Search Results

You are looking at 1 - 10 of 31 items for

  • Author or Editor: Robert J. Allen x
  • All content x
Clear All Modify Search
Robert J. Allen and Mahesh Kovilakam

Abstract

Observations show the tropical belt has widened over the past few decades, a phenomenon associated with poleward migration of subtropical dry zones and large-scale atmospheric circulation. Coupled climate models also simulate tropical belt widening, but less so than observed. Reasons for this discrepancy, and the mechanisms driving the expansion remain uncertain. Here, the role of unforced, natural climate variability—particularly natural sea surface temperature (SST) variability—in recent tropical widening is shown. Compared to coupled ocean–atmosphere models, atmosphere-only simulations driven by observed SSTs consistently lead to larger rates of tropical widening, especially in the Northern Hemisphere (NH), highlighting the importance of recent SST evolution. Assuming the ensemble mean SSTs from historical simulations accurately represent the externally forced response, the observed SSTs can be decomposed into a forced and an unforced component. Targeted simulations with the Community Atmosphere Model, version 5 (CAM5), show that natural SST variability accounts for nearly all of the widening associated with recent SST evolution. This is consistent with the similarity of the unforced SSTs to the observed SSTs, both of which resemble a cold El Niño–Southern Oscillation/Pacific decadal oscillation (ENSO/PDO)-like SST pattern, which is associated with a wider tropical belt. Moreover, CAM5 coupled simulations with observed central to eastern tropical Pacific SSTs yield more than double the rate of widening compared to analogous simulations without prescribed tropical Pacific SSTs and reproduce the magnitude of tropical widening in atmosphere-only simulations. The results suggest that the bulk of recent tropical widening, particularly in the NH, is due to unforced, natural SST variability, primarily related to recent ENSO/PDO variability.

Full access
Robert J. Allen and Steven C. Sherwood

Abstract

The utility of the thermal wind equation (TWE) in relating tropospheric (850–300 hPa) wind and temperature on climatological time scales is assessed, based on data from 59 radiosonde stations in the western tropical Pacific during 1979–2004. Observed long-term mean and seasonal variations closely obey geostrophic balance; incorporating additional (ageostrophic) terms yields negligible improvement. The authors conclude that observed winds offer a useful constraint on the horizontal structure of monthly and longer temperature variations (although the reverse is not true close to the equator where f → 0). This conclusion is also supported by general circulation model output.

Wind data show a slowing of the midlatitude jets in the Maritime Continent region since 1979, indicating that tropical thicknesses and temperature have increased less than those poleward of 25°N/S. This pattern is consistent with Microwave Sounding Unit temperature trends in the region but is exaggerated south of the equator in trends obtained directly from the temperature data. The latter are however sensitive to which stations are used and how the data are averaged, and the discrepancy is fairly small in a homogenized climatology from the Hadley Centre (HadAT). The discrepancy is most easily explained by spurious cooling at stations in the near-equatorial western Pacific.

These results support the use of the wind field as a way of overcoming heterogeneities in the temperature records in the monitoring of climate change patterns.

Full access
Robert J. Allen and Arthur T. DeGaetano

Abstract

A method to homogenize nonclimatic discontinuities in temperature extreme exceedence series is developed and evaluated. The method is based on a set of complementary tests with the application of an individual test depending on the availability of an adequate network of nearby homogeneous reference stations and the presence of significant trends in the resulting difference or original exceedence series. Given a suitable set of neighboring reference stations, a comparison of the differences in exceedences between the inhomogeneous station and neighboring sites is made for the periods before and after the documented discontinuity. In the absence of one or more reference stations, the exceedences at the inhomogeneous station are compared before and after the nonclimatic change. A method by which nonstationary series are detrended and subsequently evaluated is also presented.

When tested using homogenized data series into which an artificial discontinuity of known magnitude was introduced, as many as 80% of the ±1°F discontinuities were detected by the difference series approach. The performance of the single-station exceedence series test was less accurate. Although in a few cases, less than 40% of the ±1°F discontinuities were detected, between 60% and 76% of the ±2°F discontinuities were identified. Using both tests, the probability of falsely detecting a discontinuity (i.e., identifying an inhomogeneity when none existed) was 5%. Provided both methods deemed a documented inhomogeneity significant, the magnitude of the adjustments imposed by both methods was similar.

Full access
Arthur T. DeGaetano and Robert J. Allen

Abstract

A long-term, homogeneous set of daily maximum and minimum temperature data representing a subset of daily U.S. Historical Climatology Network stations is used to analyze trends in extreme temperature occurrence across the contiguous United States. Time series of various lengths are analyzed, with the longest spanning the period 1900–96. Trends in the annual occurrence of extreme maximum and minimum temperatures (e.g., values greater than the 90th, 95th, or 99th percentile) are strongly influenced by high exceedence counts during drought periods in the 1930s and 1950s. Peaks in exceedences during these years result in predominantly decreasing warm exceedence trends across the country during the 1930–96 period. This is uncharacteristic of recent years (1960–96) in which a large majority of stations show increases in warm extreme temperature exceedences. Significant increases in warm minimum temperature exceedences are found at nearly one-third of the stations during this period. Multiday warm temperature exceedence runs also show strong increases during this more recent period. The most rapid increases in high maximum and minimum temperature extremes occur at stations classified as urban, by satellite land use information.

