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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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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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Alan E. Stewart, Castle A. Williams, Minh D. Phan, Alexandra L. Horst, Evan D. Knox, and John A. Knox

Abstract

Prior surveys of the public indicated that a variety of meanings and interpretations exist about the probability of precipitation (PoP). Does the same variety of meanings for the PoP exist among members of the professional atmospheric science community? What do members of the professional community think that the public should know to understand the PoP more fully? These questions were examined in a survey of 188 meteorologists and broadcasters. Meteorologists were observed to express a variety of different definitions of the PoP and also indicated a high degree of confidence in the accuracy of their definitions. Differences in the definitions stemmed from the way the PoP was derived from model output statistics, parsing of a 12-h PoP over shorter time frames, and generalizing from a point PoP to a wider coverage warning area. In this regard 43% of the online survey respondents believed that there was no or very little consistency in the definition of PoP; only 8% believed that the PoP definition has been used in a consistent manner. The respondents believed that the PoP was limited in its value to the general public because, on average, those surveyed believed that only about 22% of the population had an accurate conception of the PoP. These results imply that the atmospheric science community should work to achieve a wider consensus about the meaning of the PoP. Further, until meteorologists develop a consistent conception of the PoP and disseminate it, the public’s understanding of PoP-based forecasts may remain fuzzy.

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Emily L. Pauline, John A. Knox, Lynne Seymour, and Andrew J. Grundstein

Abstract

The occurrence of extreme weather and climate events has increased in recent decades. This increasing frequency has adversely impacted economic and health outcomes, leading to an increasingly urgent need to study climate extremes. The National Centers for Environmental Information (NCEI) created the Climate Extremes Index (CEI) in 1996 to quantify climate extremes. In this article, we explore the potential for enhancing the CEI via the use of the Z-score statistic to calculate the CEI on a numerical scale, to increase usability at smaller spatial scales, and to allow the creation of a new climate Extremes Vulnerability Index (EVI). The EVI combines the results from the revised CEI with values from the Social Vulnerability Index from the Centers for Disease Control and Prevention (CDC). The EVI can be used by policy-makers, planners, and the public to understand a subregion’s vulnerability to climate extremes. This information from the EVI could then be used to implement policies and changes in infrastructure that mitigate risk in vulnerable climate divisions. In a trial application, it is found that the southeastern and portions of the central United States had the highest levels of vulnerability for the abnormal month of December 2015.

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