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Jeffrey C. Rogers

Abstract

The primary mode of North Atlantic storm track variability is identified using rotated principal component analysis (RPCA) on monthly fields of root-mean-squares of daily high-pass filtered (2–8-day periods) sea level pressures (SLP) for winters (December–February) 1900–92. It is examined in terms of its association with 1) monthly mean SLP fields, 2) regional low-frequency teleconnections, and 3) the seesaw in winter temperatures between Greenland and northern Europe. The principal storm track component is characterized by high synoptic variability preferring one of two areas at any given time. The northeastern Atlantic center (identified by positive RPCA scores) is characterized by deep cyclones in the area extending from Iceland northeastward to the Norwegian and Barents Seas, whereas the Bay of Biscay center (negative scores) is linked to cyclone activity around that area and into the Mediterranean basin. Combined principal component analysis is used to link the high-frequency storm track pressure variability with that of lower frequencies (monthly mean pressures). In this, the primary storm track pattern is linked to large monthly mean SLP variations around the Bay of Biscay and near northern Scandinavia and the Barents Sea. This pattern does not suggest a strong storm track link to the North Atlantic Oscillation (NAO). Instead, the results presented indicate that the dominant mode of variability in the storm track is associated with low-frequency SLP anomalies in the extreme northeastern Atlantic. When the component scores reach their highest positive values, both the mean Atlantic subpolar low and subtropical high are unusually strong and displaced farther northeast than normal. The pressure field intensifies to the northeast and produces strong zonal flow extending into Europe, bringing abnormally high surface air temperatures as far east as Siberia and below normal temperatures over Greenland and northern Africa (the “Greenland below” seesaw mode, GB). Besides this eastward extension of the mean pressure field, anomalously high European winter temperatures can also be somewhat less frequently caused by mild return flow around the Siberian high, which is displaced farther west than normal. In this situation the Icelandic low is in its normal Denmark Strait location and cyclones move along the more southerly storm track toward the Mediterranean basin, contributing to the synoptic forcing that helps develop the westward extended high. The NAO appears to be only indirectly linked to the European component of the GB mode of the winter surface air temperature seesaw.

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Ryan G. Lauritsen and Jeffrey C. Rogers

Abstract

Long-term (1901–2002) diurnal temperature range (DTR) data are evaluated to examine their spatial and temporal variability across the United States; the early century origin of the DTR declines; and the relative regional contributions to DTR variability among cloud cover, precipitation, soil moisture, and atmosphere/ocean teleconnections. Rotated principal component analysis (RPCA) of the Climatic Research Unit (CRU) Time Series (TS) 2.1 dataset identifies five regions of unique spatial U.S. DTR variability. RPCA creates regional orthogonal indices of cloud cover, soil moisture, precipitation, and the teleconnections used subsequently in stepwise multiple linear regression to examine their regional impact on DTR, maximum temperature (Tmax), and minimum temperature (Tmin).

The southwestern United States has the smallest DTR and cloud cover trends as both Tmax and Tmin increase over the century. The Tmin increases are the primary influence on DTR trend in other regions, except in the south-central United States, where downward Tmax trend largely affects its DTR decline. The Tmax and DTR tend to both exhibit simultaneous decadal variations during unusually wet and dry periods in response to cloud cover, soil moisture, and precipitation variability. The widely reported post-1950 DTR decline began regionally at various times ranging from around 1910 to the 1950s. Cloud cover alone accounts for up to 63.2% of regional annual DTR variability, with cloud cover trends driving DTR in northern states. Cloud cover, soil moisture, precipitation, and atmospheric/oceanic teleconnection indices account for up to 80.0% of regional variance over 1901–2002 (75.4% in detrended data), although the latter only account for small portions of this variability.