Trends in the annual occurrence of extremely cold maximum and minimum temperatures display an analogous decrease during the 1960–96 period. Here again, there is a distinct shift in the number of decreasing trends between the 1950–96 and 1960–96 periods. Based on starting decades prior to 1960, there is not a strong tendency for either increasing or decreasing trends. The period 1910–96 is an exception, with almost all stations exhibiting decreasing cold extreme occurrence trends. The extreme cold exceedence trends during the 1960–96 period are also influenced by urbanization, but to a lesser degree than the warm extremes.

Full access
Robert J. Allen and Charles S. Zender

Abstract

Throughout much of the latter half of the twentieth century, the dominant mode of Northern Hemisphere (NH) extratropical wintertime circulation variability—the Arctic Oscillation (AO)—exhibited a positive trend, with decreasing high-latitude sea level pressure (SLP) and increasing midlatitude SLP. General circulation models (GCMs) show that this trend is related to several factors, including North Atlantic SSTs, greenhouse gas/ozone-induced stratospheric cooling, and warming of the Indo-Pacific warm pool. Over the last approximately two decades, however, the AO has been decreasing, with 2009/10 featuring the most negative AO since 1900. Observational and idealized modeling studies suggest that snow cover, particularly over Eurasia, may be important. An observed snow–AO mechanism also exists, involving the vertical propagation of a Rossby wave train into the stratosphere, which induces a negative AO response that couples to the troposphere. Similar to other GCMs, the authors show that transient simulations with the Community Atmosphere Model, version 3 (CAM3) yield a snow–AO relationship inconsistent with observations and dissimilar AO trends. However, Eurasian snow cover and its interannual variability are significantly underestimated. When the albedo effects of snow cover are prescribed in CAM3 (CAM PS) using satellite-based snow cover fraction data, a snow–AO relationship similar to observations develops. Furthermore, the late-twentieth-century increase in the AO, and particularly the recent decrease, is reproduced by CAM PS. The authors therefore conclude that snow cover has helped force the observed AO trends and that it may play an important role in future AO trends.

Full access
Allen J. Lenzen, Donald R. Johnson, and Robert Atlas

Abstract

Quasi-Lagrangian diagnostics of mass, angular momentum, water vapor, and kinetic energy are evaluated for four different Goddard Laboratory for Atmospheres model simulations of the Queen Elizabeth II storm of 9–11 September 1978 to study the impact of Seasat-A satellite Scatterometer (SASS) winds and horizontal resolution in numerical prediction. In a four-way comparison, the diagnostics investigate the impact of including dealiased SASS winds in the initial conditions of the model and doubling the horizontal resolution on 36 h simulations of the QE II storm. The largest impact on the simulation stemmed from doubling the model's horizontal resolution from 4° × 5° to 2° × 2.5°. The increased resolution resulted in a storm track much closer to that observed, a much deeper surface development, a stronger mass circulation, stronger heating, and stronger increase of angular momentum. The inclusion of SASS data resulted in an approximately 2–3-mb-deeper surface cyclone for both the 2° × 2.5° and 4° × 5° resolution simulations. The inclusion also led to substantial increases in the horizontal mass circulation and heating for the 2° × 2.5° simulation. During the early explosive deepening phase of the cyclone, the inward lateral transport of water vapor in lower layers was larger in the 2° × 2.5° SASS than in the 2° × 2.5° NOSASS (exclusion of SASS surface winds) simulation. During the period of most rapid development, the results from the SASS simulation revealed a larger generation of kinetic energy throughout the troposphere and increased outward transport of kinetic energy in upper layers.

Full access
Arthur T. DeGaetano, Robert J. Allen, and Kevin P. Gallo

Abstract

A subset of stations from the daily U.S. Historical Climatology Network (HCN) is used as a basis for a historical database of temperature extreme occurrence in the United States. The dataset focuses on daily temperature occurrences that exceed (fall below) the 90th (10th) percentiles of daily maximum and minimum temperature. Using a variety of techniques, the temperature extreme occurrence data are homogenized to account for nonclimatic shifts resulting from station relocations, changes in instrument type, and variations in the time of observations. Given the daily resolution of the extreme data, these potential sources of inhomogeneity require testing and adjustment using methods other than those conventionally used with mean temperature data. A data estimation technique, specific to extremes, is also used to produce serially complete exceedence records. Stations are also identified based on their current degree of urbanization using satellite observations. The dataset is intended to provide a research-quality source of temperature extreme data, analogous and complementary to the daily HCN dataset.

