Cool-Season Regime Transition and its Impact on Northeast Precipitation Heather Archambault Heather Archambault Lance Bosart, Daniel Keyser, Anantha Aiyyer NWS Focal Point: Rich Grumm 3 November 2004 Department of Earth and Atmospheric Sciences, University at Albany, State University of New York

Research Motivation Meteorological wisdom: increased threat of a major storm during large–scale regime transitionMeteorological wisdom: increased threat of a major storm during large–scale regime transition Past research points to a connection between synoptic-scale cyclogenesis and reconfiguration of the planetary-scale flow (e.g. Colucci 1985)Past research points to a connection between synoptic-scale cyclogenesis and reconfiguration of the planetary-scale flow (e.g. Colucci 1985) Dave Groenert (CSTAR, 2002) documented an apparent tendency for increased precipitation in the Northeast during phase transitions of the North Atlantic OscillationDave Groenert (CSTAR, 2002) documented an apparent tendency for increased precipitation in the Northeast during phase transitions of the North Atlantic Oscillation

Presentation Overview Research methodologyResearch methodology Statistical look at cool-season (Nov – Apr) weather regime transitions and Northeast precipitation anomaliesStatistical look at cool-season (Nov – Apr) weather regime transitions and Northeast precipitation anomalies Composite analyses of one kind of regime transitionComposite analyses of one kind of regime transition

4 weather regimes were defined using teleconnection indices:4 weather regimes were defined using teleconnection indices: + NAO– NAO + NAO– NAO + PNA – PNA + PNA – PNA Only extreme events were used (Index anomaly ≥ |1 standard deviation|)Only extreme events were used (Index anomaly ≥ |1 standard deviation|) Objectively Defining a “Weather Regime”

Courtesy: Anantha Aiyyer & Eyad Atallah Decameters 1000 – 500 hPa Thickness Anomaly Winters ( + NAO) n = 724

Courtesy: Anantha Aiyyer & Eyad Atallah Decameters 1000 – 500 hPa Thickness Anomaly Winters ( – NAO) n = 814

Courtesy: Anantha Aiyyer & Eyad Atallah Decameters 1000 – 500 hPa Thickness Anomaly Winters ( + PNA) n = 803

Courtesy: Anantha Aiyyer & Eyad Atallah Decameters 1000 – 500 hPa Thickness Anomaly Winters ( – PNA) n = 805

Defining a Weather Regime Change Regime Change Criteria: 1. A significant change in teleconnection index magnitude (|2 stdev|) over a 7 – day period 2. Teleconnection index phase change 3. Index at start of transition must be strongly positive or negative (|Index|>1 stdev)

Creating a Teleconnection Index Database Calculated daily NAO and PNA indices for a 56-year period (1948 – 2003) using formulas outlined by the Climate Diagnostics CenterCalculated daily NAO and PNA indices for a 56-year period (1948 – 2003) using formulas outlined by the Climate Diagnostics Center Dataset: Twice-daily 500 hPa geopotential heights from 2.5° x 2.5° NCAR/NCEP reanalysisDataset: Twice-daily 500 hPa geopotential heights from 2.5° x 2.5° NCAR/NCEP reanalysis Used this database to identify all cool- season regime transitionsUsed this database to identify all cool- season regime transitions

Creating a Northeast Precipitation Index 1.Using the Unified Precipitation Dataset, calculated domain-average daily precipitation over a 56-year period (1948 –2003) Domain:Domain: Grid size: 0.25° x 0.25°

Creating a Northeast Precipitation Index Example: Top 25 cool – season Northeast precipitation events

Creating a Northeast Precipitation Index 2.Used a simple square-root transformation to normalize the daily precipitation data: y = x Calculated 7-day running sum of precipitation for each day: Sum (d+0) = Precip(d-3) + … + Precip(d+3)

Creating a Northeast Precipitation Index 4. Calculated 7-day precipitation anomaly centered on each day in the time–series: Precipitation Anomaly = [Actual Value] – [Climo Value] [Actual Value] 7-day sum – [Climo Value] 7-day sum ––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– ––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– [Climatological Standard Deviation] [Climatological Standard Deviation] 7-day sum

