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WHEN WAS THE HOTTEST SUMMER?

A State Climatologist Struggles for an Answer

John R. Christy

To answer this very common though surprisingly difficult question, a technique was developed to reconstruct a local temperature time series of summer average maximum temperatures in north-central Alabama since 1893. The results show that the warmest summer was 1925 at 34.9° ±0.4°C but that 5 other years are statistically so close they could not be eliminated as contenders. (The trend is −0.13°C decade−1.) Our insistence that this ambiguity be recognized by the inquirer, usually the media, causes confusion and reduces their interest level because they desire an absolute answer to, in their view, a very simple question.

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John R. Christy

Abstract

The International Surface Temperature Initiative is a worldwide effort to locate weather observations, digitize them for public access, and attach provenance to them. As part of that effort, this study sought documents of temperature observations for the nation of Uganda. Although scattered reports were found for the 1890s, consistent record keeping appears to have begun in 1900. Data were keyed in from images of several types of old forms as well as accessed electronically from several sources to extend the time series of 32 stations with at least 4 yr of data back as far as data were available. Important gaps still remain; 1979–93 has virtually no observations from any station. Because many stations were represented by more than one data source, a scheme is described to extract the “best guess” values for each station of monthly averages of the daily maximum, minimum, and mean temperature. A preliminary examination of the national time series indicates that, since the early twentieth century, it appears that Uganda experienced essentially no change in monthly-average daily maximum temperature but did experience a considerable rise in monthly-average daily minimum temperature, concentrated in the last three decades. Because there are many gaps in the data, it is hoped that readers with information on extant data that were not discovered for this study will contact the author or the project so that the data may be archived.

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John R. Christy

Abstract

Monthly snowfall totals from over 500 stations in California, some of which date back to 1878, are examined. Most data were accessed through the NOAA archive, but several thousand station months of data were separately keyed in from image files of original documents. Over 26 000 of these entries were new relative to the NOAA archive, generally providing data prior to 1920. The stations were then subdivided into 18 regions for the construction of representative time series of each area. There were problems with the basic data—the most difficult with which to deal was the increasing presence of “zero” totals that should have been recorded as “missing.” This and other issues reduce the confidence that the regional time series are representative of true variations and trends, especially for regions with few systematically reporting stations. Interpreting linear trends on time series with infrequent large anomalies of one sign (i.e., heavy snowfall years) and unresolved data issues should be done with caution. For those regions characterized by consistent monitoring and with the most robust statistical reproducibility, no statistically significant trends in their periods of record (up to 133 years) nor in the most recent 50 years are found. This result encompasses the main snowfall region of the western slope of the Sierra Nevada Mountains.

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John R. Christy and S. James Drouilhet Jr.

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Satellite data from the microwave sounding unit (MSU) channel 4, when carefully merged, provide daily zonal anomalies of lower-stratosphere temperature with a level of precision between 0.01° and 0.08°C per 2.5° latitude band. Global averages of these daily zonal anomalies reveal the prominent warming events due to volcanic aerosol in 1982 (El Chichón) and 1991 (Mt. Pinatubo), which are on the order of 1°C.

The quasibiennial oscillation (QBO) may be extracted from these zonal data by applying a spatial filter between 15°N and 15°S latitude, which resembles the meridional curvature. Previously published relationships between the QBO and the north polar stratospheric temperatures during northern winter are examined but were not found to be reproduced in the MSU4 data.

Sudden stratospheric warmings in the north polar region are represented in the MSU4 data for latitudes poleward of 70°N. In the Southern Hemisphere, there appears to be a moderate relationship between total ozone concentration and MSU4 temperatures, though it has been less apparent in 1991 and 1992.

In terms of empirical modes of variability, the authors find a strong tendency in EOF 1(39.2% of the variance) for anomalies in the Northern Hemisphere polar regions to be counterbalanced by anomalies equatorward of 40°N and 40°S latitudes. In addition, most of the modes revealed significant power in the 15–20 day period band.

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John R. Christy and William B. Norris

Abstract

Radiosonde datasets of temperature often suffer from discontinuities due to changes in instrumentation, location, observing practices, and algorithms. To identify temporal discontinuities that affect the VIZ/Sippican family of radiosondes, the 1979–2004 time series of a composite of 31 VIZ stations are compared to composites of collocated values of layer temperatures from two microwave sounding unit datasets—the University of Alabama in Huntsville (UAH) and Remote Sensing Systems (RSS). Discontinuities in the radiosonde time series relative to the two satellite datasets were detected with high significance and with similar magnitudes; however, some instances occurred where only one satellite dataset differed from the radiosondes. For the products known as lower troposphere (LT; surface–300 hPa) and midtroposphere (MT; surface–75-hPa layer), significant discontinuities relative to both satellite datasets were found—two cases for LT and four for MT. These are likely associated with changes in the radiosonde system. Three apparent radiosonde discontinuities were also determined for the lower-stratospheric product (LS; 150–15 hPa). Because they cannot be definitely traced to changes in the radiosonde system, they could be the result of common errors in the satellite products. When adjustments are applied to the radiosondes based independently on each satellite dataset, 26-yr trends of UAH (RSS) are consistent with the radiosondes for LT, MT, and LS at the level of ±0.06, ±0.04, and ±0.07 (±0.12, ±0.10, and ±0.10) K decade−1. Also, simple statistical retrievals based on radiosonde-derived relationships of LT, MT, and LS indicate a higher level of consistency with UAH products than with those of RSS.

