Pacific North American Pattern Research Articles

An evaluation of two GCMs: simulation of North American teleconnection indices and synoptic phenomena

We evaluate the ability of two coupled atmospheric–oceanic GCMs—the Hadley Center's third generation coupled climate model (HadCM3) and the Canadian Center for Climate Modeling and Analysis second-generation coupled model (CGCM2)—to simulate the North Atlantic Oscillation (NAO), the Pacific North American teleconnection pattern (PNA), and map patterns in the Midwest region of the United States, relative to NCEP/NCAR reanalysis (NNR) data. The observed (NNR-derived) and GCM-derived probability distributions and temporal behavior of the daily teleconnection indices exhibit agreement over the 1990–2001 reference period, and both GCMs successfully reproduce the range of 500-hPa map patterns over the study region. During the reference period, observed and modeled map patterns are similar in terms of frequency, coherence, persistence, and progression, although the most common map pattern occurs too often in HadCM3 relative to NNR and CGCM2-derived map patterns generally exhibit closer agreement with those derived from NNR data. Despite the relatively high degree of correspondence between the observed and simulated teleconnection indices and map patterns in the study area, differences between the GCM and NNR-derived map-pattern frequencies in the reference period are greater than either (1) recent historical changes in map-pattern frequencies or (2) changes in the map-pattern frequencies as derived from twenty-first century GCM simulations, indicating that changes in these phenomena over recent and approaching decades are of insufficient magnitude relative to model uncertainty to be definitively identified. Copyright © 2005 Royal Meteorological Society.

The role of synoptic‐scale circulation in the linkage between large‐scale ocean–atmosphere indices and winter surface climate in British Columbia, Canada

AbstractIn much of North America, variables such as temperature, precipitation, snowpack and streamflow are modulated by modes of large‐scale ocean‐atmosphere variability such as the Pacific Decadal Oscillation (PDO), El Niño‐Southern Oscillation (ENSO) and the Pacific North American Pattern (PNA). In this study, we test the hypothesis that the influence of these modes on air temperature and precipitation in British Columbia (BC), Canada, can be explained in relation to changes in frequencies of synoptic‐scale circulation types. A catalogue of 13 circulation types was derived by classifying daily mean sea‐level pressure (MSLP) grids from 1948 to 2003. The grids cover BC and the North Pacific and were subjected to a standard pattern recognition algorithm employing principal component analysis followed by cluster analysis on the component scores. The circulation types are generally associated with distinctive patterns of precipitation and air temperature anomalies across BC. Multiple linear regressions for selected stations in BC using the type frequencies as predictors explain up to 75% of the variance of mean winter temperature and 65% of winter precipitation. The frequencies of most circulation types vary significantly between the different phases of ENSO, PDO and PNA in a manner consistent with the temperature anomalies associated with those modes and, to a lesser extent, with the more complex precipitation anomalies. In addition, however, average temperatures and precipitation amounts for some circulation types differ systematically between phases of ENSO and PDO. Subsequent analysis revealed distinct differences among ENSO and PDO phases in the upper‐level circulation patterns associated with some surface types. A major part of the teleconnections can be explained through variations in the frequencies of synoptic‐scale circulation types, but systematic within‐type variability, particularly with PDO and PNA, can additionally influence the surface climate. Copyright © 2005 Royal Meteorological Society.

Simulation capability of tropical and extratropical seasonal climate anomalies over South America

