aug 1984, Apr 29, Dec 21 Sep 1992, Dec 1984, Oct 1987, Oct 2, Jun 1987, Dec 212, Jul 23, Feb 2, Mar 1999, Jan 211, May 1993, Aug 1996, Jan 26, Feb 21, Sep 1994, Oct 1981, Nov 1993, Feb 1984, Jan 28, Nov 1987, Jul 21, May 1988, Nov 2, Sep 1983, Jul 211, Dec 22, Feb 212, Jun 29, Jun 1992, Apr 1984, Jan 212, May 21, Mar 198, May 1983, Jul 21, Jun 199, Aug 1992, Aug 26, Aug 1991, Jun 1999 Apr 21, Jun 1983, Apr 198, Aug 1981, Aug 23, Sep 1997, Dec 212, Sep 211, Jul 1983, Mar 21, Jul 1987, Oct 22, Nov 1981, Nov 29, Mar 1999, May 1987, Jan 21, Jul 23, May 2, Nov 22, Dec 22, Jan 199, May 198, Feb 2, Nov 1987, Dec 2, May 199, Nov 2, Dec 198, Jan 198, Jun 1993, Aug 1993, Feb 21, Mar 1984, Oct 198, Oct 1984, Feb 1983, Dec 2, Feb 1984, Apr 26 Supplementary Table S3: Months of hot and cold extremes. The 4 months of largest T' (upper rows) and smallest T (lower rows) area-averaged over eight geographical regions (shown in Fig. 2). 214 Macmillan Publishers Limited. All rights reserved.

10 Wet extremes Dry extremes Region Months Jun 29, Apr 1983, Nov 1994, Aug 29, Feb 199, Jul 1993, Apr 1998, Sep 1992, Jun 1997, May 1983, Dec 1982, Feb 1983, Dec 199, Feb 1979, Sep 1987, Mar 1983, Feb 1998, Jul 1991, Jul 29, Aug 24, Oct 199, Jan 1988, May 1998, Oct 1979, Jun 199, Aug 1997, Jun 1993, Jul 1992, May 21, Nov 199, Jan 1987, Sep 1996, Mar 1987, May 27, Nov 1991, Dec 198, Oct 23, Jan 1994, Jan 1979, Oct 29 Aug 1991, Jul 1983, Apr 1996, Feb 1999, Oct 199, Jul 1997, Mar 1983, Jun 1983, Feb 1983, Jun 1997, Nov 1991, May 1987, Jan 1983, Sep 1981, Jun 212, Sep 1992, Aug 1983, Jan 1992, Oct 1982, Nov 1988, Aug 28, Mar 199, Dec 26, Jan 26, Feb 1986, Mar 1986, Dec 198, May 2, Mar 23, Feb 1998, May 1984, Jul 2, Dec 1996, Mar 27, Dec 1979, Oct 198, Sep 1997, Nov 199, Apr 1981, May 1988 Sep 1986, Jul 1993, Dec 1982, May 199, Jan 2, Jul 1992, Oct 29, Nov 198, Feb 21, Feb 199, Mar 1979, Oct 1984, Apr 211, Mar 1998, Aug 198, Dec 199, Jun 2, Aug 1993, Aug 29, Nov 1983, Apr 1999, Apr 1991, Feb 1998, Mar 1983, Aug 1987, Jun 1998, Nov 1991, May 1982, Jan 1982, Jun 1993, Dec 1984, Feb 23, Dec 27, Nov 1992, Feb 1986, Apr 1994, Jun 28, May 1991, Nov 1988, Jul 1986 Apr 1983, Jul 1996, Sep 1999, Jan 1979, Jun 1982, Jun 26, May 1984, Aug 211, Mar 1983, Dec 199, Feb 1988, Mar 1994, Nov 199, Nov 1989, Nov 1983, Mar 1999, Aug 29, Aug 1992, May 1983, Oct 2, Oct 23, Aug 1986, Feb 1996, Sep 21, Jul 