cool-season regime transition and its impact on precipitation in the northeastern united states...

Cool-Season Regime Transition and its Cool-Season Regime Transition and its Impact on Precipitation in the Impact on Precipitation in the Northeastern United StatesNortheastern United States

Heather Archambault Heather Archambault

Daniel Keyser, Lance BosartDaniel Keyser, Lance BosartDepartment of Earth and Atmospheric Sciences, Department of Earth and Atmospheric Sciences,

University at Albany, SUNYUniversity at Albany, SUNY

CSTAR Focal Point: Rich GrummCSTAR Focal Point: Rich GrummCentral Pennsylvania Weather Forecast OfficeCentral Pennsylvania Weather Forecast Office

State College, PAState College, PA

NOAA Grant NA07WA0458NOAA Grant NA07WA0458

Research MotivationResearch Motivation

• Is Northeast precipitation influenced Is Northeast precipitation influenced by large-scale weather regimes?by large-scale weather regimes?

– Positive correlation between the phase of Positive correlation between the phase of the North Atlantic Oscillation (NAO) and the North Atlantic Oscillation (NAO) and East Coast precipitation (Hurrell 1995) East Coast precipitation (Hurrell 1995)

– Weak negative correlation between the Weak negative correlation between the phase of the Pacific/North American phase of the Pacific/North American (PNA) pattern and Northeast precipitation (PNA) pattern and Northeast precipitation (Leathers et al. 1991)(Leathers et al. 1991)

– No prior studies on relationships between No prior studies on relationships between regime transitions and Northeast regime transitions and Northeast precipitationprecipitation

Research MotivationResearch Motivation

• Can synoptic-scale storms that Can synoptic-scale storms that produce significant precipitation in produce significant precipitation in the Northeast play a role in large-the Northeast play a role in large-scale weather regime transitions?scale weather regime transitions?– Synoptic-scale/planetary scale Synoptic-scale/planetary scale

interactions linked to evolution of large-interactions linked to evolution of large-scale weather regimes (e.g., Colucci scale weather regimes (e.g., Colucci 1985; Dole 1986; Cash and Lee 2001)1985; Dole 1986; Cash and Lee 2001)

– Rossby wave-breaking attributed to Rossby wave-breaking attributed to development of both phases of the North development of both phases of the North Atlantic Oscillation (Franzke et al. 2003; Atlantic Oscillation (Franzke et al. 2003; Benedict et al. 2004)Benedict et al. 2004)

Research GoalsResearch Goals

1.1. Develop objective definitions for large-scale Develop objective definitions for large-scale regimes and regime transitionsregimes and regime transitions

2.2. Establish statistical relationships between Establish statistical relationships between Northeast precipitation and large-scale Northeast precipitation and large-scale regimes/large-scale regime transitions regimes/large-scale regime transitions

3.3. Use composite analyses to explore these Use composite analyses to explore these relationshipsrelationships

4.4. Determine whether Northeast storms/regime Determine whether Northeast storms/regime transition relationships are associative or transition relationships are associative or cause and effectcause and effect

Data and Methodology: Key DefinitionsData and Methodology: Key Definitions

• Large-Scale RegimesLarge-Scale Regimes: +1/: +1/−1 standard −1 standard deviation (deviation (σσ) anomalies of the NAO or PNA) anomalies of the NAO or PNA

• Persistent Large-Scale Regimes:Persistent Large-Scale Regimes: Regimes Regimes lasting seven dayslasting seven days

• Large-Scale Regime Transitions:Large-Scale Regime Transitions: NAO or NAO or PNA index changes of 2PNA index changes of 2σσ centered on zero centered on zero occurring over a seven-day periodoccurring over a seven-day period

Data and Methodology: Large-Scale RegimesData and Methodology: Large-Scale Regimes

