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Dehai Luo, Yao Yao, and Steven B. Feldstein

Abstract

In this paper, large-scale aspects for the onset of the extreme cold European weather event in January–February 2012 are investigated. It is shown that the outbreak of this extreme cold weather event may be attributed to the transition from a positive North Atlantic Oscillation (NAO+) event to a long-lasting blocking event over the eastern Atlantic and western Europe (hereafter ENAO). A persistent decline of the surface air temperature (SAT) is seen over all of Europe during the long-lived ENAO event, while the main region of enhanced precipitation is located over southern Europe and part of central Europe, in association with the presence of a persistent double storm track: one over the Norwegian and Barents Seas and the other over southern Europe.

The NAO+ to NAO transition events are divided into NAO+ to ENAO and NAO+ to WNAO transition events [ENAO (WNAO) events correspond to eastward- (westward-) displaced NAO events whose positive center is defined to be located to the east (west) of 10°W], and a statistical analysis of the NAO+ to ENAO transition events during 1978–2012 is performed. It is found that there has been a marked increase in the frequency of the NAO+ to ENAO transition events during the period 2005–12. Composites of SAT anomalies indicate that the marked decline of the SAT observed over much of Europe is primarily associated with NAO+ to ENAO transition events. Thus, NAO+ to ENAO transition events may be more favorable for the extreme cold events over Europe observed in recent winters than other types of NAO events.

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Dehai Luo, Yao Yao, and Aiguo Dai

Abstract

In Part I of this study, it is revealed that decadal variations of European blocking, in its intensity, duration, and position, during 1978–2011 are modulated by decadal changes in the frequency of North Atlantic Oscillation (NAO) events associated with background Atlantic conditions. In Part II, reanalysis data are analyzed to first show that a T-bone-type structure of the climatological-mean blocking frequency in the Euro-Atlantic sector roughly results from a combination of the blocking frequency distributions along the southeast–northwest (SE–NW) direction associated with negative-phase NAO (NAO) events and along the southwest–northeast (SW–NE) direction associated with positive-phase NAO (NAO+) events.

A nonlinear multiscale interaction (NMI) model is then used to examine the physical processes behind the blocking frequency distributions. This model shows that the combination of eastward- and westward-displaced blocking frequency patterns along the SW–NE and SE–NW directions associated with NAO+ and NAO events leads to a T-bone-type frequency distribution, as seen in reanalysis data. Moreover, it is found that the westward migration of intense, long-lived blocking anomalies over Europe following NAO+ events is favored (suppressed) when the Atlantic mean zonal wind is relatively weak (strong). This result is held for the strong (weak) western Atlantic storm track. This helps explain the findings in Part I. In particular, long-lived blocking events with double peaks can form over Europe because of reintensification during the NAO+ decay phase, when the mean zonal wind weakens. But the double-peak structure disappears and becomes a strong single-peak structure as the mean zonal wind strengthens.

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Dehai Luo, Yao Yao, and Aiguo Dai

Abstract

Both the positive and negative phases of the North Atlantic Oscillation (NAO+ and NAO, respectively) and atmospheric blocking in the Euro-Atlantic sector reflect synoptic variability over the region and thus are intrinsically linked. This study examines their relationship from a decadal change perspective. Since the winter-mean NAO index is defined as a time average of instantaneous NAO indices over the whole winter, it is unclear how the activity of European blocking (EB) events can be related to the variation of the positive mean NAO index. Here, this question is examined by dividing the winter period 1978–2011 into two decadal epochs: 1978–94 (P1) with an increasing and high NAO index and 1995–2011 (P2) with a decreasing and low NAO index. Using atmospheric reanalysis data, it is shown that there are more intense and persistent EB events in eastern Europe during P1 than during P2, while the opposite is true for western Europe.

It is further shown that there are more NAO+ (NAO) events during P1 (P2). The EB events associated with NAO+ events extend more eastward and are associated with stronger Atlantic mean zonal wind and weaker western Atlantic storm track during P1 than during P2, but EB events associated with NAO events increase in western Europe under opposite Atlantic conditions during P2. Thus, the increase in the number of individual NAO+ (NAO) events results in more EB events in eastern (western) Europe during P1 (P2). The EB change is also associated with the increased frequency of NAO to NAO+ (NAO+ to NAO) transition events.

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Weiye Yao and Christiane Jablonowski

