Ohio Winter Precipitation and Stream Flow Associations to Pacific Atmospheric and Oceanic Teleconnection Patterns

The relationship between the Pacific/North American (PNA) atmospheric circulation teleconnection, equatorial Pacific sea surface temperature anomalies (SSTAs), and Ohio winter (DJF) precipitation and stream flow is described using data for 84 statewide climate stations and 29 rivers. Maximum correlations between the PNA index (PNAI) and station precipitation reach r = -0.7 in southwestern Ohio (n = 53) and are as high as r = +0.6 (n = 104) using a proxy North Pacific index (NPI) comprised of sea level pressures. The Ohio winter precipitation and streamflow relationship with the PNAI and NPI is strongest in southern and southwestern Ohio, generally decreasing to non-significance over northern Ohio, and particularly the northeastern snow belt. In contrast, Niño 3.4 equatorial Pacific correlations reach r = 0.5 when SSTs precede winter by one month. Wettest (driest) Ohio winters occur during relatively zonal (meridional) flow, representing PNAI negative (positive) modes when north Pacific sea level pressure is anomalously high (low). Wet winters are characterized by a 500 hPa trough across the central US east of the Rockies, with surface cyclones and associated frontal activity traversing Ohio after originating in areas such as Colorado and the western Gulf of Mexico. When the meridional flow of the PNA positive mode occurs, Ohio winters are consistently drier than normal and stream flow is typically about 50% of the PNA wet winters. Much higher variability occurs during PNA negative mode winters; precipitation and stream flow are occasionally below normal, but more typically above normal with some extraordinarily wet winters. OHIO J SCI 104 (3):51–59, 2004 Manuscript received 6 January 2003 and in revised form 9 June 2003 (#03-01). INTRODUCTION It is widely recognized that winter weather systems most often approach Ohio from the west. However, specific mechanisms by which Ohio weather is affected over many years by events occurring in distant places, such as over the Pacific Ocean, are less well understood. Long-distance linkages in weather and climate, associated with highly significant correlations between atmospheric or oceanic conditions across widely separated points on the earth, are referred to by climatologists as atmospheric “teleconnections.” This study shows the impact of Pacific atmospheric and oceanic teleconnections on Ohio winter (December–February; DJF) precipitation and stream flow. One feature in particular, the Pacific/North American teleconnection pattern (PNA), links atmospheric circulation anomalies over disparate regions such as the equatorial Pacific, the northern Pacific, northwestern Canada, and the southeastern United States (Wallace and Gutzler 1981). The PNA impact is such that atmospheric pressure over the Pacific is highly correlated to Ohio winter precipitation and stream flow. This relationship between the PNA and Ohio hydrology is quite strong and very statistically significant throughout the twentieth century, but it is little known until recently. Variations in equatorial Pacific sea surface temperature anomalies (SSTAs) are known to induce winter atmospheric circulation anomalies such as the PNA across the northern mid-latitudes (Hoskins and Karoly 1981). Winters with unusually high equatorial Pacific SSTAs (El Niño winters) tend to be characterized by a highly amplified mid-tropospheric wave pattern exhibiting unusually low pressure over the northern Pacific and the southeastern United States (US) and an amplified ridge between these locales over northwestern North America. This is the PNA positive mode (Fig. 1b). Conversely, winters with low/negative SSTAs (La Niña winters) are characterized by a zonal wave flow pattern in which high pressure occurs over the northern Pacific and southeastern US while unusually low pressure occurs over central and western North America. This is the PNA negative mode (Fig. 1a). Although often treated as being synonymous with El Niño/La Niña, the PNA mode and amplitude is also affected by other mid-latitude processes associated with perturbations growing from longitudinal gradients in jet exit regions and momentum fluxes associated with high frequency transient storms (Trenberth and others 1998). As such, the PNA is not entirely produced or forced by anomalous equatorial Pacific heating. Equatorial Pacific SSTAs also affect North American climate, particularly playing a role in regional US moisture budgets. Montroy (1997) links Niño 3.4 SSTAs, those occurring between 5° N and 5° S, 170° W to 120° W, to February and March precipitation variability around the Ohio River Valley. Montroy and others (1998) further suggest that the link is stronger in warm equatorial Pacific events than in cold events. Gershunov and Barnett (1998) find that Ohio River Valley extreme precipitation events have a lower (higher) frequency during warm (cold) equatorial Pacific SSTAs. Wang and Ting (2000) find that Ohio River Valley winter precipitation variations can be linked to SSTAs of both the equatorial Pacific and the mid-latitude Pacific. Enfield and others 52 VOL. 104 OHIO WINTER PRECIPITATION AND STREAM FLOW (2001) show that a strong Niño 3.4 signal in Mississippi River Valley winter precipitation is modulated by the Atlantic multidecadal oscillation. The seasonality and spatial distribution of the US stream flow response to El Niño is described by Kahya and Dracup (1993). Initially shown by Leathers and