Cool-Season Regime Transition and its Impact on Precipitation in the Northeastern United States Heather Archambault Daniel Keyser, Lance Bosart Department of Earth and Atmospheric Sciences, University at Albany, SUNY CSTAR Focal Point: Rich Grumm Central Pennsylvania Weather Forecast Office State College, PA NOAA Grant NA07WA0458

Research Motivation Is Northeast precipitation influenced by large-scale weather regimes?Is Northeast precipitation influenced by large-scale weather regimes? –Positive correlation between the phase of the North Atlantic Oscillation (NAO) and East Coast precipitation (Hurrell 1995) –Weak negative correlation between the phase of the Pacific/North American (PNA) pattern and Northeast precipitation (Leathers et al. 1991) –No prior studies on relationships between regime transitions and Northeast precipitation

Research Motivation Can synoptic-scale storms that produce significant precipitation in the Northeast play a role in large- scale weather regime transitions?Can synoptic-scale storms that produce significant precipitation in the Northeast play a role in large- scale weather regime transitions? –Synoptic-scale/planetary scale interactions linked to evolution of large- scale weather regimes (e.g., Colucci 1985; Dole 1986; Cash and Lee 2001) –Rossby wave-breaking attributed to development of both phases of the North Atlantic Oscillation (Franzke et al. 2003; Benedict et al. 2004)

Research Goals 1.Develop objective definitions for large-scale regimes and regime transitions 2.Establish statistical relationships between Northeast precipitation and large-scale regimes/large-scale regime transitions 3.Use composite analyses to explore these relationships 4.Determine whether Northeast storms/regime transition relationships are associative or cause and effect

Data and Methodology: Key Definitions Large-Scale Regimes: +1/−1 standard deviation (σ) anomalies of the NAO or PNALarge-Scale Regimes: +1/−1 standard deviation (σ) anomalies of the NAO or PNA Persistent Large-Scale Regimes: Regimes lasting seven daysPersistent Large-Scale Regimes: Regimes lasting seven days Large-Scale Regime Transitions: NAO or PNA index changes of 2σ centered on zero occurring over a seven-day periodLarge-Scale Regime Transitions: NAO or PNA index changes of 2σ centered on zero occurring over a seven-day period

Data and Methodology: Large-Scale Regimes − − − − Positive NAO Negative NAO 1000–500 hPa Thickness Anomalies: Winter Courtesy: Anantha Aiyyer & Eyad Atallah

− − − − Positive PNA Negative PNA Courtesy: Anantha Aiyyer & Eyad Atallah Data and Methodology: Large-Scale Regimes 1000–500 hPa Thickness Anomalies: Winter

Data and Methodology: Datasets Regimes: Following CDC method, calculated daily standardized NAO and PNA indices using 500 hPa geopotential heights from NCEP–NCAR reanalysis datasetRegimes: Following CDC method, calculated daily standardized NAO and PNA indices using 500 hPa geopotential heights from NCEP–NCAR reanalysis dataset –2.5° x 2.5° resolution, 1948–2003 Northeast Precipitation: Calculated domain- average daily Northeast precipitation from the NCEP Unified Precipitation DatasetNortheast Precipitation: Calculated domain- average daily Northeast precipitation from the NCEP Unified Precipitation Dataset –0.25° x 0.25° resolution, 1948–2003

Results: Relationships Between Large-Scale Regimes and Northeast Precipitation

Cool-Season Precipitation Anomalies During Persistent Large-Scale Regimes

Northeast precipitation is above normal during negative PNA and positive NAO regimesNortheast precipitation is above normal during negative PNA and positive NAO regimes Northeast precipitation is below normal during positive PNA regimesNortheast precipitation is below normal during positive PNA regimes

NAO, PNA Indices during Major Cool-Season Precipitation Events Are NAO and PNA indices typically positive or negative during major cool-season precipitation events in the Northeast?

