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Amir Shabbar and Walter Skinner

Abstract

Canadian summer (June–August) Palmer Drought Severity Index (PDSI) variations and winter (December– February) global sea surface temperature (SST) variations are examined for the 63-yr period of 1940–2002. Extreme wet and dry Canadian summers are related to anomalies in the global SST pattern in the preceding winter season. Large-scale relationships between summer PDSI patterns in Canada and previous winter global SST patterns are then analyzed using singular value decomposition (SVD) analysis. The matrix for the covariance eigenproblem is solved in the EOF space in order to obtain the maximum covariance between the singular values of the SST and the PDSI. The robustness of the relationship is established by the Monte Carlo technique, in which the time expansion of the primary EOF analysis is shuffled 1000 times.

Results show that the leading three SVD-coupled modes explain greater than 80% of the squared covariance between the two fields. The interannual El Niño–Southern Oscillation (ENSO), the Pacific decadal oscillation (PDO), and the interrelationship between the two play a significant role in the determination of the summer moisture availability in Canada. These Pacific Ocean processes are reflected in the second and third SVD modes, and together explain approximately 48% of the squared covariance. It is found that the warm ENSO (El Niño) events lead to a summer moisture deficit in the western two-thirds of Canada. Conversely, cold ENSO (La Niña) events produce an abundance of summer moisture, mainly in extreme western Canada and in the southeastern portions of the Canadian Prairies.

The first SVD mode strongly relates to the trend in global SSTs and multidecadal variation of the Atlantic SST, explaining approximately one-third of the squared covariance. It is reflective of both the warming trend in the global southern oceans and the influences of the Atlantic multidecadal oscillation (AMO) variability.

The 6-month lag relationship between the PDSI and large-scale SSTs provides a basis for developing long-range forecasting schemes for drought in Canada. A two-tier forecast scheme, in which the SST is predicted by an ocean model or a coupled climate model, can potentially further increase the lead time of drought forecasting.

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Amir Shabbar, Kaz Higuchi, and John L. Knox

Abstract

In Knox et al., the interannual variation of the Northern Hemisphere 50 kPa geopotential height field averaged between 30° and 80°N was investigated for the 40-year period from 1946 to 1985. We presented strong statistical evidence supporting the notion that a rather abrupt transition in the climate system took place during the early 1960s. There was no attempt to compare the spatial distribution of the 50 kPa height difference between Regime 1 (1946–62) and Regime 2 (1963–85).

As a sequel to the first paper, we investigate the spatial characteristics of the transition height field. We find that the difference in the 50 kPa height field between Regime 1 and Regime 2 is characterized by low frequency circulation modes of the Pacific/North American (PNA) teleconnection pattern, the North Atlantic Oscillation (NAO), and an Arctic oscillation. There was an increase (in the residual sense) of the frequency and amplitude of the positive phase of the PNA in Regime 2 relative to Regime 1.

Fourier analysis is applied to interpret the regime changes in terms of planetary and long waves during the winter season. The change in the Arctic circulation is primarily associated with an amplification of the wave 2 component in its normal phase location, while in the midlatitudes the primary contributor is wave 1, again in its normal location.

We also examine the 40-year time series of 50 kPa height at the three centers of the winter PNA and confirm a strong negative correlation between the first two centers and a significant positive correlation between the first and third.

To assess the current trend, the 50 kPa anomaly field averaged over the 1981–87 period is examined. The winter season shows an eastward shift of the North Pacific Ocean cooling pattern and amplified warming over most of North America, the maximum centered over western Canada. The NAO phase changed to negative.

Our results are discussed in relation to the interregime sea surface temperature change over the North Pacific Ocean and to the increase in frequency and amplitude of ENSO events during Regime 2. Overall, there is a reinforcement of the earlier evidence for the two subclimate regimes.

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William H. Klein, Amir Shabbar, and Runhua Yang

Abstract

Specification equations for monthly mean air temperature anomalies at 68 surface stations in Canada and Alaska are derived by applying a forward selection screening procedure to simultaneous monthly mean 500 mb height anomalies at 110 grid points in the Northern Hemisphere and the previous month's 1ocal temperature anomaly. For the annual average, these equations explain about 72% of the temperature variance (on dependent data) by means of only 4.3 variables, but with marked regional and seasonal differences.

