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Judah Cohen and Mathew Barlow

Abstract

The North Atlantic Oscillation (NAO) and the closely related Arctic Oscillation (AO) strongly affect Northern Hemisphere (NH) surface temperatures with patterns reported similar to the global warming trend. The NAO and AO were in a positive trend for much of the 1970s and 1980s with historic highs in the early 1990s, and it has been suggested that they contributed significantly to the global warming signal. The trends in standard indices of the AO, NAO, and NH average surface temperature for December–February, 1950–2004, and the associated patterns in surface temperature anomalies are examined. Also analyzed are factors previously identified as relating to the NAO, AO, and their positive trend: North Atlantic sea surface temperatures (SSTs), Indo–Pacific warm pool SSTs, stratospheric circulation, and Eurasian snow cover.

Recently, the NAO and AO indices have been decreasing; when these data are included, the overall trends for the past 30 years are weak to nonexistent and are strongly dependent on the choice of start and end date. In clear distinction, the wintertime hemispheric warming trend has been vigorous and consistent throughout the entire period. When considered for the whole hemisphere, the NAO/AO patterns can also be distinguished from the trend pattern. Thus the December–February warming trend may be distinguished from the AO and NAO in terms of the strength, consistency, and pattern of the trend. These results are insensitive to choice of index or dataset. While the NAO and AO may contribute to hemispheric and regional warming for multiyear periods, these differences suggest that the large-scale features of the global warming trend over the last 30 years are unrelated to the AO and NAO. The related factors may also be clearly distinguished, with warm pool SSTs linked to the warming trend, while the others are linked to the NAO and AO.

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Judah Cohen and Justin Jones

Abstract

Many tropospheric Arctic Oscillation (AO) events are preceded by stratospheric AO events and even earlier in time by anomalous upward energy flux associated with Rossby waves in the troposphere. This study identifies lower-tropospheric circulation anomalies that precede large AO events in both the troposphere and stratosphere and the anomalous upward energy flux. Compositing analysis of stratospheric warming events identifies regional tropospheric precursors, which precede stratospheric warmings. The tropospheric precursor is found to vary when compositing over polar vortex displacements and splits separately. Prior to vortex displacements the main anomaly sea level pressure center of the tropospheric precursor is located across northwest Eurasia and is associated with the Siberian high. Prior to vortex splits a similar anomaly center is identified in the tropospheric precursor but is weaker and appears to be more strongly related to a shift in the storm tracks. Differences in the sea level pressure anomalies in the North Atlantic and the North Pacific are also observed when comparing the precursors prior to vortex displacements and splits. Identification of a unique tropospheric precursor to stratospheric warming and subsequent tropospheric AO events can help to improve understanding troposphere–stratosphere coupling. Furthermore, the observational evidence presented here can be compared with model simulations of winter climate variability and lead to potential model improvements.

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Judah Cohen and David Rind

Abstract

Large-scale snow cover anomalies are thought to cause significant changes in the diabatic heating of the earth's surface in such a way as to produce substantial local cooling in the surface temperatures. This theory was tested using the GISS 3-D GCM (General Circulation Model). The results of the GCM experiment showed that snow cover caused only a short term local decrease in the surface temperature. In the surface energy budget, reduction in absorbed shortwave radiation and the increased latent heat sink of melting snow contributed to lower temperatures. However, all the remaining heating terms contribute to increasing the net heating over a snow covered surface. The results emphasize the negative feedback which limits the impact of snow cover anomalies over longer time scales.

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Judah Cohen and Kazuyuki Saito

Abstract

The use of empirical orthogonal functions (EOFs) has grown popular as a tool to determine underlying variability from the rapidly increasing volume of climate data. It has been noted that the dominant or first EOF of geopotential heights, in each hemisphere at levels from the surface through the troposphere and into the midstratosphere, appears to be zonally symmetric. It has also been suggested that all of the first EOFs, throughout the atmospheric column are barotropically coupled and annular. Moreover, such modes of variability in both hemispheres are thought to be analogous to each other. To define annularity more objectively and to facilitate comparisons both temporally and spatially, a framework has been formulated within which modes of variability may be tested for their degree of zonal symmetry or annularity. Motivated by previous choices, pressure–height fields in each hemisphere are tested for annularity, one near the surface and the other in the midstratosphere. Periods chosen coincide with times when the troposphere and stratosphere are actively coupled. According to the test for annularity on the first mode of variability, these fields can be ranked in order of degree of annularity: the first EOF of Northern Hemisphere (NH) December–January–February (DJF) 50-hPa geopotential height is annular; the first EOF of Southern Hemisphere November 50-hPa geopotential height is weakly annular; the first EOF of Southern Hemisphere November 850-hPa geopotential height is weakly nonannular; and the first EOF of NH DJF sea level pressure is nonannular.

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Judah Cohen and Christopher Fletcher

Abstract

A statistical forecast model, referred to as the snow-cast (sCast) model, has been developed using observed October mean snow cover and sea level pressure anomalies to predict upcoming winter land surface temperatures for the extratropical Northern Hemisphere. In operational forecasts since 1999, snow cover has been used for seven winters, and sea level pressure anomalies for three winters. Presented are skill scores for these seven real-time forecasts and also for 33 winter hindcasts (1972/73–2004/05). The model demonstrates positive skill over much of the eastern United States and northern Eurasia—regions that have eluded skillful predictions among the existing major seasonal forecast centers. Comparison with three leading dynamical forecast systems shows that the statistical model produces superior skill for the same regions. Despite the increasing complexity of the dynamical models, they continue to derive their forecast skill predominantly from tropical atmosphere–ocean coupling, in particular from ENSO. Therefore, in the Northern Hemisphere extratropics, away from the influence of ENSO, the sCast model is expected to outperform the dynamical models into the foreseeable future.