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Jeffrey C. Rogers and Robert V. Rohli

Abstract

Severe Florida citrus freezes since 1880 are identified and described in terms of the horticultural damage, overall frequency of occurrence, and association with polar anticyclone outbreaks in the plains of southern Canada and the United States. The most severe “advective” freezes are associated with strong cold anticyclones having tracks southward across the plains to Texas with subsequent northeastward movement. Other anticyclones move in a track somewhat east of this and ultimately pass over Florida or the eastern Gulf of Mexico. Over 80% of the worst Florida citrus freezes are associated with anticyclones with central pressures in excess of 1045 mb moving along these paths. However, anticyclones of similar intensity with more zonally oriented paths across higher latitudes are associated with minor citrus damage. The major freezes tend to be clustered in time in the 1890s and since 1977. On interdecadal time scales, the recent freezes are not linked to higher winter mean pressure in the northern plains, and there has not been an unusually high frequency of strong anticyclones in recent decades, compared to earlier this century. Compared to the freeze-free period of 1948–57, the winters of 1977–86 are characterized by a more amplified 500-mb mean standing wave pattern across North America. This is linked to changes in the Pacific/North American upper-air teleconnection pattern, the index for which had much lower values (characterized by zonal flow) prior to 1958.

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Chung-Chieh Wang and Jeffrey C. Rogers

Abstract

General characteristics of the dynamical and thermal structure and evolution of strong explosive cyclones in the northwestern Atlantic near North America (18 cases) and the extreme northeastern Atlantic near Iceland (19 cases) are compared and contrasted through a composite study. Twice-daily gridded analyses from the European Centre for Medium-Range Weather Forecasts at 2.5° resolution from January 1985 to March 1996 are used. In the process of case selection, it is found that the frequency of rapid cyclogenesis in the Greenland–Iceland region is higher than previously thought, and some of the events can be extremely violent.

Many dynamically consistent differences are found when composite cyclones in the two sectors of the North Atlantic are compared. The upper-level forcing that triggers the development in the northeast Atlantic (NEA) is no less intense at the onset of rapid deepening. The NEA cyclones are also associated with lower static stability and locally concentrated but shallower thermal gradient, with less overall environmental baroclinicity. These factors lead to rapid depletion of available potential energy and result in a faster evolution and a shorter life cycle. Therefore, low-level thermal gradient and upper-level forcing components all weaken immediately after rapid deepening. The low-level incipient low in the NEA composite is also stronger, with a distinct potential vorticity (PV) anomaly visible at least 24 h prior to most rapid deepening, and the development produces a more pronounced warm core seclusion. Explosive cyclones in the northwest Atlantic, on the other hand, tend to have a higher stability and a greater amount of environmental baroclinicity, with temperature gradients in a broader area and deeper layers. These factors correspond to slower evolution and a longer life cycle.

For cases in the NEA near Iceland, it appears that both upper-level forcing and initial system strengths affect the maximum deepening rate. The close proximity of this region to the high PV reservoir in the lower stratosphere is helpful in the generation of very strong forcing and a violent development under favorable synoptic conditions, when a “parent cyclone” with appreciable strength exists to the north/northeast of the incipient system.

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Jeffrey C. Rogers and Marilyn N. Raphael

Abstract

The geographical distribution of meridional eddy sensible heat transport in the extremes of the Pacific/North American (PNA) teleconnection pattern is examined and compared to heat transport occurring in conjunction with other regional teleconnections. The heat fluxes are estimated using 700-mb air temperatures and geostrophic winds during 12 winter months when the PNA index reaches its highest values (large-amplitude standing ridge and trough pattern over North America) and during 12 months when it is lowest (relatively zonal flow across the continent). The standing wave fluxes are generally largest in the positive PNA phase, especially across latitudes 45°–55°N, although the flux between 60°–75°N is not as great as in the negative phase, when poleward heat transport is strong over northern Canada and near Iceland. The largest spatial heat flux variations in the extremes of the PNA occur in areas with long-term climatological flux maxima and relatively large long-term standard deviations. These include eastern Asia, the northeastern Pacific, western North America, and over the Atlantic Ocean, although in the latter region the maxima are split between Newfoundland during positive PNA index months and Iceland in negative PNA index months. In the extremes of the PNA, there is a strong tendency for the relative magnitudes of the standing and transient eddy fluxes to be out of phase in many areas of the hemisphere. This characteristic is not predominant in other regional teleconnections although it occurs in the western Pacific pattern. In other teleconnections the eddy fluxes are generally in phase, contributing directly to the total eddy flux, and centers of flux maxima do not generally correspond to those appearing in the long-term means.