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Roy W. Spencer and John R. Christy

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In Part I of this study, monthly 2.5° gridpoint anomalies in the TIROS-N satellite series Microwave Sounding Unit (MSU) channel 2 brightness temperatures during 1979–88 are evaluated with multiple satellites and radiosonde data for their climate temperature monitoring capability. The MSU anomalies are computed about a 10-year mean annual cycle at each grid point, with the MSUs intercalibrated to a common arbitrary level. The intercalibrations remove relative biases between instruments of up to several tenths of a degree celsius. The monthly gridpoint anomaly agreement between concurrently operating satellites reveals single-satellite precision generally better than 0.07°C in the tropics and better than 0.15°C at higher latitudes. Monthly anomalies in radiosonde channel 2 brightness temperatures computed with the radiative transfer equation compare very closely to the MSU measured anomalies in all climate zones, with correlations generally from 0.94 to 0.98 and standard errors of 0.15°C in the tropics to 0.30°C at high latitudes. Simplification of these radiative transfer calculations to a static weighting profile applied to the radiosonde temperature profile leads to an average degradation of only 0.02° in the monthly skill. In terms of a more traditionally measured quantity, the MSU channel 2 anomalies match best with either the radiosonde 100–20-kPa or 100–15-kPa layer anomalies. No significant spurious trends were found in the 10-yr satellite dataset compared to the radiosondes that would indicate a calibration drift in either system. Thus, sequentially launched, overlapping passive microwave radiometers provide a useful system for monitoring intraseasonal to interannual climate anomalies and offer hope for monitoring of interdecadal trends from space. The Appendix includes previously unpublished details of the MSU gridpoint anomaly dataset construction. Part II of this study addresses the removal from channel 2 of the temperature influence above the 30-kPa level, providing a sharper and thus potentially more useful weighting function for monitoring lower tropospheric temperatures.

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Roy W. Spencer and John R. Christy

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Microwave Sounding Unit channel 4 data from the TIROS-N series of NOAA satellites are intercalibrated to provide a continuous global record of deep-layer averaged lower stratospheric temperatures during 1979–1991. A 13-year record of temperature anomalies is time averaged into pentads and months on a 2.5° grid. The monthly gridpoint anomalies are validated with ten years of radiosonde data during 1979–88. The calibration stability of each satellite's measurements is evaluated during satellite overlap periods, the longest of which reveal no measurable instrumental drift at the level of 0.01°C yr−1. Intercomparisons between NOAA-6 and NOAA-7 anomalies indicate monthly gridpoint precision of 0.05°C in the tropics to around 0.10°C in the extratropies, and signal-to-noise ratios generally over 500, while global monthly precision is 0.01° to 0.02°C. These precision and stability statistics are much better than have been previously reported by other investigators for MSU channel 4. Pentad precision is about 0.10°C in the tropics to around 0.25°C at high latitudes and signal-to- noise ratios generally over 250 in the tropics and high latitude but 100–200 in the middle latitudes. Radiosonde comparisons to the monthly gridpoint anomalies have correlations ranging from 0.90 in the tropics (when the interannual variability is smallest) to as high as 0.99 at high-latitude stations. The corresponding standard error of estimate is generally around 0.3°C.

A significant difference in decadal trends is found between the satellite and radiosonde systems, with a step change of 0.217°C (sondes cooler) compared to the satellite measurements. Investigations of the possible sources of the discrepancy lead us to suspect that the gradual transition from on-site calibration of sondes with thermometers to factory calibration of sondes around 1982 might have caused a change in the calibration, although this conclusion must be viewed as tentative.

The largest globally averaged temperature variations during 1979–91 occur after the El Chichón (1982) and Pinatubo (1991) volcanic eruptions. These warm events are superimposed upon a net downward trend in temperatures during the period. This cooling trend has more of a step function than linear character, with the step occurring during the El Chichón warm event. It is strongest in polar regions and the Northern Hemisphere middle latitudes. These characteristics are qualitatively consistent with radiative adjustments expected to occur with observed ozone depictions.

