scholarly journals The Connection between the Southern Annular Mode and a Feature-Based Perspective on Southern Hemisphere Midlatitude Winter Variability

2020 ◽  
Vol 33 (1) ◽  
pp. 115-129 ◽  
Author(s):  
Clemens Spensberger ◽  
Michael J. Reeder ◽  
Thomas Spengler ◽  
Matthew Patterson
Keyword(s):  
Storm Track ◽  
Principal Component ◽  
Southern Annular Mode ◽  
Strong Correlations ◽  
Sea Level Pressure ◽  
Annular Mode ◽  
Rossby Wave Breaking ◽  
The Pacific ◽  
Feature Based ◽  
Consistent Picture

AbstractThis article provides a reconciling perspective on the two main, but contradictory, interpretations of the southern annular mode (SAM). SAM was originally thought to characterize meridional shifts in the storm track across the entire hemisphere. This perspective was later questioned, and SAM was interpreted as a statistical artifact depending on the choice of base region for the principal component analysis. Neither perspective, however, fully describes SAM. We show that SAM cannot be interpreted in terms of midlatitude variability, as SAM merely modulates the most poleward part of the cyclone tracks and only marginally influences the distribution of other weather-related features of the storm track (e.g., position of jet axes and Rossby wave breaking). Instead, SAM emerges as the leading pattern of geopotential variability due to strong correlations of sea level pressure around the Antarctic continent. As SAM correlates strongly both with the pan-Antarctic mean temperature and the meridional heat flux through 65°S, we hypothesize that SAM can be interpreted as a measure of the degree of the (de)coupling between Antarctica and the southern midlatitudes. As an alternative way of characterizing southern midlatitude variability, we seek domains in which the leading EOF patterns of both the geopotential and storm-track features yield a dynamically consistent picture. This approach is successful for the South Pacific. Here the leading variability patterns are closely related to the Pacific–South America pattern and point toward an NAO-like variability.

scholarly journals Antarctic ice mass variations from 1979 to 2017 driven by anomalous precipitation accumulation

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Byeong-Hoon Kim ◽  
Ki-Weon Seo ◽  
Jooyoung Eom ◽  
Jianli Chen ◽  
Clark R. Wilson
Keyword(s):  
Mass Loss ◽  
Principal Component ◽  
Southern Annular Mode ◽  
Mass Change ◽  
Abrupt Increase ◽  
Future Projections ◽  
Steady Rate ◽  
The Pacific ◽  
Ice Mass Balance ◽  
Antarctic Precipitation

AbstractAntarctic ice mass balance is determined by precipitation and ice discharge, and understanding their relative contributions to contemporary Antarctic ice mass change is important to project future ice mass loss and resulting sea level rise. There has been evidence that anomalous precipitation affects Antarctic ice mass loss estimates, and thus the precipitation contribution should be understood and considered in future projections. In this study, we revisit changes in Antarctic ice mass over recent decades and examine precipitation contributions over this period. We show that accumulated (time-integrated) precipitation explains most inter-annual anomalies of Antarctic ice mass change during the GRACE period (2003–2017). From 1979 to 2017, accumulated Antarctic precipitation contributes to significant ice mass loss acceleration in the Pacific sector and deceleration in the Atlantic-Indian Sectors, forming a bi-polar spatial pattern. Principal component analysis reveals that such a bi-polar pattern is likely modulated by the Southern Annular Mode (SAM). We also find that recent ice mass loss acceleration in 2007 is related to a variation in precipitation accumulation. Overall ice discharge has accelerated at a steady rate since 1992, but has not seen a recent abrupt increase.

scholarly journals Decadal Variability of the ENSO Teleconnection to the High-Latitude South Pacific Governed by Coupling with the Southern Annular Mode*

2006 ◽  
Vol 19 (6) ◽  
pp. 979-997 ◽  
Author(s):  
Ryan L. Fogt ◽  
David H. Bromwich
Keyword(s):  
High Latitude ◽  
Decadal Variability ◽  
Southern Oscillation ◽  
Phase Relationship ◽  
South Pacific ◽  
Southern Annular Mode ◽  
Height Anomaly ◽  
Austral Spring ◽  
Annular Mode ◽  
The Pacific

