tion on the two sides of the Antarctic Peninsula, a region where the trends in the SAM are also known to have had a signi ﬁcant impact on SAT in this season (Marshall et al., 2006).
Climate model projections forecast an increase in Antarcticprecipitation as global temperatures rise. Indeed, modeling studies suggest that a distinct pattern of austral summer drying and moistening observed in SouthernHemisphere midlatitudes and high latitudes, respectively, over the last few decades is likely to have a strong anthropogenic component (Fyfe et al., 2012). If the patterns of circulation variability examined here change on low-frequency time scales —as is the case for the SAM and to a lesser extent the PSA patterns— there will be a response in the associated precipitation pattern, which in turn will have an impact on the regional surface mass balance. Current global coupled-climate models, which are employed to make projec- tions of future climate change, are generally able to reproduce these atmosphericpatterns but struggle with correctly replicating their regional impacts on Antarctic climate (e.g., Marshall & Bracegirdle, 2015). Therefore, an examination of how the main patterns of atmosphericcirculation are likely to change in the future may prove more useful in predicting how regional-scale Antarcticprecipitation will evolve than a direct analysis of changes in modeled precipitation itself.
spaced geographically, are relatively sparse, and are not all equally well dated, nor do they all have the same set of measurements. Notwithstanding, Antarctic ice core reconstructions offer consid- erable guidance in understanding past changes in, for example, temperature, winds (i.e., atmosphericcirculation), precipitation, and sea ice extent. In this paper we utilized highly smoothed ice core records to avoid issues of dating quality. As noted above, ex- amination of the last ~2000 years of ice core sodium demonstrates that West Antarctica has been in a continuing state of deglaciation since at least ~1500 years ago, and likely as of ~6000 e7000 years ago based on longer ice core and glacial geologic records ( Conway et al., 1999 ). As deglaciation continued it was accompanied by poleward contraction of the zonal westerlies and ASL, and increased frequencies of inland penetration of marine source (warm, moist) air masses, albeit with increased cooling in regions dominated by offshore air ﬂow (e.g., westward of the ASL). West Antarctica and to a substantially lesser extent other Antarctic re- gions, experienced intensi ﬁcation in marine air mass intrusion ~600 years ago during the onset of a globally distributed LIA event, with cooling and warming dependent on the location and intensity of low pressure centers such as the ASL and Icelandic Low. This period was followed, until recent decades, by weakening of the ASL
Atmosphere 2020, 11, 77 7 of 16
3. Results from Reanalysis 3.1. Canonical El Niño
For the Canonical El Niño, OLR correlation maps were characterized by strong negative correlation values over SSA (Figure 2 a), suggesting an increase of precipitation when the positive phase of the Canonical El Niño takes place. Figure 2 b shows that the upper level circulation anomaly pattern was characterized by an anomalous cyclonic circulation around (35 ◦ S, 265 ◦ E) and an anomalous anticyclonic circulation over northeastern SSA around (28 ◦ S, 315 ◦ E). The combination of these two circulation anomalies induced a strong geopotential height gradient over SSA and favored advection of cyclonic vorticity over there (in agreement with References [ 2 , 17 , 32 ]). Comparing with Figure 3 a, where the composite map of the Canonical El Niño events was plotted, it was possible to see that the distribution of the extratropical z200 anomalies was similar with both metrics. This result suggested that the atmospheric response associated with Canonical El Niño is highly linear. Figure 3 a also shows the F s field, which suggested the presence of a large Rossby wave propagating from the central-western subtropical Pacific, around (40 ◦ S, 200 ◦ E), toward the southeast. However, z200 anomalies were not significant at extratropical latitudes. Additionally, it was possible to see a short Rossby wave train excited in the central-tropical Pacific, around (35 ◦ S, 270 ◦ E), that propagated eastward inducing an anomalous anticyclonic circulation over northeastern SSA around (25 ◦ S, 310 ◦ E). As in the correlation map (Figure 2 b), these circulation anomalies induced a strong geopotential height gradient over SSA and favored the advection of cyclonic vorticity (Figure 3 a), in agreement with References [ 2 , 17 , 32 ].