An ensemble of 20 extended integrations of the atmospheric model CSIRO Mark 2, forced with the sea-surface temperature observed during the 1986–1998 period, was performed to analyze the simulation capability of seasonal climate anomalies over South America and adjacent oceanic areas. Variations of the simulation skill within the region and during the experimental period were assessed through standard statistical measures and compared to the signal-to-noise ratio distribution. Before the skill assessment, model systematic errors were thoroughly evaluated. The results confirm that the simulation skill is very high in tropical oceanic areas, and decreases rapidly towards middle and high latitudes. Model performance at mid and high atmospheric levels is substantially better than at low levels. Relatively high simulation capability was found over the Pacific Ocean between the equator and the Antarctic coast, which is coherent with the presence of three relative maximums in the signal-to-noise ratio, similar to the increase of the forced variance found by several authors over much of the Pacific–North American pattern region. Rainfall rate and second-order moments associated with the cyclonic activity and the meridional eddy fluxes of heat and humidity are better simulated in a narrow strip parallel to the SPCZ and extending further southeast into mid latitudes of the continent. The simulation skill noticeably improves during the warm and cold ENSO phases, in correspondence with an intensification of the signal-to-noise ratio, and useful rainfall anomaly simulations can be obtained over the Amazonas and Rio de la Plata river basins.

The Circum-Pacific Teleconnection Pattern in meridional wind in the high troposphere

The Circum-Pacific Teleconnection Pattern (CPTP) is revealed in the meridional wind in the high troposphere via an emprirical orthogonal function (EOF) and correlation analysis on NCEP/NCAR re-analysis data. The CPTP is found to be composed of the North Pacific-North American teleconnection pattern (PNA), the South Pacific-South American teleconnection pattern (PSA), and the teleconnection patterns over the tropical western Pacific and the tropical eastern Pacific (or, Central America, or, tropical Atlantic). There is substantial interannual variability of the CPTP and a typical CPTP can be detected in some years. It is speculated that the zonal wind anomalies over the equatorial region in the western and eastern sides of the Pacific may play a role in linking the two hemispheres. The anomalous convection activities in the Tropics are plausible triggering factors for the zonal wind anomalies that are responsible for the composition of the CPTP.

A New Look at the Pacific Storm Track Variability: Sensitivity to Tropical SSTs and to Upstream Seeding

Abstract There is a fairly well defined stationary wave and storm track response to El Niño SSTs over the Pacific. In this paper, the case is made that this response is a direct result of increased baroclinicity in the central Pacific and that changes in the stationary wave pattern farther east are primarily forced by changes in these transient eddies. There is also a lot of natural variability that is not associated with El Niño. The paper also stresses the point that much of the variability can be understood as forced by variations in the upstream seeding of the storm track. The question of whether these seeding variations should be thought of as chaotic noise or forced by identifiable mechanisms is not addressed. Thus, the claim is that the storm track variability and its feedback to the quasi-stationary circulation depends on two key parameters: mid-Pacific baroclinicity, controlled by SSTs, and the strength of the upstream seeding. The approach is to first examine the effect of storm track seeding by waves entering from the Asian continent during normal years (non-ENSO years). The results show that two mechanisms operate to distribute eddy energy along the storm track: downstream development and baroclinic development. The large effect on baroclinic development at the storm track entrance results from a combination of factors: surface baroclinicity, land–sea contrast, and strong moist fluxes from the western subtropics. Experiments show that sensitivity to the seeding amplitude is large. The larger the seeding amplitude, the closer the more intense baroclinic waves flux energy downstream to upper-level waves. These barotropic waves tend to break anticyclonically and produce a ridge in the eastern Pacific. Sensitivity to SST anomalies shows qualitative and quantitative similarity with the observed anomalies. Simulations show increased mid-Pacific baroclinicity because stronger convection in the midtropical Pacific enhances a large pool of warm air over the entire mideastern subtropical ocean. Waves with sources at the storm track entrance break anticyclonically and produce the ridge in the eastern Pacific. On the other hand, baroclinic waves generated or regenerated in the mid-Pacific tend to break cyclonically, produce a trough tendency, and reduce the eastern ridge amplitude in the Pacific–North American (PNA) sector. These results strongly suggest thatthe variability of the quasi-permanent circulation indeed could be produced by the high-frequency eddy feedback, andtwo mechanisms are primarily responsible for the forcing of the quasi-permanent circulation: downstream development from the western Pacific and the anomalous baroclinicity in the mideastern Pacific. The intensity of these counteracting forcings gives the different flavors of the El Niño response over the PNA region. Regardless of the SST anomaly strength, the PNA patterns seem unique but obviously have different intensities.