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Maurice J. McHugh and Jeffrey C. Rogers

Abstract

The relationship between the North Atlantic oscillation (NAO) and austral summer (December–February) rainfall variability over southeastern Africa is described. Thirty-one stations in 0°–16°S and 25°–40°E have statistically significant correlations to the NAO index over varying periods of record starting since 1895 and form a regional normalized rainfall index of southeast African rainfall (SEAR) correlated to the NAO index (NAOI) at r = −0.48 over 1894/95–1989/90, although the relationship is r = −0.70 since 1958. The spectrum of the SEAR index has significant amplitude at 7.6 yr, a periodicity commonly associated with the NAO, and the NAOI/SEAR cospectrum has its largest power at this periodicity. NCEP–NCAR reanalysis data, extending from 1958/59 to 1995/96 are used to evaluate moisture and circulation field variations associated with both NAO and SEAR indices. Precipitable water over southeastern Africa varies significantly such that anomalously high (low) convective rainfall occurs over southeast Africa when the NAO is weak (strong). Unusually wet summers are associated with anomalous equatorial westerly flow originating in the subtropical Atlantic and traversing the continent. Relatively dry summers are associated with increased southeasterly monsoon flow originating over the subtropical Indian Ocean. The NAO linkage to southeastern African rainfall is especially pronounced in 300-hPa zonal winds where five highly significant elongated bands of alternating zonal wind anomalies extend from the Atlantic Arctic to equatorial Africa. The latter 300-hPa equatorial band exhibits westerly (easterly) flow during wet (dry) austral summers and undergoes regional divergence (convergence) over southeastern Africa. The westerly flow, along with orographic uplift, has an element of instability due to the vertical component of the Coriolis parameter that assists rain production during wet summer. Potential interactions between the NAO and ENSO in producing regional latitudinal ITCZ shifts are discussed.

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Jill S. M. Coleman and Jeffrey C. Rogers

Abstract

A synoptic climatological weather classification scheme incorporating both surface and upper-air data is developed for the central United States based on an automated two-step cluster analysis. It employs daily NCEP–NCAR reanalysis data over all seasons of 57 yr (1948–2004) in creating synoptic types from surface and upper-air (925, 850, 700, and 500 hPa) temperature and humidity data as well as sea level pressure, geopotential heights, and winds aloft. The cluster analysis creates 10 synoptic types exhibiting distinct seasonal preferences, with three each that occur primarily in summer and winter, and four that occur primarily in winter and the transition seasons, particularly spring. The typing scheme generates synoptic patterns largely characterized by distinctive surface circulations, baroclinic vertical structure, and thermal advection. Interannual variations occur in the frequencies of the synoptic types, some of which are out of phase with each other. The annual frequencies of two winter synoptic types, associated respectively with strong zonal and meridional flow, are highly correlated (r ≫ 0.63–0.73) to the phase of the Pacific–North American teleconnection pattern, while Niño-3.4 equatorial Pacific sea surface temperatures are linked to a synoptic type producing low pressure around the Gulf Coast.