+

+

+

+

−

−

−

−

Positive NAOPositive NAO Negative NAONegative NAO

10001000––500 hPa Thickness Anomalies: Winter500 hPa Thickness Anomalies: Winter

Courtesy: Anantha Aiyyer & Eyad Atallah

+

++

+ −−

−

−

Positive PNAPositive PNA Negative PNANegative PNA

Courtesy: Anantha Aiyyer & Eyad Atallah

Data and Methodology: Large-Scale RegimesData and Methodology: Large-Scale Regimes

10001000––500 hPa Thickness Anomalies: Winter500 hPa Thickness Anomalies: Winter

Data and Methodology: DatasetsData and Methodology: Datasets

• RegimesRegimes: Following CDC method, calculated : Following CDC method, calculated daily standardized NAO and PNA indices daily standardized NAO and PNA indices using 500 hPa geopotential heights from using 500 hPa geopotential heights from NCEPNCEP––NCAR reanalysis datasetNCAR reanalysis dataset

– 2.52.5°° x 2.5° resolution, x 2.5° resolution, 19481948–2003–2003

• Northeast PrecipitationNortheast Precipitation: Calculated domain-: Calculated domain-average daily Northeast precipitation from average daily Northeast precipitation from the NCEP Unified Precipitation Datasetthe NCEP Unified Precipitation Dataset

– 0.250.25°° x 0.25° resolution, x 0.25° resolution, 19481948–2003–2003

Results:Results:

Relationships Between Large-Scale Relationships Between Large-Scale Regimes and Northeast PrecipitationRegimes and Northeast Precipitation

Cool-Season Precipitation Anomalies Cool-Season Precipitation Anomalies During Persistent Large-Scale RegimesDuring Persistent Large-Scale Regimes

Cool-Season Precipitation Anomalies Cool-Season Precipitation Anomalies During Persistent Large-Scale RegimesDuring Persistent Large-Scale Regimes

• Northeast precipitation is above normal during Northeast precipitation is above normal during negative PNA and positive NAO regimesnegative PNA and positive NAO regimes

• Northeast precipitation is below normal during Northeast precipitation is below normal during positive PNA regimespositive PNA regimes

NAO, PNA Indices during NAO, PNA Indices during Major Cool-Season Precipitation EventsMajor Cool-Season Precipitation Events

Are NAO and PNA indices typically positive or Are NAO and PNA indices typically positive or negative during major cool-season negative during major cool-season precipitation events in the Northeast?precipitation events in the Northeast?

NAO Index During Top 25 NAO Index During Top 25 Cool-Season Precipitation EventsCool-Season Precipitation Events

6 events > +1.0 NAO Index

5 events < −1.0 NAO Index

NAO Index During Top 25 NAO Index During Top 25 Cool-Season Precipitation EventsCool-Season Precipitation Events

28 13 16

2517

7 11

12

15 20

NAO/Precipitation RelationshipsNAO/Precipitation Relationships

• Major precipitation events in the Northeast Major precipitation events in the Northeast appear to occur independently of the phase of appear to occur independently of the phase of the NAOthe NAO

• This finding is consistent with results showing This finding is consistent with results showing that cool-season precipitation is marginally that cool-season precipitation is marginally enhanced during persistent positive NAO enhanced during persistent positive NAO regimesregimes

PNA Index During Top 25 PNA Index During Top 25 Cool-Season Precipitation EventsCool-Season Precipitation Events

PNA Index During Top 25 PNA Index During Top 25 Cool-Season Precipitation EventsCool-Season Precipitation Events

7 events > +1.0 PNA Index

1 event < −1.0 PNA Index

2

4

513

20 22

2425

PNA/Precipitation RelationshipsPNA/Precipitation Relationships

• Major precipitation events in the Northeast Major precipitation events in the Northeast appear to occur preferentially during the appear to occur preferentially during the positivepositive phase of the PNA phase of the PNA

• Shown earlier: Cool-season precipitation is Shown earlier: Cool-season precipitation is enhanced in the Northeast during persistent enhanced in the Northeast during persistent negative negative PNA regimes and suppressed during PNA regimes and suppressed during positive PNA regimespositive PNA regimes

• How are these results consistent?How are these results consistent?


