Drought and Failure of the
Pacific-North American Monsoon
during the Holocene from Varved
Sediments at DSDP Site 480 in
the Gulf of California
Ann Marie Harris and Robert E. Karlin
Dept. of Geologic Sciences, University of Nevada, Reno
DSDP Site 480
Modern Monsoonal
Circulation
• Winter-spring
•
•
upwelling season due
to NW wind forcing
Summer
SE monsoon;
increased storminess
and precipitation
Fall-winter
slackening of SE
monsoon; cooling of
surface waters
(Schrader and Baumgartner, 1983)
Modern Sedimentation at
DSDP Site 480
• Winter
– Primarily biogenic material (diatoms)
– Minor terrigenous material
• Summer
– High terrigenous content
– Less biogenic material
Water Masses in the Gulf
Laminated
Sediments
Bioturbated
Sediments
6.1 cm
Sharply defined
laminations
are measurable
using
computerassisted
methods
16.1 cm
Measuring Laminae Thickness using
CoralXDS
Transects Spliced Together to Make
Continuous Record
Slab transects
Half core transect
Eighty Intercalated Speckled Beds
(An Earthquake Shaking Record?)
Interpretation of Speckled Bed
Formation – Effects on Age Model
Continuous sedimentation
age model
RadiocarbonRadiocarbon-based
age model
Instantaneous sedimentation
age model
Magnetic Secular Variation
Site 480 Correlation to Saanich Inlet
ODP 1033B
00
Age (cal kyrs BP)
10
15
5
20
25
disturbed
Core 1
5
laminated
Core 2
10 bioturbated
laminated
Core 3
15
bioturbated/homogeneous
20
Oxygen Isotope Stage 1
Core 4
no recovery
Oxygen Isotope Stage 2
DSDP Site 480
Radiocarbon-Based
Age Model
Composite age curve
Microfossil abundance ties
Age estimates from 18O stratigraphy
Ties to ODP1033 SMV curve
Calibrated radiocarbon dates
Age estimate from 18O stratigraphy and sedimentation rates
Radiocarbon error envelope
DSDP Site 480 Age Model
age control point from radiocarbon-based age model
0
2
Varve age model (Core 2 adjusted
upwards 0.3 m)
Radiocarbon-based age model (Core 2
adjusted upwards 0.3m)(calibrated
using CALIB 4.4 and a reservoir
age of 802+-34 years[Reimer,2001])
Unadjusted radiocarbon ages
Core 1
Radiocarbon error envelope
4
6
Core 2
8
no core recovery
Core 3
10
0
2
distorted by coring
4
6
8
10
12
The Site 480 Holocene Proxy
Record
• Lamina-pairs are varves
• Monsoonal circulation dominated the Gulf
region throughout the Holocene
• The Site 480 record provides the
opportunity to study climate on the subannual to decadal level
The Annual Record Is
Highly Variable
Varve Thickness
Thickness (cm)
0.4
0.3
0.2
0.1
0
0
2
4
6
Age (cal kyr BP)
8
10
12
Linear Trend on 250-year Averages
0.14
weighted curve fit
using 250 points
0.1
0.08
Winter
0.06
0.04
Summer
0
1
2
3
4
5
6
7
Age (cal kyr BP)
8
9
10
11
0.02
12
Thickness (cm)
0.12
Annual
Millennial- to Multi-Centennial Scale Patterns
0.14
0.1
0.08
Winter
0.06
0.04
Summer
0
1
2
3
4
5
6
7
Age (cal kyr BP)
8
9
10
• Linear trend suggests that the ITCZ has moved northward
•
•
11
0.02
12
weighted curve fit
using 250 points
through the Holocene
Large scale oscillations in the ITCZ and jet stream cause
thickness are due to millennial to multi-centennial chages in
thickness
Large-scale changes in the two dominant seasonal wind fields
are coupled
Thickness (cm)
0.12
Annual
Modern “Normal” Patterns
Winter
Summer
AzoresAzoresBermuda
High
track of ‘normal’ jet stream
prevailing wind direction over GOC
limit of trade winds
Unique geography of Gulf
region allows formation of
a thermal low pressure
trough parallel to Gulf
What is driving upwelling and
monsoonal circulation?