Seasonal Forecasting with a Simple General Circulation Model: Predictive Skill in the AO and PNA

Abstract A primitive equation dry atmospheric model is used to perform ensemble seasonal predictions. The predictions are done for 51 winter seasons [December–January–February (DJF)] from 1948 to 1998. Ensembles of 24 forecasts are produced, with initial conditions of 1 December plus small perturbations. The model uses a forcing field that is calculated empirically from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses. The forcing used to forecast a given winter is the sum of its winter climatological forcing plus an anomaly. The anomalous forcing is obtained as that of the month prior to the start of the forecast (November), which is also calculated from NCEP data. The predictions are thus made without using any information about the season to be predicted. The ensemble-mean predictions for the 51 winters are verified against the NCEP–NCAR reanalyses. Comparisons are made with the results obtained with a full GCM. It is found that the skill of the simple GCM is comparable in many ways to that of the full GCM. The skill in predicting the amplitude of the main patterns of Northern Hemisphere mean-seasonal variability, the Arctic Oscillation (AO) and the Pacific–North American (PNA) pattern is also discussed. The simple GCM has skill not only in predicting the PNA pattern during winters with strong ENSO forcing, but it also has skill in predicting the AO in winters without appreciable ENSO forcing.

Temporal and spatial variabilities of atmospheric polychlorinated biphenyls (PCBs), organochlorine (OC) pesticides and polycyclic aromatic hydrocarbons (PAHs) in the Canadian Arctic: Results from a decade of monitoring

The Northern Contaminants Program (NCP) baseline monitoring project was established in 1992 to monitor for persistent organic pollutants (POPs) in Arctic air. Under this project, weekly samples of air were collected at four Canadian and two Russian arctic sites, namely Alert, Nunavut; Tagish, Yukon; Little Fox Lake, Yukon; Kinngait, Nunavut; Dunai Island, Russia and Amderma, Russia. Selected POPs, including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and organochlorine (OC) pesticides, were analyzed in both the gas and particulate phases. This paper summarizes results obtained from this project in the past 5 years. Temporal trends were developed for atmospheric PCBs and OCs observed at Alert using a digital filtration (DF) technique. It was found that trends developed with 5 years of data (1993–1997) did not differ significantly from those determined with 7 years of data (1993–1999). This implies that with the DF technique, long-term trends can still be developed with less than 10 years of data. An acceleration in decline of OC and PCB air concentrations was noted in 1999 for some compounds, although the reason is unknown. Monitoring efforts must continue to assess the effect of this decline on the long-term trends of POPs in the Canadian Arctic. Occasional high trans-/ cis-chlordane ratios and heptachlor air concentrations measured at Alert between 1995 and 1997 suggests sporadic fresh usage of chlordane-based pesticides. However, significant decreasing trends of chlordanes along with their chemical signatures has provided evidence that emission of old soil residues is replacing new usage as an important source to the atmosphere. Measurements of OC air concentrations conducted at Kinngait in 1994–1995 and 2000–2001 indicated faster OC removal at this location than at Alert. This may be attributed to the proximity of Kinngait to temperate regions where both biotic and abiotic degradation rates are faster. The PAH concentrations observed at Alert mimic those at mid-latitudes and are consistent with long-range transport to the Arctic, particularly for the lighter PAHs. A decline in particulate PAH was observed, similar to atmospheric sulphate aerosol and can be attributed to the collapse of industrial activity in the former Soviet Union between 1991 and 1995. Spatial comparisons of OC seasonality at Alert, Tagish, Dunai and Kinngait show elevated air concentrations of some compounds in spring. However, elevated spring concentrations were observed for different compounds at different sites. Potential causes are discussed. Further investigation in the atmospheric flow pattern in spring which is responsible for the transport of POPs into the Arctic is required. OC and PCB air concentrations at Alert were found to be influenced by two climate variation patterns, the North Atlantic Oscillation (NAO) and the Pacific North American (PNA) pattern. Planetary atmospheric patterns must be taken into account in the global prediction and modelling of POPs in the future.