The average properties of the specification equations closely resemble those derived previously from the field of 700 mb heights. The 500 mb equation however, explain about 2% more of the temperature variance by means of 0.7 fewer terms in the equations. This small but consistent superiority is evident during each month of the year and for equations based on heights only, as well as those including previous local temperature. The superiority is produced by higher correlations of temperature with local heights and suggests that the 500 mb level is more equivalent barotropic than 700 mb.

The specification equations are tested by means of several verification statistics computed for 5 years of independent data at 51 Canadian stations. The results show that equations based on concurrent 500 mb heights and previous local temperature perform slightly better than those based on heights only and much better than climatology or month-to-month persistence.

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Xuebin Zhang, Jian Sheng, and Amir Shabbar

Abstract

The multichannel singular spectrum analysis has been used to characterize the spatio–temporal structures of interdecadal and interannual variability of SST over the Pacific Ocean from 20°S to 58°N. Using the Comprehensive Ocean–Atmosphere Data Set from 1950 to 1993, three modes with distinctive spatio-temporal structures were found. They are an interdecadal mode, a quasi-quadrennial (QQ) oscillation with a period of 51 months, and a quasi-biennial (QB) oscillation with a period of 26 months. The interdecadal mode is a standing mode with opposite signs of SST anomalies in the North Pacific and in the tropical Pacific. The amplitude of this mode is larger in the central North Pacific than in the tropical Pacific. This mode contributes 11.4% to the total variance. It is associated with cooling in the central North Pacific and warming in the equatorial Pacific since around 1976–77. The QQ oscillation exhibits propagation of SST anomalies northeastward from the Philippine Sea and then eastward along 40°N, but behaves more like a standing wave over the tropical Pacific. It explains nearly 20% of the total variance. The QB oscillation is localized in the Tropics and is characterized by the westward propagation of SST anomalies near the equator. This mode accounts for 7.4% of the total variance. Since the interdecadal mode is apparently independent of QB and QQ oscillations, it may play an important role in configuring the state of the tropical SST anomalies, which in turn affects the strength of the El Niño–Southern Oscillation phenomenon. It seems likely that the higher phase of the interdecadal mode since 1976–77 has raised the background SST state, on which the superposition of the QQ and QB oscillations produced the strongest warm event on record in 1982–83, as well as more frequent warm events since 1976.

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Aiming Wu, William W. Hsieh, and Amir Shabbar

Abstract

Nonlinear projections of the tropical Pacific sea surface temperature anomalies (SSTAs) onto North American winter (November–March) surface air temperature (SAT) and precipitation anomalies have been performed using neural networks. During El Niño, the linear SAT response has positive anomalies centered over Alaska and western Canada opposing weaker negative anomalies centered over the southeastern United States. In contrast, the nonlinear SAT response, which is excited during both strong El Niño and strong La Niña, has negative anomalies centered over Alaska and northwestern Canada and positive anomalies over much of the United States and southern Canada.

For precipitation, the linear response during El Niño has a positive anomaly area stretching from the east coast to the southwest coast of the United States and another positive area in northern Canada, in opposition to the negative anomaly area over much of southern Canada and northern United States, and another negative area over Alaska. In contrast, the nonlinear precipitation response, which is excited during both strong El Niño and strong La Niña, displays positive anomalies over much of the United States and southern Canada, with the main center on the west coast at around 45°N and a weak center along the southeast coast, and negative anomalies over northwestern Canada and Alaska.

The nonlinear response accounts for about one-fourth and one-third as much variance as the linear response of the SAT and precipitation, respectively. A polynomial fit further verifies the nonlinear response of both the SAT and precipitation to be mainly a quadratic response to ENSO. Both the linear and nonlinear response patterns of the SAT and precipitation are basically consistent with the circulation anomalies (the 500-mb geopotential height anomalies), detected separately by nonlinear projection. A cross-validation test shows that including the nonlinear (quadratic) response can potentially contribute to additional forecast skill over North America.