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Judah Cohen and Justin Jones
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Justin E. Jones and Judah Cohen

Abstract

Strong anticyclones have a significant impact on the cool season climate over mid- and high-latitude landmasses as they are typically accompanied by arctic air masses that can eventually move into populated midlatitude regions. Composite analyses of Alaskan and Siberian strong anticyclones based on sea level pressure (SLP) thresholds of 1050 and 1060 hPa, respectively, were performed to diagnose large-scale dynamical and thermodynamical parameters associated with the formation of strong anticyclones over these two climatologically favorable regions. The anticyclone composite analyses indicate the presence of moderate-to-high-amplitude ridge–trough patterns associated with anticyclogenesis. These ridge–trough patterns are critical as they lead to dynamically favorable circumstances for rapid anticyclogenesis.

The strong Alaskan anticyclone develops downstream of a highly amplified upper-tropospheric ridge and is associated with a region of strong tropospheric subsidence due to differential anticyclonic vorticity advection and cold-air advection over the anticyclone center. The strong Siberian anticyclone is associated with an upper-tropospheric pattern of lesser amplitude, suggesting that these dynamical factors, while still important, are less critical to its development. The relative location of elevated terrain features also appears to contribute greatly to the overall evolution of each of these anticyclones.

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Leonard M. Druyan, Patrick Lonergan, and Judah Cohen

Abstract

African wave disturbances (AWDs), an important trigger of Sahel summer rainfall, are studied using ECMWF gridded datasets for July and August 1987 and 1988. Power spectra of time series of 700-mb meridional winds near Niamey taken from analyses at both 2° × 2.5° and 4° × 5° horizontal resolution are compared to spectra based on Niamey station data. In addition, spatial distributions of meteorological fields at both resolutions are discussed for three case studies, including the synoptic features of several AWDs. Additional examples are presented from GCM simulations at comparable horizontal resolutions. While vertical motion and divergence centers were more extreme at 2° × 2.5°, many of the deduced characteristics of an AWD were similar at both resolutions. The higher-resolution analyses and simulation show a sharp transition across wave troughs between lower-tropospheric convergence (uplift) on the west and divergence (subsidence) on the east for several AWDs. For the two more southerly AWDs analyzed here, uplift associated with the convergence ahead of the trough appears to be displaced to the southwest at midtropospheric altitudes. Twice-daily July–September precipitation at Niamey is weakly, but significantly, correlated with corresponding time series of ECMWF analyzed vertical motion at nearby grid points.

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Gavin Gong, Dara Entekhabi, and Judah Cohen

Abstract

Numerous studies have hypothesized that surface boundary conditions or other external mechanisms drive the hemispheric mode of atmospheric variability known as the Arctic Oscillation (AO), or its regional counterpart, the North Atlantic Oscillation (NAO). However, no single external factor has emerged as the dominant forcing mechanism, which has led, in part, to the characterization of the AO–NAO as a fundamental internal mode of the atmospheric system. Nevertheless, surface forcings may play a considerable role in modulating, if not driving, the AO–NAO mode. In this study, a pair of large-ensemble atmospheric GCM experiments (with SST climatology), one with prescribed climatological snow mass and another with freely varying snow mass, is conducted to investigate the degree to which the AO–NAO is modulated by interannual variability of surface snow conditions. Statistical analysis of the results indicates that snow anomalies are not required to produce the AO–NAO mode of variability. Nevertheless, interannual variations in snow mass are found to exert a modulating influence on the AO–NAO mode. Snow variations excite the AO pattern over the North Atlantic sector, produce correlated hemispheric AO features throughout the troposphere and stratosphere, and generate autumn sea level pressure anomalies over Siberia that evolve into the winter AO–NAO. These numerical modeling results are consistent with previous observational analyses that statistically link the AO–NAO mode with the Siberian high and associated snow cover variations.

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Kazuyuki Saito, Judah Cohen, and Dara Entekhabi

Abstract

Recently it has been shown that the area extent of Eurasian snow cover during September–October–November (SON) and the principal component of the leading mode of extratropical Northern Hemisphere (NH) climate variability in the following winter are statistically correlated. In this paper, physical linkages between SON Eurasian snow cover and the wintertime climate variability in the NH atmosphere are postulated. And in order to test the proposed hypotheses, comprehensive analyses of satellite-based observations for snow cover and reanalysis data for geopotential heights and sea level pressure are used.

The magnitude of the correlation between snow cover and climate variability is found to be inversely proportional to the height suggesting that snow cover may act as a lower boundary forcing to the tropospheric circulation. Conversely, however, an index constructed to capture the downward propagating circulation anomaly from the lower stratosphere to the middle troposphere is shown to be as highly correlated with snow cover variability as the Arctic oscillation derived from sea level pressure.

A mechanism involving the vertical propagation of Rossby waves is proposed to explain this apparent contradiction. Anomalous fall snow cover variability not only alters near-surface temperatures but also impacts upward propagating Rossby waves. Changes forced in the stratosphere by anomalous snow cover are not realized until later in the winter season when the troposphere and stratosphere are actively coupled.

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