Abstract Decadal variability of the El Niño–Southern Oscillation (ENSO) teleconnection to the high-latitude South Pacific is examined by correlating the European Centre for Medium-Range Weather Forecasts (ECMWF) 40-yr Re-Analysis (ERA-40) and observations with the Southern Oscillation index (SOI) over the last two decades. There is a distinct annual contrast between the 1980s and the 1990s, with the strong teleconnection in the 1990s being explained by an enhanced response during austral spring. Geopotential height anomaly composites constructed during the peak ENSO seasons also demonstrate the decadal variability. Empirical orthogonal function (EOF) analysis reveals that the 1980s September–November (SON) teleconnection is weak due to the interference between the Pacific–South American (PSA) pattern associated with ENSO and the Southern Annular Mode (SAM). An in-phase relationship between these two modes during SON in the 1990s amplifies the height and pressure anomalies in the South Pacific, producing the strong teleconnections seen in the correlation and composite analyses. The in-phase relationship between the tropical and high-latitude forcing also exists in December–February (DJF) during the 1980s and 1990s. These results suggest that natural climate variability plays an important role in the variability of SAM, in agreement with a growing body of literature. Additionally, the significantly positive correlation between ENSO and SAM only during times of strong teleconnection suggests that both the Tropics and the high latitudes need to work together in order for ENSO to strongly influence Antarctic climate.

The North Pacific Oscillation–West Pacific Teleconnection Pattern: Mature-Phase Structure and Winter Impacts

2008 ◽  
Vol 21 (9) ◽  
pp. 1979-1997 ◽  
Author(s):  
Megan E. Linkin ◽  
Sumant Nigam
Keyword(s):  
Sea Level ◽  
Pacific Northwest ◽  
Geopotential Height ◽  
Storm Track ◽  
Teleconnection Pattern ◽  
Sea Level Pressure ◽  
Level Pressure ◽  
The North ◽  
Mature Phase ◽  
The Pacific

Abstract The North Pacific Oscillation (NPO) in sea level pressure and its upper-air geopotential height signature, the west Pacific (WP) teleconnection pattern, constitute a prominent mode of winter midlatitude variability, the NPO/WP. Its mature-phase expression is identified from principal component analysis of monthly sea level pressure variability as the second leading mode just behind the Pacific–North American variability pattern. NPO/WP variability, primarily on subseasonal time scales, is characterized by a large-scale meridional dipole in SLP and geopotential height over the Pacific and is linked to meridional movements of the Asian–Pacific jet and Pacific storm track modulation. The hemispheric height anomalies at upper levels resemble the climatological stationary wave pattern attributed to transient eddy forcing. The NPO/WP divergent circulation is thermal wind restoring, pointing to independent forcing of jet fluctuations. Intercomparison of sea level pressure, geopotential height, and zonal wind anomaly structure reveals that NPO/WP is a basin analog of the NAO, which is not surprising given strong links to storm track variability in both cases. The NPO/WP variability is influential: its impact on Alaskan, Pacific Northwest, Canadian, and U.S. winter surface air temperatures is substantial—more than that of PNA or ENSO. It is likewise more influential on the Pacific Northwest, western Mexico, and south-central Great Plains winter precipitation. Finally, and perhaps, most importantly, NPO/WP is strongly linked to marginal ice zone variability of the Arctic seas with an influence that surpasses that of other Pacific modes. Although NPO/WP variability and impacts have not been as extensively analyzed as its Pacific cousins (PNA, ENSO), it is shown to be more consequential for Arctic sea ice and North American winter hydroclimate.

scholarly journals Rainfall Changes over Southwestern Australia and Their Relationship to the Southern Annular Mode and ENSO

2014 ◽  
Vol 27 (15) ◽  
pp. 5801-5814 ◽  
Author(s):  
Bhupendra A. Raut ◽  
Christian Jakob ◽  
Michael J. Reeder
Keyword(s):  
Southern Oscillation ◽  
Summer Rainfall ◽  
Southern Annular Mode ◽  
Winter Rainfall ◽  
Sea Level Pressure ◽  
Annular Mode ◽  
Southwestern Australia ◽  
Synoptic Conditions ◽  
The Mean ◽  
Rainfall Patterns

