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K. V. Suneeth - One of the best experts on this subject based on the ideXlab platform.
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Zonally resolved water vapour coupling with tropical tropopause temperature: Seasonal and interannual variability, and influence of the Walker Circulation
Climate Dynamics, 2020Co-Authors: K. V. SuneethAbstract:Global stratospheric water vapour is strongly coupled to the tropical cold-point tropopause temperatures. We quantified the coupling between temperature and water vapour in the TTL on seasonal and interannual time scale using 10 years of satellite based observations, focusing on the zonal asymmetries arising from the Walker Circulation. Tropical region (15° N–15° S) shows near perfect correlation values in the TTL (between ~ 15.5 km to CPT) at ascending branches of Walker Circulation, indicating a strong coupling between annual cycles of cold-point and water vapour variability. Descending branches of Walker Circulation are characterized by weak temperature-water vapour coupling and local lapse-rate minima at about 1–1.5 km below CPT level. The correlation pattern of deseasonalized anomalies of temperature and water vapour shows a strong zonal asymmetry, with maximum correlation values (r ~ 0.7 to 0.9) in the TTL (between 15 km and cold-point level) over Maritime Continents including western Pacific Ocean (90° E–180° E), and Atlantic Ocean (0–50° W) and minimum values (r ~ 0.2 to 0.4) over eastern Pacific and Indian Ocean. The QBO amplitude in CPT-T is more or less zonally symmetric, with peak value over the Maritime Continent and Atlantic Ocean. During ENSO event, CPT-T over the western Pacific and Maritime Continent are largely influenced with warm anomaly. Associate with this CPT-T variation, large enhancement in WMR_100 is noticed over the Indian Ocean between 20° N–20° S latitude. The difference between zonal mean deseasonalized anomalies of WMR_100 and SMR_cpt show significant fluctuations (± 0.3 ppmv) that are roughly coincide with the Oceanic Niño Index with a 6 month lag.
John C H Chiang - One of the best experts on this subject based on the ideXlab platform.
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deconstructing atlantic intertropical convergence zone variability influence of the local cross equatorial sea surface temperature gradient and remote forcing from the eastern equatorial pacific
Journal of Geophysical Research, 2002Co-Authors: John C H Chiang, Yochanan Kushnir, Alessandra GianniniAbstract:(1) We investigate causes of interannual variability in Atlantic Intertropical Convergence Zone (ITCZ) convection using a monthly mean global precipitation data set spanning 1979-1999. Starting from the hypothesis of two dominant influences on the ITCZ, namely, the cross-equatorial gradient in tropical Atlantic sea surface temperature (SST) and the anomalous Walker Circulation due to the rearrangement of tropical Pacific convection associated with the El Nino-Southern Oscillation, we analyze anomaly composites over the 1979-1999 period that best isolate the effects of each mechanism. Our results suggest that to first order, a strong anomalous Walker Circulation suppresses precipitation over the tropical Atlantic, whereas an anomalous warm north/cool south SST gradient shifts the meridional location of maximum ITCZ convection anomalously north. We examined the processes underlying each of the two mechanisms. For the anomalous Walker Circulation we find consistency with the idea of suppression of convection through warming of the tropical troposphere brought about by anomalous convective heating in the eastern equatorial Pacific. For the SST gradient mechanism our results confirm previous studies that link convection to cross-equatorial winds forced by meridional SST gradients. We find that positive surface flux feedback brought about through the cross-equatorial winds is weak and confined to the deep tropics. On the basis of the results of this and other studies we propose an expanded physical picture that explains key features of Atlantic ITCZ variability, including its seasonal preference, its sensitivity to small anomalous SST gradients, and its role in the context of tropical Atlantic SST gradient variability. INDEX TERMS: 4215 Oceanography: General: Climate and interannual variability (3309), 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504), 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854), 3374 Meteorology and Atmospheric Dynamics: Tropical meteorology, 4522 Oceanography: Physical: Ocean Optics; KEYWORDS: Tropical Atlantic, precipitation, climate variability, El Nino-Southern Oscillation, Ocean-atmosphere interaction
Alessandra Giannini - One of the best experts on this subject based on the ideXlab platform.