Summer
Winter
From Brown, 1996
Precipitation and Boundaries of the
Pacific-North American Monsoon
North American
Monsoon boundary
Intermittent NAM
precipitation boundary
Pacific influence
v. Gulf of Mexico
prevailing
moisture-bearing
wind
Site 480
GPCP - precipation (mm/d)
prevailing
moisture-bearing
wind
(modified from Bordoni et al., 2004)
Controls on Winter
Sedimentation
• Winter northwesterly wind direction,
strength, and persistence
– Position of Aleutian Low and Pacific High
– Pineapple Express
• Affects intensity and duration of upwelling
Controls on Summer
Sedimentation
• Summer southerly wind and storm activity
– Shifting position of Pacific High and ITCZ
– Affects Strength and position of Thermal Low
– Changes in sea surface temperature
– Controlled by insolation changes
• Affects aeolian and/or fluvial input
– Strength and number of convective events
– Vegetation stabilization?
Dust Fronts
Yuma, AZ
June 6, 2006
Phoenix, AZ
http://www.azsinglescene.com/weather.htm
Episodic Winter Perturbations
track of ‘normal’ jet stream
prevailing wind direction over GOC
Climate Prediction Center/NCEP
limit of trade winds
El Nino (3-7 yrs; 7-9 month duration)
“Enhanced El Nino” Warm Phase PDO (20-50 yrs)
Causes More Southerly Position of Pacific High
Warm and
Cool Phase
ENSO and
PDO
warm
cool
Typical wintertime
anomaly patterns
Sea Surface Temperature (colors)
Sea Level Pressure (contours)
Surface windstress (arrows)
Mantua, 2003
Effects of PDO on Sedimentation
• Warm phase PDO enhances El Nino
– Light and variable winds
– reduced upwelling and aeolian input
• Cool phase PDO enhances La Nina
– increases northwesterly wind intensity and
storminess
– Increased winter upwelling; greater aeolian
input; higher sedimentation
Solar Insolation Variability
• Periodic variations - Swabe 11-yr cycle;
Hale 22-yr cycle
• Results in:
– Increased monsoonal strength
– More intense upwelling
– Increased convection -> greater aeolian input
– Increased sedimentation
Multi-decadal to Centennial
Scale Variability
Annual
0.14
weighted curve fit
using 50 points
0.12
0.1
W inter
0.08
0.06
0.04
Summer
0
1
2
3
4
5
6
7
8
9
Age (cal kyr BP)
50 year averages
10
11
0.02
12
Multi-decadal Drought Proxy
Lower periods of Sedimentation
0.14
varve
0.12
0.1
winter
0.08
0.1
0.06
0.08
0.04
summer
0.02
0.06
0.04
0
2
4
6
Age (calkyrs BP)
8
10
0.02
12
Biogenic Silica (Barron, 2005)
Wt %
YD
Wt %
Annual
weighted curve fit
using 50 points
Winter
0.12
0.1
0.08
0.06
Summer
0
1
Thickness (cm)
0.14
0.04
2
3
4
5
6
7
Age (cal kyr BP)
8
9
10
11
0.02
12
Biogenic opal data from Barron et al., 2005
Annually resolved Holocene records
older than 2000 yr BP
• White Mountains, NV tree rings
• Santa Barbara Basin marine cores
• Great Plains tree rings
• Carioca Basin marine cores
Limits of
Pacific-North American Monsoon
North American
Monsoon boundary
White Mtns
prevailing
moisture-bearing
wind
Intermittent NAM
precipitation boundary
Pacific influence
v. Gulf of Mexico
El Malpais
Chihuahuan
Desert
Santa
Barbara
Basin
Site 480
GPCP - precipation (mm/d)
prevailing
moisture-bearing
wind
(modified from Bordoni et al., 2004)
White Mtns Modern Precipitation
Regime
• Most of precipitation received as snowpack
during winter months
• Some moisture from Gulf surges in
summer
• Major changes are due to position of
winter storm track
Multi-Decadal Droughts
Tree Ring Records
White Mountains, NV
22
22
20
20
Drought
18
18
0
1
2
3
4
Age (cal yrs BP)
5
6
7
Precipitation (cm)
24
24
Reconstructed July-June Precipitation
16
16
8
Drought defined as <19 cm/yr by Hughes and Gramlich (199
Filtered tree ring data from
ftp://ftp.ncdc.noaa.gov/pub/data/paleo/treering/reconstructions/nevada/mwkreconfilt.dat
Thickness (cm)
Site 4800.20.2
0.10.1
0
Age
(cal yrs BP)
24
24
?
White Mountains
?
22
22
Drought
20
20
18
18
0
1
2
3
4
5
Age (cal yrs Before 2000AD)
6
7
16
16
8
Precipitation (cm)
0
Tune in next year
for the
Site 480 World Tour
Regional Multi-decadal Droughts
0.2
Site 480
0.1
0
0
2
4
6
8
10
12
• At this point, do I compare to olther
records or just summarize and stop?