Abstract Since the 1970s, winter rainfall over coastal southwestern Australia (SWA) has decreased by 10%–20%, while summer rainfall has been increased by 40%–50% in the semiarid inland area. In this paper, a K-means algorithm is used to cluster rainfall patterns directly as opposed to the more conventional approach of clustering synoptic conditions (usually the mean sea level pressure) and inferring the associated rainfall. It is shown that the reduction in the coastal rainfall during winter is mainly due to fewer westerly fronts in June and July. The reduction in the frequency of strong fronts in June is responsible for half of the decreased rainfall in June–August (JJA), whereas the reduction in the frequency of weaker fronts in June and July accounts for a third of the total decrease. The increase in rainfall inland in December–February (DJF) is due to an increased frequency of easterly troughs in December and February. These rainfall patterns are linked to the southern annular mode (SAM) index and Southern Oscillation index (SOI). The reduction in coastal rainfall and the increase in rainfall inland are both related to the predominantly positive phase of SAM, especially when the phase of ENSO is neutral.

On the Summertime Strengthening of the Northern Hemisphere Pacific Sea Level Pressure Anticyclone

2009 ◽  
Vol 22 (5) ◽  
pp. 1174-1192 ◽  
Author(s):  
Sumant Nigam ◽  
Steven C. Chan
Keyword(s):  
Sea Level ◽  
Northern Hemisphere ◽  
Asian Monsoon ◽  
Storm Track ◽  
Hadley Cell ◽  
Winter Storm ◽  
Sea Level Pressure ◽  
Level Pressure ◽  
Mascarene High ◽  
The Pacific

Abstract This study revisits the question posed by Hoskins on why the Northern Hemisphere Pacific sea level pressure (SLP) anticyclone is strongest and maximally extended in summer when the Hadley cell descent in the northern subtropics is the weakest. The paradoxical evolution is revisited because anticyclone buildup to the majestic summer structure is gradual, spread evenly over the preceding 4–6 months, and not just confined to the monsoon-onset period, which is interesting, as monsoons are posited to be the cause of the summer vigor of the anticyclone. Anticyclone buildup is moreover found focused in the extratropics, not the subtropics, where SLP seasonality is shown to be much weaker, generating a related paradox within the context of the Hadley cell’s striking seasonality. Showing this seasonality to arise from, and thus represent, remarkable descent variations in the Asian monsoon sector, but not over the central-eastern ocean basins, leads to the resolution of this paradox. Evolution of other prominent anticyclones is analyzed to critique the development mechanisms: the Azores high evolves like the Pacific one, but without a monsoon to its immediate west. The Mascarene high evolves differently, peaking in austral winter. Monsoons are not implicated in both cases. Diagnostic modeling of seasonal circulation development in the Pacific sector concludes this inquiry. Of the three forcing regions examined, the Pacific midlatitudes are found to be the most influential, accounting for over two-thirds of the winter-to-summer SLP development in the extratropics (6–8 hPa), with the bulk coming from the abatement of winter storm-track heating and transients. The Asian monsoon contribution (2–3 hPa) is dominant in the Pacific (and Atlantic) subtropics. The modeling results resonate with observational findings and attest to the demise of winter storm tracks as the principal cause of the summer vigor of the Pacific anticyclone.

scholarly journals On the Leading Patterns of Northern Hemisphere Sea Level Pressure Variability*

2015 ◽  
Vol 72 (9) ◽  
pp. 3469-3486 ◽  
Author(s):  
Brian V. Smoliak ◽  
John M. Wallace
Keyword(s):  
Time Series ◽  
Sea Level ◽  
Northern Hemisphere ◽  
Surface Air Temperature ◽  
Sea Level Pressure ◽  
Annular Mode ◽  
Level Pressure ◽  
The Pacific ◽  
Reversed Polarity ◽  
A Chain