January northern hemisphere circumpolar vortex variability and its relationship with hemispheric temperature and regional teleconnections

Variability in the hemispheric-scale atmospheric circulation can be directly linked to variations in surface environmental features, such as temperature, precipitation, salinity of water bodies, and pollutant transport. One indicator of the behavior of the hemispheric-scale circulation is the circumpolar vortex (CPV). This research utilizes a geographic information system approach to characterize variability in the Northern Hemispheric (NH) CPV. Specifically, the area, shape, and centroid of the January NHCPV are analyzed for 1959–2001 because it may provide insight about relationships between hemispheric-scale circulation and global temperature change. We also use a new means of characterizing the hemispheric-scale circulation using a ‘circularity ratio’ (Rc). Results suggest that the January NHCPV has exhibited no long-term trends in area or shape, and that the mean centroid is positioned at approximately 85.3°N, 178.0°W. Regional patterns emerge, which suggest that the area and circularity are associated with variability in surface temperature and moist static energy. Furthermore, the area of the January NHCPV is associated with variability in the Arctic Oscillation, while the shape is tied to variability in the Pacific-North American teleconnection pattern. These results will facilitate understanding of the relationship between hemispheric-scale circulation, regional circulation, and local temperatures. Copyright © 2005 Royal Meteorological Society.

Information Theory and Predictability for Low-Frequency Variability

AbstractA predictability framework, based on relative entropy, is applied here to low-frequency variability in a standard T21 barotropic model on the sphere with realistic orography. Two types of realistic climatology, corresponding to different heights in the troposphere, are used. The two dynamical regimes with different mixing properties, induced by the two types of climate, allow the testing of the predictability framework in a wide range of situations. The leading patterns of empirical orthogonal functions, projected onto physical space, mimic the large-scale teleconnections of observed flow, in particular the Arctic Oscillation, Pacific–North American pattern, and North Atlantic Oscillation. In the ensemble forecast experiments, relative entropy is utilized to measure the lack of information in three different situations: the lack of information in the climate relative to the forecast ensemble, the lack of information by using only the mean state and variance of the forecast ensemble, and information flow—the time propagation of the lack of information in the direct product of marginal probability densities relative to joint probability density in a forecast ensemble. A recently developed signal–dispersion–cross-term decomposition is utilized for climate-relative entropy to determine different physical sources of forecast information. It is established that though dispersion controls both the mean state and variability of relative entropy, the sum of signal and cross-term governs physical correlations between a forecast ensemble and EOF patterns. Information flow is found to be responsible for correlated switches in the EOF patterns within a forecast ensemble.

Characterizing regional‐scale variations in monthly and seasonal surface air temperature over Mexico

AbstractMonthly and seasonal variations in surface air temperature (SAT) over Mexico have not received much research attention, a situation partly reflecting the lack of a coherent historical data set. As a step toward rectifying the data gap, this study outlines the development of a gridded monthly (2.5° × 2.5°lat.–long.) SAT data set (1940–2001) for Mexico. Using the data set, we investigate several basic dimensions of SAT variability. Our analysis demonstrates that much of the variability can be compactly expressed in terms of four regions which are physically plausible with respect to the country's climatology. Not surprisingly, persistence is an important component of regional SAT variability. Evaluated month to month, persistence tends to be greatest during the warm season, whereas across seasons there is evidence for persistence of warm season anomalies into the following cool season, behaviour that is consistent with positive feedback relationships between SAT, rainfall and land surface conditions. The regional time series display longer period variability that is partially linked to the state of the large‐scale, slowly evolving climate modes of the Atlantic multidecadal oscillation and the Pacific decadal oscillation. Analyses are also presented to describe teleconnections between SAT and the El Niño–southern oscillation phenomena, and SAT and other large‐scale atmospheric modes, such as the Pacific North American pattern and the North Atlantic oscillation. Copyright © 2004 Royal Meteorological Society