The NAO and Northeast Precipitation

The phase of the NAO does not appear to be correlated with Northeast precipitationThe phase of the NAO does not appear to be correlated with Northeast precipitation The tendency of the NAO may be correlated with Northeast precipitationThe tendency of the NAO may be correlated with Northeast precipitation  18 of 25 top Northeast precipitation events: negative NAO tendencies  average NAO tendency: over 7 days Illustrates the motivation behind this researchIllustrates the motivation behind this research

The PNA and Northeast Precipitation

 17 of 25 events: positive PNA transitions  17 of 25 events: positive PNA transitions Top 20 Precip. Events – 500 hPa Heights

The PNA and Northeast Precipitation As with the NAO, tendency of the PNA may be more importantAs with the NAO, tendency of the PNA may be more important Top 25 Northeast Precipitation Events  Average PNA tendency: over 7 days  3 events involved PNA “regime changes”

Regime Change and Northeast Precipitation Looked at teleconnection indices during major Northeast precipitation eventsLooked at teleconnection indices during major Northeast precipitation events Now, another perspective:Now, another perspective: What is climatological Northeast precipitation anomaly during cool-season regime change?What is climatological Northeast precipitation anomaly during cool-season regime change?

NAO Regime Change and Precipitation

Signals:Signals:  “Negative” NAO regime transitions:  “Negative” NAO regime transitions: a somewhat wetter than normal period  strongest signal: late fall – mid-winter  strongest signal: late fall – mid-winter  a weakening North Atlantic jet may be associated with enhanced Northeast precipitation  “Positive” NAO regime transitions: a somewhat drier than normal period  a strengthening North Atlantic jet may be associated with drier weather in the Northeast  a strengthening North Atlantic jet may be associated with drier weather in the Northeast

PNA Regime Change and Precipitation

Signals:  “Positive” PNA regime transitions: in Fall and Spring, tend to have enhanced precip  Neither + nor – PNA transitions are associated with enhanced precipitation in Winter  “Negative” PNA regime transitions: a drier than normal period  associated with a building ridge in the East  associated with a building ridge in the East

Composite Analyses Goal: analyze forcing for ascent during negative NAO regime transitions occurring in November, December, and JanuaryGoal: analyze forcing for ascent during negative NAO regime transitions occurring in November, December, and January Only included regime transitions that were exactly 7 days long: 20 casesOnly included regime transitions that were exactly 7 days long: 20 cases  Analyses don’t contain the most extreme regime transitions  Analyses don’t contain the most extreme regime transitions

Composite Analyses Sutcliffe approximation of omega:Sutcliffe approximation of omega:  advection of vorticity by the thermal wind (shown by hPa thickness)  advection of vorticity by the thermal wind (shown by hPa thickness)

Composite Cases: Day -3: 500 hPa

Negative NAO Regime Change Composite Relatively weak mean offshore flowRelatively weak mean offshore flow Weak surface trough over NortheastWeak surface trough over Northeast Days +0 to +3: block develops in the eastern Atlantic and sharpens the northwesterly flow over the Northeast:Days +0 to +3: block develops in the eastern Atlantic and sharpens the northwesterly flow over the Northeast:  offshore flow shuts off precipitation *No “smoking gun” storm affecting the Northeast…in the mean*No “smoking gun” storm affecting the Northeast…in the mean

Conclusions Statistical Results: Negative NAO regime transition corresponds to enhanced precipitation in the Northeast from fall to mid-winterNegative NAO regime transition corresponds to enhanced precipitation in the Northeast from fall to mid-winter Positive PNA regime transition corresponds to enhanced precipitation in the Northeast during fall and springPositive PNA regime transition corresponds to enhanced precipitation in the Northeast during fall and spring d/dt |index|  more important than indexd/dt |index|  more important than index

Conclusions (cont.) Composite Analysis: Many different flavors to a negative NAOMany different flavors to a negative NAO No big storm over the Northeast in compositeNo big storm over the Northeast in composite

Caveats and Conclusions PNA and NAO used as first order approximations of weather regimes: clear limitations PNA and NAO used as first order approximations of weather regimes: clear limitations Advantage: strong correlation between observed NAO, PNA and 7 – 10 day MRF ensemble forecastsAdvantage: strong correlation between observed NAO, PNA and 7 – 10 day MRF ensemble forecasts  “Regime changes” are fairly well-predicted  Models forecasting a negative NAO/positive PNA transition should be a heads-up to forecasters