Variability of the North American Monsoon Associated with Preceding Winter’s Snowmass and Pacific SSTs Nabeel Keblawi METO 658N Dr. Sumant Nigam Dr. Alfredo.

Variability of the North American MonsoonAssociated with Preceding Winter’s Snowmass and Pacific SSTs

Nabeel KeblawiMETO 658NDr. Sumant NigamDr. Alfredo Ruiz-Barradas

Overview

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

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.

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.

Rationale

Given all these contradictions, there’s 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.

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 ERA40’s coarse resolution. Snow depth varies greatly over small spatial areas… E.g. windward slopes vs. lee slopes on a mountain

range

Variables

Snow Depth Units: mm SWE (Snow Water Equivalent)

Surface Convective Precipitation Units: mm/day

ENSO & NPO indices Standardized Indices from NOAA-NCEP

Spatial Domain

North America: Latitudes: 20°N-60°N Longitudes: 140°W-100°W

Areas of highest variability are selected by taking the standard deviation maximums of the anomalies of snow depth and convective rainfall.

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 >0.5 mm/day for July rainfall

Indices: Area-averaged anomalies within selected areas divided by one standard deviation.

Statistical Analysis: Correlations/Regressions

SWE Climatology & Variability: April

US#1

US#2

CAN#1

CAN#2

Precipitation Climatology

Precipitation Variability (areas are fixed throughout JAS)

North Zone

South Zone

North & South Zones are fixed throughout JAS

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

ENSO & PDO Time Series

The correlation between ENSO & PDO is r = 0.79

Correlation: Pacific SSTs & SWE

Notice the opposite correlations of US vs. Canadian Rockies

Regression: Pacific SSTs on SWE

US#1

US#2

CAN#1

CAN#2

ENSO – strongest influence on snowfall all over the Rockies

Correlation: Pacific SSTs & July Precip

In the monsoon zones: little or no correlation for July, but…

Correlation: Pacific SSTs & August Precip

… significant positive correlations merely one month later! remarkable

Correlation: Pacific SSTs & Sept Precip

Positive correlations still there…

Regression: Pacific SSTs on July Precip

Looking at the bars, ENSO has a stronger effect on US precipitation than the PDO.

Regression: Pacific SSTs on August Precip

A noticeable change from previous month; moisture associated with ENSO/PDO events surges northward well into the US…

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.

US1 SWE Regressed on JAS Precipitation

Similar pattern to the SWE-monsoon correlation table; no effect in July, then the linkage becomes stronger.

US2 SWE Regressed on JAS Precipitation

Unlike US1, US2 not significantly linked with the monsoon.

C1 SWE Regressed on JAS Precipitation

C1 shows inverse relationship with monsoon except perhaps for Sept…

C2 SWE Regressed on JAS Precipitation

C2 is an interesting case: consistent inverse relationship with monsoon… opposite of US1!

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.

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…

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.

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 1+2, 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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