; Truc et al., 2013). The latter can be discerned as a major research gap in southernmost South Africa. Due to the strongly erosive Agulhas Current, very few marine records from the Agulhas Bank exist and the influence of SSTs on the regional precipitationpatterns therefore remains under dis- pute. On the one hand, it has been suggested that decreased SSTs on the southwestern coast of South Africa lead to de- creasing precipitation in southernmost South Africa (Rouault et al., 2003) and positive SST anomalies along the east coast associated with Agulhas strengthening enhance summer pre- cipitation in the eastern South Africa (Jury et al., 1993; Dupont et al., 2011; Scott et al., 2012). On the other hand, the oxygen isotope composition of marine mollusk shells preserved in Nelson Bay archaeological cave deposits indi- cates that during periods of wetter conditions over the south- ern African interior, the Agulhas surface water temperatures were actually lower than during arid periods (Cohen and Tyson, 1995). Based on these data in combination with in- terannual observations, a conceptual model relating oceanic and atmosphericcirculation systems of southern Africa was
Abstract We investigate the impact that the four principal large-scale patterns of SouthernHemisphere (SH) atmosphericcirculation variability have on Antarctic surface air temperature (SAT): (1) the southern baroclinic annular mode (BAM), which is associated with variations in extratropical storm amplitude; (2) the Southern Annular Mode (SAM), associated with latitudinal shifts in the midlatitude jet; and (3) the two Paci ﬁc-South American patterns (PSA1 and PSA2), which are characterized by wave trains originating in the tropical Paci ﬁc that extend across the SH extratropics. A key aspect is the use of 35 years of daily observations and reanalysis data, which affords a suf ﬁciently large sample size to assess the signatures of the circulationpatterns in both the mean and variability of daily mean SAT anomalies. The BAM exerts the weakest in ﬂuence on Antarctic SAT, albeit it is still important over select regions. Consistent with previous studies, the SAM is shown to in ﬂuence SAT across most of the continent throughout the year. The PSA1 also affects SAT across almost all of Antarctica. Regionally, both PSA patterns can exert a greater impact on SAT than the SAM but also have a signi ﬁcantly weaker in ﬂuence during summer, reﬂecting the seasonality of the SH response to El Niño–Southern Oscillation. The SAM and PSA patterns have distinct signatures in daily SAT variance that are physically consistent with their signatures in extratropical dynamic variability. The broad-scale climate linkages identi ﬁed here provide benchmarks for interpreting the Antarctic climate response to future changes in tropical sea surface temperatures, ozone recovery, and greenhouse gas increases.
The atmosphericcirculation in the SouthernHemisphere (SH) has a large impact on SH climate and sea ice cover through complex air-ocean-sea ice interaction. Previous studies have identified the hemispheric-scale at- mospheric variability in SH comprising of a dominant mode characterized by zonally symmetric see-saw pattern between the mid- and high-latitudes. This mode appears as the leading mode of many atmospheric variables, e.g., mean sea level pressure (MSLP), geopotential heights, zonal winds -, which has been referred to as Antarctic Oscillation (AAO) or Southern Annular Mode (SAM) and dominates the region poleward of 20˚S    . Second and third modes are identified as the El Niño-Southern Oscillation (ENSO)-driven Pacific- South America (PSA) teleconnection patterns . These modes are part of a stationary wave train generated by tropical convection that travels towards Polar regions.
Changes in stationary waves also induce changes in sur- face temperature. Using a simple ice-sheet model based on an idealized geometry coupled with a stationary-wave model, Roe and Lindzen (2001a, b) highlighted the importance of accounting for the feedbacks between ice sheets and the tem- peratures induced by stationary waves to properly simulate the evolution of an ice sheet. In the same way, with a three- dimensional stationary wave model, Liakka et al. (2011) showed that the southern margin of ice sheets strongly de- pends on the temperature anomalies due to stationary waves. All these studies illustrate the existence of feedbacks be- tween ice sheets and atmosphericcirculation. This suggests that the construction of a given ice sheet (e.g. the North American ice sheet) may influence the growth or the decay of the other one (e.g. the Eurasian ice sheet) through changes in atmosphericcirculation that induce modifications in both temperature and precipitationpatterns. In turn, these modifi- cations directly influence the surface mass balance of the ice sheets.
