Influence of the Pacific Decadal Oscillation on Winter ... - AMS Journals

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WEATHER AND FORECASTING

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Influence of the Pacific Decadal Oscillation on Winter Precipitation and Drought during Years of Neutral ENSO in the Western United States GREGORY B. GOODRICH Department of Geography and Geology, Western Kentucky University, Bowling Green, Kentucky (Manuscript received 3 February 2006, in final form 27 June 2006) ABSTRACT The influence of the Pacific decadal oscillation (PDO) on important hydroclimatic variables during years of neutral ENSO for 84 climate divisions in the western United States is analyzed from 1925 to 1998. When the 34 neutral ENSO years are split by cold (12 yr) and warm (22 yr) PDOs, the resulting winter precipitation patterns are spatially similar to those that occur during years of La Niña–cold PDO and, to a lesser extent, years of El Niño–warm PDO, respectively, although the characteristic ENSO dipole is not evident. The PDO influence is similar when the winter Palmer drought severity index is analyzed, although the core area of influence moves from the Southwest to the northern Rockies. Correlations between Niño-3.4 SSTs and the hydroclimatic variables reverse sign when the neutral ENSO years are split by PDO phase. The greatest difference between correlations occurs in the characteristic dipole between the Pacific Northwest and the desert Southwest. Since seasonal forecast guidance based on ENSO conditions in the tropical Pacific often yields a forecast of “equal chances” during years of neutral ENSO, forecasters may be able to improve their forecasts for the southwestern United States depending on if the PDO is known to be in the cold (drier than normal) or warm (wetter than normal) phase. However, this can be difficult to implement considering the current uncertainty of the phase of the PDO.

1. Introduction Understanding the hydroclimatic complexity of the western United States has become an important area of research in light of recent drought, climate projections that suggest increasing warmth, and a fast-growing population (Barnett et al. 2004). The relatively wellunderstood relationship between the El Niño–Southern Oscillation (ENSO) and winter precipitation in the West has allowed for the development of seasonal climate models that allow fairly accurate seasonal forecasts of several months (Barnston et al. 1999). El Niño events that occur during the late-summer/fall are associated with dry winters in the Pacific Northwest and wet winters in the Southwest while La Niña events are associated with the opposite precipitation distribution (Ropelewski and Halpert 1986). Areas in between have weak and variable relationships with ENSO that have

Corresponding author address: Gregory B. Goodrich, Dept. of Geography and Geology, Western Kentucky University, No. 31066, 1906 College Heights Blvd., Bowling Green, KY 421011066. E-mail: [email protected] DOI: 10.1175/WAF983.1 © 2007 American Meteorological Society

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been shown to shift over time (Brown and Comrie 2004). Complicating matters in the late 1990s was the identification of multidecadal variability of ENSO impacts on winter precipitation due to low-frequency changes in sea surface temperatures (SSTs) in the North Pacific Ocean (Gershunov and Barnett 1998; Gutzler et al. 2002). This Pacific decadal oscillation (PDO), which has warm and cold phases that last up to 30 yr, has similar influences on precipitation patterns as does ENSO (Mantua et al. 1997). Gershunov and Barnett (1998) were the first to show that when PDO and ENSO are in phase (El Niño–warm PDO, La Niña– cold PDO), the ENSO climate signal is stronger and more stable with regard to winter precipitation in the western United States. Out-of-phase relationships between PDO and ENSO (El Niño–cold PDO, La Niña– warm PDO) have a weaker climate signal. McCabe and Dettinger (1999) showed that correlations between climate division precipitation in the western United States and ENSO have changed in the last century mostly due to changes in PDO phase. While the dynamics of ENSO have been well known for some time, there is still much debate as to the underlying mechanics of the PDO as

