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The surface mass balance of the Greenland ice sheet has decreased in recent decades with important implications for global sea-level rise. Here, a climate reanalysis model is used to examine observed circulation variability and changes in precipitation across southern Greenland to gain insight into the future climate in the region. The influence on precipitation from the North Atlantic Oscillation (NAO), Atlantic Multidecadal Oscillation (AMO), Icelandic Low, Azores High, regional blocking patterns, as well as near-surface temperature and winds are explored. Statistically significant correlations are higher between precipitation and the Icelandic Low and near-surface winds (0.5–0.7; p < 0.05) than correlations between precipitation and either the NAO or AMO climate indices (southwest Greenland: r = 0.12 and 0.28, respectively; and southeast Greenland: r = 0.25 and -0.07, respectively). Moreover, the recent enhanced warming in the Arctic (Arctic amplification) and the increase in the Greenland Blocking Index coincide with increased mean annual precipitation and interannual variability in southwest Greenland.

The Southeast (SE) U.S. has experienced several severe droughts over the past 30 years, with the most recent drought during 2006–2008 causing agricultural impacts of $1 billion. However, the mechanisms that lead to droughts over the region and their persistence have been poorly understood due to the region's humid coastal environment and its complex climate. In this study, we carry out a multiscale analysis of drought mechanisms for the SE U.S. over 1979–2008 using the North American Regional Reanalysis (NARR) to identify conditions associated with drought and contrast with those associated with pluvials. These conditions include land surface drought propagation, land-atmosphere feedbacks, regional moisture sources, persistent atmospheric patterns, and larger-scale oceanic conditions. Typical conditions for SE U.S. droughts (pluvials) are identified as follows: (1) weaker (stronger) southerly meridional fluxes and weaker (stronger) westerly zonal fluxes, (2) strong moisture flux divergence (convergence) by transient eddies, and (3) strong (weak) coupling between the land surface and atmosphere. The NARR demonstrates that historic SE droughts are mainly derived from a combination of a strong North Atlantic subtropical high (NASH) and Icelandic Low (IL) during summer and winter, respectively, which peak 1 month earlier than the onset of the drought. The land surface plays a moderate role in drought occurrence over the SE via recycling of precipitation, and the oceans show an asymmetric influence on droughts and pluvials depending on the season. This study suggests that the NASH and IL can be used as a predictor for SE droughts at 1 month lead despite the overall that it represents an atmospheric forcing.

The ω equation method based on an equivalent isobaric geopotential height in σ coordinates has been used to retrieve the precipitation over Greenland. This approach is designed to accurately represent the topographic effects of the Greenland Ice Sheet on atmospheric motion and precipitation. The 11 year mean precipitation from 1985 to 1996 over all of Greenland is 376 mm yr−1, which is close to the long-term mean precipitation of 346 mm yr−1 estimated from glaciological data. The precipitation over all of Greenland shows that the largest value in 1986 is 472 mm yr−1 and the smallest value in 1995 is 309 mm yr−1. The major interannual variability of the atmospheric circulation in the North Atlantic can be represented by the variation of the North Atlantic Oscillation (NAO) index, which is most pronounced during winter. It is found that if the NAO index increases, the total precipitation over Greenland decreases, and vice versa. The correlation coefficient between these two series for 1985–1995 is −0.75. The mean precipitation over southern Greenland, where the majority of precipitation falls, is more closely related to the NAO index in winter, and their correlation coefficient is −0.80. This relationship can be understood from the composite maps of sea level pressure and Greenland precipitation for the high and low index months. During months of high NAO index values, the Icelandic Low is strong. During months of low NAO index values, the monthly mean low is located to the southwest of Greenland over the Labrador Sea. Precipitation amounts over the southeast coast of Greenland are about 100 mm larger during the low NAO index months than the high NAO index months. Precipitation over all of Greenland during the low NAO index months is higher. There are significant downward trends in annual precipitation from 1985–1995 for all of Greenland and its southern and central west coastal regions, amounting to about 3% per year.