DOI: 10.4236/acs.2018.82017 264 Atmospheric and Climate Sciences concentrated in the southern part of Sahara with tropical rainfall dependence . This study conclude that the prevailing of mid-latitude jet stream enhances the winter precipitation, while the African Easterly Jet (AEJ) and lower layer moisture both subjected to the West African monsoon (WAM), affect summer rainfall. The transition period results from the changes in rainfall types (i.e. the change from extratropical to tropical rainfall types) is related to the changes in moisture and zonal wind in the Northern Hemisphere (NH) . However, the previous studies strived to understand the characteristics of Sahara Desert pre- cipitation and its connection to general circulation but the variability on the large time scale and the climate change nexus need further research. Thus, this study focused on the long-term variability in the Sahara precipitation and its re- lationship to the climate change including an existence of an abrupt change and the changes in atmospheric general circulation as the root of recent increase in Sahara precipitation observed in the last four decades of 1971-2010. This will lead to the further understanding of the characteristics of spatial and temporal variability of Sahara precipitation and significantly improve our understanding of from global change responses to the regional climates changes. It will also en- hance our understanding to the role of climate change in arid regions particu- larly, the vulnerability of hyper-arid climate.
with T_polar and T_equator. The climate change of U10 is much larger in the ERA40 reanalysis than the model simulation (GFDL_B), and the response is quite linear with the stratospheric temperature forcing (as previously mentioned, the ERA40 T_polar is close to double the temperature in the GFDL_B runs (e.g., ). For both ERA40 and GFDL_B, regression with T_polar better describes the climatic change. Moreover, the fact that GFDL_A does not show any significant climate change confirms the fact that the zonal mean surface westerlies are controlled mainly by ozone depletion that mostly produces the stratospheric temperature anomalies . It should be noted that the responses to T_equator for GFDL_B and GFDL_A are similar-although small, they tend to be out of phase with observed climatic change of U10.  find a similar, but smaller, response in runs with greenhouse gas changes that do not include ozone variability. Changes in upper tropospheric effects from CO 2 and other gases may also influence the circulation in
The aim is to identify a classification in which the set of classes defining atmospheric pressure fields can explain the occurrence of wave events at a specified location. There are many ways in which classification algorithms can be constructed. Classifications can be subjective, objective or a mixture of both (Bárdossy, 2010; Huth et al., 2008). Ob- jective classification algorithms employ a self-learning tech- nique whereby atmospheric classes are derived through an optimization procedure (for examples see Bárdossy, 2010; Bárdossy et al., 2002; Huth et al., 2008; Hewitson and Crane, 2002). Since the goal is to gain insight into the drivers of a re- gional wave climate, it follows that the wave climate should be included within the optimization procedure (see Sect. 2.4). Then the set of CP classes that are derived have strong links to the regional wave climate. Furthermore, classes linked with these variables explain, as best possible, their occur- rences. This is a useful tool in guiding the algorithm to an
There are quite a few suggested methods of MOS in the lit- erature (for an overview see Maraun et al., 2010). The type of MOS method most suited for a specific impact study will ultimately depend on the application objective/s and in this case, high river flow events were of most interest to quan- tify potential flood inducing events. Therefore, two meth- ods that downscale the precipitation distribution were se- lected, (1) a quantile-quantile mapping (QM) correction as described by Bo´e et al. (2007) and (2) a distribution-based scaling (DBS) of the modelled and observed precipitation (Yang et al., 2010), but also a simpler factor change method, (3) the direct method (DM; Lenderink et al., 2007), was in- cluded as a benchmark. The rationale for choosing the first 2 methods are that they are sensitive to changes in the tails of the distribution and are therefore potentially more useful in hydrological impact studies. The direct method is merely correcting for the mean bias, and does not transfer any infor- mation on any modelled shift in the precipitation distribution from the RCM.