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well as the complexity of its relationship to ENSO. Miller and Schneider (2000) outline many of the possible options for multidecadal variability in the North Pacific Ocean, which range from ocean–atmosphere feedback loops to tropical–extratropical interactions. More recently, it has been suggested that ENSO may be the forcing mechanism for the PDO (Gedalof et al. 2002; Newman et al. 2003). Regardless of the relationship between the PDO and ENSO, it has been well established that the usefulness of ENSO as a seasonal predictive tool does depend on the phase of the PDO. While the PDO influence on high-index events such as El Niño and La Niña have been analyzed many times (Gershunov and Barnett 1998; Gutzler et al. 2002), the low-index events, or times of neutral ENSO, have not been studied in as much detail. Unfortunately, while strong ENSO events like the 1998 El Niño provide optimal forecast skill (Barnston et al. 1999), current seasonal forecast models offer less guidance during lowindex events, which often leads to seasonal forecasts not much different from climatological averages. Goodrich (2004) examined the influence of PDO on Arizona winter precipitation during times of neutral ENSO. It was found that Arizona winter precipitation and its predictability are significantly influenced by PDO phase. Winters that follow late-summer and fall neutral ENSO–cold PDO conditions are nearly as dry as winters that follow late-summer and fall La Niña–cold PDO conditions and actually provide greater predictability using skill scores. Neutral ENSO–warm PDO conditions are associated with wet winters but are of a lesser magnitude than El Niño–warm PDO. Since latesummer and fall neutral ENSO conditions occur nearly half of the time, this additional forecast skill could prove useful to forecasters making winter-season forecasts during the fall. What remains unanswered is the strength and spatial scale of the influence of PDO on winter precipitation during years of neutral ENSO for the remainder of the western United States. Also unknown is whether this low-index PDO influence can be found in the drought signal. While drought and precipitation are well correlated, some drought indices such as the Palmer drought severity index (PDSI; Palmer 1965) utilize the output side (evapotranspiration, runoff, etc.) of the water balance equation in addition to the input side (precipitation), which may alter the PDO influence. The objective of this research is to analyze the influence of the PDO on precipitation and PDSI during years of neutral ENSO in the western United States from 1925 to 1998. A number of statistical techniques will be used to determine the strength and spatial variability of neutral ENSO modulation. The varying na-

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ture of correlations between ENSO and precipitation during different phases of the PDO will also be examined. A primary goal of this study is to expand the initial findings of Goodrich (2004) in order to improve seasonal forecasts during years of neutral ENSO.

2. Data a. Precipitation data Time series of summed winter precipitation and average winter PDSI from 84 climate divisions in 11 western states represent the hydroclimatic variables used in this study. To follow along with the earlier works of Gutzler et al. (2002) and Goodrich (2004), winter comprises the months of December–March (DJFM), and the year associated with each winter is that of the JFM period. Climate division data are used largely because of the availability of relatively long-term time series and for comparative purposes to Goodrich (2004). Data from each of the climate divisions represents a simple unweighted average from all representative stations within that division (Guttman and Quayle 1996). A regression technique based on available U.S. Department of Agriculture (USDA) statewide averages was used to create the climate division data from 1895 to 1930, resulting in reduced variance for these years (Guttman and Quayle 1996). Since there has been debate (Keim et al. 2003) over the quality of the 1895–1930 climate division data, the analysis of precipitation and PDSI starts in 1925 to minimize the use of pre-1931 data while capturing the beginning of the 1925–46 warm phase of the PDO. While changes in station location over time in each climate division have the potential to create inhomogeneity issues, Gutzler et al. (2002) show that most individual stations in the Southwest have long-term patterns that are very similar to their respective climate divisions. The time series of precipitation and PDSI used in this study are available from the National Climatic Data Center (NCDC). The PDSI was the first drought index to use both sides of the water balance equation and remains one of the most comprehensive drought indexes available. The PDSI is also one of the main drought variables used by the weekly Drought Monitor, a collaborative effort between the National Drought Mitigation Center, the U.S. Department of Agriculture, and the National Oceanic and Atmospheric Administration (NOAA) (Svoboda et al. 2002). The index varies roughly between ⫺6.0 (exceptionally dry) and ⫹6.0 (exceptionally wet), and while the index can go higher, most values are between ⫺4.0 and ⫹4.0. See Heim (2002) for a detailed analysis regarding the creation of the PDSI. Limitations of the PDSI are described in detail by Alley (1984),

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Karl and Knight (1985), and Guttman (1991). Both the precipitation and PDSI time series are available online (http://www.cdc.noaa.gov/Timeseries/). Since some of the statistical techniques used in this study assume that the data are normally distributed (an approximate Gaussian distribution), a square root transformation was used to normalize the precipitation data.