NAO Index During Top 25 Cool-Season Precipitation Events

6 events > +1.0 NAO Index 5 events < −1.0 NAO Index NAO Index During Top 25 Cool-Season Precipitation Events

NAO/Precipitation Relationships Major precipitation events in the Northeast appear to occur independently of the phase of the NAOMajor precipitation events in the Northeast appear to occur independently of the phase of the NAO This finding is consistent with results showing that cool-season precipitation is marginally enhanced during persistent positive NAO regimesThis finding is consistent with results showing that cool-season precipitation is marginally enhanced during persistent positive NAO regimes

PNA Index During Top 25 Cool-Season Precipitation Events

7 events > +1.0 PNA Index 1 event < −1.0 PNA Index

PNA/Precipitation Relationships Major precipitation events in the Northeast appear to occur preferentially during the positive phase of the PNAMajor precipitation events in the Northeast appear to occur preferentially during the positive phase of the PNA Shown earlier: Cool-season precipitation is enhanced in the Northeast during persistent negative PNA regimes and suppressed during positive PNA regimesShown earlier: Cool-season precipitation is enhanced in the Northeast during persistent negative PNA regimes and suppressed during positive PNA regimes How are these results consistent?How are these results consistent?

PNA/Precipitation Relationships Contrast top cool-season precipitation events that occurred during strong +PNA regimes with top events that occurred during strong −PNA regimesContrast top cool-season precipitation events that occurred during strong +PNA regimes with top events that occurred during strong −PNA regimes (PNA > +2.0 vs. PNA +2.0 vs. PNA < −2.0) Use three types of composite analyses (centered at midpoints of events) that show:Use three types of composite analyses (centered at midpoints of events) that show: 1. The large-scale pattern 2. The upper-level jet structure 3. Dynamical forcing for ascent

Top Negative PNA Precipitation Events

Top 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) 500 hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 2.3 cm / 24 hComposite PNA: +2.3 std. dev.

Mean Northeast Precipitation: 1.5 cm / 24 h Composite Negative PNA Storms (Top 5) 300 hPa Height (dam), Wind Speed (m s -1 ) Composite PNA: −2.3 std. dev.

Composite Positive PNA Storms (Top 5) 300 hPa Height (dam), Wind Speed (m s -1 ) Mean Northeast Precipitation: 2.3 cm / 24 hComposite PNA: +2.3 std. dev.

Mean Northeast Precipitation: 1.5 cm / 24 hComposite PNA: −2.3 std. dev. Composite Negative PNA Storms (Top 5) 1000–500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Mean Northeast Precipitation: 2.3 cm / 24 hComposite PNA: +2.3 std. dev. Composite Positive PNA Storms (Top 5) 1000–500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

PNA/Precipitation Relationships Better dynamics for storms during +PNA regimes than −PNA regimesBetter dynamics for storms during +PNA regimes than −PNA regimes Why, then, is precipitation enhanced during persistent −PNA regimes?Why, then, is precipitation enhanced during persistent −PNA regimes? During −PNA regimes, above-normal southwesterly flow from the Gulf of Mexico may enhance Northeast precipitationDuring −PNA regimes, above-normal southwesterly flow from the Gulf of Mexico may enhance Northeast precipitation During +PNA regimes, the jet stream is suppressed to the south and typically allows cool, dry air to prevail over the Northeast (Leathers et al. 1991)During +PNA regimes, the jet stream is suppressed to the south and typically allows cool, dry air to prevail over the Northeast (Leathers et al. 1991)

Summary of Relationships between Large- Scale Regimes and Northeast Precipitation Positive NAO and negative PNA regimes are associated with enhanced Northeast precipitationPositive NAO and negative PNA regimes are associated with enhanced Northeast precipitation The PNA is typically positive during major precipitation events in the NortheastThe PNA is typically positive during major precipitation events in the Northeast Major precipitation events in the Northeast appear to occur independently of the phase of the NAOMajor precipitation events in the Northeast appear to occur independently of the phase of the NAO