Several conclusions can be drawn from these results. The estimated I –D parameters for summer and fall differ mainly on parameter α, with I –D for fall being associated with higher value. The sampling uncertainty appears significant mostly for the α parameter and is higher for fall. However, it can only partially explain the seasonal difference in param- eter α. Comparison of I –D thresholds for different weather types shows that DF cases associated with meridional north pattern, are characterized by significantly higher (than in all other weather types) values of both I –D parameters. Results for zonal west and mixed patterns are associated with very low values for parameter β in comparison with the other cases, but also with reference to other thresholds reported in literature (see Guzzetti et al., 2007). Despite the associated uncertainties due to sampling size, overall the results pre- sented (Table 3, Figs. 9 and 10) show that classification of DF according to season or weather type can lead to consid- erably different thresholds. Therefore, these findings suggest that such a classification-based approach could prove bene- ficial for the operational use of rainfall thresholds. However, quantification of the significance of improvement would re- quire the comparison of such scheme (i.e., class-based I –D) with a static (i.e., independent of season and weather type) threshold.
Son et al., 2010; Lee and Feldstein, 2013). One possible ex- planation for this difference is that the earlier studies mainly examined the austral summer season focusing on the re- sponse of the linear trend in the zonal mean circulation to- wards the positive SAM phase and the poleward shift of the Hadley cell (Son et al., 2010; Polvani et al., 2011; Previdi and Polvani, 2014). Our study focuses on attributing systematic changes in the circulation over the latter half of the NCEP reanalysis period employing a data-driven methodology that can infer causation (as explained above, FEM-BV-VARX is a non-stationary extension of the Granger causality inference and can describe the standard Granger causality as a particu- lar stationary case). Moreover, we are not simply considering changes to the zonal SAM index in the austral summer in iso- lation but are explicitly attributing changes to the entire SH circulation including coherent features to all possible com- binations of the relevant radiative forcings. Previous studies mainly analyzed changes in the mean state and not in the fre- quency of occurrence or changes in structure. This might also partly explain why our findings differ from previous studies. To increase the confidence in our results, we systemati- cally examined the sensitivity of our results to the treatment of the ozone data. Considering a 365-day-averaged and time- lagged seasonally varying OMD leads to more robust results because we account for the strong annual cycle of strato- spheric ozone and its delayed impact on the tropospheric cir- culation. Ozone has a strong seasonal component with OMD known to impact the tropospheric circulation (from the ob- servational record) in December–January. Thus, we repeated our analysis using lagged (by 0, 1, 2 and 3 months), season- ally varying and 365-running-mean OMD data. While we did find some sensitivity to lag interval, our results were qualita- tively unchanged.
Correspondence to: H. Angot (email@example.com)
Received: 20 May 2014 – Published in Atmos. Chem. Phys. Discuss.: 3 June 2014
Revised: 19 September 2014 – Accepted: 22 September 2014 – Published: 30 October 2014
Abstract. Although essential to fully understand the cy- cling of mercury at the global scale, mercury species records in the SouthernHemisphere are scarce. Under the frame- work of the Global Mercury Observation System (GMOS) project, a monitoring station has been set up on Ams- terdam Island (37 ◦ 48 0 S, 77 ◦ 34 0 E) in the remote southern Indian Ocean. For the first time in the Southern Hemi- sphere, a 2-year record of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particle-bound mercury (PBM) is presented. GEM concentrations were remarkably steady (1.03 ± 0.08 ng m − 3 ) while RGM and PBM concentrations were very low and exhibited a strong variability (mean: 0.34 pg m − 3 , range: < detection limit– 4.07 pg m − 3 ; and mean: 0.67 pg m − 3 , range: < detection limit–12.67 pg m − 3 , respectively). Despite the remoteness of the island, wind sector analysis, air mass back trajectories and the observation of radonic storms highlighted a long- range contribution from the southern African continent to the GEM and PBM budgets from July to September dur- ing the biomass burning season. Low concentrations of GEM were associated with southerly polar and marine air masses from the remote southern Indian Ocean. This unique data set provides new baseline GEM concentrations in the South- ern Hemisphere midlatitudes while mercury speciation along with upcoming wet deposition data will help to improve our understanding of the mercury cycle in the marine boundary layer.