26, Dec 1983, May 1997, Jan 1994, Sep 1979, Sep 1987, Jan 1998, Dec 28, Feb 1984, Feb 1998, Nov 1988, Feb 28, Nov 22, Dec 1993, Dec 24, Jun 21 Aug 26, Dec 1981, Apr 1998, Jun 1997, Jan 1988, May 1984, Sep 199, Dec 1999, May 27, Feb 1979, Nov 22, Nov 2, Sep 1994, Sep 21, Sep 1993, Sep 1984, Jul 211, Mar 1979, May 1996, Feb 199, Jul 22, Feb 199, Jan 199, Nov 1997, Feb 21, Jan 1986, May 1983, Sep 1998, Mar 21, Jun 27, Jun 198, Feb 27, Apr 1989, Apr 1994, Oct 198, Aug 24, Mar 28, Nov 29, Apr 212, Sep 1999 May 1991, Oct 1997, Sep 1996, Feb 198, Jun 1997, Jul 24, Nov 1994, Jan 24, Mar 1998, Jan 1987, Sep 199, Mar 1981, Oct 1989, Dec 21, Oct 22, Dec 212, Dec 29, Apr 1997, Feb 21, Jun 1988, Oct 23, Aug 1987, Jul 1979, Mar 2, Nov 21, Jun 211, Aug 1999, Sep 1987, Aug 24, Nov 1992, Oct 1992, Jun 2, Apr 198, Jun 198, Mar 29, Feb 1999, Jan 1979, Apr 1987, Jul 23, Jul 1993 Jun 22, Feb 1998, Jul 199, Sep 1987, Dec 1991, Nov 1994, May 27, Mar 22, Apr 1987, Oct 1987, Aug 1991, Aug 1992, Mar 21, Sep 1992, Apr 1998, Jan 1992, May 1988, Nov 212, Jul 23, Jan 1988, Mar 1993, Jul 1993, Aug 2, Jun 211, Feb 21, Jun 1987, Oct 21, Oct 1982, Jan 22, May 1987, Dec 1986, Mar 199, Jun 29, Apr 22, Aug 1986, Feb 1993, Oct 1979, Oct 26, Dec 1994, Apr 1992 Apr 1983, Jan 22, Aug 1981, Mar 27, Dec 198, Jul 1991, Nov 212, Dec 21, May 1998, Jun 199, Mar 29, Jun 29, Oct 1991, Oct 212, Feb 199, Sep 199, Oct 22, Nov 199, May 21, May 24, Jul 212, Nov 26, Oct 1981, Jan 21, Jun 1996, Jan 198, Feb 21, Apr 22, Feb 1979, Nov 23, Sep 1992, Mar 1987, Sep 1983, Mar 21, Aug 1987, Sep 211, Feb 198, Feb 1994, Dec 212, Sep 198 Jan 211, Mar 211, May 1989, Mar 1996, Jun 23, Dec 1988, Nov 211, Apr 1989, Jan 1981, Dec 1984, Jul 1999, Nov 27, Feb 1982, Oct 1988, Jan 28, Jul 1984, Aug 27, Aug 1983, Nov 1986, Apr 1997, Jun 199, Sep 212, Sep 29, Aug 2, Nov 2, May 22, Feb 2, May 21, Sep 1998, Sep 2, Oct 27, Sep 1988, Jul 1989, Sep 1979, Jul 21, Dec 1998, Jun 2, Oct 1983, Apr 1982, Apr 29 Oct 22, Feb 1989, Sep 1993, Jul 21, Jan 1979, Dec 29, Apr 1989, Jun 1982, Jan 1991, Nov 198, Jan 198, Nov 1979, Jun 24, Sep 212, Mar 1996, May 24, Nov 2, Apr 29, Dec 2, Feb 23, Dec 198, Aug 212, Jul 198, Apr 1979, Oct 26, Sep 199, Apr 24, Sep 1998, Jun 23, Aug 27, Jul 29, Mar 28, Sep 28, Dec 211, Aug 21, Mar 1992, Mar 1979, Jul 1981, Jan 1981, May 198 Jun 1988, Jan 1981, Mar 21, Apr 1989, Jun 212, Oct 1999, May 1988, Jan 1986, Apr 