southward shift
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2021 ◽  
pp. 1-43
Author(s):  
TAO WANG ◽  
WENSHOU TIAN ◽  
TAO LIAN ◽  
CHENG SUN ◽  
FEI XIE ◽  
...  

AbstractChanges in the meridional position of the sea surface temperature (SST) anomalies (SSTAs) associated with the interannual component (PC1-I) of the principal component 1 (PC1) of the first leading mode of the North Pacific SST (referred here as PC1-I-related SSTAs) are investigated using reanalysis products and climate model output. It is found that the PC1-I-related SSTAs (or PC1-I anomaly) significantly shift southward at a rate of 1.04°/decade and have moved southward by 4.4 degrees since the 1960s. Our further analysis indicates that the southward shift of the PC1-I-related SSTAs is due to changes in ENSO teleconnections. Compared to the 1950–1975 period (PRE era), the meridional width of the ENSO-induced tropical positive geopotential height (GH) anomaly is narrower during the 1991–2016 period (POST era), inducing a southward shift of the subtropical westerly anomaly over the North Pacific through geostrophic wind relations. This southward shift of the westerly anomaly favors the southward shift of the ENSO-induced negative GH anomaly (cyclonic circulation anomaly) over the North Pacific by positive vorticity forcing of the zonal wind shear. The southward-shifting GH anomaly associated with ENSO further forces the PC1-I anomaly to shift southward. Furthermore, the contraction of the ENSO-induced tropical positive GH anomaly is related to the contraction of the meridional width of ENSO. The modeling results support that the decrease in the ENSO meridional width favors the contraction of the ENSO-induced tropical positive GH anomaly and the southward shift of ENSO teleconnections over the North Pacific, contributing to the southward shift of the PC1-I anomaly.


2021 ◽  
pp. 1-45
Author(s):  
Yuhan Gong ◽  
Tim Li

AbstractThe cause of southward shift of anomalous zonal wind in the central equatorial Pacific (CEP) during ENSO mature winter was investigated through observational analyses and numerical model experiments. Based on an antisymmetric zonal momentum budget diagnosis using daily ERA-Interim data, a two-step physical mechanism is proposed. The first step involves advection of the zonal wind anomaly by the climatological mean meridional wind. The second step involves the development of an antisymmetric mode in the CEP, which promotes a positive contribution to the observed zonal wind tendency by the pressure gradient and Coriolis forces. Two positive feedbacks are responsible for the growth of the antisymmetric mode. The first involves the moisture–convection–circulation feedback, and the second involves the wind–evaporation–SST feedback. General circulation model experiments further demonstrated that the boreal winter background state is critical in generating the southward shift, and a northward shift of the zonal wind anomaly is found when the same SST anomaly is specified in boreal summer background state.


2021 ◽  
pp. 1-16
Author(s):  
Scott A. Reynhout ◽  
Michael R. Kaplan ◽  
Esteban A. Sagredo ◽  
Juan Carlos Aravena ◽  
Rodrigo L. Soteres ◽  
...  

Abstract In the Cordillera Darwin, southernmost South America, we used 10Be and 14C dating, dendrochronology, and historical observations to reconstruct the glacial history of the Dalla Vedova valley from deglacial time to the present. After deglacial recession into northeastern Darwin and Dalla Vedova, by ~16 ka, evidence indicates a glacial advance at ~13 ka coeval with the Antarctic Cold Reversal. The next robustly dated glacial expansion occurred at 870 ± 60 calendar yr ago (approximately AD 1150), followed by less-extensive dendrochronologically constrained advances from shortly before AD 1836 to the mid-twentieth century. Our record is consistent with most studies within the Cordillera Darwin that show that the Holocene glacial maximum occurred during the last millennium. This pattern contrasts with the extensive early- and mid-Holocene glacier expansions farther north in Patagonia; furthermore, an advance at 870 ± 60 yr ago may suggest out-of-phase glacial advances occurred within the Cordillera Darwin relative to Patagonia. We speculate that a southward shift of westerlies and associated climate regimes toward the southernmost tip of the continent, about 900–800 yr ago, provides a mechanism by which some glaciers advanced in the Cordillera Darwin during what is generally considered a warm and dry period to the north in Patagonia.