Updated 6–11-Month Prediction of Atlantic Basin Seasonal Hurricane Activity

An updated statistical scheme for forecasting seasonal tropical cyclone activity in the Atlantic basin by 1 December of the previous year is presented. Previous research by Gray and colleagues at Colorado State University showed that a statistical forecast issued on 1 December of the previous year could explain up to about 50% of the jackknife hindcast variance for the 1950–90 time period. Predictors utilized in the original forecast scheme included a forward extrapolation of the quasi-biennial oscillation (QBO) and two measures of West African rainfall. This forecast has been issued since 1991 but has shown little skill because of the as yet unexplained failure of the West African rainfall predictors during the 1990s. The updated scheme presented in this paper does not utilize West African rainfall predictors. It employs the new NCEP–NCAR reanalysis data and involves predictors that span the globe. Much experimentation has led to the choosing of conditions associated with the El Niño-Southern Oscillation (ENSO), the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), the Pacific–North American pattern (PNA), and the QBO. This new statistical scheme shows similar cross-validated (jackknifed) hindcast skill to the original scheme (explaining up to about 50% of the variance) but is developed over a decade longer time period (1950–2001). In addition, since all predictors are taken directly from the NCEP–NCAR reanalysis, one does not need to consider rainfall collection issues that have caused major problems with the West African rainfall data since 1995. Hypothetical physical linkages between the predictors and the following year's hurricane activity are also presented. Based on the net tropical cyclone (NTC) activity prediction and a weighted average of North Atlantic sea surface temperatures, forecasts of U.S. hurricane landfall probability that showed considerable hindcast skill over the 52-yr period of 1950–2001 can also be issued.

Interdecadal Trend of Prediction Skill in an Ensemble AMIP-Type Experiment

Abstract This study addresses the interdecadal trend in potential skill score as estimated from the 500-hPa height temporal correlation coefficient (TCC), based on a 50-yr 10-member ensemble GCM integration with observed SST. The skill scores are based on the perfect model assumption, in which one of the members of the ensemble is assumed to be true. A distinct decadal positive trend in the TCC in boreal winter (December–January–February) was found. This trend is shown to be consistent with the positive trend in the interdecadal time-scale temporal variance of SST. The geographical pattern of the differences of the TCC between each decade and the 50-yr period resembles the Matsuno–Gill pattern, suggesting that the increase in the TCC is due to the Rossby wave excitation induced by the anomalous diabatic heating caused by the anomalous SST. Similar interdecadal trends in the variance of the Southern Oscillation index and Pacific–North American pattern were found in both the observation and the simulation. ...

How do climate fluctuations affect persistent organic pollutant distribution in North America? Evidence from a decade of air monitoring.

Interannual variations of persistent organic pollutant (POP) air concentrations from the Great Lakes region and the Arctic during the 1990s are strongly associated with atmospheric low-frequency fluctuations, notably the North Atlantic Oscillation (NAO), the El Niño-Southern Oscillation (ENSO), and the Pacific North American (PNA) pattern. This suggests interactions between climate variation and the global distribution of POPs. Atmospheric concentrations of hexachlorocyclohexanes (HCHs), hexachlorobenzene (HCB), and several lighter polychlorinated biphenyls (PCBs) measured around the Great Lakes basin increased during the positive phases of NAO and ENSO in the spring. This implies that anomalous high air temperatures associated with NAO and ENSO enhance volatilization of POPs from reservoirs on the Earth's surface accumulated in the past. These compounds are then available for transport from source regions to more pristine regions such as the Arctic under favorable flow patterns associated with global climate variations.

