Variability of the North American Monsoon Associated with Preceding Winter

1
Variability of the North American
MonsoonAssociated with Preceding Winters
Snowmass and Pacific SSTs

• Nabeel Keblawi
• METO 658N
• Dr. Sumant Nigam
• Dr. Alfredo Ruiz-Barradas

2
Overview

• Introduction (brief)
• Data Methodology
• Climatology/Variability
• SWE/Monsoon/Pacific SSTs
• Discussion
• Conclusion

3
Background Information

• Indian Monsoon
• North American Monsoon
• Connection with neighboring snow cover of the
  preceding winter.
• Blanford Hypothesis Winter/Spring snow cover in
  the Himalayas mountain region has an inverse
  relationship with June-September Indian rainfall.

4
Contradictory results

• In recent years there have been contradictions
  against the Blanford Hypothesis.
• Fusallo (2004), Bamzai Shukla (1999) and Ye
  Bao (2001)
• Positive correlation between snow cover and
  monsoonal rainfall anomalies
• These results are at odds with the Blanford
  Hypothesis for the spatial distribution of snow
  cover.

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Rationale

• Given all these contradictions, theres the need
  to determine the driving factors of the monsoon,
  whether they are
• Antecedent Pacific SST Anomalies ENSO PDO
• ENSO Equatorial SST Anomalies in Pacific Ocean
• PDO ENSO-like SST Anomalies in North Pacific
• Antecedent Snow Depth Anomalies SWE
• Other factors involved?
• Monsoons in North America and India often cause
  flash flooding that is hazardous and disruptive.
• Areas that are prone to severe drought.
• Implications on the regional water supply.

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Data

• Snow Depth Precipitation
• NARR 1 x 1 1979-2002
• ERA40 2.5 x 2.5 1979-2002
• ENSO-MEI PDO
• NOAA-NCEP
• Initial comparison between NARR ERA40 ruled out
  any differences in correlation between SWE
  Precip anomalies due to time differences and
  resolution.
• However, ERA40 snow depth data was concluded to
  be of lesser quality than the NARR data due to
  ERA40s coarse resolution.
• Snow depth varies greatly over small spatial
  areas
• E.g. windward slopes vs. lee slopes on a mountain
  range

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Variables

• Snow Depth
• Units mm SWE (Snow Water Equivalent)
• Surface Convective Precipitation
• Units mm/day
• ENSO NPO indices
• Standardized Indices from NOAA-NCEP

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Spatial Domain

• North America
• Latitudes 20N-60N
• Longitudes 140W-100W
• Areas of highest variability are selected by
  taking the standard deviation maximums of the
  anomalies of snow depth and convective rainfall.

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Methodology

• Snow depth (mm SWE) April 1
• Representative of total winter snow accumulations
• Convective Precipitation (mm/day) JAS
• Climatologies/Anomalies of each variable
• Std. dev gt0.5 mm/day for July rainfall
• Indices Area-averaged anomalies within selected
  areas divided by one standard deviation.
• Statistical Analysis Correlations/Regressions

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SWE Climatology Variability April
CAN2
CAN1
US1
US2
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Precipitation Climatology
12
Precipitation Variability (areas are fixed
throughout JAS)
North Zone
South Zone
North South Zones are fixed throughout JAS
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NARR SWE-Monsoon Correlation
July US 1 US 2 CAN 1 CAN 2
North Zone 0.19 0.29 -0.02 -0.13
South Zone 0.01 -0.15 -0.18 -0.34

Aug
North Zone 0.53 -0.08 0.05 -0.22
South Zone 0.30 0.03 -0.27 -0.20

Sept
North Zone 0.39 0.05 0.21 0.01
South Zone 0.55 -0.08 0.05 -0.09
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ENSO PDO Time Series
The correlation between ENSO PDO is r 0.79
15
Correlation Pacific SSTs SWE
Notice the opposite correlations of US vs.
Canadian Rockies
16
Regression Pacific SSTs on SWE
CAN2
CAN1
US1
US2
ENSO strongest influence on snowfall all over
the Rockies
17
Correlation Pacific SSTs July Precip
In the monsoon zones little or no correlation
for July, but
18
Correlation Pacific SSTs August Precip
significant positive correlations merely one
month later! remarkable
19
Correlation Pacific SSTs Sept Precip
Positive correlations still there
20
Regression Pacific SSTs on July Precip
Looking at the bars, ENSO has a stronger effect
on US precipitation than the PDO.
21
Regression Pacific SSTs on August Precip
A noticeable change from previous month moisture
associated with ENSO/PDO events surges northward
well into the US
22
Regression Pacific SSTs on Sept Precip
Influence from the PDO abates somewhat in
September, while that from ENSO stays to the
north throughout the last month of monsoon.
23
US1 SWE Regressed on JAS Precipitation
Similar pattern to the SWE-monsoon correlation
table no effect in July, then the linkage
becomes stronger.
24
US2 SWE Regressed on JAS Precipitation
Unlike US1, US2 not significantly linked with the
monsoon.
25
C1 SWE Regressed on JAS Precipitation
C1 shows inverse relationship with monsoon except
perhaps for Sept
26
C2 SWE Regressed on JAS Precipitation
C2 is an interesting case consistent inverse
relationship with monsoon opposite of US1!
27
Discussion

• Snow depth in the US Rockies 1 show strong and
  consistent positive correlation with the North
  American Monsoon.
• But the antecedent ENSO is also positively
  correlated with the snow depth in US1, and
  subsequently with the monsoon.
• ENSO ? SWE ? Monsoon
• ENSO ? Monsoon
• Negative correlation between monsoon and Canadian
  Rockies snow depth.
• Correlation/Regression figures of antecedent
  Pacific SSTs on Canadian Rockies SWE anomalies
  supports this with strong negative correlation
  also.

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Discussion

• These results indicate that Pacific SSTs may be
  one factor driving the monsoon.
• Appears unlikely that SWE is driving the monsoon.
• Possible Physical Explanation -- Suppose we have
  anomalously warm Pacific SSTs
• 1) Through increased evaporation rates overlying
  anomalously warm waters, this anomalously moist
  air eventually flows into the monsoon region
• 2) Then we have increased moisture flux
  convergence in the monsoon region -- moisture
  advection and increased thermodynamic instability
  atmos. column
• 3) Orographical effects may also force vertical
  motion and locally enhance rainfall

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Discussion

• Canadian Rockies SWE correlation with monsoon
  opposite of US Rockies SWE.
• And the effect of Pacific SSTs on the US Rockies
  vs. the Canadian Rockies also appears to be
  opposite of each other
• US Rockies SWE above normal in warm ENSO/PDO.
• Canadian SWE below normal in warm ENSO/PDO.
• This may explain the contradictory results
  between SWE and the monsoon they make sense
  when we consider that the Pacific SSTs are
  driving the monsoon, not SWE anomalies.
• However, this says nothing about the onset of the
  monsoon, of which the SWE anomalies may still
  influence via land-sea temperature gradients.

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Potential for Future Research

• The NAM-Pacific SST relationship may be further
  investigated to determine if the Pacific SSTs are
  stronger factors in driving the monsoon.
• Not just ENSO or PDO but also the Pacific Warm
  Pool, ENSO 12, ENSO3.4, MJO, etc
• The relationship between SWE and the monsoon may
  also be investigated on daily-weekly time scales
  to determine its influence on the onset of the
  monsoon.
• Other variables, such as total column
  precipitable water, soil moisture, and
  geopotential height may also be used to further
  investigate moisture flux convergence, wind
  strength/direction, and the effects of SWE and
  Pacific SSTs on the onset and strength of the
  monsoon.

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