Two solitudes - how seasonality biases drought from tree rings

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Scott St. George

Graphics prepared for an upcoming talk at the University of Arizona's Laboratory of Tree-Ring Research

Tecnología Meditación

TWO
SOLITUDES

how seasonality biases
drought records from tree rings

why are Canadian streamﬂow reconstructions so lousy?

100%

80%

60%
explained
variance
40%

20%

0%

Canada USA

100%

Woodhouse et al., 2006
Meko et al., 2001
80% Sacramento River
Upper Colorado River

Meko et al., 2007
Colorado River
Woodhouse, 2007
Middle Boulder Creek

60%
explained
variance
40%

20%

0%

Canada USA

why are Canadian tree rings not connected to climate modes?

Janice Lough and Hal Fritts
The Southern Oscillation and tree rings: 1600 - 1961
Journal of Applied Meteorology 24, 1985

Roseanne D’Arrigo and Gordon Jacoby
A thousand year record of northwestern New
Mexico winter precipitation reconstructed from tree
rings and its relation to El Niño and the Southern
Oscillation
The Holocene 1, 1991

David Stahle and collaborators
Experimental dendroclimatic reconstruction of the
Southern Oscillation Stahle et al., 1998

Bulletin of the American Meteorological Society 79, 1998

Prairie ringwidth records
138
Signiﬁcant correlation with ENSO
4
Expected from random chance
6.9

Trees, seasonality and climate forcings
The southwestern US

Trees, seasonality and climate forcings
The Canadian Prairies

DROUGHT AND SEASONALITY
How does the connection between reconstructed drought
1
and seasonal precipitation vary across North America?

DROUGHT AND SEASONALITY
How does the connection between reconstructed drought
1 and seasonal precipitation vary across North America?

CLIMATE MODES AND TREE RINGS
In which regions are drought reconstructions most strongly
2 connected to remote climate forcings (ENSO, PDO, AMO)?

The North American
Drought Atlas

Cook et al., 2007, Earth Science Reviews

Cook et al., 2007, Earth Science Reviews

PART ONE
tree rings, drought and seasonality

“
...the regional summer precipitation signal is strong in
Pseudotsuga menziesii latewood-width [in southeastern
Arizona].

David Meko and Chris Baisan
Pilot study of latewood-width of conifers as an indicator of variability of summer rainfall in
the North American monsoon region
International Journal of Climatology 21, 2001

Medicine Hat, Alberta

Source: Environment Canada, Adjusted Historical Canadian Climate Data, 1895 – 2006

Prairie ringwidth records
138
Signiﬁcant correlation with ENSO
4
Expected from random chance
6.9 45

PART TWO
drought records and climate modes

are these signals present in reconstructed drought?

Julia Cole and Ed Cook
The changing relationship between ENSO variability
and moisture balance in the continental United States
Geophysical Research Letters 25, 1998

COMPOSITES
where is it wet or dry when ENSO is strong?

LOW FREQUENCIES (>7yr)
TREE-RINGS and PDO

0.5

0.25

mean squared 61
0 coherence

3 Mexican PDSI 10

2
5
1

0 0

-1
-5
-2

-3 62
-10
1900 1920 1940 1960 1980 2000

3 PDO index Mexican PDSI 10

2
5
1

0 0

-1
-5
-2

-3 63
-10
1900 1920 1940 1960 1980 2000

IMPLICATIONS

what/where OK

what/where/why NOT OK

IMPLICATIONS

tree ring model output

Celine Herwiejer, Richard Seager and Ed Cook
North American droughts of the mid to late nineteenth century:
a history, simulation and implication for Mediaeval drought
The Holocene 16, 2006

M A N Y TH AN KS

Dave Meko
Ed Cook
Hal Fritts
Toby Ault

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Even after more than a century of coordinated monitoring, instrumental weather observations are still too short to adequately constrain decadal or multidecadal behavior in the Earth’s climate system. Leading climatologists and climate modelers have called for the wider application of high-resolution proxy records to decadal variability and prediction studies, and our community has responded by producing new paleoclimate products that specifically target this type of ‘intermediate-term’ behavior. But we now also know our medium changes that message: the biological and geological systems that encode climate information into natural archives often also alter the original ‘input’, usually due to either seasonal filtering or non-climatic persistence. In this talk, we’ll discuss some of the challenges inherent to the use of high-resolution proxies to study decadal or multi-decadal climate variability, and suggest strategies that might clarify how climate acts on those timescales. And we’ll also present a new theoretical framework that could help paleo-scientists evaluate competing ideas about the causes of decadal- or multi-decadal events known to have occurred during the past one or two millennia.

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We know from tree rings and other natural drought records that the western United States has been affected by several 'megadroughts' during the past millennium. But are these exceptionally long-lasting droughts due to unusual external forcings, or are they inevitable given a sufficiently long period of time? Here we present a statistical model that combines sea surface temperature records and drought severity statistics from the western USA, and use that tool to set out an expectation for megadrought, given no other changes in the climate system. Even though this model was trained using only modern climate data (and incorporates no information from tree rings or other proxies), it still produced megadroughts. Moreover, those simulated megadroughts were just as long-lasting, covered as large an area, and were just as severe as real megadroughts estimated from tree rings. That result means that megadroughts can occur in the western United States even if nothing else changes in the climate -- they really are just a matter of time. On the other hand, the only aspect of real-world megadroughts that the model cannot duplicate was the high number of these events during the so-called Medieval Climate Anomaly (800 to 1300 CE). So that cluster of megadroughts may have been caused by some sort of unusual climate circumstances that have not been observed by us during the past few decades. The proxy record tells us that many different kinds of exceptional or unusual climate events happened in the past. But it is often difficult to determine what caused those exceptional events because even, within a period of a thousand years, we still have very few cases. So besides being an aid to understand the causes of past megadroughts, we hope this approach can be applied to other paleoclimate records to distinguish between real interrelations between separate components of the climate system and simple coincidences.