Abstract The leading patterns of variability of the monthly mean Northern Hemisphere (NH) sea level pressure (SLP) field, as derived from empirical orthogonal teleconnection (EOT) analysis of a 93-yr (1920–2012) record of NOAA–CIRES 20th Century Reanalyses, are presented and discussed, with emphasis on wintertime patterns. The analysis yields nine or more highly reproducible wintertime hemispheric EOTs, the first six of which closely resemble EOF1 or EOF2 in their respective sectors of the hemisphere. Collectively, the first nine wintertime patterns account for 70% of the variance of NH SLP, 40% of the variance of NH surface air temperature (SAT), and 52% of the variance of the time series of NH-mean SAT poleward of 20°N. Wintertime EOT1 corresponds to the NH annular mode (NAM) and EOT2 corresponds to the SLP expression of the Pacific–North America pattern. The remaining wintertime EOT patterns are monopoles arranged like the links of a chain wrapped around the primary center of action of the annular mode. The NH summertime and Southern Hemisphere patterns are arranged in a similar manner. The continental NH wintertime patterns exhibit strong temperature anomalies of reversed polarity to their respective SLP monopoles. The interannual variability of wintertime EOTs 3–9 and summertime EOTs 2–9 is dominated by sampling fluctuations. Over the 93-yr record, the more prominent continental wintertime patterns exhibit weak trends toward falling SLP and rising SAT, particularly over Russia and Alaska. The interpretation of shorter-term trends is more ambiguous.

scholarly journals On the Relationship between Circulation Patterns, the Southern Annular Mode, and Rainfall Variability in Western Cape

Atmosphere ◽  
2021 ◽  
Vol 12 (6) ◽  
pp. 753
Author(s):  
Chibuike Chiedozie Ibebuchi
Keyword(s):  
South Africa ◽  
High Probability ◽  
Climate Models ◽  
Rainfall Variability ◽  
Principal Component ◽  
Southern Annular Mode ◽  
Average Frequency ◽  
Frequency Of Occurrence ◽  
Western Cape ◽  
Annular Mode

This study investigates circulation types (CTs) in Africa, south of the equator, that are related to wet and dry conditions in the Western Cape, the statistical relationship between the selected CTs and the Southern Annular Mode (SAM), and changes in the frequency of occurrence of the CTs related to the SAM under the ssp585 scenario. Obliquely rotated principal component analysis applied to sea level pressure (SLP) was used to classify CTs in Africa, south of the equator. Three CTs were found to have a high probability of being associated with wet days in the Western Cape, and four CTs were equally found to have a high probability of being associated with dry days in the Western Cape. Generally, the dry/wet CTs feature the southward/northward track of the mid-latitude cyclone, adjacent to South Africa; anti-cyclonic/cyclonic relative vorticity, and poleward/equatorward track of westerlies, south of South Africa. One of the selected wet CTs was significantly related to variations of the SAM. Years with an above-average SAM index correlated with the below-average frequency of occurrences of the wet CT. The results suggest that through the dynamics of the CT, the SAM might control the rainfall variability of the Western Cape. Under the ssp585 scenario, the analyzed climate models indicated a possible decrease in the frequency of occurrence of the aforementioned wet CT associated with cyclonic activity in the mid-latitudes, and an increase in the frequency of the occurrence of CT associated with enhanced SLP at mid-latitudes.

The South Pacific Pressure Trend Dipole and the Southern Blob

2021 ◽  
pp. 1-54
Author(s):  
René D. Garreaud ◽  
Kyle Clem ◽  
José Miguel Vicencio
Keyword(s):  
Decadal Variability ◽  
Storm Track ◽  
South Pacific ◽  
Southern Annular Mode ◽  
Hadley Cell ◽  
Positive Trend ◽  
West Antarctica ◽  
The South ◽  
Sea Level Pressure ◽  
The Tropics