We have observed that the most relevant data for the ana- logue method can be found where specific atmospheric cir- culation patterns appear concomitantly with heavy precipi- tation events. Those skilled regions are coherent with the atmospheric flows illustrated, for example, by means of the back trajectories of air masses. Indeed, the circulation recur- rently diverges from the climatology during days with strong precipitation on the southern part of the alpine Rhˆone catch- ment. We have found that for over 152 days with precipita- tion amount above 50 mm at the Binn station, only 3 did not show a trajectory of a southerly flow, meaning that such a circulation was present for 98 % of the events.
ABSTRACT: This study has used TOMS AI as well as the reanalysis dataset of thirty- four years (1979-2012) to investigate the influence of atmosphericcirculation on dust transport during the Harmattan period in West Africa, using Aerosol Index (AI) data, obtained from various satellite sensors. Changes in Inter-Tropical Discontinuity (ITD), Sea Surface Temperature (SST) over the Gulf of Guinea, and North Atlantic Oscillation (NAO) during Harmattan period (November-March) have been analyzed on daily basis with Harmattan dust mobilization as well as atmosphericcirculation pattern being evaluated via a kernel density estimate that shows the relation between the two variables. The study has found out that strong north-easterly (NE) trade winds were over most of the Sahelian region of West Africa during the winter months with the maximum wind speed reaching 8.61 m/s in January. The strength of NE winds determines the extent of dust transport to the coast of Gulf of Guinea during winter. This study has also confirmed that the occurrence of the Harmattan chiefly depends on SST in Atlantic Ocean as well as ITD position, not to mention the strength of low level winds. However, it has been noted that NAO has limited effects on dust mobilization in West Africa, in shear contrast to North Africa where NAO is a strong factor in dust mobilization.
have been the result of upwelling processes, giving additional support to the hypothesis of Prézelin et al. (2000). Although these authors correlated the up- welling with ‘unusual strong winds’ and suggested that these events may be rare, observation of the same phenomena in 2 different years suggest that upwelling may be repeated every year in this region. The bottom topography is characterized by a deep channel in this region (Hofmann et al. 1996, Prézelin et al. 2000), probably providing a direct connection between this site and the outer shelf and favoring upwelling pro- cesses. During 1997, the abundance of both krill and salps was relatively high at stations with this phyto- plankton assemblage (Fig. 8, Table 4), as also seen in this region during January 1993 (Ross et al. 1998). Thus, it appears that upwelling processes may have implications for the trophic-chain dynamics in re- stricted sites of the western Antarctic Peninsula as stated by Prézelin et al. (2000).
in a continuous path some 22,000 kilometres long. The ACC consists of two main fronts, the Polar and Subantarctic Fronts (Figure 1). Fronts in the ocean are boundaries between waters with different temperature, salinity and density characteristics. Density changes across fronts result in jets of water along a front, transporting water masses and their properties, such as heat, salt and dissolved carbon dioxide. This current effi ciently blends waters from the Atlantic, Indian and Pacifi c Oceans (Naveira-Garabato et alii, 2003), creating a global circulation. The Subantarctic Front marks the northern boundary of the Southern Ocean, where cold surface water from the south sinks beneath warmer subtropical waters. Around its circumpolar path the ACC meanders northward and southward defl ected by submarine topography: particularly the Scotia Arc; Kerguelen Plateau; Macquarie Ridge complex; and the Pacifi c Antarctic Ridge (Figure 1). Thus, the northern edge of the Southern Ocean varies in latitude, controlled strongly by topography. We will see later that submarine mountains play a key role in the dynamics of the zonal and meridional circulations.