• Contrast top cool-season precipitation events Contrast top cool-season precipitation events that occurred during strong +PNA regimes with that occurred during strong +PNA regimes with top events that occurred during strong top events that occurred during strong −−PNA PNA regimes regimes

(PNA > +2.0 vs. PNA < (PNA > +2.0 vs. PNA < −2.0)−2.0)

• Use three types of composite analyses Use three types of composite analyses (centered at midpoints of events) that show:(centered at midpoints of events) that show:1. The large-scale pattern1. The large-scale pattern2. The upper-level jet structure2. The upper-level jet structure3. Dynamical forcing for ascent 3. Dynamical forcing for ascent

Top Negative PNA Precipitation EventsTop Negative PNA Precipitation Events

Top Positive PNA Precipitation EventsTop Positive PNA Precipitation Events

Top Positive PNA Precipitation EventsTop Positive PNA Precipitation Events

#2

Mean Northeast Precipitation: 1.5 cm / 24 h

Composite Negative PNA Storms (Top 5)

500 hPa Height (dam), Height Anomaly (dam)

Composite PNA: −2.3 std. dev.

Composite Positive PNA Storms (Top 5)


Mean Northeast Precipitation: 2.3 cm / 24 h Composite PNA: +2.3 std. dev.

Mean Northeast Precipitation: 1.5 cm / 24 h


300 hPa Height (dam), Wind Speed (m s-1)

Composite PNA: −2.3 std. dev.

Mean Northeast Precipitation: 1.5 cm / 24 h Composite PNA: −2.3 std. dev.


1000–500 hPa Thick (dam), 700 hPa Abs. Vort. (10-5 s-1), MSLP (hPa)


• Better dynamics for storms during +PNA Better dynamics for storms during +PNA regimes than regimes than −−PNA regimes PNA regimes

• Why, then, is precipitation enhanced during Why, then, is precipitation enhanced during persistent persistent −−PNA regimes?PNA regimes?

• During −PNA regimes, above-normal During −PNA regimes, above-normal southwesterly flow from the Gulf of Mexico southwesterly flow from the Gulf of Mexico may enhance Northeast precipitationmay enhance Northeast precipitation

• During +PNA regimes, the jet stream is During +PNA regimes, the jet stream is suppressed to the south and typically allows suppressed to the south and typically allows cool, dry air to prevail over the Northeast cool, dry air to prevail over the Northeast (Leathers et al. 1991) (Leathers et al. 1991)

Summary of Relationships between Large-Summary of Relationships between Large-Scale Regimes and Northeast PrecipitationScale Regimes and Northeast Precipitation

• Positive NAO and negative PNA regimes are Positive NAO and negative PNA regimes are associated with enhanced Northeast associated with enhanced Northeast precipitationprecipitation

• The PNA is typically positive during The PNA is typically positive during major major precipitation events in the Northeastprecipitation events in the Northeast

• Major precipitation events in the Northeast Major precipitation events in the Northeast appear to occur independently of the phase of appear to occur independently of the phase of the NAOthe NAO

Relationships Between Large-Scale Regime Relationships Between Large-Scale Regime Transitions and Northeast PrecipitationTransitions and Northeast Precipitation

Cool-Season Precipitation Anomalies Cool-Season Precipitation Anomalies During Large-Scale Regime TransitionsDuring Large-Scale Regime Transitions

Cool-Season Precipitation Anomalies During Cool-Season Precipitation Anomalies During Large-Scale Regime TransitionsLarge-Scale Regime Transitions

Above-normal Northeast precipitation:Above-normal Northeast precipitation:• Transition periods from negative PNA to Transition periods from negative PNA to

positive PNApositive PNA• Transition periods from positive NAO to Transition periods from positive NAO to

negative NAOnegative NAO

Below-normal Northeast precipitation:Below-normal Northeast precipitation:• Transition periods from positive PNA to Transition periods from positive PNA to

negative PNAnegative PNA

NAO Index Trends During Major NAO Index Trends During Major Cool-Season Precipitation EventsCool-Season Precipitation Events

How does the NAO behaveHow does the NAO behave during periods during periods surrounding major cool-season precipitation surrounding major cool-season precipitation events in the Northeast?events in the Northeast?

