scholarly journals Large shift of the Pacific Walker Circulation across the Cenozoic

2021 ◽  
Author(s):  
Qing Yan
Keyword(s):  
Late Miocene ◽  
Coupled Model ◽  
Walker Circulation ◽  
Tropical Pacific ◽  
South American ◽  
Early Eocene ◽  
Large Shift ◽  
The Pacific ◽  
Large Spread ◽  
Eastern Edge

<p>Fluctuations in the Pacific Walker circulation (PWC), a zonally-oriented overturning cell across the tropical Pacific, can cause widespread climatic and biogeochemical perturbations. It remains unknown how the PWC developed during the Cenozoic era, with its substantial changes in greenhouse gases and continental positions. Through a suite of coupled model simulations on tectonic timescales, we demonstrate that the PWC was ~38º broader and ~5% more intense during the Early Eocene relative to present. As the climate cooled from the Early Eocene to the Late Miocene, the width of the PWC shrank, accompanied by an increase in intensity that was tied to the enhanced Pacific zonal temperature gradient. However, the locations of the western and eastern branches behave differently from the Early Eocene to the Late Miocene, with the western edge remained steady with time due to the relatively stable geography of the western tropical Pacific; the eastern edge migrates westward with time as the South American continent moves northwest. A transition occurs in the PWC between the Late Miocene and Late Pliocene, manifested by an eastward shift (both the western and eastern edges migrate eastward by >12º) and weakening (by ~22%), which we show here is linked with the closure of the tropical seaways. Moreover, our results suggest that rising CO<sub>2</sub> favors a weaker PWC under the same land-sea configurations, a robust feature across the large spread of Cenozoic climates considered here, supporting a weakening of the PWC in a warmer future.</p>

scholarly journals Large shift of the Pacific Walker Circulation across the Cenozoic

2020 ◽  
Author(s):  
Qing Yan ◽  
Robert Korty ◽  
Zhongshi Zhang ◽  
Chris Brierley ◽  
Xiangyu Li ◽  
...  
Keyword(s):  
Late Miocene ◽  
Coupled Model ◽  
Walker Circulation ◽  
Tropical Pacific ◽  
South American ◽  
Early Eocene ◽  
Large Shift ◽  
The Pacific ◽  
Large Spread ◽  
Eastern Edge

Abstract Fluctuations in the Pacific Walker Circulation (PWC), a zonally oriented overturning cell across the tropical Pacific, can cause widespread climatic and biogeochemical perturbations. It remains unknown how the PWC developed during the Cenozoic era, with its substantial changes in greenhouse gases and continental positions. Through a suite of coupled model simulations on tectonic timescales, we demonstrate that the PWC was ∼38° broader and ∼5% more intense during the Early Eocene relative to present. As the climate cooled from the Early Eocene to the Late Miocene, the width of the PWC shrank, accompanied by an increase in intensity that was tied to the enhanced Pacific zonal temperature gradient. However, the locations of the western and eastern branches behave differently from the Early Eocene to the Late Miocene, with the western edge remaining steady with time due to the relatively stable geography of the western tropical Pacific; the eastern edge migrates westward with time as the South American continent moves northwest. A transition occurs in the PWC between the Late Miocene and Late Pliocene, manifested by an eastward shift (both the western and eastern edges migrate eastward by >12°) and weakening (by ∼22%), which we show here is linked with the closure of the tropical seaways. Moreover, our results suggest that rising CO2 favors a weaker PWC under the same land-sea configurations, a robust feature across the large spread of Cenozoic climates considered here, supporting a weakening of the PWC in a warmer future.

scholarly journals Impact of Resolution on the Tropical Pacific Circulation in a Matrix of Coupled Models

2009 ◽  
Vol 22 (10) ◽  
pp. 2541-2556 ◽  
Author(s):  
Malcolm J. Roberts ◽  
A. Clayton ◽  
M.-E. Demory ◽  
J. Donners ◽  
P. L. Vidale ◽  
...  
Keyword(s):  
Coupled Model ◽  
Walker Circulation ◽  
Ocean Model ◽  
Tropical Pacific ◽  
Coupled Models ◽  
Model Stability ◽  
The Mean ◽  
The Impact ◽  
Mean State ◽  
The Tropical Pacific