• Other records –
– Santa Barbara
Cariaco
GISP2
IRD
ODP 893
0.2
0.
0.1
2
Site 480
0.
0
1
0
Varve thickness 30-yr
weighted average
2
4
6
8
10
12
8
10
12
Age (cal kyrs BP)
0
0
2
4
6
Pacific-North American Monsoon
North American
Monsoon boundary
White Mtns
prevailing
moisture-bearing
wind
possible
intermittent
precipitation
boundary
G. California
influence
v. G. Mexico
El Malpais
Chihuahuan
Desert
Santa
Barbara
Basin
Site 480
prevailing
moisture-bearing wind
GPCP - precipation (mm/d)
(modified from Bordoni et al., 2004)
Findings - summary
• What does this mean
• Link clouds precip dust
• Low sedimentation = prolonged drought?
• Increase irradiance = colder eastern equa
Pacif + stronger zonal temp gradient at
equator
Warm and cool PDO Phases
Warm phase
Cool phase
Typical wintertime Sea Surface Temperature (colors),
Sea Level Pressure (contours) and surface windstress (arrows) anomaly patterns
during warm and cool phases of PDO
Multi-Decadal Droughts:
Failure of the
Pacific-North American
Monsoon during the
Holocene
Ann Marie Harris and Robert E. Karlin
Dept. of Geologic Sciences, University of Nevada, Reno
Trends in Biogenic and Lithic
Lamima Thickness
Deformation Model
Modified from Kawakami and Kawamura (2000)
Links between Gulf sedimentation
and precipitation
The varved Holocene sedimentary record at DSDP Site 480 is a particularly unique record in that it preserves a sub-annual to decadal scale record in a climatologically sensitive region. Sedimentation at the
site is controlled by regional shifts of wind direction, intensity, and persistence associated with the North
American Monsoon. Based on modern observations, previous studies, and climate models, these changes in winds should be strongly affected by several related factors:
• Migration of the ITCZ on an annual to millennial time scale
• Relative positions and strengths of the North Pacific High, Aleutian Low, and storm tracks
• Existence and strength of a low pressure trough over the Gulf region
• Existence and position of a continental high
Winds would also be affected by changes in seasonal insolation which would also affect sea surface temperature (SST) and any land-sea temperature contrasts. These factors also affect the duration of the
summer circulation regime which strongly correlates to summer precipitation. Because of the known relationship between aeolian dust entrainment and summer storminess and monsoonal precipitation, a history
of summer monsoonal precipitation can be inferred from the Site 480 record.
• A similar inference cannot be based on winter sedimentation.
What can be inferred is the position of the storm track and
the North Pacific High. When
Possible inconsistencies with other
Proxy Records
• Packrat Middens
• Santa Barbara Basin
• New Mexico & Great Plains tree records
• Cariaco Basin
• Ice Cores
• Pack rat midden
– seasonality question
– no vegetation difference essentially in
Sonoran desert
• Santa Barbara Basin
– Outside PNAM region
• Great Plains
– East of Intermittent Moisture Boundary
Seasonal Changes in Atmosp Circ
• The ITCZ
– As the seasons change, the region with the most insolation changes,
hence the position of the ITCZ changes
– Over the ocean the shift is moderate ~10 degrees
– Over land, because of large seasonal changes in temperature, large
shift occurs particularly over South America and less so over North
America
• North American Monsoon
– In the summer, high insolation warms the continent and produces lowpressure
• Winds blow from the ocean to the land and then rise
• These warm and moist air brings heavy precipitation
– In the winter, the continent cools quickly, producing high-pressure
• Winds blow from the continent to the ocean
• Dry conditions prevail
What is the NAM?
• Distinct shift in mid-level winds accompanied by
an increase in rainfall
• Occurs over NW Mexico and SW United States
• Onset usually in July and decays in September
• Great deal of variability at every timescale
NAM Moisture Source
• Moisture source current consensus
– low-level moisture Æ Gulf of California (GoC)
– mid-level moisture Æ Gulf of Mexico (GoM)
• Transport of low-level moisture by gulf surges
(one way)
• Induced by passage of tropical easterly waves
(TEWs) over GoC and/or outflow
boundaries/gust fronts
Gulf surges
Hales (1972) and Brenner
(1974)
• Cooler temps, increased
dewpoints, pressure rise,
southerly wind
• Increase in convection
• Shallow vertical extent
• Loss of definition upon entering
desert SW
Adams and
Comrie (1997)
Changes in the Jet Stream
Position
Displaced and split jet stream
(Kutzbach and Wright,
1985; COHMAP Members,
1988)
Jet stream migration
in response to ice
sheet retreat
(Negrini, 2002)