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J. M. Gregory

Abstract

Climate change resulting from the enhanced greenhouse effect of increasing atmospheric CO2 concentrations is expected to bring about global and local changes in sea level. A global rise in sea level would result from thermal expansion of seawater and from melting of land ice, while changes in ocean dynamics and atmospheric pressure patterns could alter relative sea surface topography. Global and local sea level changes have been diagnosed from a 75-yr experiment with a version of the U.K. Meteorological Office coupled ocean-atmosphere general circulation model in which the CO2 concentration increases at 1% per year. Over the final decade, the component of mean global average sea level rise caused by thermal expansion is 90 mm; on this time scale, a significant contribution is expected from melting of mountain glaciers, but the model does not represent these. Sea level rises over practically the entire ocean area, but there is considerable variation in the magnitude, showing that the global figure by itself gives only a rough idea of the local effect; the largest rises are found in the northwest Atlantic. Here it is illustrated how this local variation makes it difficult to estimate global sea level rise from a limited number of coastal stations, as must usually be done in practice.

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J. M. Gregory and N. J. Dowrick

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Gregory J. McCabe and David M. Wolock

Abstract

Singular value decomposition (SVD) is used to identify the variability common to global sea surface temperatures (SSTs) and water-balance-modeled water-year (WY) runoff in the conterminous United States (CONUS) for the 1900–2012 period. Two modes were identified from the SVD analysis; the two modes explain 25% of the variability in WY runoff and 33% of the variability in WY SSTs. The first SVD mode reflects the variability of the El Niño–Southern Oscillation (ENSO) in the SST data and the hydroclimatic effects of ENSO on WY runoff in the CONUS. The second SVD mode is related to variability of the Atlantic multidecadal oscillation (AMO). An interesting aspect of these results is that both ENSO and AMO appear to have nearly equivalent effects on runoff variability in the CONUS. However, the relatively small amount of variance explained by the SVD analysis indicates that there is little covariation between runoff and SSTs, suggesting that SSTs may not be a viable predictor of runoff variability for most of the conterminous United States.

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M. Kathleen Brennan and Gregory J. Hakim

Abstract

Arctic sea ice decline in recent decades has been dramatic; however, few long-term records of Arctic sea ice exist to put such a decline in context. Here we employ an ensemble Kalman filter data assimilation approach to reconstruct Arctic sea ice concentration over the last two millennia by assimilating temperature-sensitive proxy records with ensembles drawn from last millennium climate model simulations. We first test the efficacy of this method using pseudoproxy experiments. Results show good agreement between the target and reconstructed total Arctic sea ice extent (R 2 value and coefficient of efficiency values of 0.51 and 0.47 for perfect model experiments and 0.43 and 0.43 for imperfect model experiments). Imperfect model experiments indicate that the reconstructions inherit some bias from the model prior. We assimilate 487 temperature-sensitive proxy records with two climate model simulations to produce two gridded reconstructions of Arctic sea ice over the last two millennia. These reconstructions show good agreement with satellite observations between 1979 and 1999 CE for total Arctic sea ice extent with an R 2 value and coefficient of efficiency of about 0.60 and 0.50, respectively, for both models. Regional quantities derived from these reconstructions show encouraging similarities with independent reconstructions and sea ice sensitive proxy records from the Barents Sea, Baffin Bay, and East Greenland Sea. The reconstructions show a positive trend in Arctic sea ice extent between around 750 and 1820 CE, and increases during years with large volcanic eruptions that persist for about 5 years. Trend analysis of total Arctic sea ice extent reveals that for time periods longer than 30 years, the satellite era decline in total Arctic sea ice extent is unprecedented over the last millennium.