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Jill S. M. Coleman and Jeffrey C. Rogers

Abstract

The relationship between the Pacific–North American (PNA) teleconnection pattern and Ohio River Valley (ORV) winter precipitation and hydrology is described. The PNA is significantly linked to moisture variability in an area extending from southeastern Missouri, northeastward over states adjacent to the Ohio River through Ohio. Maximum correlation between the PNA index and station precipitation peaks in southern Indiana at r = −0.71, making the circulation/climate teleconnection one of the strongest in the Northern Hemisphere. The North Pacific index (NPI), a Pacific basin sea level pressure index that is highly correlated to the PNA, confirms a strong circulation–ORV precipitation relationship extending back to 1899. In contrast, measures such as the Tahiti–Darwin Southern Oscillation index (SOI) and Niño-3.4 (5°S–5°N, 120°–170°W) sea temperatures are not significantly correlated to ORV winter precipitation. Wettest (driest) winters occur with zonal (meridional) flow with the PNA negative (positive) and North Pacific sea level pressure anomalously high (low). Moisture flux convergence extends much farther north from the Gulf of Mexico in the wet winters, compared to dry, and excess of precipitation over evaporation (moisture budget) is over 100 mm larger throughout much of the ORV. Wet winters, particularly those of 1949 and 1950 changed the ORV hydrology to one of extensive wet conditions, as measured by the Palmer hydrologic drought index (PHDI). Unusually dry winters, however, appear to have less impact on the index; many ORV climate divisions remain moist through the winter despite low precipitation. Winter mean streamflow along the Ohio River and its tributaries varies significantly between PNA extremes, with river discharges up to 100% higher in PNA-negative winters as opposed to PNA-positive winters.

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Jeffrey C. Rogers, Sheng-Hung Wang, and Jill S. M. Coleman

Abstract

A 124 (1882–2005) summer record of total surface energy content consisting of time series of surface equivalent temperature (TE) and its components T (mean air temperature) and Lq/cp (moist enthalpy, denoted Lq) is developed, quality controlled, and analyzed for Columbus, Ohio, where long records of monthly dewpoint temperature are available. The analysis shows that the highest TE occurs during the summer of 1995 when both T and Lq were very high, associated with a severe midwestern heat wave. That year contrasts with the hot summers of 1930–36, when Lq and TE had relatively low or negative anomalies (low humidity) compared to those of T. Following the 1930–36 summers, T and Lq departures are much more typically the same sign in individual summers, and the two parameters develop a statistically significant high positive correlation into the twenty-first century. Mean T and Lq departures from the long-term normal have opposite signs, however, when summers are stratified either by seasonal total rainfall amounts or by the Palmer drought severity soil moisture index. Normalized trends of T, Lq, and TE are downward from 1940 to 1964 with those of TE exceeding T. Since 1965, however, significant positive T trends slightly exceed TE in magnitude and those of dewpoint temperature and Lq are comparatively lower. A highly significant upward trend in minimum temperatures especially dominates the T variability, creating a significant downward trend in the temperature range that dominates recent summer climate variability more than moisture trends. Regional moisture flux variations are largest away from Columbus, over the upper Midwest and western Atlantic Ocean, during its seasonal extremes in total surface energy.

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Hongshuo Wang, Jeffrey C. Rogers, and Darla K. Munroe

Abstract

Soil moisture shortages adversely affecting agriculture are significantly associated with meteorological drought. Because of limited soil moisture observations with which to monitor agricultural drought, characterizing soil moisture using drought indices is of great significance. The relationship between commonly used drought indices and soil moisture is examined here using Chinese surface weather data and calculated station-based drought indices. Outside of northeastern China, surface soil moisture is more affected by drought indices having shorter time scales while deep-layer soil moisture is more related on longer index time scales. Multiscalar drought indices work better than drought indices from two-layer bucket models. The standardized precipitation evapotranspiration index (SPEI) works similarly or better than the standardized precipitation index (SPI) in characterizing soil moisture at different soil layers. In most stations in China, the Z index has a higher correlation with soil moisture at 0–5 cm than the Palmer drought severity index (PDSI), which in turn has a higher correlation with soil moisture at 90–100-cm depth than the Z index. Soil bulk density and soil organic carbon density are the two main soil properties affecting the spatial variations of the soil moisture–drought indices relationship. The study may facilitate agriculture drought monitoring with commonly used drought indices calculated from weather station data.

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