Dennis J. Shea

A New Sea Surface Temperature and Sea Ice Boundary Dataset for the Community Atmosphere Model

A new surface boundary forcing dataset for uncoupled simulations with the Community Atmosphere Model is described. It is a merged product based on the monthly mean Hadley Centre sea ice and SST dataset version 1 (HadISST1) and version 2 of the National Oceanic and Atmospheric Administration (NOAA) weekly optimum interpolation (OI) SST analysis. These two source datasets were also used to supply ocean surface information to the 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40). The merged product provides monthly mean sea surface temperature and sea ice concentration data from 1870 to the present: it is updated monthly, and it is freely available for community use. The merging procedure was designed to take full advantage of the higher-resolution SST information inherent in the NOAA OI.v2 analysis.

Atlantic hurricanes and natural variability in 2005

[1] The 2005 North Atlantic hurricane season (1 June to 30 November) was the most active on record by several measures, surpassing the very active season of 2004 and causing an unprecedented level of damage. Sea surface temperatures (SSTs) in the tropical North Atlantic (TNA) region critical for hurricanes (10° to 20°N) were at record high levels in the extended summer (June to October) of 2005 at 0.9°C above the 1901-70 normal and were a major reason for the record hurricane season. Changes in TNA SSTs are associated with a pattern of natural variation known as the Atlantic Multi-decadal Oscillation (AMO). However, previous AMO indices are conflated with linear trends and a revised AMO index accounts for between 0 and 0.1°C of the 2005 SST anomaly. About 0.45°C of the SST anomaly is common to global SST and is thus linked to global warming and, based on regression, about 0.2°C stemmed from after-effects of the 2004-05 El Nino.

On the Evolution of the Southern Oscillation

Abstract The evolution of the Southern Oscillation (SO) is examined in the time domain by computing lagged cross correlations between sea level pressures at Darwin and sea level or surface pressures at selected stations. Also, in the Northern Hemisphere, the historical and U.S. Navy sea level pressure analyses are used. All monthly time series are low-pass filtered to retain periodicities greater than 20 months in order to highlight the interannual fluctuations which are primarily associated with the SO. A detailed analysis of the post-1941 period results in plotted maps of the phase (lead or lag) and magnitude of the maximum cross correlations with Darwin, in a manner analogous to a broadband coherence and phase spectrum. The relationships within the SO are further examined, where possible, back to 1882 using time series of running decadal cross covariances. The dominant pattern reveals the two poles of the traditional standing oscillation or seesaw of the SO, with centers of opposite sign over Indonesia...

A Global Monthly Sea Surface Temperature Climatology

Abstract A new global 2°×2° monthly sea surface temperature (SST) climatology, primarily derived from a 1950–1979-based SST climatology from the Climate Analysis Center (CAC), is presented and described. The CAC climatology has been modified by using data from the Comprehensive Ocean-Atmosphere Data Set to improve the SST estimates in the regions of the Kuroshio and the Gulf Stream. This results in considerably larger and more realistic SST gradients in these regions. This modified climatology is smoothed in time using a truncated Fourier series to eliminate mean annual cycle fluctuations of three months or less, and finally, some spatial smoothing is applied over the high-latitude southern oceans. This new SST climatology, which we call the Shea-Trenberth-Reynolds (STR) climatology, is compared with the Alexander and Mobley (AM) SST climatology often used as a lower boundary condition by general circulation models. Significant differences are noted. Generally, the STR climatology is warmer in the Norther...

The Hurricane’s Inner Core Region. I. Symmetric and Asymmetric Structure

Abstract Observational information from 533 radial flight legs executed by the National Hurricane Research Laboratory over a 13-year period (1957–69) is used to present the structural characteristics and the variability of the hurricane’s inner core region. Tangential and radial winds, D-values, and adjusted temperatures are composited with respect to the Radius of Maximum Wind (RMW) in order to construct a five-level mean symmetric storm and a five-level mean asymmetric storm. The slope of the RMW with height and the position of the RMW relative to the inner cloud wall are presented. Utilizing these results, an idealized, steady-state schematic model of the flow conditions in the inner hurricane core is presented. Storms are stratified by deepening and filling tendency, intensity and storm speed. Finally, the variations of the RMW with latitude, maximum wind, inner radar radius, central pressure, and other features are discussed. Many significant features are noted: 1) storm inflow is confined almost exc...

The Southern Oscillation. Part IV: The Precursors South of 15°S to the Extremes of the Oscillation

Abstract The year before a Warm Event takes place in the Southern Oscillation the trough in the westerlies at the surface over the South Pacific Ocean fails to amplify to its normal size in the latitudes north of 45°S during the southern fall and winter. There is therefore an anomalous northerly wind in these months over the Pacific Ocean between 15°S and 45°S, west of 140°W. In contrast, the troughs amplitude is above normal in the fall and winter of the following year when the Warm Event takes place, and one therefore observes an anomalous southerly wind where a northerly anomaly occurred the previous year. Consistent with the different wind anomalies, the temperature of the surface water is higher in the year before the Warm Event than in the year of the event between 15°S and 45°S, from Australia to 140°W. We propose that when the South Pacific Convergence Zone expands toward the south as usual in the southern spring of the year before a Warm Event, the convection in the Convergence Zone is enhanced ...