Significance Statement

Areal coverage of Arctic sea ice is a critical aspect of the climate system that has been changing rapidly in recent decades. Prior to the advent of satellite observations, sparse observations of Arctic sea ice make it difficult to put the current changes in context. Here we reconstruct annual averages of Arctic sea ice coverage for the last two millennia by combining temperature-sensitive proxy records (i.e., ice cores, tree rings, and corals) with climate model simulations using a statistical technique called data assimilation. We find large interannual changes in Arctic sea ice coverage prior to 1850 that are associated with volcanic eruptions, with a steady rise in Arctic sea ice coverage between 750 and 1820 CE. The satellite-period loss of sea ice has no analog during the last millennium.

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Gregory J. Stossmeister and Gary M. Barnes

Abstract

Observations in the boundary layer by the NOAA AOC WP-3D aircraft from 8 to 10 October 1985 document the development of a second vortex, which evolves into the circulation center for Tropical Storm Isabel. The new circulation develops just outside the radius of maximum winds and is associated with intensifying convection 90 km from the original center. The original center loses its identity as convection dissipates around it.

Low surface pressure, warm, dry air, and low equivalent potential temperature are found in the new center near its formation time. The new center is found beneath the downwind anvil of the intense convection in the rainband and appears to form over a period of 3–6 h, although significant changes in the storm-scale airflow north of the original center are occurring over the proceeding 24 h. The new center moves with a speed and direction similar to that of the original center. The observations of Isabel are compared to beat bursts, subsidence, and midlevel mesovortices that have been observed in tropical and midlatitude mesoscale convective systems. It is hypothesized that subsidence warming beneath the anvil, in the appropriate environment, could lower the pressure by several millibars and serve as an incipient perturbation for a tropical cyclone.

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Steven M. Cavallo and Gregory J. Hakim

Abstract

Tropopause polar vortices (TPVs) are commonly observed, coherent circulation features of the Arctic with typical radii as large as approximately 800 km. Intensification of cyclonic TPVs has been shown to be dominated by infrared radiation. Here the hypothesis is tested that while radiation alone may not be essential for TPV genesis, radiation has a substantial impact on the long-term population characteristics of cyclonic TPVs.

A numerical model is used to derive two 10-yr climatologies of TPVs for both winter and summer: a control climatology with radiative forcing and an experimental climatology with radiative forcing withheld. Results from the control climatology are first compared to those from the NCEP–NCAR reanalysis project (NNRP), which indicates sensitivity to both horizontal grid resolution and the use of polar filtering in the NNRP. Smaller horizontal grid resolution of 60 km in the current study yields sample-mean cyclonic TPV radii that are smaller by a factor of ~2 compared to NNRP, and vortex track densities in the vicinity of the North Pole are considerably larger compared to NNRP. The experimental climatologies show that winter (summer) vortex maximum amplitude is reduced by 22.3% (38.0%), with a net tendency to weaken without radiation. Moreover, while the number and lifetime of cyclonic TPVs change little in winter without radiation, the number decreases 12% and the mean lifetime decreases 19% during summer without radiation. These results suggest that dynamical processes are primarily responsible for the genesis of the vortices, and that radiation controls their maximum intensity and duration during summer, when the destructive effect of ambient shear is weaker.

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T. Toniazzo, J. M. Gregory, and P. Huybrechts

Abstract

Warmer climate conditions persisting for a period of many centuries could lead to the disappearance of the Greenland ice sheet, with a related 7-m rise in sea level. The question is addressed of whether the ice sheet could be regenerated if preindustrial climate conditions were reestablished after its melting. The HadCM3 coupled atmosphere–ocean GCM is used to simulate the global and regional climate with preindustrial atmospheric greenhouse gas composition and with the Greenland ice sheet removed. Two separate cases are considered. In one, the surface topography of Greenland is given by that of the bedrock currently buried under the ice sheet. In the other, a readjustment to isostatic equilibrium of the unloaded orography is taken into account, giving higher elevations. In both cases, there is greater summer melting than in the current climate, leading to partially snow-free summers with much higher temperatures. On the long-term average, there is no accumulation of snow. The implication of this result is that the removal of the Greenland ice sheet due to a prolonged climatic warming would be irreversible.

