Andrew Orr

Non-annular atmospheric circulation change induced by stratospheric ozone depletion and its role in the recent increase of Antarctic sea ice extent

Based on a new analysis of passive microwave satellite data, we demonstrate that the annual mean extent of Antarctic sea ice has increased at a statistically significant rate of 0.97% dec−1 since the late 1970s. The largest increase has been in autumn when there has been a dipole of significant positive and negative trends in the Ross and Amundsen‐Bellingshausen Seas respectively. The autumn increase in the Ross Sea sector is primarily a result of stronger cyclonic atmospheric flow over the Amundsen Sea. Model experiments suggest that the trend towards stronger cyclonic circulation is mainly a result of stratospheric ozone depletion, which has strengthened autumn wind speeds around the continent, deepening the Amundsen Sea Low through flow separation around the high coastal orography. However, statistics derived from a climate model control run suggest that the observed sea ice increase might still be within the range of natural climate variability.

The Impact of a Changing Southern Hemisphere Annular Mode on Antarctic Peninsula Summer Temperatures

Since the mid-1960s, rapid regional summer warming has occurred on the east coast of the northern Antarctic Peninsula, with near-surface temperatures increasing by more than 2 degrees C. This warming has contributed significantly to the collapse of the northern sections of the Larsen Ice Shelf. Coincident with this warming, the summer Southern Hemisphere Annular Mode (SAM) has exhibited a marked trend, suggested by modeling studies to be predominantly a response to anthropogenic forcing, resulting in increased westerlies across the northern peninsula. Observations and reanalysis data are utilized to demonstrate that the changing SAM has played a key role in driving this local summer warming. It is proposed that the stronger summer westerly winds reduce the blocking effect of the Antarctic Peninsula and lead to a higher frequency of air masses being advected eastward over the orographic barrier of the northern Antarctic Peninsula. When this occurs, a combination of a climatological temperature gradient across the barrier and the formation of a fohn wind on the lee side typically results in a summer near-surface temperature sensitivity to the SAM that is 3 times greater on the eastern side of the peninsula than on the west. SAM variability is also shown to play a less important role in determining summer temperatures at stations west of the barrier in the northern peninsula (similar to 62 degrees S), both at the surface and throughout the troposphere. This is in contrast to a station farther south (similar to 65 degrees S) where the SAM exerts little influence.

The Influence of the Amundsen–Bellingshausen Seas Low on the Climate of West Antarctica and Its Representation in Coupled Climate Model Simulations

AbstractIn contrast to earlier studies, the authors describe the climatological deep low pressure system that exists over the South Pacific sector of the Southern Ocean, referred to as the Amundsen–Bellingshausen Seas low (ABSL), in terms of its relative (rather than actual) central pressure by removing the background area-averaged mean sea level pressure (MSLP). Doing so removes much of the influence of large-scale variability across the ABSL sector region (e.g., due to the southern annular mode), allowing a clearer understanding of ABSL variability and its effect on the regional climate of West Antarctica. Using ECMWF Interim Re-Analysis (ERA-Interim) fields, the annual cycle of the relative central pressure of the ABSL for the period from 1979 to 2011 shows a minimum (maximum) during winter (summer), differing considerably from the earlier studies based on actual central pressure, which suggests a semiannual oscillation. The annual cycle of the longitudinal position of the ABSL is insensitive to the ba...

Improved Middle Atmosphere Climate and Forecasts in the ECMWF Model through a Nonorographic Gravity Wave Drag Parameterization

Abstract In model cycle 35r3 (Cy35r3) of the ECMWF Integrated Forecast System (IFS), the momentum deposition from small-scale nonorographic gravity waves is parameterized by the Scinocca scheme, which uses hydrostatic nonrotational wave dynamics to describe the vertical evolution of a broad, constant, and isotropic spectrum of gravity waves emanating from the troposphere. The Cy35r3 middle atmosphere climate shows the following: (i) an improved representation of the zonal-mean circulation and temperature structure; (ii) a realistic parameterized gravity wave drag; (iii) a reasonable stationary planetary wave structure and stationary wave driving in July and an underestimate of the generation of stationary wave activity in the troposphere and stationary wave driving in January; (iv) an improved representation of the tropical variability of the stratospheric circulation, although the westerly phase of the semiannual oscillation is missing; and (v) a realistic horizontal distribution of momentum flux in the ...

Characteristics of summer airflow over the Antarctic Peninsula in response to recent strengthening of westerly circumpolar winds

Abstract Summer near-surface temperatures over the northeast coast of the Antarctic Peninsula have increased by more than 2°C over the past 40 years, a temperature increase 3 times greater than that on the northwest coast. Recent analysis has shown a strong correlation between this striking warming trend and significant change in the summer Southern Hemisphere annular mode (SAM), which has resulted in greatly increased summer westerlies across the northern peninsula. It has been proposed that the strengthening westerlies have resulted in increased vertical deflection of relatively warm maritime air over the northern peninsula, contributing significantly to the observed warming and the recent collapse of northern sections of the Larsen Ice Shelf. In this study, laboratory and numerical modeling of airflow incident to the peninsula are employed to further understand this mechanism. It is shown that the effect of the strengthening westerlies has led to a distinct transition from a “blocked” regime to a “flow...