2021 ◽  
Author(s):  
Jürgen Mey ◽  
Juluis Jara ◽  
Manfred R. Strecker

<p><span><span>The Dead Sea depression features exceptionally well preserved lacustrine sedimentary sequences and fossil lake-level markers that attest to a much more extensive lake with a maximum highstand water level of more than 200 m above the modern Dead Sea. Lake-level reconstructions based on sedimentary sequences places this highstand phase within the interval of 15-29 ka. Regional paleoclimatic records, however, indicate arid conditions during this time. This apparent contradiction has been explained by spatially heterogeneous moisture delivery resulting from a southward shift of the Westerly wind system and a change in the path and intensity of winter storms. A newly established lake level-chronology based on </span><sup><span>14</span></sup><span>C- and U/Th-dating of fossil stromatolites has provided contrasting results with respect to previous investigations. Accordingly, the paleolake-highstand was of much shorter duration and occurred at least 10 ka earlier than previously suggested</span><span>. The new lake-level curve agrees with evidence of arid glacial and humid interglacial periods in the Levant.  </span><span>In this study we compared these different lake-level reconstructions quantitatively, using a distributed hydrological balance model. This model computes evaporation based on an aerodynamic- /mass-transfer approach. Calibration and validation of this model is achieved by using ~30 years of pre-anthropogenic lake-level observations combined with interpolated climate surfaces based on weather-station records. In the paleo-hydrological reconstruction we account for parameter uncertainties using Monte-Carlo simulations. Our preliminary results show a pronounced sensitivity of the lake-level to precipitation, wind speed, and surface roughness.</span></span></p>


2020 ◽  
Vol 33 (12) ◽  
pp. 5081-5101
Author(s):  
Jiabao Wang ◽  
Hyemi Kim ◽  
Daehyun Kim ◽  
Stephanie A. Henderson ◽  
Cristiana Stan ◽  
...  

AbstractIn an assessment of 29 global climate models (GCMs), Part I of this study identified biases in boreal winter MJO teleconnections in anomalous 500-hPa geopotential height over the Pacific–North America (PNA) region that are common to many models: an eastward shift, a longer persistence, and a larger amplitude. In Part II, we explore the relationships of the teleconnection metrics developed in Part I with several existing and newly developed MJO and basic state (the mean subtropical westerly jet) metrics. The MJO and basic state diagnostics indicate that the MJO is generally weaker and less coherent and propagates faster in models compared to observations. The mean subtropical jet also exhibits notable biases such as too strong amplitude, excessive eastward extension, or southward shift. The following relationships are found to be robust among the models: 1) models with a faster MJO propagation tend to produce weaker teleconnections; 2) models with a less coherent eastward MJO propagation tend to simulate more persistent MJO teleconnections; 3) models with a stronger westerly jet produce stronger and eastward shifted MJO teleconnections; 4) models with an eastward extended jet produce an eastward shift in MJO teleconnections; and 5) models with a southward shifted jet produce stronger MJO teleconnections. The results are supported by linear baroclinic model experiments. Our results suggest that the larger amplitude and eastward shift biases in GCM MJO teleconnections can be attributed to the biases in the westerly jet, and that the longer persistence bias is likely due to the lack of coherent eastward MJO propagation.


2020 ◽  
Author(s):  
Kwatra Sadhvi ◽  
Iyyappan Suresh ◽  
Izumo Takeshi ◽  
Jerome Vialard ◽  
Matthieu Lengaigne ◽  
...  

<p>The Great Whirl (GW) is a quasi-permanent anticyclonic eddy that forms off the horn of Africa in the western Arabian Sea. It generally appears in June, peaks in July-August, and dissipates in September. While the annual cycle of the GW has been described by past literature, its year-to-year variability has not yet been thoroughly explored. Satellite sea-level observations reveal that the leading mode of interannual variability (half of the interannual summer variance in the GW region) is associated with a typically ~100-km GW northward or southward shift. This meridional shift is associated with coherent sea surface temperature (SST) and surface chlorophyll signals, with warmer SST and reduced marine primary productivity in regions with positive sea level anomalies (and vice versa). Eddy-resolving (~10-km resolution) simulations with an ocean general circulation model capture those observed patterns reasonably well, even in the absence of interannual variations in the surface forcing. Interannual surface forcing variations enhance the GW interannual variability, but do not constrain its phase. Our results hence indicate that year-to-year variations in the Somalia upwelling SST and productivity associated with the GW are thus not a deterministic response to surface forcing, but largely arise from oceanic internal instabilities.</p>