The Southern Oscillation. Part VI: Anomalies of Sea Level Pressure on the Southern Hemisphere and of Pacific Sea Surface Temperature during the Development of a Warm Event

Abstract The paper shows the discrete, mean three-month anomalies of sea level pressure on the Southern Hemisphere during the year before and the year of a Warm Event in the Southern Oscillation, together with associated anomalies of sea surface temperature in the South Pacific 0cean. The two sets anomalies develop in a parallel and physically logical sequence over the South Pacific Ocean in conjunction with changes in the South Pacific Convergence Zone. Nearly all of the Southern Hemisphere responds to the Southern Oscillation, but the response is largest in the Australia-South Pacific sector. Large anomalies of sea level pressure form well ahead of any on the Northern Hemisphere, and this observation together with the conspicuous anomalies in the region of Australia and the South Pacific suggest that the origin of the Southern Oscillation must be sought in this region.

The Hurricane’s Inner Core Region. II. Thermal Stability and Dynamic Characteristics

Abstract This is the second (Paper II) of two papers on the characteristics of the hurricane’s inner core region as revealed by the research flight data of the National Hurricane Research Laboratory. This paper presents information on the thermal stability and the dynamic characteristics of the hurricane’s inner core region from information derived from Paper I. Discussion is given on the hurricane’s inner core vertical stability, divergence, vertical motion, heating mechanism, wind-pressure acceleration, thermal wind balances, and other features. It is shown that large vertical moist instability is present in the eye-wall cloud. Large super-gradient winds are present at the radius of maximum winds. Substantial mixing occurs between eye and eye wall and the average hurricane eye ventilates itself by about half of its mass during the time it takes to move the distance of its eye diameter. Maximum heating does not occur at the radius of maximum updraft. Inner core heating comes from the sinking motion withi...

Bayesian Climate Change Assessment

Abstract A Bayesian fingerprinting methodology for assessing anthropogenic impacts on climate was developed. This analysis considers the effect of increased CO2 on near-surface temperatures. A spatial CO2 fingerprint based on control and forced model output from the National Center for Atmospheric Research Climate System Model was developed. The Bayesian approach is distinguished by several new facets. First, the prior model for the amplitude of the fingerprint is a mixture of two distributions: one reflects prior uncertainty in the anticipated value of the amplitude under the hypothesis of “no climate change.” The second reflects behavior assuming“climate change forced by CO2.” Second, within the Bayesian framework, a new formulation of detection and attribution analyses based on practical significance of impacts rather than traditional statistical significance was presented. Third, since Bayesian analyses can be very sensitive to prior inputs, a robust Bayesian approach, which investigates the ranges of...

Correlations between solar activity and the atmosphere: An unphysical explanation

A wide array of atmospheric properties has recently been shown to be correlated to solar activity. The disparate behavior characterizing those properties makes their relationship to solar activity difficult to explain with a unified physical argument. Statistical considerations related to sampling and the length of records available to correlate atmospheric behavior against solar activity are able to explain several of the essential features of solar-atmospheric correlations. Stratifying wintertime data according to the phase of the quasi-biennial oscillation (QBO) leaves undersampled most of the variability in the polar stratosphere operating on quasi-biennial time scales. Aliasing of low frequencies by that undersampled variability introduces spurious covariance that appears to be responsible for the dramatic increase in correlation when data are stratified according to the QBO. Correlations with unstratified data are not explained by aliasing from undersampled behavior. However, uncertainties in their interpretation and reliability leave these quantities open to question as well. The relatively short lengths of atmospheric records limit the ability of cross covariance properties to discriminate to solar activity and hence to distinguish such from other forms of interannual variability. Likewise, the discrete nature of the cross spectrum with solar activity admits only a few statistical degrees of freedom, which limits the reliability with which correlations can be determined. In fact, when the distribution of cross-spectral power is taken into account, none of the correlations considered is significant. Coherence and correlation with sea level pressure both decrease with increasing record length and fall beneath the 90% level of statistical significance when records are extended back to the turn of the 20th century. The physical significance of such properties is considered in statistics generated from artificial solar variability, which demonstrate that behavior like that observed is not unique to the solar period. Over a wide range of period, false solar variability leads to correlations and coherences that are as high or higher than those produced by actual solar variability. A steady increase in these properties with false solar period suggests that time scales typical of solar activity are not represented in the limited atmospheric records adequately enough to permit a reliable calculation of covariance statistics.