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John R. Christy and Richard T. McNider

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Three time series of average summer [June–August (JJA)] daily maximum temperature (TMax) are developed for three interior regions of Alabama from stations with varying periods of record and unknown inhomogeneities. The time frame is 1883–2014. Inhomogeneities for each station’s time series are determined from pairwise comparisons with no use of station metadata other than location. The time series for the three adjoining regions are constructed separately and are then combined as a whole assuming trends over 132 yr will have little spatial variation either intraregionally or interregionally for these spatial scales. Varying the parameters of the construction methodology creates 333 time series with a central trend value based on the largest group of stations of −0.07°C decade−1 with a best-guess estimate of measurement uncertainty from −0.12° to −0.02°C decade−1. This best-guess result is insignificantly different (0.01°C decade−1) from a similar regional calculation using NOAA’s divisional dataset based on daily data from the Global Historical Climatology Network (nClimDiv) beginning in 1895. Summer TMax is a better proxy, when compared with daily minimum temperature and thus daily average temperature, for the deeper tropospheric temperature (where the enhanced greenhouse signal is maximized) as a result of afternoon convective mixing. Thus, TMax more closely represents a critical climate parameter: atmospheric heat content. Comparison between JJA TMax and deep tropospheric temperature anomalies indicates modest agreement (r 2 = 0.51) for interior Alabama while agreement for the conterminous United States as given by TMax from the nClimDiv dataset is much better (r 2 = 0.86). Seventy-seven CMIP5 climate model runs are examined for Alabama and indicate no skill at replicating long-term temperature and precipitation changes since 1895.

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Roy W. Spencer and John R. Christy

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TIROS-N satellite Microwave Sounding Unit (MSU) channel 2 data from different view angles across the MSU man swath are combined to remove the influence of the lower stratosphere and much of the upper troposphere on the measured brightness temperatures. The retrieval provides a sharper averaging kernel than the raw channel 2 weighting function, with a peak lowered from 50 kPa to 70 kPa and with only slightly more surface influence than raw channel 2. Monthly 2.5° gridpoint anomalies of this tropospheric retrieval compared between simultaneously operating satellites indicate close agreement, 0.15°C in the tropics to around 0.30°C over much of the higher latitudes. The agreement is not as close as with raw channel 2 anomalies because synoptic-scale temperature gradient information across the 2000-km swath of the MSU is lost in the retrieval procedure and because the retrieval involves the magnification of a small difference between two large numbers. Single gridpoint monthly anomaly correlations between the satellite measurements and the radiosonde calculations range from around 0.95 at high latitudes to below 0.8 in the tropical west Pacific, with standard errors of estimate of 0.16°C at Guam to around 0.50°C at high-latitude continental stations. Calculation of radiosonde temperature with a static weighting function instead of the radiative transfer equation degrades the standard errors by an average of less than 0.04°C. Of various standard tropospheric layers, the channel 2 retrieval anomalies correlate best with radiosonde 100–50- or 100–40-kPa-thickness anomalies. A comparison between global and hemispheric anomalies computed for raw channel 2 data versus the tropospheric retrieval show a correction in the 1979–90 time series for the volcano-induced stratospheric warming of 1982–83, which was independently observed by MSU channel 4. This correction leads to a slightly greater tropospheric warming trend in the 12-year time series (1979–90) for the tropospheric retrieval [0.039°C (±0.03°C) per decade] than for channel 2 alone [0.022°C (±0.02°C) per decade].

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John R. Christy, Kevin E. Trenberth, and John R. Anderson

Abstract

Seven years of daily global surface pressure (Ps,) analyses derived from European Centre for Medium Range Forecasts (ECMWF) data are examined to describe more fully interhemispheric mass exchanges and intraseasonal variability. Extreme events in hemispheric mean Ps are determined, and composited grid point differences show that hemispheric anomalies are mainly determined by pressures in the North Pacific, western North Atlantic, northern Asia and the Southern Hemisphere (SH) circumpolar trough. Seasonal differences in the composites indicate that the regional anomalies occur farther poleward in the winter hemisphere, and the tropical anomalies tend to have the same sign as that of the summer hemispheric mean anomaly.

Long-lasting, localized, extreme Ps anomalies are identified in 18 significant events of hemispheric mass imbalance, and are found to be highly favored when the hemispheric mean departs significantly from normal. The result implies that regionally persistent anomalies are related to global-scale mass redistributions, rather than being totally the result of more localized redistributions.

The global atmospheric angular momentum exhibits significant changes during interhemispheric mass imbalances that exceed one standard deviation (about 0.4 mb). There is a strong tendency for the hemisphere in which a deficit of mass occurs to experience, on average, a 5% increase in hemispheric angular momentum.

Zonal complex empirical orthogonal functions are used to describe the Ps cos ϕ anomalies, filtered for 30–75 day fluctuations. Dominant modes are found in which each hemisphere, independently, produced intrahemispheric exchanges between polar and temperate latitudes. An interhemispheric mode indicates exchanges of mass between the midlatitudes of the Northern Hemisphere and the entire tropics plus the SH subtropics. The interhemispheric mode displays a southward propagation of anomalies from the tropical belt into the SH.

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