2020 ◽  
Author(s):  
Prodromos Zanis ◽  
Dimitris Akritidis ◽  
Aristeidis K. Georgoulias ◽  
Robert J. Allen ◽  
Susanne E. Bauer ◽  
...  

<p>We present an analysis of the fast responses on pre-industrial climate due to present-day aerosols in a multi-model study based on Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations from 10 Earth System Models (ESMs) and General Circulation Models (GCMs). The aforementioned simulations were implemented within the framework of the Aerosol Chemistry Model Intercomparison Project (AerChemMIP). All models carried out two sets of simulations; a control experiment with all forcings set to the year 1850 and a perturbation experiment with all forcings identical to the control, except for aerosols with precursor emissions set to the year 2014. The perturbation by the present-day aerosols indicates negative top of the atmosphere (TOA) effective radiative forcing (ERF) values around the globe, especially over continental regions of the Northern Hemisphere in summer, with the largest negative values appearing over East Asia. Simulations in 3 models (CNRM-ESM2-1, MRI-ESM2-0 and NorESM2-LM) with individual perturbation experiments using present day SO<sub>2</sub>, BC and OC emissions show the dominating role of sulfates in all-aerosols ERF. In response to the pattern of all aerosols ERF, the fast temperature responses are characterised by cooling over the continental areas, especially in the Northern Hemisphere, with the largest cooling over East Asia and India and sulfate being the dominant aerosol surface temperature driver for present-day emissions. The largest fast precipitation responses are seen in the tropical belt regions, generally characterized by  a reduction over continental regions and a southward shift of the tropical rain belt. This is a characteristic and robust feature among most models in this study, associated with a southward shift of the Intertropical convergence zone (ITCZ) and a weakening of the monsoon systems around the globe (Asia, Africa and America) in response to hemispherically asymmetric cooling from a Northern Hemisphere aerosol perturbation. An interesting feature in aerosol induced circulation changes is a characteristic dipole pattern with intensification of the Icelandic Low and an anticyclonic anomaly over Southeastern Europe, inducing warm air advection towards the northern polar latitudes in winter.</p><p>This research was funded by the project "PANhellenic infrastructure for Atmospheric Composition and climatE change" (MIS 5021516) which is implemented under the Action "Reinforcement of the Research and Innovation Infrastructure", funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).</p>


2020 ◽  
Vol 10 (4) ◽  
pp. 109-114
Author(s):  
Saadia Sultan Wahla ◽  
Safdar Ali Shirazi ◽  
Sohail Abbas ◽  
Mian Sabir Hussain ◽  
Mareena Khurshid

The purpose of present study is to investigate district and division wise annual rainfall variations over theprovince of Punjab, which is the largest in terms of population size as well important contributor in agricultural producein Pakistan. The results revealed that the rainfall trend has shifted from upper and lower Punjab towards the south,west, north and east respectively. The statistical analysis has inferred an overall increasing trend for the period 1990 to2000 and a decreasing trend during period 2001 to 2010 in Punjab province. The Z test value differences in the averagerainfall for each district level meteorological station have detected three increasing and two decreasing trends duringsummers of 1981-2015.The data revealed a significant changed seasonal trend observed in Murree and Sialkot(northern Punjab), Faisalabad and Lahore (central Punjab). Particularly, the changes have been observed in the southPunjab and over the central Punjab, while the same rainfall variations have shown a southward shift. The trend of therainfall had shifted in the Faisalabad division with the significant positive trend. In the Central Punjab, the positivetrend has also been found in all the districts. In the lower Punjab, Multan, Dera Ghazi Khan districts has shown anincreasing trend of rainfall. The findings are significant in changing agro-climatic zones in the Punjab and consequentshifting patterns of agriculture therefore can have net impact on the food security situation in the Punjab-Pakistan.


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