California has experienced devastating autumn wildfires in recent years. These autumn wildfires have coincided with extreme fire weather conditions during periods of strong offshore winds coincident with unusually dry vegetation enabled by anomalously warm conditions and late onset of autumn precipitation. In this study, we quantify observed changes in the occurrence and magnitude of meteorological factors that enable extreme autumn wildfires in California, and use climate model simulations to ascertain whether these changes are attributable to human-caused climate change. We show that state-wide increases in autumn temperature (∼1 °C) and decreases in autumn precipitation (∼30%) over the past four decades have contributed to increases in aggregate fire weather indices (+20%). As a result, the observed frequency of autumn days with extreme (95th percentile) fire weather—which we show are preferentially associated with extreme autumn wildfires—has more than doubled in California since the early 1980s. We further find an increase in the climate model-estimated probability of these extreme autumn conditions since ∼1950, including a long-term trend toward increased same-season co-occurrence of extreme fire weather conditions in northern and southern California. Our climate model analyses suggest that continued climate change will further amplify the number of days with extreme fire weather by the end of this century, though a pathway consistent with the UN Paris commitments would substantially curb that increase. Given the acute societal impacts of extreme autumn wildfires in recent years, our findings have critical relevance for ongoing efforts to manage wildfire risks in California and other regions.

The interference between transient eddies and climatological stationary eddies in the Northern Hemisphere is investigated. The amplitude and sign of the interference is represented by the stationary wave index (SWI), which is calculated by projecting the daily 300-hPa streamfunction anomaly field onto the 300-hPa climatological stationary wave. ERA-Interim data for the years 1979 to 2013 are used. The amplitude of the interference peaks during boreal winter. The evolution of outgoing longwave radiation, Arctic temperature, 300-hPa streamfunction, 10-hPa zonal wind, Arctic sea ice concentration, and the Arctic Oscillation (AO) index are examined for days of large SWI values during the winter. Constructive interference during winter tends to occur about one week after enhanced warm pool convection and is followed by an increase in Arctic surface air temperature along with a reduction of sea ice in the Barents and Kara Seas. The warming of the Arctic does occur without prior warm pool convection, but it is enhanced and prolonged when constructive interference occurs in concert with enhanced warm pool convection. This is followed two weeks later by a weakening of the stratospheric polar vortex and a decline of the AO. All of these associations are reversed in the case of destructive interference. Potential climate change implications are briefly discussed.

The wintertime (December – February) 1990 - 2016 Arctic surface air temperature (SAT) trend is examined using self-organizing maps (SOMs). The high dimensional SAT dataset is reduced into nine representative SOM patterns, with each pattern exhibiting a decorrelation time scale about 10 days and having about 85% of its variance coming from intraseasonal timescales. The trend in the frequency of occurrence of each SOM pattern is used to estimate the interdecadal Arctic winter warming trend associated with the SOM patterns. It is found that trends in the SOM patterns explain about one-half of the SAT trend in the Barents and Kara Seas, one-third of the SAT trend around Baffin Bay and two-thirds of the SAT trend in the Chukchi Sea. A composite calculation of each term in the thermodynamic energy equation for each SOM pattern shows that the SAT anomalies grow primarily through the advection of the climatological temperature by the anomalous wind. This implies that a substantial fraction of Arctic amplification is due to horizontal temperature advection that is driven by changes in the atmospheric circulation. An analysis of the surface energy budget indicates that the skin temperature anomalies as well as the trend, although very similar to that of the SAT, are produced primarily by downward longwave radiation.

The response to the Madden-Julian oscillation (MJO) over the Pacific-North American (PNA) region is investigated. In addition, the sensitivity of this response to the interaction between Madden-Julian oscillation forcing and the extratropical initial flow is explored. First, a simple dynamical model is run with an ensemble of 100 randomly selected initial conditions from ERA-Interim data, with no heating, MJO phase-1-like heating, and MJO phase-5-like heating. The 300-hPa geopotential height field is separated into a part that would evolve without an active MJO present, and a part that is a consequence of the MJO heating. A negative 300-hPa geopotential height anomaly centered over northeastern China bounded by positive anomalies on its equatorward and poleward flanks is found to be followed by a large amplitude negative PNA-like response for MJO phase 1 and a positive PNA-like response for phase 5.A similar study is carried out using observational data. An analog approach-using projections to determine the analogs-is used to approximate the part of the flow that results from the MJO heating. The composite initial flow that corresponds to a large MJO response in observational data somewhat matches that in the model, although there is more variability between phases. Finally, the analog method is used to examine questions related to predictability and the MJO. It is found that predictability is improved by taking into account the presence of the MJO and by choosing analogs with high projections. The presence of an active MJO also increases predictability.