b. ENSO–PDO time series SST data from the Niño-3.4 region (5°N–5°S, 120°– 170°W) are used to represent ENSO. The NOAA method to determine various ENSO events states that when the 3-month moving average of Niño-3.4 anomalies exceeds ⫹0.5 (⫺0.5) for three consecutive months, an El Niño (La Niña) event is said to occur. All other periods are considered neutral ENSO. It has been shown that the strongest lagged relationship between Niño-3.4 SSTs and winter precipitation in the western United States occurs from September to November (Harshburger et al. 2002). Therefore, if the center of the 3-month moving average designating the ENSO event occurred during any part of the September– November period, the following winter was classified accordingly. Using this method, there are 33 neutral ENSO years, 23 El Niño years, and 18 La Niña years during the 74-yr period of record. The dataset used in this study is the Kaplan extended Niño-3.4 dataset (Kaplan et al. 1998) and was obtained from the International Research Institute for Climate prediction (IRI) data library (available online at http://iridl.ldeo.columbia. edu/SOURCES/.Indices/.nino/.EXTENDED/. NINO34/). A small adjustment was made to the time series to change the base period climatology from 1951– 1980 to 1971–2000. The Pacific decadal oscillation (PDO) characterizes low-frequency changes in the SST field in the Pacific Ocean with a period of approximately 50 yr. The PDO index is the leading principal component or eigenvector of the mean monthly SSTs in the Pacific Ocean north of 20°N (Mantua et al. 1997). Positive values of the index are associated with above normal SSTs along the west coast of North America and below normal SSTs in the central and western North Pacific around 45°N. During the positive phase of the PDO, the Aleutian low strengthens and winter precipitation increases in the southwestern United States (Mantua et al. 1997). Since the late 1890s, there have only been two complete cycles of the PDO (Mantua and Hare 2002). The cold phase of the PDO persisted from 1890 to 1924 and from 1947 to 1976 while the warm phase occurred from 1925 to 1946 and from 1977 through at least the late 1990s. While several earlier works (Hare and Mantua 2000; Schwing and Moore 2000) have suggested there may

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TABLE 1. List of ENSO–PDO combinations from 1925 to 1998: L, La Niña; E, El Niño; N, neutral ENSO; W, warm PDO; and C, cold PDO.

192x 193x 194x 195x 196x 197x 198x 199x

0

1

2

3

4

5

6

7

8

9

EW NW LC NC EC NW NW

EW EW LC NC LC NW EW

NW EW EC NC LC NW EW

NW LW NC NC EC EW NW

LW NW EC EC LC NW EW

LW NW LW NC LC LC LW EW

EW NW NW LC EC LC NW LW

NW NW NC LC NC EW EW NW

NW NW NC EC NC EW EW EW

NW LW NC NC EC NW LW

El Niño–warm PDO (15 yr), El Niño–cold PDO (8 yr), La Niña– warm PDO (8 yr), La Niña–cold PDO (10 yr), and Neutral–warm PDO (21 yr), neutral–cold PDO (12 yr).

have been a return to a cold phase at the end of the 1997–98 El Niño, the current phase of the PDO is uncertain as the index has displayed greater interannual variability than usual since that time. The uncertainty in the current multidecadal phase of the PDO will limit the analysis through the winter of 1998. The PDO dataset used in this study was obtained from the Joint Institute for the Study of Atmosphere and Oceans (JISAO) at the University of Washington (available online at http://jisao.washington.edu/pdo/PDO.latest).

3. Methodology and results a. Difference of means Regional variability in the strength of the ENSO signal on precipitation and PDSI is well documented by Hidalgo and Dracup (2003) and Brown and Comrie (2004), among others. These regional differences can be useful when interpreting results from regional climate simulations as described in Barnett et al. (2004) and Christensen et al. (2004). To discuss these regionalscale differences in this study, the 74 yr from 1925 to 1998 were first stratified into six PDO–ENSO combinations (Table 1). The percentage deviation from normal winter precipitation and PDSI is shown in Figs. 1 and 2. Rather than using a 30-yr average to denote “normal” in this study, the entire 74-yr period was used to eliminate any temporal bias from the PDO. Table 2 lists the average values for each of the six combinations along with the number of climate divisions that were greater or less than zero. As in Goodrich (2004), years of neutral ENSO during the cold phase of the PDO have the greatest drought signal of the six subcategories. Across the western United States, precipitation is 8.9% below normal and 68 of 84 (81%) climate divisions are drier than normal. For the PDSI, 61 of 84

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FIG. 1. Winter precipitation relative to normal for each ENSO–PDO combination: ⬎15% (darkest), 0% to 15%, ⫺15% to 0%, and ⬍⫺15% (lightest).