Relationships Between Large-Scale Regime Transitions and Northeast Precipitation

Cool-Season Precipitation Anomalies During Large-Scale Regime Transitions

Above-normal Northeast precipitation: Transition periods from negative PNA to positive PNATransition periods from negative PNA to positive PNA Transition periods from positive NAO to negative NAOTransition periods from positive NAO to negative NAO Below-normal Northeast precipitation: Transition periods from positive PNA to negative PNATransition periods from positive PNA to negative PNA

NAO Index Trends During Major Cool-Season Precipitation Events How does the NAO behave during periods surrounding major cool-season precipitation events in the Northeast?

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

events > +1.0 NAO Tendency 8 events < − 1.0 NAO Tendency

Relationships between NAO Changes and Northeast Precipitation The phase of the NAO typically becomes more negative in the seven-day period surrounding a major precipitation event in the NortheastThe phase of the NAO typically becomes more negative in the seven-day period surrounding a major precipitation event in the Northeast This result is consistent with earlier results showing cool-season precipitation is enhanced in the Northeast during regime transitions from the positive to negative phase of the NAOThis result is consistent with earlier results showing cool-season precipitation is enhanced in the Northeast during regime transitions from the positive to negative phase of the NAO

1963 Top Precipitation Events Occurring During Negative NAO Transitions

1963 Top Precipitation Events Occurring During Negative NAO Transitions #3

Composite 500 hPa Heights, Anomalies During Transitions from +NAO to −NAO

Mean Northeast Precipitation: 0.4 cm / 24 hComposite NAO: +1.2 std. dev. Composite Negative NAO Transition: Day hPa Height (dam), Height Anomaly (dam)

Composite Negative NAO Transition: Day 2 ) 500 hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 0.2 cm / 24 hComposite NAO: +1.0 std. dev.

Composite Negative NAO Transition: Day hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 0.8 cm / 24 hComposite NAO: +0.9 std. dev.

Composite Negative NAO Transition: Day hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 2.0 cm / 24 hComposite NAO: +0.6 std. dev.

Composite Negative NAO Transition: Day hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 0.7 cm / 24 hComposite NAO: −0.3 std. dev.

Composite Negative NAO Transition: Day hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 0.1 cm / 24 hComposite NAO: −1.3 std. dev.

Composite Negative NAO Transition: Day hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 0.3 cm / 24 hComposite NAO: −1.5 std. dev.

Composite 300 hPa Heights, Wind Speeds During Transitions from +NAO to −NAO

Mean Northeast Precipitation: 0.4 cm / 24 hComposite NAO: +1.2 std. dev. Composite Negative NAO Transition: Day hPa Height (dam), Wind Speed (m s -1 )

Mean Northeast Precipitation: 0.2 cm / 24 hComposite NAO: +1.0 std. dev. Composite Negative NAO Transition: Day hPa Height (dam), Wind Speed (m s -1 )

Mean Northeast Precipitation: 0.8 cm / 24 hComposite NAO: +0.9 std. dev. Composite Negative NAO Transition: Day hPa Height (dam), Wind Speed (m s -1 )

Mean Northeast Precipitation: 2.0 cm / 24 hComposite NAO: +0.6 std. dev. Composite Negative NAO Transition: Day hPa Height (dam), Wind Speed (m s -1 )

Mean Northeast Precipitation: 0.7 cm / 24 hComposite NAO: −0.3 std. dev. Composite Negative NAO Transition: Day hPa Height (dam), Wind Speed (m s -1 )

Mean Northeast Precipitation: 0.1 cm / 24 hComposite NAO: −1.3 std. dev. Composite Negative NAO Transition: Day hPa Height (dam), Wind Speed (m s -1 )

Mean Northeast Precipitation: 0.3 cm / 24 hComposite NAO: −1.5 std. dev. Composite Negative NAO Transition: Day hPa Height (dam), Wind Speed (m s -1 )

1000–500 hPa Thickness, 700 hPa Absolute Vorticity, and Sea Level Pressure During Transitions from +NAO to −NAO

Composite Negative NAO Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa) Mean Northeast Precipitation: 0.4 cm / 24 hComposite NAO: +1.2 std. dev.