Abstract Results are presented from a matrix of coupled model integrations, using atmosphere resolutions of 135 and 90 km, and ocean resolutions of 1° and 1/3°, to study the impact of resolution on simulated climate. The mean state of the tropical Pacific is found to be improved in the models with a higher ocean resolution. Such an improved mean state arises from the development of tropical instability waves, which are poorly resolved at low resolution; these waves reduce the equatorial cold tongue bias. The improved ocean state also allows for a better simulation of the atmospheric Walker circulation. Several sensitivity studies have been performed to further understand the processes involved in the different component models. Significantly decreasing the horizontal momentum dissipation in the coupled model with the lower-resolution ocean has benefits for the mean tropical Pacific climate, but decreases model stability. Increasing the momentum dissipation in the coupled model with the higher-resolution ocean degrades the simulation toward that of the lower-resolution ocean. These results suggest that enhanced ocean model resolution can have important benefits for the climatology of both the atmosphere and ocean components of the coupled model, and that some of these benefits may be achievable at lower ocean resolution, if the model formulation allows.

scholarly journals Precipitation in the Amazon and its relationship with moisture transport and tropical Pacific and Atlantic SST from the CMIP5 simulation

2015 ◽  
Vol 12 (1) ◽  
pp. 671-704 ◽  
Author(s):  
G. Martins ◽  
C. von Randow ◽  
G. Sampaio ◽  
A. J. Dolman
Keyword(s):  
Moisture Transport ◽  
General Circulation ◽  
Regional Analysis ◽  
Coupled Model ◽  
General Circulation Models ◽  
Moisture Convergence ◽  
Cmip5 Models ◽  
Wet Season ◽  
The Pacific ◽  
Eastern Edge

Abstract. Studies on numerical modeling in Amazonia show that the models fail to capture important aspects of climate variability in this region and it is important to understand the reasons that cause this drawback. Here, we study how the general circulation models of the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulate the inter-relations between regional precipitation, moisture convergence and Sea Surface Temperature (SST) in the adjacent oceans, to assess how flaws in the representation of these processes can translate into biases in simulated rainfall in Amazonia. Using observational data (GPCP, CMAP, ERSST.v3, ERAI and evapotranspiration) and 21 numerical simulations from CMIP5 during the present climate (1979–2005) in June, July and August (JJA) and December, January and February (DJF), respectively, to represent dry and wet season characteristics, we evaluate how the models simulate precipitation, moisture transport and convergence, and pressure velocity (omega) in different regions of Amazonia. Thus, it is possible to identify areas of Amazonia that are more or less influenced by adjacent ocean SSTs. Our results showed that most of the CMIP5 models have poor skill in adequately representing the observed data. The regional analysis of the variables used showed that the underestimation in the dry season (JJA) was twice in relation to rainy season as quantified by the Standard Error of the Mean (SEM). It was found that Atlantic and Pacific SSTs modulate the northern sector of Amazonia during JJA, while in DJF Pacific SST only influences the eastern sector of the region. The analysis of moisture transport in JJA showed that moisture preferentially enters Amazonia via its eastern edge. In DJF this occurs both via its northern and eastern edge. The moisture balance is always positive, which indicates that Amazonia is a source of moisture to the atmosphere. Additionally, our results showed that during DJF the simulations in northeast sector of Amazonia have a strong bias in precipitation and an underestimation of moisture convergence due to the higher influence of biases in the Pacific SST. During JJA, a strong precipitation bias was observed in the southwest sector associated, also with a negative bias of moisture convergence, but with weaker influence of SSTs of adjacent oceans. The poor representation of precipitation-producing systems in Amazonia by the models and the difficulty of adequately representing the variability of SSTs in the Pacific and Atlantic oceans may be responsible for these underestimates in Amazonia.

scholarly journals Local and remote SST variability contribute to the westward shift of the Pacific Walker circulation during 1979–2015

2021 ◽  
Vol 8 (1) ◽  
Author(s):  
Xichen Li ◽  
Xinyue Wang ◽  
Tao Lian ◽  
Nathaniel C. Johnson ◽  
Jiang Zhu ◽  
...  
Keyword(s):  
Model Simulation ◽  
Global Climate ◽  
Coupled Model ◽  
Decisive Role ◽  
Walker Circulation ◽  
Western Pacific ◽  
Coupled Models ◽  
Central Pacific ◽  
Easterly Wind ◽  
The Pacific

AbstractDuring the modern satellite era since 1979, the Pacific Walker circulation (PWC) experienced an intensification and a westward shift, which has broad impacts on the global climate variability. While the strengthening of the PWC has been shown to be driven by both the regional Pacific sea surface temperature (SST) and the remote forcing from other basins, its westward shift is primarily attributed to the phase change of the Atlantic Multidecadal variability. In this study, we investigate the potential effect of the remote SST forcing from the Atlantic and the Indian Oceans on the westward shift of the PWC, through statistical analysis and numerical experiments using atmospheric and coupled models. Results show that the tropical Atlantic warming plays a key (decisive) role in driving the PWC westward shift by triggering a Gill–Matsuno-type circulation anomaly in the tropics. This circulation response drives anomalous surface westerlies over the eastern Pacific and subsidence over the central Pacific that weakens the eastern part of the PWC, meanwhile generating easterly wind anomalies over the central-western Pacific and anomalous atmospheric convection over the western Pacific that intensifies the western part of the PWC. This direct forcing contributes ~ 32% of the observed PWC movement, while the Atlantic-induced inter-basin SST changes contribute another ~ 36% of its westward shift according to coupled model simulation results. Our results reinforce the importance of the inter-basin interactions in adjusting the tropical climate variabilities, and have broad implication for projecting the global climate.