Seasonal back-diffused δ18O time series, reconstructed for nine shallow firn cores from the Greenland Summit for 1959–1987, are evaluated in terms of (1) their spatial and temporal variability and (2) their association to the atmospheric circulation. Rotated principal component analysis evaluates spatial and temporal patterns of organization in the back-diffused δ18O ice core time series. Mean sea level pressure and 500-hPa height fields are obtained for winter and summer seasons that fall in the isotopically warmest and coldest thirds of the seasonal data. The winter back-diffused δ18O data indicate that some degree of longitudinal separation takes place between isotopic records from western and eastern Greenland in agreement with coastal station climatic data. Temporal variability is linked with statistical significance to the variability of the atmospheric circulation over the Denmark Strait, as well as over the Norwegian and Barents Seas. The North Atlantic Oscillation (NAO) and the mean Icelandic low in the Denmark Strait affect isotopic variability over the cores west of the ice divide, just as they have been shown to prominently affect west coast air temperature records. Isotopic variability at some ice cores east of the divide is influenced by pressure variability over the Norwegian and Barents Seas, although statistical significance is lower. The NAO appears insignificant in summer isotopic variability, as the circulation in that season is very much localized and dominated by either ridging (warm cases) or troughs (cold cases) over eastern Greenland and the Arctic Ocean. Results suggest that long-term winter δ18O variability can potentially be linked to climate variation over Europe, especially from the Mediterranean basin northward to about 55°N and as far east as central Siberia.

We report on an abrupt springtime temperature rise (ASTR) occurring over the Greenland ice cap. The abrupt 12°-15°C temperature rise typically occurs in May over a 24-36 hour period and is accompanied by a 16-22 hPa pressure rise that begins 2-3 days earlier. The Greenland ASTR is examined using composites of pressure and temperature data from Automatic Weather Stations at Cathy and Kenton for the years 1987 through 1990 and 1992 and 1993. Comparative analysis of the ASTR at coastal weather stations is made and contemporaneous atmospheric circulation variations are examined with gridded hemispheric sea level pressure and 500 hPa data. At the ice cap summit the ASTR is accompanied by an average 360 m rise in 500 hPa geopotential heights, strong surface winds, and high relative humidities. During an ASTR, concomitant air temperatures along the western coast of Greenland and on Baffin Island also rise, but with a magnitude only half that at the ice cap summit. The contemporaneous ASTR pressure rise at coastal stations is of equivalent magnitude to that at the summit and is most prominent at Greenland stations and on Iceland. Evidence is presented that the ASTR is linked to a seasonal readjustment in the regional Atlantic atmospheric circulation in which the westerlies become weakest in May and migrate northward into the Arctic. A meridional flow regime is established with southwesterly flow advecting mild air over Greenland. Blocking highs form over the Atlantic and the surface mean Icelandic low is weak, or replaced by mean high pressure. The nature and timing of the ASTR is important for planning field activities on the Greenland ice sheet. Further study of the ASTR should lead to a better understanding of boundary layer processes over the ice sheet.

[1] The Greenland Ice Sheet (GrIS) is one of the most dominant orographic obstacles for atmospheric flow in the Northern Hemisphere. Within an idealized framework, we investigate the potential impact of a reduced Greenland topography upon precipitation, snow accumulation, and atmospheric circulation over the GrIS. Using the global atmospheric model ECHAM5-HAM at about 1° spatial resolution (T106) and with present-day climatological mean conditions, we perform four 16-year sensitivity experiments that are identical except for the height of the GrIS topography: one control simulation with the present-day Greenland topography, and three simulations with topographies reduced to 75%, 50%, and 25% of the present-day height. This simple reduction of the GrIS topography (as compared to realistic melt dynamics) leads to an overall increase in annual total precipitation and snow accumulation, composed of significant increases in eastern, northern, and central Greenland and decreases on the western slopes. In principle, this gain in snow accumulation raises the possibility of a negative feedback that would stabilize the height of the GrIS. However, this feedback is likely overcompensated by enhanced ablation (positive feedback), as surface air temperatures strongly increase with reduced topographic height. The analysis of changes in circulation patterns indicates that flatter topographies allow the atmospheric flow to penetrate farther inland, enabling precipitation in regions that are presently desert-like. Prominent circulation features change, in particular the all-season Greenland Anticyclone and wintertime Icelandic Low become weaker with lower topography.