Atmospheric circulation influences on seasonal precipitation patterns in Alaska during the latter 20th century

A set of long, nearly complete daily precipitation series for Alaska spanning the latter half of the 20th century has been analyzed for seasonal relationships between variations in mean, heavy, and extreme precipitation and large‐scale atmospheric circulation variations at interannual, decadal, and secular timescales. Relationships with four candidate predictors (the Pacific North American (PNA), Arctic Oscillation (AO), Pacific Decadal Oscillation (PDO), and Nino3 indices) are used for insights into possible large‐scale climate forcing. Winter precipitation (mean and extreme) variability and trends along the south coast and interior of Alaska appear to be closely related to variations in the PNA pattern over this timeframe, while El Nino/Southern Oscillation (ENSO) influences, through the Nino3 index, appear to be significant along the south coast alone. Along the south coast the PNA and ENSO exert opposing influences on extreme (and mean) precipitation. Within interior Alaska the positive PNA pattern tends to suppress precipitation owing to orographic factors. Summer variations appear more closely related to the influence of the AO and PDO. The north slope region of Alaska appears to be too far removed from the influences of any of the examined predictors for any clear relationship to be evident.

The impact of varying atmospheric forcing on the thickness of arctic multi‐year sea ice

A 1‐D thermodynamic sea ice model, forced with North Pole Drift Station observations from 1954–91, is used to study the effect of changing atmospheric forcing on multi‐year Arctic sea ice. From 1954–70, most seasons show positive trends in calculated sea ice thickness over much of the Arctic. A dip in calculated ice thickness takes place between 1971–77 over most of the Arctic. Following the North Pacific regime shift in 1976–1977, the period 1978–91 reveals large negative trends in calculated sea ice thickness in all seasons. The results indicate that an important part of the variability and trends in Arctic sea ice thickness is thermodynamically‐driven. Of the total variance in multi‐year sea ice thickness, 10 to 20% is explained by variations in the Arctic Oscillation and Pacific North American patterns. The multi‐year ice thickness response to a positive wintertime Arctic Oscillation anomaly occurs the following summer and persists for more than a year.

Empirical patterns of variability in atmospheric and oceanic excitation of polar motion

Excitation of polar motion is related in large measure to the redistribution of atmospheric and oceanic mass and to circulation changes. The atmosphere and ocean exhibit spatial patterns that may be isolated by a principal-component type of analysis as fundamental modes explaining their variability. These patterns contribute to polar motion excitation. Here atmospheric excitation functions χ A are computed in equal area sectors (equivalent to 5°×10° boxes at the equator) for the period 1983–2000, based on the reanalyses of the US National Centers for Environmental Prediction–National Center for Atmospheric Research. Oceanic excitation functions χ O are calculated in sectors of comparable resolution, based on the near-global ocean model of Ponte and Stammer (1999) [Geophys. Res 104 23,293,409] for the period 1985–1997. To find patterns of variability in χ A and χ O we examined modes obtained using complex Empirical Orthogonal Function analysis designed to mathematically isolate independent types of variability. EOF analysis, commonly used in studies of climate, separates types of variability that often are caused by an underlying physical mechanism. The first mode is clearly significant in both χ A and χ O and their associated time series have strong annual oscillations with temporally varying amplitudes. The first mode of χ A has strong signals over central Asia, Greenland, Australia, and the southern tips of South America and Africa, and those of χ O are over the mid-latitude North Pacific and North Atlantic as well as areas of the Southern Ocean. Others modes, though only marginally significant, have elements of noted climate signals such as the North Atlantic Oscillation or Pacific-North American patterns.