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FIG. 2. Winter PDSI for each ENSO–PDO combination: ⬎1.0 (darkest), 0 to 1.0, ⫺1.0 to 0, and ⬍⫺1.0 (lightest).

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TABLE 2. Average values for each ENSO–PDO combination for precipitation (relative to normal) and PDSI with number of climate divisions greater or less than zero (out of 84). Precipitation Phase

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Avg

El Niño–warm PDO 0.5% El Niño–cold PDO 1.8% La Niña–warm PDO ⫺4.1% La Niña–cold PDO ⫺2.3% Neutral–warm PDO 6.7% Neutral–cold PDO ⫺8.9%

⬎0 ⬍0 34 52 27 42 69 16

50 32 57 42 15 68

PDSI Avg ⫺0.04 0.28 ⫺0.02 0.28 0.07 ⫺0.45

⬎0 ⬍0

DA

40 58 35 52 47 23

1.3 4.5 ⫺0.1 ⫺0.4 1.3 ⫺6.4

44 26 49 32 37 61

DA: Drought area calculated by summing PDSI ⫻ area for each climate division divided by 105.

(73%) climate divisions have an average PDSI less than zero and the drought area severity statistic (summation of PDSI ⫻ area of each climate division divided by 105) is the driest (⫺6.4) for the neutral ENSO–cold PDO combination. This contrasts starkly with winters of neutral ENSO that occur during the warm phase of the PDO, which have average precipitation 6.7% above normal and 69 of 84 (82%) of climate divisions wetter than normal. Figure 3 shows the significance at different levels of t tests between the various ENSO–PDO combinations from Figs. 1 and 2. The Southwest and the Great Basin are where the neutral ENSO–PDO relationship is most significant, suggesting that knowledge of PDO phase during years of neutral ENSO may be most beneficial to seasonal forecasters in these regions. It is worth noting that the “modulation effect” of the PDO on neutral ENSO (39) is significant to at least the 80% level in more of the 84 climate divisions than during either El Niño (22) or La Niña (36). The influence of the PDO on years of El Niño and La Niña is similar to those noted in previous studies (Gershunov and Barnett 1998, among others), but the spatial pattern of moisture anomalies from one PDO phase to the other is not as dramatic as it is during years of neutral ENSO. In general, the larger moisture anomalies found in the Pacific Northwest and Southwest during the in-phase relationships are reduced to nearnormal anomalies during the out-of-phase relationship (Figs. 1 and 2). Another influence of the PDO during El Niño and La Niña is that the noted ENSO dipole between the Pacific Northwest and the Southwest (Redmond and Koch (1991) disappears when ENSO and PDO are out of phase. This is also shown in Fig. 3, where the dipole relationship between the Pacific Northwest and Southwest appears in the map of significance levels, especially in the PDSI. While the influence of the PDO on years of La Niña and El Niño is similar with respect to a reduction of

moisture anomalies toward normal during the out-ofphase relationship, there seems to be more of a dry signal than wet signal during years of neutral ENSO. As previously described, years of neutral ENSO–warm PDO are generally wet in 82% of the western U.S. climate divisions, but there are very few climate divisions that have PDSI ⬎ 1.0 or precipitation ⬎15% above normal. This is in contrast to years of neutral ENSO–cold PDO, where the dry signal is similar to the in-phase La Niña–cold PDO relationship (Figs. 1 and 2). The number of climate divisions experiencing mild drought (PDSI ⬍ ⫺1.0) during years of neutral ENSO– cold PDO (17) is only slightly less than during La Niña– cold PDO (24), as is the number of stations with precipitation ⬎15% below normal (21 to 27). This suggests that prolonged droughts in the Southwest that can occur during the cold PDO can be attributed to both years of La Niña and neutral ENSO (which together occur nearly 75% of the time during cold PDO), rather than just La Niña as previously reported (Cole et al. 2002).

b. Multiple correlation coefficients Since knowledge of PDO index values does not explain the influence of PDO phase on precipitation and PDSI during years of neutral ENSO, El Niño, or La Niña (Gutzler et al. 2002), it was worth considering how correlations between Niño-3.4 SSTs and hydroclimatic variables change during years of neutral ENSO from one PDO phase to another (Roy et al. 2003). A test statistic suggested by Kleinbaum and Kupper (1978), computed as 0.5 ln Z⫽

