Jeffrey C. Rogers

Icelandic Low Cyclone Activity: Climatological Features, Linkages with the NAO, and Relationships with Recent Changes in the Northern Hemisphere Circulation

Abstract Output from a cyclone detection and tracking algorithm, applied to twice-daily sea level pressure (SLP) fields for the period 1966–93, is used to examine the characteristics of cyclone activity associated with the locus of the mean Icelandic low (IL), variability during extremes of the North Atlantic oscillation (NAO), and recent changes in relation to circulation over the Northern Hemisphere. Cyclone events within the climatological IL display a modest seasonal cycle with a winter maximum. However, winter systems are considerably deeper than their summer counterparts with much larger maximum deepening rates. During the cold season (October–March), IL cyclone intensities are typical of oceanic systems but exhibit lower maximum deepening rates. During the warm season (April–September), intensities are typical of Northern Hemisphere values with deepening characteristics similar to those for all extratropical oceans. Depending on the month, 10%–15% (13%–18%) of cyclone events in the IL region repres...

North Atlantic Storm Track Variability and Its Association to the North Atlantic Oscillation and Climate Variability of Northern Europe

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 pre...

Greenland Ice Sheet Surface Mass Balance Variability (1988–2004) from Calibrated Polar MM5 Output*

Regional climate model runs using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesocale Model modified for use in polar regions (Polar MM5), calibrated by independent in situ observations, demonstrate coherent regional patterns of Greenland ice sheet surface mass balance (SMB) change over a 17-yr period characterized by warming (1988–2004). Both accumulation and melt rates increased, partly counteracting each other for an overall negligible SMB trend. However, a 30% increase in meltwater runoff over this period suggests that the overall ice sheet mass balance has been increasingly negative, given observed meltwater-induced flow acceleration. SMB temporal variability of the whole ice sheet is best represented by ablation zone variability, suggesting that increased melting dominates over increased accumulation in a warming scenario. The melt season grew in duration over nearly the entire ablation zone by up to 40 days, 10 days on average. Accumulation area ratio decreased by 3%. Albedo reductions are apparent in five years of the Moderate Resolution Imaging Spectroradiometer (MODIS) derived data (2000–04). The Advanced Very High Resolution Radiometer (AVHRR)-derived albedo changes (1988–99) were less consistent spatially. A conservative assumption as to glacier discharge and basal melting suggests an ice sheet mass loss over this period greater than 100 km 3 yr 1 , framing the Greenland ice sheet as the largest single glacial contributor to recent global sea level rise. Surface mass balance uncertainty, quantified from residual random error between model and independent observations, suggests two things: 1) changes smaller than approximately 200 km 3 yr 1 would not satisfy conservative statistical significance thresholds (i.e., two standard deviations) and 2) although natural variability and model uncertainty were separated in this analysis, the magnitude of each were roughly equivalent. Therefore, improvements in model accuracy and analysis of longer periods (assuming larger changes) are both needed for definitive mass balance change assessments.

The North Atlantic Oscillation signature in deuterium and deuterium excess signals in the Greenland Ice Sheet Project 2 Ice Core, 1840–1970

The Greenland Ice Sheet Project 2 (GISP2) core can enhance our understanding of the relationship between parameters measured in the ice in central Greenland and variability in the ocean, atmosphere, and cryosphere of the North Atlantic Ocean and adjacent land masses. Seasonal (summer, winter) to annual responses of δD and deuterium excess isotopic signals in the GISP2 core to the seesaw in winter temperatures between West Greenland and northern Europe from A.D. 1840 to 1970 are investigated. This seesaw represents extreme modes of the North Atlantic Oscillation, which also influences sea surface temperatures (SSTs), atmospheric pressures, geostrophic wind strength, and sea ice extents beyond the winter season. Temperature excursions inferred from the δD record during seesaw/extreme NAO mode years move in the same direction as the West Greenland side of the seesaw. Symmetry with the West Greenland side of the seesaw suggests a possible mechanism for damping in the ice core record of the lowest decadal temperatures experienced in Europe from A.D. 1500 to 1700. Seasonal and annual deuterium excess excursions during seesaw years show negative correlation with δD. This suggests an isotopic response to a SST/ land temperature seesaw. The isotopic record from GISP2 may therefore give information on both ice sheet and sea surface temperature variability. Cross-plots of δD and d show a tendency for data to be grouped according to the prevailing mode of the seesaw, but do not provide unambiguous identification of individual seesaw years. A combination of ice core and tree ring data sets may allow more confident identification of GA and GB (extreme NAO mode) years prior to 1840.

Florida Citrus Freezes and Polar Anticyclones in the Great Plains

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 northe...

Aspects of climate variability in the North Atlantic sector: Discussion and relation to the Greenland Ice Sheet Project 2 high-resolution isotopic signal

This paper reviews aspects of climate variability in the North Atlantic sector that may influence the seasonal to decadal scale isotopic signal in the Greenland Ice Sheet Project 2 (GISP2) ice core. Interpretation of the isotopic signal and its spatial applicability at the seasonal level requires investigation into synoptic scale climatology. We discuss possible climatic influences of (1) likely source regions of precipitation reaching the GISP2 site, (2) the characteristics of cyclone activity over the North Atlantic sector, and (3) changes in major atmospheric features such as the mean sea level Icelandic Low and Azores High pressure systems, the North Atlantic Oscillation, and the Baffin trough. Next, we evaluate correlations between the GISP2 deuterium isotopic signal and coastal temperatures with atmospheric pressure patterns, thereby deriving climatic interpretations of the high-resolution isotopic record. The GISP2 site is influenced by both the Icelandic Low to the southeast and Davis Strait/Baffin Bay storms to the southwest and west. The North Atlantic Oscillation influences the GISP2 isotopic signal through the seesaw in winter temperatures between west Greenland and northern Europe. Agreement in excursion directions of GISP2 isotopes and east Greenland and Iceland temperature records is associated with different positions of the Baffin trough in winter and also with anomalous 500-mbar net geostrophic flow. Linkages between isotopic excursion direction and atmospheric variability need to be explored further by comparison with additional seasonal data sets from other Greenland ice cores.

Ohio River Valley Winter Moisture Conditions Associated with the Pacific–North American Teleconnection Pattern

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 Nino-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 (posit...

A Composite Study of Explosive Cyclogenesis in Different Sectors of the North Atlantic. Part I: Cyclone Structure and Evolution

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 2.58 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.

North Atlantic Oscillation Influence on Precipitation Variability around the Southeast African Convergence Zone

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 south...

U.S. Diurnal Temperature Range Variability and Regional Causal Mechanisms, 1901–2002

AbstractLong-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 Un...