others (1991), the PNA influence on winter precipitation across the Ohio River Valley is more thoroughly described by Coleman and Rogers (2003). They show that the PNA/precipitation correlation is in excess of absolute r = 0.60 at many stations across the Midwest and Ohio River Valley, a relatively strong long-term relationship between the atmospheric circulation and climate. Their work also suggests that El Niño/Southern Oscillation signal is not as pronounced in midwestern winter precipitation as is the PNA. The purpose of this study is to increase the number of Ohio weather stations and incorporate a full set of FIGURE 1. The mean 500 hPa height (c) differences (m) occurring between winters when the PNA index is (a) negative and (b) positive in Table 1. Areas are shaded where the differences are significant with 95% and 99% confidence, using a two-tailed t-test and dashed contours indicate negative differences. Ohio stream flow records in expanding the analysis of Coleman and Rogers (2003). This study provides more detail of the impact of these Pacific teleconnections in Ohio’s winter hydrology and climate, permitting more comparison between the PNA and equatorial SSTA influences. These links between the Pacific and Ohio winter precipitation have not been described in detail previously and are likely of interest to a wider audience beyond climatologists. MATERIALS AND METHODS Monthly precipitation data at 84 Ohio weather stations with varying periods of record over 1896-2002 were obtained from the National Climatic Data Center and Midwestern Climate Center. We used cooperative weather observer stations, National Weather Service offices, and airport data having the longest records and minimal missing data. Data accuracy and consistency has been evaluated using a variety of checks and procedures by the National Climatic Data Center. Long-term data sets were created at 14 stations by combining intermediatelength records at stations having the same city name and similar monthly precipitation totals and standard deviations over their periods of record. In these cases the precipitation data were transformed into departures from normal, separately for each, and combined to form one final station data record consisting only of the departures from normal. For the purpose of the correlation and compositing methods used in this study, departures from normal were just as useful as the raw data available for the other 70 stations. Monthly mean stream flow records for 29 gauging stations were compiled from the US Geological Survey’s Hydro-Climatic Data Network (HCDN) (Slack and Landwehr 1992). Stations were selected based on long record length (>50 years) and a paucity of missing data up to the HCDNs end of record in 1988. The HCDN rivers and creeks chosen by Slack and Landwehr had minimal impact from human activities in the form of water flow diversion or augmentation, large reservoirs or dams, and land use changes. Three indices of the atmospheric and oceanic circulation teleconnection patterns were used. Equatorial Pacific SSTAs are available for the “Niño 3.4” area extending from 5° N to 5° S, 170° W to 120° W (Pielke and Landsea 1999). Niño 3.4 is often linked to US moisture variability (Montroy 1997; Montroy and others 1998; Enfield and others 2001; Schmidt and others 2001). The extreme warm (El Niño) and cold (La Niña) winters since 1920 are listed (Table 1) when SSTAs exceed absolute 0.75° C. The PNA index (PNAI) was calculated using 500 hPa geopotential heights at 4 key teleconnection centers (Wallace and Gutzler 1981). These centers are illustrated in Figure 1c, which shows the result obtained by subtracting the mean 500 hPa height fields of the negative (Fig. 1a) and positive (Fig. 1b) PNA modes. Upper air data for this index are only available since 1947, and the highest and lowest quintile PNAI winters are listed in Table 1. The winter mean PNAI/Niño 3.4 correlation (r = 0.43 from 1947-1999) has a coefficient of variation of only 19%, illustrating that a majority of the OHIO JOURNAL OF SCIENCE 53 J. C. ROGERS AND J. S. M. COLEMAN variance is unexplained and potentially originating from other mid-latitude phenomena. The North Pacific index (NPI) is the averaged mean sea level pressure over the Pacific Ocean between 30° N – 65° N, 160° E – 140° W (Trenberth and Hurrell 1994). This area generally corresponds to the large 500 hPa northern North Pacific center of the PNA (Fig. 1c). The PNAI/NPI correlation is r = -0.94 over 1947-1999 (see also Trenberth and Hurrell 1994). NPI data are available since 1899 and represent an excellent PNAI proxy prior to 1946. Positive (negative) NPI mode winters include those in which mean northern Pacific pressure is above 1011 hPa (below 1006 hPa) (Table 1). The Pacific SLP of the NPI is negatively correlated to the 500 hPa PNAI because the PNA phase is considered positive (Fig. 1b) when northern Pacific 500 hPa heights, and sea level pressures (NPI), are below normal. Conversely, high pressure (positive NPI) occurs across the northern Pacific in the negative phase of the PNA. In addition to correlation analyses, composite mean winter precipitation and stream flow data were obtained for years with positive and negative extremes in the PNAI, NPI, and Niño 3.4. Mean differences were TABLE 1 Winters used in the precipitation and stream flow composites created for negative and positive PNA and NPI modes and for warm and cold SST anomalies in the equatorial Pacific Niño 3.4 region. Winters are dated by the year in which January and February fall.