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Two solitudes - how seasonality biases drought from tree rings

1. TWO SOLITUDES how seasonality biases drought records from tree rings

10.

11.

12. TWO SOLITUDES how seasonality biases drought records from tree rings

13.

14. why are Canadian streamﬂow reconstructions so lousy?

15. 100% 80% 60% explained variance 40% 20% 0% Canada USA

16. 100% Woodhouse et al., 2006 Meko et al., 2001 80% Sacramento River Upper Colorado River Meko et al., 2007 Colorado River Woodhouse, 2007 Middle Boulder Creek 60% explained variance 40% 20% 0% Canada USA

17. 100% Woodhouse et al., 2006 Meko et al., 2001 80% Sacramento River Upper Colorado River Meko et al., 2007 Colorado River Woodhouse, 2007 Middle Boulder Creek 60% Case and MacDonald, 2003 explained Saskatchewan River Beriault and Sauchyn, 2006 Churchill River variance Bonin and Burn, 2005 Athabasca River 40% Watson and Luckman, 2005 Gedalof et al., 2007 Bow River Columbia River 20% 0% Canada USA

18. why are Canadian tree rings not connected to climate modes?

19. Janice Lough and Hal Fritts The Southern Oscillation and tree rings: 1600 - 1961 Journal of Applied Meteorology 24, 1985 Roseanne D’Arrigo and Gordon Jacoby A thousand year record of northwestern New Mexico winter precipitation reconstructed from tree rings and its relation to El Niño and the Southern Oscillation The Holocene 1, 1991 David Stahle and collaborators Experimental dendroclimatic reconstruction of the Southern Oscillation Stahle et al., 1998 Bulletin of the American Meteorological Society 79, 1998

20. Prairie ringwidth records 138 Signiﬁcant correlation with ENSO 4 Expected from random chance 6.9

21.

22.

23.

24. seasonality

25. Trees, seasonality and climate forcings The southwestern US

26. Trees, seasonality and climate forcings The Canadian Prairies

27.

28. DROUGHT AND SEASONALITY How does the connection between reconstructed drought 1 and seasonal precipitation vary across North America?

29. DROUGHT AND SEASONALITY How does the connection between reconstructed drought 1 and seasonal precipitation vary across North America? CLIMATE MODES AND TREE RINGS In which regions are drought reconstructions most strongly 2 connected to remote climate forcings (ENSO, PDO, AMO)?

30. The North American Drought Atlas Cook et al., 2007, Earth Science Reviews

31. Cook et al., 2007, Earth Science Reviews

32. CORRELATION COMPOSITES COHERENCE

33. PART ONE tree rings, drought and seasonality

34.

35. signiﬁcant (α=0.05)

36. instrumental tree rings

37. SUMM ER PRECIP ITAT ION

38.

39. WINTER PRECIPITAT ION

40.

41. do tree rings replicate these patterns?

42.

43.

44. +

45. summer mixed winter

46. DOMINANT not exclusive

47. “ ...the regional summer precipitation signal is strong in Pseudotsuga menziesii latewood-width [in southeastern Arizona]. David Meko and Chris Baisan Pilot study of latewood-width of conifers as an indicator of variability of summer rainfall in the North American monsoon region International Journal of Climatology 21, 2001

48. Medicine Hat, Alberta Source: Environment Canada, Adjusted Historical Canadian Climate Data, 1895 – 2006

49. Prairie ringwidth records 138 Signiﬁcant correlation with ENSO 4 Expected from random chance 6.9 45

50. ENSO and winter precipitation

51. PART TWO drought records and climate modes

52. ENSO PDSI from tree rings PDO AMO

53. are these signals present in reconstructed drought?

54. Julia Cole and Ed Cook The changing relationship between ENSO variability and moisture balance in the continental United States Geophysical Research Letters 25, 1998

55. ENSO PDSI from tree rings PDO AMO

56.

57.

58. COMPOSITES where is it wet or dry when ENSO is strong?

59.

60.

61. ENSO PDSI from tree rings PDO AMO

62.

63.

64. LOW frequency

65. LOW FREQUENCIES (>7yr) TREE-RINGS and PDO 0.5 0.25 mean squared 61 0 coherence

66. LOW FREQUENCIES (>7yr) TREE-RINGS and PDO 0.5 0.25 mean squared 61 0 coherence

67. 3 Mexican PDSI 10 2 5 1 0 0 -1 -5 -2 -3 62 -10 1900 1920 1940 1960 1980 2000

68. 3 PDO index Mexican PDSI 10 2 5 1 0 0 -1 -5 -2 -3 63 -10 1900 1920 1940 1960 1980 2000

69.

70. ENSO PDSI from tree rings PDO AMO

71.

72. SEAS ONALITY AND D ROUGHT

73. summer mixed winter

74. IMPLICATIONS 1590s

75. IMPLICATIONS what/where OK

76. IMPLICATIONS what/where OK what/where/why NOT OK

77. IMPLICATIONS tree ring model output Celine Herwiejer, Richard Seager and Ed Cook North American droughts of the mid to late nineteenth century: a history, simulation and implication for Mediaeval drought The Holocene 16, 2006

78. summer mixed winter

79. M A N Y TH AN KS Dave Meko Ed Cook Hal Fritts Toby Ault

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