The Role of the Dry Static Stability for the Recent Change in the Pacific Walker Circulation

2016 ◽  
Vol 29 (8) ◽  
pp. 2765-2779 ◽  
Author(s):  
Byung-Ju Sohn ◽  
Sukyoung Lee ◽  
Eui-Seok Chung ◽  
Hwan-Jin Song
Keyword(s):  
Climate Models ◽  
Lower Boundary ◽  
Walker Circulation ◽  
Static Stability ◽  
Recent Change ◽  
Tropical Pacific ◽  
Cmip5 Models ◽  
Atmospheric Models ◽  
Lower Boundary Condition ◽  
The Pacific

Abstract There is an uncertainty in how the Pacific Walker circulation (PWC) will change in response to increased greenhouse gas (GHG) warming. On average, climate models predict that the PWC will weaken. Observational evidence is mixed, with some evidence supporting the models while others do not. In this study, insight into the PWC trend is provided by examining the tropical dry static stability, a quantity that is inversely proportional to the strength of the PWC. For the 1979–2012 period, the static stability increased markedly in all phase 5 of the Coupled Model Intercomparison Project (CMIP5) models, far more so than in the satellite and global reanalysis data, which show a strengthening of the PWC. The stabilization is greater for a subset of models that simulate a significant weakening of the PWC. With the observed sea surface temperature as the lower boundary condition, over the western tropical Pacific, atmospheric models that belong to the weakening-PWC-CMIP5 group produce greater stabilization than those that belong to the strengthening-PWC-CMIP5 group. Compared with the latter group, the former group of atmospheric models simulates weaker trade winds over the western and central tropical Pacific and, consistent with the Bjerknes mechanism, the corresponding CMIP5 models produce a weaker west–east gradient in tropical SST. Given that the models’ convective parameterizations overstabilize the atmosphere compared with an explicit convection, the findings here suggest that the models’ representations of tropical convection and stability contribute to the models’ tendency to simulate a weakening of the PWC and an El Niño–like SST.

scholarly journals Robust Strengthening and Westward Shift of the Tropical Pacific Walker Circulation during 1979–2012: A Comparison of 7 Sets of Reanalysis Data and 26 CMIP5 Models

2016 ◽  
Vol 29 (9) ◽  
pp. 3097-3118 ◽  
Author(s):  
Shuangmei Ma ◽  
Tianjun Zhou
Keyword(s):  
Vertical Section ◽  
Coupled Model ◽  
Walker Circulation ◽  
Reanalysis Data ◽  
Tropical Pacific ◽  
La Niña ◽  
Cmip5 Models ◽  
La Nina ◽  
Independent Observations ◽  
The Tropical Pacific

Abstract In this study, the zonal mass streamfunction Ψ, which depicts intuitively the tropical Pacific Walker circulation (PWC) structure characterized by an enclosed and clockwise rotation cell in the zonal–vertical section over the equatorial Pacific, was used to study the changes of PWC spatial structure during 1979–2012. To examine the robustness of changes in PWC characteristics, the linear trends of PWC were evaluated and compared among the current seven sets of reanalysis data, along with a comparison to the trends of surface climate variables. The spatial pattern of Ψ trend exhibited a strengthening and westward-shifting trend of PWC in all reanalysis datasets, with the significantly positive Ψ dominating the western Pacific and negative Ψ controlling the eastern Pacific. This kind of change is physically in agreement with the changes of the sea level pressure (SLP), surface winds, and precipitation derived from both the reanalyses and independent observations. Quantitative analyses of the changes in the PWC intensity and western edge, defined based on the zonal mass streamfunction, also revealed a robust strengthening and westward-shifting trend among all reanalysis datasets, with a trend of 15.08% decade−1 and 3.70° longitude decade−1 in the ensemble mean of seven sets of reanalysis data, with the strongest (weakest) intensification of 17.53% decade−1 (7.96% decade−1) in the Twentieth Century Reanalysis (NCEP-2) and largest (smallest) westward shift of −4.68° longitude decade−1 (−2.55° longitude decade−1) in JRA-55 (JRA-25). In response to the recent observed La Niña–like anomalous SST forcing, the ensemble simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), with 26 models in the ensemble, reasonably reproduced the observed strengthening and westward-shifting trend of PWC, implying the dominant forcing of the La Niña–like SST anomalies to the recent PWC change.