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Gregory J. McCabe and David M. Wolock

Abstract

A monthly snow accumulation and melt model was used with monthly Precipitation-elevation Regressions on Independent Slopes Model (PRISM) temperature and precipitation data to generate time series of 1 April snow water equivalent (SWE) for 1900 through 2008 in the western United States. Averaged across the western United States, SWE generally was higher than long-term (1900–2008) average conditions during the periods 1900–25, 1944–55, and 1966–82; SWE was lower than long-term average conditions during the periods 1926–43, 1957–65, and 1984–2008. During the period 1900–2008, the temporal pattern in winter precipitation exhibited wetter-than-average and drier-than-average decadal-scale periods with no long-term increasing or decreasing trend. Winter temperature generally was below average from 1900 to the mid-1950s, close to average from the mid-1950s to the mid-1980s, and above average from the mid-1980s to 2008. In general, periods of higher-than-average SWE have been associated with higher precipitation and lower temperature. Since about 1980, western U.S. winter temperatures have been consistently higher than long-term average values, and the resultant lower-than-average SWE values have been only partially offset by periods of higher-than-average precipitation. The post-1980 lower-than-average SWE conditions in the western United States are unprecedented within the context of twentieth-century climate and estimated SWE.

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Gregory J. McCabe and David M. Wolock

Abstract

Extensive and severe droughts have substantial effects on water supplies, agriculture, and aquatic ecosystems. To better understand these droughts, we used tree-ring-based reconstructions of the Palmer drought severity index (PDSI) for the period 1475–2017 to examine droughts that covered at least 33% of the conterminous United States (CONUS). We identified 37 spatially extensive drought events for the CONUS and examined their spatial and temporal patterns. The duration of the extensive drought events ranged from 3 to 12 yr and on average affected 43% of the CONUS. The recent (2000–08) drought in the southwestern CONUS, often referred to as the turn-of-the-century drought, is likely one of the longest droughts in the CONUS during the past 500 years. A principal components analysis of the PDSI data from 1475 through 2017 resulted in three principal components (PCs) that explain about 48% of the variability of PDSI and are helpful to understand the temporal and spatial variability of the 37 extensive droughts in the CONUS. Analyses of the relations between the three PCs and well-known climate indices, such as indices of El Niño–Southern Oscillation, indicate statistically significant correlations; however, the correlations do not appear to be large enough (all with an absolute value less than 0.45) to be useful for the development of drought prediction models.

Significance Statement

To better understand the variability of spatially extensive U.S. droughts through time and across space, we examined tree-ring-based reconstructions of a relative dryness/wetness index for the period 1475–2017. We identified 37 extensive drought events with durations that ranged from 3 to 12 years and that on average affected 43% of the conterminous United States. Also, three of the seven longest droughts occurred after 1900. Because associations between indices of climatic conditions and drought are weak, use of climatic indices for predictive models of drought seems tenuous.

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Steven M. Cavallo and Gregory J. Hakim

Abstract

Long-lived coherent vortices located near the tropopause are often found over polar regions. Although these vortices are a commonly observed feature of the Arctic, and can have lifetimes longer than one month, little is known about the mechanisms that control their evolution. This paper examines mechanisms of intensity change for a cyclonic tropopause polar vortex (TPV) using an Ertel potential vorticity (EPV) diagnostic framework. Results from a climatology of intensifying cyclonic TPVs suggest that the essential dynamics are local to the vortex, rather than a consequence of larger-scale processes. This fact motivates a case study using a numerical model to investigate the role of diabatic mechanisms in the growth and decay of a particular cyclonic vortex. A component-wise breakdown of EPV reveals that cloud-top radiational cooling is the primary diabatic mechanism that intensifies the TPV during the growth phase. Increasing amounts of moisture become entrained into the vortex core at later times near Hudson Bay, allowing the destruction of potential vorticity near the tropopause due to latent heat release to become comparable to the radiational tendency to create potential vorticity.

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