Tropical precipitation anomalies associated with El Niño and Madden–Julian oscillation (MJO) phase 1 (La Niña and MJO phase 5) are characterized by a tripole, with positive (negative) centers over the Indian Ocean and central Pacific and a negative (positive) center over the warm pool region. However, their midlatitude circulation responses over the North Pacific and North America tend to be of opposite sign. To investigate these differences in the extratropical response to tropical convection, the dynamical core of a climate model is used, with boreal winter climatology as the initial flow. The model is run using the full heating field for the above four cases, and with heating restricted to each of seven small domains located near or over the equator, to investigate which convective anomalies may be responsible for the different extratropical responses. An analogous observational study is also performed. For both studies, it is found that, despite having a similar tropical convective anomaly spatial pattern, the extratropical response to El Niño and MJO phase 1 (La Niña and MJO phase 5) is quite different. Most notably, responses with opposite-signed upper-tropospheric geopotential height anomalies are found over the eastern North Pacific, northwestern North America, and the southeastern United States. The extratropical response for each convective case most closely resembles that for the domain associated with the largest-amplitude precipitation anomaly: the central equatorial Pacific for El Niño and La Niña and the warm pool region for MJO phases 1 and 5.

The dynamical core of a dry global model is used to investigate the role of central Pacific versus warm pool tropical convection on the extratropical response over the North Pacific and North America. A series of model runs is performed in which the amplitude of the warm pool (WP) and central Pacific (CP) heating anomalies associated with the MJO and El Niño–Southern Oscillation (ENSO) is systematically varied. In addition, model calculations based on each of the eight MJO phases are performed, first using stationary heating, and then with heating corresponding to a 48-day MJO cycle and to a 32-day MJO cycle. In all model runs, the extratropical response to tropical convection occurs within 7–10 days of the convective heating. The response is very sensitive to the relative amplitude of the heating anomalies. For example, when heating anomalies in the WP and CP have similar amplitude but opposite sign, the amplitude of the extratropical response is much weaker than is typical for the MJO and ENSO. For the MJO, when the WP heating anomaly is much stronger than the CP heating anomaly (vice versa for ENSO), the extratropical response is amplified. For the MJO heating, it is found that the extratropical responses to phases 4 and 8 are most distinct. A likely factor contributing to this distinctiveness involves the relative amplitude of the two heating anomalies. The stationary and moving (48- and 32-day) heating responses are very similar, revealing a lack of sensitivity to the MJO phase speed.

The tropospheric response to Sudden Stratospheric Warmings (SSWs) is associated with an equatorward shift in the midlatitude jet and associated storm tracks, while Strong Polar Vortex (SPV) events elicit a contrasting response. Recent analyses of the North Atlantic jet using probability density functions of a jet latitude index have identified three preferred jet latitudes, raising the question of whether the tropospheric response to SSWs and SPVs results from a change in relative frequencies of these preferred jet regimes rather than a systematic jet shift. We explore this question using atmospheric reanalysis data from 1979 to 2018 (26 SSWs and 33 SPVs), and a 202‐years integration of the Whole Atmosphere Community Climate Model (92 SSWs and 68 SPVs). Following SSWs, the northern jet regime becomes less common and the central and southern regimes become more common. These changes occur almost immediately following “split” vortex events, but are more delayed following “displacement” events. In contrast, the northern regime becomes more frequent and the southern regime less frequent following SPV events. Following SSWs, composites of 500‐hPa geopotential heights, surface air temperatures, and precipitation most closely resemble composites of the southern jet regime, and are generally opposite in sign to the composites of the northern jet regime. These comparisons are reversed following SPVs. Thus, one possible interpretation is that the two southernmost regimes appear to be favored following SSWs, while the southernmost regime becomes less common following SPVs.

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. This paper was prepared for the 46th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, held in New Orleans, Oct. 3–6, 1971. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal, provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract Solutions to the equations describing flow in porous media under sinusoidal pressure conditions are available in the pressure conditions are available in the literature. Very little use has been made of this theory and the experimental applications reported have dealt with gases and no flow through the media. Frequency response data from sinusoidal pressure experiments using liquids can be pressure experiments using liquids can be used to characterize the porous media and fluid properties. The lack of this type of data is probably due to the difficulty of generating the sinusoidal disturbances having sufficient frequency range and amplitude. Measurements were made using a synthetic oil in an unconsolidated medium. Equipment was designed to generate pressure sine wave whose upper frequency limit was in excess of 1,000 rad./sec. The core was equipped with high speed pressure transducers at various positions along the medium which measured the positions along the medium which measured the pressure response at a known flow rate. pressure response at a known flow rate. The experiments were designed to study the effect of pressure amplitude and frequency on the response of the system. Comparison of the theory and these experiments indicates that there are definite frequency limits which may be used if the equations are to adequately describe the data. As the frequency and or permeability increases, the inertial terms become important. This study shows that practical use can be made of existing theory to increase our ability to describe the flow of fluids in porous media. porous media Introduction The measurement of pressure transient responses is one of the few methods available for the reservoir to communicate its rock and fluid properties to an observer. Frequency response methods have been used by several investigators to determine rock and fluid properties. Experimental studies have been reported which used sinusoidal pressure inputs in gas filled porous systems with storage pores. Other porous systems with storage pores. Other investigators have used pluse testing methods to determine reservoir properties. Several methods have been proposed for describing the behavior of pressure transients in porous media.