AbstractDuring the last four decades, the sea level pressure has been decreasing over the Amundsen-Bellingshausen Sea (ABS) region and increasing between 30-40°S from New Zealand to Chile, thus forming a pressure trend dipole across the South Pacific. The trends are strongest in austral winter and have influenced the climate of West Antarctica and South America. The pressure trends have been attributed to decadal variability in the tropics, expansion of the Hadley cell and an associated positive trend of the Southern Annular Mode, but these mechanisms explain only about half of the pressure trend dipole intensity. Experiments conducted with two atmospheric models indicate that upper ocean warming over the subtropical southwest Pacific (SSWP), termed the Southern Blob, accounts for about half of the negative pressure trend in the ABS region and nearly all the ridging /drying over the eastern subtropical South Pacific, thus contributing to the central Chile megadrought. The SSWP warming intensifies the pressure trend dipole through warming the troposphere across the sub-tropical South Pacific and shifting the mid-latitude storm track poleward into the ABS. Multi-decadal periods of strong SSWP warming also appears in fully coupled pre-industrial simulations, associated with a pressure trend dipole and reduction in rainfall over the central tropical Pacific, thus suggesting a natural origin of the Southern Blob and its teleconnection. However, the current warming rate exceeds the range of natural variability, implying a likely additional anthropogenic contribution.

scholarly journals A Station-Based Southern Annular Mode Index from 1884 to 2005

2009 ◽  
Vol 22 (4) ◽  
pp. 940-950 ◽  
Author(s):  
Martin Visbeck
Keyword(s):  
Mass Conservation ◽  
Southern Annular Mode ◽  
Sea Level Pressure ◽  
Subtropical Zone ◽  
Annular Mode ◽  
Continuous Surface ◽  
Pressure Anomaly ◽  
Antarctic Continent ◽  
The Antarctic ◽  
Atmospheric Mass

Abstract Atmospheric pressure observations from the Southern Hemisphere are used to estimate monthly and annually averaged indexes of the southern annular mode (SAM) back to 1884. This analysis groups all relevant observations in the following four regions: one for Antarctica and three in the subtropical zone. Continuous surface pressure observations are available at a number of locations in the subtropical regions since the end of the nineteenth century. However, year-round observations in the subpolar region near the Antarctic continent began only during the 1940–60 period. The shorter Antarctic records seriously compromise the length of a traditionally estimated SAM index. To improve the situation “proxy” estimates of Antarctic sea level pressure anomalies are provided based on the concept of atmospheric mass conservation poleward of 20°S. This allows deriving a longer SAM index back to 1884. Several aspects of the new record, its statistical properties, seasonal trends, and the regional pressure anomaly correlations, are presented.

scholarly journals Seasonal Variations of Subtropical Precipitation Associated with the Southern Annular Mode

2014 ◽  
Vol 27 (9) ◽  
pp. 3446-3460 ◽  
Author(s):  
Harry H. Hendon ◽  
Eun-Pa Lim ◽  
Hanh Nguyen
Keyword(s):  
Seasonal Variation ◽  
Seasonal Variations ◽  
Meridional Circulation ◽  
Storm Track ◽  
Phase Speed ◽  
Southern Annular Mode ◽  
Subtropical Climate ◽  
Flux Divergence ◽  
Annular Mode ◽  
Subtropical Jet

Abstract Seasonal variations of subtropical precipitation anomalies associated with the southern annular mode (SAM) are explored for the period 1979–2011. In all seasons, high-polarity SAM, which refers to a poleward-shifted eddy-driven westerly jet, results in increased precipitation in high latitudes and decreased precipitation in midlatitudes as a result of the concomitant poleward shift of the midlatitude storm track. In addition, during spring–autumn, high SAM also results in increased rainfall in the subtropics. This subtropical precipitation anomaly is absent during winter. This seasonal variation of the response of subtropical precipitation to the SAM is shown to be consistent with the seasonal variation of the eddy-induced divergent meridional circulation in the subtropics (strong in summer and weak in winter). The lack of an induced divergent meridional circulation in the subtropics during winter is attributed to the presence of the wintertime subtropical jet, which causes a broad latitudinal span of eddy momentum flux divergence due primarily to higher phase speed eddies breaking poleward of the subtropical jet and lower speed eddies not breaking until they reach the equatorward flank of the subtropical jet. During the other seasons, when the subtropical jet is less distinctive, the critical line for both high and low speed eddies is on the equatorward flank of the single jet and so breaking in the subtropics occurs over a narrow range of latitudes. The implications of these findings for the seasonality of future subtropical climate change, in which a shift to high SAM in all seasons is expected to be promoted, are discussed.

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