Secular variability of ENSO events in a 1000-year climatic simulation

The CSIRO Mark 2 coupled global climatic model has been used to generate 1000 years of simulated climatic variability corresponding to present climatic conditions. A small climatic drift was noted during the simulation, and all results presented are based on detrended conditions. The emphasis here is on El Nino/Southern Oscillation (ENSO) events and their secular variability during the simulation. A number of features of the simulated ENSO climatology are presented and compared with observations. These demonstrate that the model reproduced the major characteristics of observed ENSO variability. A variety of analyses is given to illustrate secular variability of ENSO-related climate. Thus, while the simulated Southern Oscillation is shown to be a robust feature of the model climatology, decadal and oscillatory time variations occurred. Time-smoothing of the Southern Oscillation Index revealed underlying multi-decadal episodes, with associated climatic anomalies. Examination of 'strong' ENSO events highlighted an asymmetry between El Nino and La Nina events, similar to that observed, with clear secular variability in the occurrence rate of La Nina events. Persistent (multi-annual) ENSO events, such as occurred in the 1990s, were found to exist within the simulation, suggesting that these may be attributable to natural climatic variability. An investigation of the temporal variation of the anomalous depth of the 20 °C isotherm in the Pacific Ocean, revealed secular variability, which, if predictable, could identify decadal-length climatic trends. Other analyses involved the behaviour of the Pacific North American Oscillation and its secular variability. Distinct climatic impacts over North America were identified with this variability. Importantly, the simulation identified a number of features of ENSO-related climatic variability which have been poorly, if at all, documented in observations to date.

Ohio River Valley Winter Moisture Conditions Associated with the Pacific–North American Teleconnection Pattern

Abstract The relationship between the Pacific–North American (PNA) teleconnection pattern and Ohio River Valley (ORV) winter precipitation and hydrology is described. The PNA is significantly linked to moisture variability in an area extending from southeastern Missouri, northeastward over states adjacent to the Ohio River through Ohio. Maximum correlation between the PNA index and station precipitation peaks in southern Indiana at r = −0.71, making the circulation/climate teleconnection one of the strongest in the Northern Hemisphere. The North Pacific index (NPI), a Pacific basin sea level pressure index that is highly correlated to the PNA, confirms a strong circulation–ORV precipitation relationship extending back to 1899. In contrast, measures such as the Tahiti–Darwin Southern Oscillation index (SOI) and Nino-3.4 (5°S–5°N, 120°–170°W) sea temperatures are not significantly correlated to ORV winter precipitation. Wettest (driest) winters occur with zonal (meridional) flow with the PNA negative (posit...

Cold Front Variability in the Southern United States and the Influence of Atmospheric Teleconnection Patterns

The variability of cold front passages in the southern United States and the effects of three teleconnection patterns on cold front frequencies are analyzed in this study. The Southern Oscillation is weakly correlated with cold front frequencies. However, cold front passage frequencies during El Niño and La Niña events are not significantly different. The Pacific/North American (PNA) and North Atlantic Oscillation (NAO) teleconnection patterns exert a significant influence on cold front variability in this region. The PNA pattern is most strongly tied to variations in cold front passage frequencies in Florida. Meridional (zonal) flow conditions are associated with a southward (northward) displacement of storm tracks over the eastern United States, resulting in an increase (decrease) in cold front passages in Florida. Various parts of the southern United States are affected by the NAO. High NAO months are characterized by an expanded Atlantic Subtropical High which often results in fewer cold front passages in the eastern portions of the southern United States. During low NAO months, much of Texas experiences a higher frequency of cold front passages, a likely result of increased upper-air trough development over the region.

An Analysis of Tornado Days in Missouri for the Period 1950-2002

Tornado analysis using a "tornado-day" approach was made for the State of Missouri, and its climatic divisions, for the period 1950-2002. Although tornado days have not increased in Missouri, the NW Prairie climatic division recorded a statistically significant decline in tornado days during that time period. Seasonal patterns in tornado days were examined for each of the 115 counties in Missouri using cluster analysis. It revealed that the duration of the "tornado season" varies somewhat across Missouri, several southern counties being affected by a bimodal seasonal pattern in tornado days. The interannual tornado-day time series was examined for Missouri, spectral density estimates showing periodicities of 2.1, 2.8, 8.5, 11.6, and 12.8 yrs. Cross-amplitude analysis revealed that these tornado-day periodicities appear to be connected to fluctuations affecting the Pacific-North American patterns (PNA)—(8.5 yrs.), and quasi periodicities associated with the Southern Oscillation and the El Niño 3.4 indexes (11.6 to 12.8 yrs.).