scholarly journals Global-scale interdecadal variability a skillful predictor at decadal-to-multidecadal timescales for Sahelian and Indian Monsoon Rainfall

2022 ◽  
Vol 5 (1) ◽  
Author(s):  
Manish K. Joshi ◽  
Archana Rai ◽  
Ashwini Kulkarni
Keyword(s):  
Climate Models ◽  
Physical Mechanism ◽  
Coupled Model ◽  
Walker Circulation ◽  
Atlantic Multidecadal Oscillation ◽  
Global Scale ◽  
Substantial Improvement ◽  
Interdecadal Variability ◽  
The Pacific ◽  
Temporal Variance

AbstractIn the present study, a sea surface temperature-based index named global-scale interdecadal variability (GIV) encompassing the combined variability of Atlantic multidecadal oscillation (AMO) and interdecadal Pacific oscillation (IPO) has been proposed. The warm phase of GIV exhibits a “cold AMO-like” pattern in the Atlantic basin and a “warm IPO-like” pattern in the Pacific basin. About 84% (R ~−0.914) of Sahelian and 42% (R ~−0.647) of Indian rainfall’s temporal variance is attributed to GIV, showing substantial improvement compared to the variance explained by AMO and IPO individually. The physical mechanism for GIV-rainfall teleconnection is related to a modification of the Walker circulation. Although there is a substantial degree of uncertainty in the current generation of state-of-the-art climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), some still replicate the observed GIV’s spatial structure, its teleconnection, and associated physical mechanism. The results presented herein advance our knowledge about rainfall’s interdecadal variability and have imperative ramifications for developing skillful decadal predictions.

scholarly journals How Robust Is the Weakening of the Pacific Walker Circulation in CMIP5 Idealized Transient Climate Simulations?

2017 ◽  
Vol 31 (1) ◽  
pp. 81-97 ◽  
Author(s):  
Elina Plesca ◽  
Verena Grützun ◽  
Stefan A. Buehler
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Walker Circulation ◽  
Internal Variability ◽  
Sea Level Pressure ◽  
The Pacific ◽  
Indexing Method ◽  
Climate Simulations ◽  
Large Spread ◽  
The Moment

Abstract The tropical overturning circulations are likely weakening under increased CO2 forcing. However, insufficient understanding of the circulations’ dynamics diminishes the full confidence in such a response. Based on a CMIP5 idealized climate experiment, this study investigates the changes in the Pacific Walker circulation under anthropogenic forcing and the sensitivity of its weakening response to internal variability, general circulation model (GCM) configuration, and indexing method. The sensitivity to internal variability is analyzed by using a 68-member ensemble of the MPI-ESM-LR model, and the influence of model physics is analyzed by using the 28-member CMIP5 ensemble. Three simple circulation indices—based on mean sea level pressure, 500-hPa vertical velocity, and 200-hPa velocity potential—are computed for each member of the two ensembles. The study uses the output of the CMIP5 idealized transient climate simulations with 1% yr−1 CO2 increase from preindustrial level, and investigates the detected circulation response until the moment of CO2 doubling (70 yr). Depending on the indexing method, it is found that 50%–93% of the MPI-ESM-LR and 54%–75% of the CMIP5 ensemble members project significant negative trends in the circulation’s intensity. This large spread in the ensembles reduces the confidence that a weakening circulation is a robust feature of climate change. Furthermore, the similar magnitude of the spread in both ensembles shows that the Walker circulation response is strongly influenced by natural variability, even over a 70-yr period.

Growing Coconut Palms in the Pacific Islands: Colliding Knowledge and Values

2020 ◽  
Author(s):  
Judith A. Bennett
Keyword(s):  
Pacific Islands ◽  
Social Values ◽  
Coconut Oil ◽  
Tropical Pacific ◽  
Experiential Knowledge ◽  
The West ◽  
Passive Resistance ◽  
The People ◽  
The Pacific ◽  
The Tropical Pacific

Coconuts provided commodities for the West in the form of coconut oil and copra. Once colonial governments established control of the tropical Pacific Islands, they needed revenue so urged European settlers to establish coconut plantations. For some decades most copra came from Indigenous growers. Administrations constantly urged the people to thin old groves and plant new ones like plantations, in grid patterns, regularly spaced and weeded. Local growers were instructed to collect all fallen coconuts for copra from their groves. For half a century, the administrations’ requirements met with Indigenous passive resistance. This paper examines the underlying reasons for this, elucidating Indigenous ecological and social values, based on experiential knowledge, knowledge that clashed with Western scientific values.

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