冉

冊

冉

1 ⫹ R1 1 ⫹ R2 ⫺ 0.5 ln 1 ⫺ R1 1 ⫺ R2

冑

1 1 ⫹ N1 ⫺ 3 N2 ⫺ 3

冊

,

where R1 and R2 are the multiple correlation coefficients from the two different equations and N1 and N2 are the number of cases in the development of each equation, was used. The value of Z is compared to critical values of t with a total N size of N1 ⫹ N2 ⫺ 6. For much of the interior western United States, there was little change in correlations between the neutral ENSO values and both precipitation and PDSI regardless of PDO phase. For the Pacific Northwest and the far desert Southwest however, this was not the case. Correlations between neutral ENSO values of Niño-3.4 SSTs and precipitation during the warm phase of the PDO were similar to that of the entire record, with a negative (positive) relationship in the Pacific Northwest (Southwest). What is interesting is that the sign, but not the strength, of the correlation changes across these regions

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FIG. 3. Significance of t tests between the various ENSO–PDO combinations from Figs. 1 and 2: least 95% significance (dark), 90% significance (middle), and 80% significance (light).

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FIG. 4. Significance of multiple correlation coefficients test between neutral ENSO–warm PDO and neutral ENSO–cold PDO years with precipitation and PDSI: at least 95% significance (dark), 90% significance (middle), and 80% significance (light).

during the cold phase of the PDO. The resulting correlations are opposite of that during the entire record. Climate divisions in the Southwest (Pacific Northwest) have strong negative (positive) correlations (⬇⫾0.6) during the cold phase of the PDO. For example, in Washington’s climate division 4, the driest winters during neutral ENSO–warm PDO were the “near–El Niño” winters that had Niño-3.4 SSTs near the 0.5 threshold, while the driest winters during neutral ENSO–cold PDO were “near–La Niña.” The reason for this correlation reversal is uncertain. Figure 4 shows the results of the Kleinbaum–Kupper test, which determines the significance of the correlation reversal from one PDO phase to the other. Since the strength of the PDO index has been shown to not be the driving factor for this relationship, there must be other atmospheric teleconnections forced by the location of PDO anomalies in the North Pacific during years of neutral ENSO.

4. Conclusions In this study, the years from 1925 to 1998 were split into six ENSO–PDO combinations to determine the influence of PDO phase on years of neutral ENSO in the western United States. As found previously in Arizona, years of neutral ENSO–cold PDO had the most widespread drought signal. More than 80% of western U.S. climate divisions were drier than normal during neutral ENSO–cold PDO winters and the drought area severity statistic shows more drought severity during these years than in any other ENSO–PDO combination. This contrasts with years of neutral ENSO–warm PDO, which were wetter than normal in 82% of the climate divisions. The influence of PDO on years of neutral ENSO was different from the influence of PDO

during years of La Niña and El Niña in that the dry signal was enhanced during the cold PDO while the wet signal was not as evident during the warm PDO. Correlations between low-index Niño-3.4 SSTs and hydroclimatic variables changed sign but not strength depending on the phase of the PDO. A dipole pattern between low-index Niño-3.4 SSTs and precipitation during years of neutral ENSO was observed. These results suggest that decades of cold PDO should be more prone to drought in the Southwest because of the strong dry signal apparent during years of neutral ENSO in addition to years of La Niña. Since nearly 75% of the years during the cold phase of the PDO are either neutral ENSO or La Niña, the southwestern United States is predisposed to extended periods of drought similar to the current decade-long drought and the historical droughts of the 1890s and 1950s. The Pacific Northwest, which is prone to drought during years of El Niño, should not be as susceptible to multiyear droughts since years of neutral ENSO are not associated with drought in this region. The Southwest and Great Basin are where the neutral ENSO–PDO relationship is most significant, suggesting that knowledge of PDO phase during years of neutral ENSO may be most beneficial to seasonal forecasters in these regions. This research suggests that if traditional forecast guidance yields a forecast of “equal chances,” forecasters may wish to skew their winter forecasts to drier (wetter) than normal following fall neutral ENSO–cold PDO (neutral ENSO–warm PDO) conditions for these areas. Because knowledge of the present state of the PDO is necessary for implementation, this method should not be used until the current state of the PDO can be determined with confidence.

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