The Relationship between the Southern Hemisphere Annular Mode and Antarctic Peninsula Summer Temperatures : Analysis of a High-Resolution Model Climatology

Abstract The large regional summer warming on the east coast of the northern Antarctic Peninsula (AP), which has taken place since the mid-1960s, has previously been proposed to be caused by a trend in the Southern Hemisphere Annular Mode (SAM). The authors utilize a high-resolution regional atmospheric model climatology (14-km grid spacing) to study the mechanisms that determine the response of the near-surface temperature to an increase in the SAM (ΔT/ΔSAM). Month-to-month variations in near-surface temperature and surface pressure are well represented by the model. It is found that north of ∼68°S, ΔT/ΔSAM is much larger on the eastern (lee) side than on the western (windward) side of the barrier. This is because of the enhanced westerly flow of relatively warm air over the barrier, which warms (and dries) further as it descends down the lee slope. The downward motion on the eastern side of the barrier causes a decrease in surface-mass balance and cloud cover. South of ∼68°S, vertical deflection across ...

Atmosphere‐ocean‐ice interactions in the Amundsen Sea Embayment, West Antarctica

Over recent decades outlet glaciers of the Amundsen Sea Embayment (ASE), West Antarctica, have accelerated, thinned and retreated, and are now contributing approximately 10% to global sea level rise. All the ASE glaciers flow into ice shelves, and it is the thinning of these since the 1970s, and their ungrounding from “pinning points” that is widely held to be responsible for triggering the glaciers’ decline. These changes have been linked to the inflow of warm Circumpolar Deep Water (CDW) onto the ASEs continental shelf. CDW delivery is highly variable, and is closely related to the regional atmospheric circulation. The ASE is south of the Amundsen Sea Low (ASL), which has a large variability and which has deepened in recent decades. The ASL is influenced by the phase of the Southern Annular Mode, along with tropical climate variability. It is not currently possible to simulate such complex atmosphere-ocean-ice interactions in models, hampering prediction of future change. The current retreat could mark the beginning of an unstable phase of the ASE glaciers that, if continued, will result in collapse of the West Antarctic Ice Sheet, but numerical ice-sheet models currently lack the predictive power to answer this question. It is equally possible that the recent retreat will be short-lived and that the ASE will find a new stable state. Progress is hindered by incomplete knowledge of bed topography in the vicinity of the grounding line. Furthermore, a number of key processes are still missing or poorly represented in models of ice-flow.

Evaluation of four global reanalysis products using in-situ observations in the Amundsen Sea Embayment, Antarctica

The glaciers within the Amundsen Sea Embayment (ASE), West Antarctica, are amongst the most rapidly retreating in Antarctica. Meteorological reanalysis products are widely used to help understand and simulate the processes causing this retreat. Here we provide an evaluation against observations of four of the latest global reanalysis products within the ASE region—the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-I), Japanese 55-year Reanalysis (JRA-55), Climate Forecast System Reanalysis (CFSR), and Modern Era Retrospective-Analysis for Research and Applications (MERRA). The observations comprise data from four automatic weather stations (AWSs), three research vessel cruises, and a new set of 38 radiosondes all within the period 2009–2014. All four reanalyses produce 2 m temperature fields that are colder than AWS observations, with the biases varying from approximately −1.8°C (ERA-I) to −6.8°C (MERRA). Over the Amundsen Sea, spatially averaged summertime biases are between −0.4°C (JRA-55) and −2.1°C (MERRA) with notably larger cold biases close to the continent (up to −6°C) in all reanalyses. All four reanalyses underestimate near-surface wind speed at high wind speeds (>15 m s−1) and exhibit dry biases and relatively large root-mean-square errors (RMSE) in specific humidity. A comparison to the radiosonde soundings shows that the cold, dry bias at the surface extends into the lower troposphere; here ERA-I and CFSR reanalyses provide the most accurate profiles. The reanalyses generally contain larger temperature and humidity biases, (and RMSE) when a temperature inversion is observed, and contain larger wind speed biases (~2 to 3 m s−1), when a low-level jet is observed.

A Predominant Reversal in the Relationship between the SAM and East Antarctic Temperatures during the Twenty-First Century

AbstractThe scientific literature portrays a temporally invariant spatial relationship between the phase of the southern annular mode (SAM) and the sign of surface air temperature (SAT) anomalies across Antarctica. However, here the authors describe a predominant switch from a negative to positive SAM–temperature relationship (STR) across East Antarctica in austral summer/autumn during the first decade of the twenty-first century, when the SAM was generally weakly positive. Of the nine years that had a positive regional STR from 1957 to 2010, seven occurred during the last decade. This reversal appears to be a response to anomalous high pressure over East Antarctica, resulting from variability in the phase and amplitude of the local component of the zonal wavenumber 3 pressure pattern. In years when a reversed (positive) regional STR exists the anomalous circulation is such that there is greater energy flux into the region, while enhanced katabatic drainage across the continental interior disrupts the sur...

Future circulation changes off West Antarctica: Sensitivity of the Amundsen Sea Low to projected anthropogenic forcing

The Amundsen Sea Low (ASL) is a major driver of West Antarctic climate variability, with the potential to accelerate the loss of glacial ice. Using the 11 global climate models which most reliably simulate the seasonality in ASL location, we assess the ASL sensitivity to projected future changes using the CMIP5 historical (1951–2000) and representative concentration pathway experiment RCP8.5 (2051–2100). For the first time, we show that the future ASL will likely migrate poleward in summer (December, January, and February) and autumn (March, April, and May), and eastward in autumn and winter (June, July, and August). The autumn-winter changes drive colder southerly winds over the Ross Sea and warmer northerly winds toward the Antarctic Peninsula. This is consistent with recent trends in ERA-Interim reanalysis meridional winds (1979–2014) and reconstructed temperature (1957–2006), suggesting that the impact of anthropogenic forcing on the ASL is likely to play an important role on both past and future patterns of West Antarctic climate variability.