1987, Feb 26, May 1994, Oct 21, Apr 198, Sep 212, Nov 198, Nov 29, Feb 21, Sep 1998, Jun 26, Apr 1982, Jan 1984, Sep 1979, Mar 1999, Mar 21, Nov 27, Dec 1989, Aug 212, Jan 23, Nov 1999, Oct 1992, May 198, Aug 1984, Oct 1987, Jul 212, Jul 198, Aug 1983, Mar 1996, Apr 2, Jul 27, Oct 1988 Dec 1988, Mar 26, Nov 21, Sep 198, Jan 21, Apr 198, Jul 1999, Oct 1984, Jun 2, Apr 21, Apr 21, Oct 1994, Oct 1997, Jul 2, May 27, May 1986, Nov 212, Mar 199, Oct 1982, Aug 198, Feb 2, Jan 24, May 1982, Jul 1987, Aug 1996, May 1999, Feb 1991, Aug 22, Oct 1998, Sep 29, May 21, Jul 1993, Jun 212, Aug 21, Mar 212, Dec 1989, Dec 26, Jan 211, Feb 212, Feb 29 Oct 198, Mar 212, Oct 199, Jul 1983, May 1989, Feb 1993, Aug 23, May 199, Jan 1989, Dec 21, Mar 1997, Dec 1991, Mar 23, Jun 23, Jun 2, Jul 1984, Sep 198, Apr 27, Nov 1988, Jul 1994, Nov 1981, Nov 211, Apr 1982, Jul 26, Jan 1997, Jan 2, May 21, Dec 1988, Feb 28, Jun 1996, Oct 27, Apr 211, Dec 27, Jun 2, Aug 1991, Apr 1984, Sep 198, Jan 1992, Aug 2, Feb 1992 Mar 1986, Apr 1989, Jul 21, Feb 1984, Aug 198, Dec 1984, Mar 198, Jan 1996, Jun 1979, Sep 1994, Nov 2, Aug 21, Dec 26, Apr 1986, Jan 28, May 1979, Oct 2, Feb 1982, Apr 1999, Jul 21, Jul 1996, Nov 1996, Aug 27, Apr 1994, May 1984, Dec 28, Jun 1999, Feb 1989, May 198, Jun 1981, Oct 1993, May 22, Apr 29, Sep 1998, Nov 211, May 1996, Aug 1996, Nov 1982, Sep 212, Jun 1998 Dec 1981, Jan 211, Apr 1997, Dec 2, Jan 1984, Aug 21, Apr 2, Jun 1982, Nov 1988, Mar 1989, Mar 1996, Apr 1986, Oct 2, Jan 28, Aug 22, Sep 198, May 21, Nov 1986, Nov 1981, Apr 1982, May 26, Sep 1997, Feb 1999, Aug 1984, Jul 1989, Oct 198, May 24, Aug 1989, Feb 1986, Jul 2, Jun 199, Mar 198, Feb 1997, Jun 1981, Sep 26, May 28, Jul 1983, Nov 199, Sep 211, Nov 1989 Mar 211, Jan 28, Sep 22, Jun 1982, Nov 27, Feb 2, Apr 1997, Nov 1984, Jul 27, Aug 1991, Sep 26, Jul 1997, Jan 27, Oct 2, Apr 21, Aug 1989, May 1979, Jun 2, Oct 1986, May 1996, Jan 211, Aug 21, May 22, Mar 1993, May 1989, Oct 1992, Jun 21, Nov 21, Dec 1981, Apr 1996, Feb 28, Jul 198, Mar 1982, Jun 23, Oct 211, Dec 1983, Jan 1981, Dec 23, May 1984, Sep 29 Supplementary Table S4: Months of wet and dry extremes. The 4 months of largest P' (upper rows) and smallest P (lower rows) area-averaged over eight geographical regions (shown in Fig. 2). 214 Macmillan Publishers Limited. All rights reserved.

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