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Amir Shabbar, Barrie Bonsal, and Madhav Khandekar

Abstract

Precipitation responses over Canada associated with the two extreme phases of the Southern Oscillation (SO), namely El Niño and La Niña, are identified. Using the best available precipitation data from 1911 to 1994, both the spatial and temporal behavior of the responses are analyzed from the El Niño/La Niña onset to several seasons afterward. Composite and correlation analyses indicate that precipitation over a large region of southern Canada extending from British Columbia, through the prairies, and into the Great Lakes region is significantly influenced by the SO phenomenon. The results show a distinct pattern of negative (positive) precipitation anomalies in this region during the first winter following the onset of El Niño (La Niña) events. During this same period, significant positive precipitation anomalies occur over the northern prairies and southeastern Northwest Territories in association with El Niño events. Statistical significance of the responses is tested by the Student’s t-test and the Wilcoxon rank-sum test, while field significance is established through the Monte Carlo procedure. All of the significant precipitation anomalies can be explained by the associated upper-atmospheric flow patterns, which during the first winter following the onset of El Niño (La Niña) events resemble the positive (negative) phase of the Pacific–North American (PNA) pattern. Significant correlations between Southern Oscillation index (SOI) values and the observed precipitation anomalies over southern Canada suggest the possibility of developing a long-range forecasting technique for Canadian precipitation based on the occurrence and evolution of the various phases of the SO.

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John L. Knox, Kaz Higuchi, Amir Shabbar, and Neil E. Sargent

Abstract

There is accumulating evidence in the literature that different short-period climate regimes (subclimates) may have characterized the Northern Hemisphere during the past 40 years. We, therefore, investigate the 40-yr record of 50 kPa height (1946–85) and analyze the time series of zonal anomalies stratified by season. We find that there appears to be two contiguous regimes-with a rather abrupt transition during the early 1960s—which had significantly different means, trends and degrees of variability. The results are compared with those from recent investigations of Northern Hemisphere surface and/or tropospheric temperature variation. The possibility of a “climatic jump” during the early 1960s is discussed. Our results raise the question of an appropriate period to use for determining “normals,” whether for standard level surfaces or, more generally, for calculating the statistics of the general circulation, both in the horizontal and vertical.

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H. M. Van den Dool, Peitao Peng, Åke Johansson, Muthuvel Chelliah, Amir Shabbar, and Suranjana Saha

Abstract

The question of the impact of the Atlantic on North American (NA) seasonal prediction skill and predictability is examined. Basic material is collected from the literature, a review of seasonal forecast procedures in Canada and the United States, and some fresh calculations using the NCEP–NCAR reanalysis data.

The general impression is one of low predictability (due to the Atlantic) for seasonal mean surface temperature and precipitation over NA. Predictability may be slightly better in the Caribbean and the (sub)tropical Americas, even for precipitation. The NAO is widely seen as an agent making the Atlantic influence felt in NA. While the NAO is well established in most months, its prediction skill is limited. Year-round evidence for an equatorially displaced version of the NAO (named ED_NAO) carrying a good fraction of the variance is also found.

In general the predictability from the Pacific is thought to dominate over that from the Atlantic sector, which explains the minimal number of reported Atmospheric Model Intercomparison Project (AMIP) runs that explore Atlantic-only impacts. Caveats are noted as to the question of the influence of a single predictor in a nonlinear environment with many predictors. Skill of a new one-tier global coupled atmosphere–ocean model system at NCEP is reviewed; limited skill is found in midlatitudes and there is modest predictability to look forward to.

There are several signs of enthusiasm in the community about using “trends” (low-frequency variations): (a) seasonal forecast tools include persistence of last 10 years’ averaged anomaly (relative to the official 30-yr climatology), (b) hurricane forecasts are based largely on recognizing a global multidecadal mode (which is similar to an Atlantic trend mode in SST), and (c) two recent papers, one empirical and one modeling, giving equal roles to the (North) Pacific and Atlantic in “explaining” variations in drought frequency over NA on a 20 yr or longer time scale during the twentieth century.

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