Mean Northeast Precipitation: 0.2 cm / 24 hComposite NAO: +1.0 std. dev. Composite Negative NAO Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Mean Northeast Precipitation: 0.8 cm / 24 hComposite NAO: +0.9 std. dev. Composite Negative NAO Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Mean Northeast Precipitation: 2.0 cm / 24 hComposite NAO: +0.6 std. dev. Composite Negative NAO Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Mean Northeast Precipitation: 0.7 cm / 24 hComposite NAO: −0.3 std. dev. Composite Negative NAO Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Mean Northeast Precipitation: 0.1 cm / 24 hComposite NAO: −1.3 std. dev. Composite Negative NAO Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Mean Northeast Precipitation: 0.3 cm / 24 hComposite NAO: −1.5 std. dev. Composite Negative NAO Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Negative NAO Regime Transitions Overall pattern amplification occurs during transition from positive NAO to negative NAO regimeOverall pattern amplification occurs during transition from positive NAO to negative NAO regime Warm air advection associated with surface cyclogenesis near the Northeast causes downstream high-latitude ridge eruption (blocking over the western North Atlantic)Warm air advection associated with surface cyclogenesis near the Northeast causes downstream high-latitude ridge eruption (blocking over the western North Atlantic)

PNA Index Trends During Major Cool-Season Precipitation Events How does the PNA pattern behave during periods surrounding major cool-season precipitation events in the Northeast?

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

8 events > +1.0 PNA Tendency 2 events < −1.0 PNA Tendency Seven-Day PNA Index Tendency During Top 25 Cool-Season Precipitation Events

Relationships between PNA Changes and Northeast Precipitation The phase of the PNA typically becomes more positive in the seven-day period surrounding a major precipitation event in the NortheastThe phase of the PNA typically becomes more positive in the seven-day period surrounding a major precipitation event in the Northeast This is consistent with earlier results showing cool-season precipitation is enhanced in the Northeast during regime transitions from the negative to positive phase of the PNAThis is consistent with earlier results showing cool-season precipitation is enhanced in the Northeast during regime transitions from the negative to positive phase of the PNA

Top Precipitation Events Occurring During Positive PNA Transitions

#14

Composite 500 hPa Heights, Anomalies During Transitions from −PNA to +PNA

Mean Northeast Precipitation: 0.3 cm / 24 hComposite PNA: −1.3 std. dev. Composite Positive PNA Transition: Day hPa Height (dam), Height Anomaly (dam)

Composite Positive PNA Transition: Day hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 0.2 cm / 24 hComposite PNA: −1.0 std. dev.

Composite Positive PNA Transition: Day hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 0.3 cm / 24 hComposite PNA: −0.3 std. dev.

Composite Positive PNA Transition: Day hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 1.6 cm / 24 hComposite PNA: +0.1 std. dev.

Composite Positive PNA Transition: Day hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 0.8 cm / 24 hComposite PNA: +0.4 std. dev.

Composite Positive PNA Transition: Day hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 0.3 cm / 24 hComposite PNA: +1.0 std. dev.

Composite Positive PNA Transition: Day hPa Height (dam), Height Anomaly (dam) Mean Northeast Precipitation: 0.2 cm / 24 hComposite PNA: +1.4 std. dev.

Composite 300 hPa Heights, Wind Speeds During Transitions from −PNA to +PNA

Composite Positive PNA Transition: Day hPa Height (dam), Wind Speed (m s -1 ) Mean Northeast Precipitation: 0.3 cm / 24 hComposite PNA: −1.3 std. dev.

Composite Positive PNA Transition: Day hPa Height (dam), Wind Speed (m s -1 ) Mean Northeast Precipitation: 0.2 cm / 24 hComposite PNA: −1.0 std. dev.