Seven-Day NAO Index Tendency During Seven-Day NAO Index Tendency During Top 25 Cool-Season Precipitation EventsTop 25 Cool-Season Precipitation Events

Seven-Day NAO Index Tendency During Seven-Day NAO Index Tendency During Top 25 Cool-Season Precipitation EventsTop 25 Cool-Season Precipitation Events

11

16

21 25

23

22

1514

13

12

9

3

4 events > +1.0 NAO Tendency

8 events < − 1.0 NAO Tendency

Relationships between NAO Changes Relationships between NAO Changes and Northeast Precipitationand Northeast Precipitation

• The phase of the NAO typically becomes more The phase of the NAO typically becomes more negative in the seven-day period surrounding negative in the seven-day period surrounding a major precipitation event in the Northeasta major precipitation event in the Northeast

• This result is consistent with earlier results This result is consistent with earlier results showing cool-season precipitation is enhanced showing cool-season precipitation is enhanced in the Northeast during regime transitions from in the Northeast during regime transitions from the positive to negative phase of the NAOthe positive to negative phase of the NAO

196319631963

Top Precipitation Events Occurring Top Precipitation Events Occurring During Negative NAO TransitionsDuring Negative NAO Transitions

196319631963

Top Precipitation Events Occurring Top Precipitation Events Occurring During Negative NAO TransitionsDuring Negative NAO Transitions

#3

Composite 500 hPa Heights, Anomalies Composite 500 hPa Heights, Anomalies

During Transitions from +NAO to During Transitions from +NAO to −NAO−NAO

Mean Northeast Precipitation: 0.4 cm / 24 h Composite NAO: +1.2 std. dev.

Composite Negative NAO Transition: Day 1



500 hPa Height (dam), Height Anomaly (dam))




Mean Northeast Precipitation: 0.7 cm / 24 h Composite NAO: −0.3 std. dev.

Composite 300 hPa Heights, Wind SpeedsComposite 300 hPa Heights, Wind Speeds


10001000–500 hPa Thickness–500 hPa Thickness,,

700 hPa Absolute Vorticity, and700 hPa Absolute Vorticity, and

Sea Level PressureSea Level Pressure


Negative NAO Regime TransitionsNegative NAO Regime Transitions

• Overall pattern amplification occurs during Overall pattern amplification occurs during transition from positive NAO to negative NAO transition from positive NAO to negative NAO regimeregime

• Warm air advection associated with surface Warm air advection associated with surface cyclogenesis near the Northeast causes cyclogenesis near the Northeast causes downstream high-latitude ridge eruption downstream high-latitude ridge eruption (blocking over the western North Atlantic)(blocking over the western North Atlantic)

PNA Index Trends During Major PNA Index Trends During Major Cool-Season Precipitation EventsCool-Season Precipitation Events

How does the PNA pattern behaveHow does the PNA pattern behave during during periods surrounding major cool-season periods surrounding major cool-season precipitation events in the Northeast?precipitation events in the Northeast?