Abstract

The paper demonstrates that quasi-biennial oscillation (QBO)-like oscillations can be simulated in an ensemble of dry GCM dynamical cores that are driven by a simple Held–Suarez temperature relaxation and low-level Rayleigh friction. The tropical stratospheric circulations of four dynamical cores, which are options in NCAR’s Community Atmosphere Model, version 5 (CAM5), are intercompared. These are the semi-Lagrangian (SLD) and Eulerian (EUL) spectral transform, finite-volume (FV), and spectral element (SE) dynamical cores. The paper investigates how the model design choices impact the wave generation, propagation, and dissipation mechanisms in the equatorial region. SLD, EUL, and SE develop spontaneous QBO-like oscillations in the upper equatorial stratosphere, whereas FV does not sustain the oscillation. Transformed Eulerian-mean (TEM) analyses reveal that resolved waves are the dominant drivers of the QBOs. However, the Eliassen–Palm flux divergence is strongly counteracted by the TEM momentum budget residual, which represents the forcing by diffusion and thermal damping. Interestingly, a reversed Brewer–Dobson circulation accelerates the downward propagation of the SLD’s QBO, whereas the EUL’s and SE’s QBOs are slowed by a mean ascent. Waves are abundant in the SLD’s, EUL’s, and SE’s tropical atmosphere despite the absence of moist convection as a typical wave trigger. Dynamic instabilities are suggested as a wave-triggering mechanism in the troposphere and wave-dissipation process in the stratosphere. In particular, there are indications that the increased occurrences of strongly negative instability indicators in SLD, EUL, and SE are related to more vigorous wave activities and higher magnitudes of the resolved wave forcing in comparison to FV.

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Weiye Yao and Christiane Jablonowski

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The paper demonstrates that sudden stratospheric warmings (SSWs) can be simulated in an ensemble of dry dynamical cores that miss the typical SSW forcing mechanisms like moist processes, land–sea contrasts, or topography. These idealized general circulation model (GCM) simulations are driven by a simple Held–Suarez–Williamson (HSW) temperature relaxation and low-level Rayleigh friction. In particular, the four dynamical cores of NCAR’s Community Atmosphere Model, version 5 (CAM5), are used, which are the semi-Lagrangian (SLD) and Eulerian (EUL) spectral-transform models and the finite-volume (FV) and the spectral element (SE) models.

Three research themes are discussed. First, it is shown that SSW events in such idealized simulations have very realistic flow characteristics that are analyzed via the SLD model. A single vortex-split event is highlighted that is driven by wavenumber-1 and -2 wave–mean flow interactions. Second, the SLD simulations are compared to the EUL, FV, and SE dynamical cores, which sheds light on the impact of the numerical schemes on the circulation. Only SLD produces major SSWs, while others only exhibit minor stratospheric warmings. These differences are caused by SLD’s more vigorous wave–mean flow interactions in addition to a warm pole bias, which leads to relatively weak polar jets in SLD. Third, it is shown that tropical quasi-biennial oscillation (QBO)–like oscillations and SSWs can coexist in such idealized HSW simulations. They are present in the SLD dynamical core that is used to analyze the QBO–SSW interactions via a transformed Eulerian-mean (TEM) analysis. The TEM results provide support for the Holton–Tan effect.

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Augustine Y. M. Yao

Abstract

The beta distribution is the model for frequency distributions of relative humidity observations. The standard chi-square (χ2) test measures the goodness of fit of the beta distribution model to the observed data base. Ninety distributions of daily, mean 5-, 10- and 15-day, and mean monthly relative humidity data sets from 19 U. S. stations form the basis for this report. At the 0.05 level of significance the tests reject only 5% of the mean monthly and 3% of the daily and the mean 5-, 10- and 15-day distributions. The hypothesis that relative humidity follows a beta distribution is not rejected.

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Yao Ha and Zhong Zhong

Abstract

This study investigates the decadal change in tropical cyclone (TC) activity over the South China Sea (SCS) in the boreal summer (June–August) since the early 1990s and explores possible causes behind it. Results show that the SCS TC activity experienced an abrupt decadal decrease at around 2003/03. Compared to the TC activities from the early 1990s to 2002, the number of TCs formed in the SCS markedly decreased from 2003 through the early 2010s. Moreover, most of the TCs were primarily confined within the SCS basin during this period. The TCs that formed during the period of 2003–11 usually moved west-northwestward and rapidly weakened after making landfall. It is found that a significant decadal-scale sea surface temperature (SST) warming occurred in the northern Indian Ocean and the western Pacific Ocean after 2002 while convection intensified over the tropical regions between 60° and 80°E and around 150°E, respectively. The warm SST anomalies induced an anomalous subsiding flow over the SCS basin via the Walker-like (zonal) circulation. Meanwhile, anomalously dry, sinking air around 5°–20°N derived from local Hadley (meridional) circulation reinforced the subsiding flow of the zonal circulation. The above circulation patterns suppressed TC genesis over the northern SCS, leading to the decadal decrease in TC activity that occurred around 2002/03. In addition, in conjunction with the local anomalous easterly flow, the intraseasonal atmospheric variability over the SCS has decreased since the early 2000s. This is unfavorable for the development of synoptic-scale disturbances and may also contribute to the decadal decrease in TC activity.

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C. S. Yao

Abstract

Statistical methods of dichotomous variables are suggested in order to analyze the historical climatic records in ancient writings. From historical descriptive records of floods and droughts, we calculate the variability, persistence and interchangeability, cyclic period and period, and index of floods and droughts.

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C. S. Yao
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C. S. Yao

Abstract

Methods of fitting a linear autoregressive model to a stationary time series are summarized. Parameters of the linear autoregressive model were estimated by the Durbin stepwise procedure and the order of this model was chosen by means of a t-test or F-test. An illustrative example used to forecast the monthly rainfall is also presented.

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