Composite Positive PNA Transition: Day hPa Height (dam), Wind Speed (m s -1 ) Mean Northeast Precipitation: 0.3 cm / 24 hComposite PNA: −0.3 std. dev.

Composite Positive PNA Transition: Day hPa Height (dam), Wind Speed (m s -1 ) Mean Northeast Precipitation: 1.6 cm / 24 hComposite PNA: +0.1 std. dev.

Composite Positive PNA Transition: Day hPa Height (dam), Wind Speed (m s -1 ) Mean Northeast Precipitation: 0.8 cm / 24 hComposite PNA: +0.4 std. dev.

Composite Positive PNA Transition: Day hPa Height (dam), Wind Speed (m s -1 ) Mean Northeast Precipitation: 0.3 cm / 24 hComposite PNA: +1.0 std. dev.

Composite Positive PNA Transition: Day hPa Height (dam), Wind Speed (m s -1 ) Mean Northeast Precipitation: 0.2 cm / 24 hComposite PNA: +1.4 std. dev.

1000–500 hPa Thickness, 700 hPa Absolute Vorticity, and Sea Level Pressure During Transitions from −PNA to +PNA

Mean Northeast Precipitation: 0.3 cm / 24 hComposite PNA: −1.3 std. dev. Composite Positive PNA Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Mean Northeast Precipitation: 0.2 cm / 24 hComposite PNA: −1.0 std. dev. Composite Positive PNA Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Mean Northeast Precipitation: 0.3 cm / 24 hComposite PNA: −0.3 std. dev. Composite Positive PNA Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Mean Northeast Precipitation: 1.6 cm / 24 hComposite PNA: +0.1 std. dev. Composite Positive PNA Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Mean Northeast Precipitation: 0.8 cm / 24 hComposite PNA: +0.4 std. dev. Composite Positive PNA Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Mean Northeast Precipitation: 0.3 cm / 24 hComposite PNA: +1.0 std. dev. Composite Positive PNA Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Mean Northeast Precipitation: 0.2 cm / 24 hComposite PNA: +1.4 std. dev. Composite Positive PNA Transition: Day –500 hPa Thick (dam), 700 hPa Abs. Vort. (10 -5 s -1 ), MSLP (hPa)

Positive PNA Regime Transitions Trough associated with mid-transition Northeast storm creates favorable conditions for deeper trough in eastern US by end of the periodTrough associated with mid-transition Northeast storm creates favorable conditions for deeper trough in eastern US by end of the period Amplification of the pattern from negative to positive PNA linked to warm air advection over western Canada during second half of the transition periodAmplification of the pattern from negative to positive PNA linked to warm air advection over western Canada during second half of the transition period

Conclusions Cool-season Northeast precipitation may be more influenced by NAO and PNA regime transitions than by NAO and PNA regimesCool-season Northeast precipitation may be more influenced by NAO and PNA regime transitions than by NAO and PNA regimes Transitions from positive to negative NAO regimes and from negative to positive PNA regimes are associated with enhanced Northeast precipitationTransitions from positive to negative NAO regimes and from negative to positive PNA regimes are associated with enhanced Northeast precipitation Composite analyses help confirm that synoptic-scale systems can play a critical role in large-scale regime transitionsComposite analyses help confirm that synoptic-scale systems can play a critical role in large-scale regime transitions

Future Work Study how Northeast precipitation is influenced by simultaneous or lagged PNA/NAO regime transitionsStudy how Northeast precipitation is influenced by simultaneous or lagged PNA/NAO regime transitions Investigate the predictability of large-scale regime transitionsInvestigate the predictability of large-scale regime transitions

Acknowledgements Lance Bosart and Dan KeyserLance Bosart and Dan Keyser Rich GrummRich Grumm Anantha AiyyerAnantha 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 students past and presentTom Galarneau, Joe Kravitz, and all grad students past and present My parentsMy parents