Two analyses are presented that illustrate the influence of adjustment. The change in temperature extreme occurrence with time reverses at between 15% and 20% of the HCN stations depending upon whether adjusted or unadjusted series is used. Changes in the distribution of extreme occurrences during drought and nondrought years are also shown to occur.

Full access
Robert J. Allen, Amato T. Evan, and Ben B. B. Booth

Abstract

Through the latter half of the twentieth century, meridional shifts in tropical precipitation have been associated with severe droughts. Although linked to a variety of causes, the origin of these shifts remains elusive. Here, it is shown that they are unlikely to arise from internal variability of the climate system alone, as simulated by coupled ocean–atmosphere climate models. Similar to previous work, the authors find that anthropogenic and volcanic aerosols are the dominant drivers of simulated twentieth-century tropical precipitation shifts. Models that include the cloud-albedo and lifetime aerosol indirect effects yield significantly larger shifts than models that lack aerosol indirect effects and also reproduce most of the southward tropical precipitation shift in the Pacific. However, all models significantly underestimate the magnitude of the observed shifts in the Atlantic sector, unless driven by observed SSTs. Mechanistically, tropical precipitation shifts are driven by interhemispheric sea surface temperature variations, which are associated with hemispherically asymmetric changes in low-latitude surface pressure, winds, and low clouds, as well as the strength, location, and cross-equatorial energy transport of the Hadley cells. Models with a larger hemispheric aerosol radiative forcing gradient yield larger hemispheric temperature contrasts and, in turn, larger meridional precipitation shifts. The authors conclude that aerosols are likely the dominant driver of the observed southward tropical precipitation shift in the Pacific. Aerosols are also significant drivers of the Atlantic shifts, although one cannot rule out a contribution from natural variability to account for the magnitude of the observed shifts.

Full access
Kenneth H. Brink, J. S. Allen, and Robert L. Smith

Abstract

An analysis is presented of low-frequency (<0.4 cpd) fluctuations in currents, temperature and tide gage data collected during the March-September 1976 segment of the CUEA JOINT-II experiment off the coast of Peru. The observations were made near 15°S, a region of particularly strong and persistent coastal upwelling. Conclusions about the dynamics of motions over the continental shelf and slope are reached by means of correlations, empirical orthogonal functions and other indicators. It is found that flow over the shelf, where stratification was weak, was generally dominated by vertical turbulent frictional effects and was strongly coupled to the effectively inviscid, baroclinic flow over the slope. The momentum balance was three dimensional, with the alongshore pressure gradient playing an important role. In contrast to behavior in other coastal upwelling regions, the alongshore velocity field over the shelf and slope was evidently not strongly driven by the local alongshore component of the wind stress. The mean wind stress throughout the period was equatorward (upwelling favorable), whereas the mean alongshore currents over the shelf were poleward. The alongshore current fluctuations, which propagated poleward along the coast, were initially poorly correlated with the local wind stress, but during the course of the experiment, the wind stress increased in magnitude and gained in importance as a driving mechanism. The temperature and onshore-offshore current fluctuations over the shelf and, therefore, presumably the upwelling circulation were, however, correlated with the local wind stress throughout the experiment.

Full access
Pijush K. Kundu, J. S. Allen, and Robert L. Smith

Abstract

The low-frequency [ω<0.5 cycle per day (cpd)] current fluctuations at four depths in 100 m of waterhave been investigated for two stations on the continental shelf off the coast of Oregon. One station, DB-7,was maintained during the summer of 1972 as part of the Coastal Upwelling Experiment-1 (CUE-I), and theother station, Carnation, was maintained during the summer of 1973 as part of CUE-II. A decomposition ofthe north-south (almost alongshore) v and the east-west (onshore-offshore) u components of the current hasbeen performed in terms of two types of modal structures in the vertical direction: (i) dynamic modes determined by the separable solutions of the appropriate equations of motion, and (ii) empirical orthogonal modeswhich are the eigenvectors of the correlation matrix and depend only on the statistics of the data. For thealongshore currents, the standard deviation of the dynamic barotropic mode is found to be twice as large asthat of the first baroclinic mode. The barotropic part is found to be correlated with the north-south component of the wind stress τw and the sea level, whereas the first mode baroclinic part is found to be correlatedwith the temperature fluctuations. The first empirical eigenmode accounts for about 91% of the energy andis fairly depth-independent, whereas the second empirical eigenmode accounts for about 7% of the energyand resembles the first dynamic baroclinic mode. Spectral analysis shows high mutal coherence between thebarotropic modes for the u and v components and the wind stress nr at the frequencies 0.06 cpd in 1973 and0.14 cpd in 1972. Results from a theoretical model show that the observed values of the phase relations atthese frequencies are consistent with a resonant condition between the wind stress and forced, long, barotropic continental shelf waves.

Full access