Seven-Day PNA Index Tendency During Seven-Day PNA Index Tendency During Top 25 Cool-Season Precipitation EventsTop 25 Cool-Season Precipitation Events

8 events > +1.0 PNA Tendency

2 events < −1.0 PNA Tendency

Seven-Day PNA Index Tendency During Seven-Day PNA Index Tendency During Top 25 Cool-Season Precipitation EventsTop 25 Cool-Season Precipitation Events

39

14

15

18

20

2224

5 8

Relationships between PNA Changes Relationships between PNA Changes and Northeast Precipitationand Northeast Precipitation

• The phase of the PNA typically becomes more The phase of the PNA typically becomes more positive in the seven-day period surrounding a positive in the seven-day period surrounding a major precipitation event in the Northeastmajor precipitation event in the Northeast

• This is consistent with earlier results showing This is consistent with earlier results showing cool-season precipitation is enhanced in the cool-season precipitation is enhanced in the Northeast during regime transitions from the Northeast during regime transitions from the negative to positive phase of the PNAnegative to positive phase of the PNA

Top Precipitation Events Occurring Top Precipitation Events Occurring During Positive PNA TransitionsDuring Positive PNA Transitions

Top Precipitation Events Occurring Top Precipitation Events Occurring During Positive PNA TransitionsDuring Positive PNA Transitions

#14

Composite 500 hPa Heights, Anomalies Composite 500 hPa Heights, Anomalies

During Transitions from During Transitions from −−PNA to PNA to +PNA+PNA


Composite Positive PNA Transition: Day 1


Composite 300 hPa Heights, Wind SpeedsComposite 300 hPa Heights, Wind Speeds

During Transitions from During Transitions from −PNA to +PNA−PNA to +PNA

10001000–500 hPa Thickness–500 hPa Thickness,,

700 hPa Absolute Vorticity, and700 hPa Absolute Vorticity, and

Sea Level PressureSea Level Pressure

During Transitions from During Transitions from −PNA to +PNA−PNA to +PNA

Positive PNA Regime TransitionsPositive PNA Regime Transitions

• Trough associated with mid-transition Trough associated with mid-transition Northeast storm creates favorable conditions Northeast storm creates favorable conditions for deeper trough in eastern US by end of the for deeper trough in eastern US by end of the periodperiod

• Amplification of the pattern from negative to Amplification of the pattern from negative to positive PNA linked to warm air advection over positive PNA linked to warm air advection over western Canada during second half of the western Canada during second half of the transition periodtransition period

ConclusionsConclusions

• Cool-season Northeast precipitation may be Cool-season Northeast precipitation may be more influenced by NAO and PNA regime more influenced by NAO and PNA regime transitionstransitions than by NAO and PNA regimes than by NAO and PNA regimes

• Transitions from positive to negative NAO Transitions from positive to negative NAO regimes and from negative to positive PNA regimes and from negative to positive PNA regimes are associated with enhanced regimes are associated with enhanced Northeast precipitationNortheast precipitation

• Composite analyses help confirm that Composite analyses help confirm that synoptic-scale systems can play a critical role synoptic-scale systems can play a critical role in large-scale regime transitionsin large-scale regime transitions

Future WorkFuture Work

• Study how Northeast precipitation is Study how Northeast precipitation is influenced by simultaneous or lagged influenced by simultaneous or lagged PNA/NAO regime transitionsPNA/NAO regime transitions

• Investigate the predictability of large-scale Investigate the predictability of large-scale regime transitionsregime transitions

AcknowledgementsAcknowledgements

• Lance Bosart and Dan KeyserLance Bosart and Dan Keyser• Rich GrummRich Grumm• Anantha Aiyyer Anantha Aiyyer • Alicia WasulaAlicia Wasula• Kevin Tyle and David KnightKevin Tyle and David Knight• Celeste, Sharon, Diana, Sally, and LynnCeleste, Sharon, Diana, Sally, and Lynn• Scott RunyonScott Runyon• Tom Galarneau, Joe Kravitz, and all grad Tom Galarneau, Joe Kravitz, and all grad

students past and presentstudents past and present• My parentsMy parents

cool-season regime transition and its impact on precipitation in the northeastern united states...

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