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deconstructing atlantic intertropical convergence zone variability influence of the local cross equatorial sea surface temperature gradient and remote forcing from the eastern equatorial pacific
Journal of Geophysical Research, 2002Co-Authors: John C H Chiang, Yochanan Kushnir, Alessandra GianniniAbstract:(1) We investigate causes of interannual variability in Atlantic Intertropical Convergence Zone (ITCZ) convection using a monthly mean global precipitation data set spanning 1979-1999. Starting from the hypothesis of two dominant influences on the ITCZ, namely, the cross-equatorial gradient in tropical Atlantic sea surface temperature (SST) and the anomalous Walker Circulation due to the rearrangement of tropical Pacific convection associated with the El Nino-Southern Oscillation, we analyze anomaly composites over the 1979-1999 period that best isolate the effects of each mechanism. Our results suggest that to first order, a strong anomalous Walker Circulation suppresses precipitation over the tropical Atlantic, whereas an anomalous warm north/cool south SST gradient shifts the meridional location of maximum ITCZ convection anomalously north. We examined the processes underlying each of the two mechanisms. For the anomalous Walker Circulation we find consistency with the idea of suppression of convection through warming of the tropical troposphere brought about by anomalous convective heating in the eastern equatorial Pacific. For the SST gradient mechanism our results confirm previous studies that link convection to cross-equatorial winds forced by meridional SST gradients. We find that positive surface flux feedback brought about through the cross-equatorial winds is weak and confined to the deep tropics. On the basis of the results of this and other studies we propose an expanded physical picture that explains key features of Atlantic ITCZ variability, including its seasonal preference, its sensitivity to small anomalous SST gradients, and its role in the context of tropical Atlantic SST gradient variability. INDEX TERMS: 4215 Oceanography: General: Climate and interannual variability (3309), 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504), 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854), 3374 Meteorology and Atmospheric Dynamics: Tropical meteorology, 4522 Oceanography: Physical: Ocean Optics; KEYWORDS: Tropical Atlantic, precipitation, climate variability, El Nino-Southern Oscillation, Ocean-atmosphere interaction
Shang-ping Xie - One of the best experts on this subject based on the ideXlab platform.
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Slowdown of the Walker Circulation driven by tropical Indo-Pacific warming
Nature, 2012Co-Authors: Hiroki Tokinaga, Shang-ping Xie, Clara Deser, Yu Kosaka, Yuko M. OkumuraAbstract:Changes in the Walker Circulation, an enormous east–west atmospheric Circulation over the equatorial Pacific Ocean, are shown to be driven by changes in zonal sea surface temperature gradients rather than by changes in the hydrological cycle, as previously suggested. The Walker Circulation is a large-scale east–west atmospheric Circulation over the equatorial Pacific that is closely tied to the El Nino–Southern Oscillation. Over the past six decades the Walker Circulation has become weaker, and it has been suggested that this is linked to changes in the hydrological cycle. Here Hiroki Tokinaga et al. conduct a comprehensive analysis of observations and models, and show that instead, the slowdown is driven by changes in zonal sea-surface temperature gradients in the tropical Indo-Pacific Ocean. Global mean sea surface temperature (SST) has risen steadily over the past century1,2, but the overall pattern contains extensive and often uncertain spatial variations, with potentially important effects on regional precipitation3,4. Observations suggest a slowdown of the zonal atmospheric overturning Circulation above the tropical Pacific Ocean (the Walker Circulation) over the twentieth century1,5. Although this change has been attributed to a muted hydrological cycle forced by global warming5,6, the effect of SST warming patterns has not been explored and quantified1,7,8. Here we perform experiments using an atmospheric model, and find that SST warming patterns are the main cause of the weakened Walker Circulation over the past six decades (1950–2009). The SST trend reconstructed from bucket-sampled SST and night-time marine surface air temperature features a reduced zonal gradient in the tropical Indo-Pacific Ocean, a change consistent with subsurface temperature observations8. Model experiments with this trend pattern robustly simulate the observed changes, including the Walker Circulation slowdown and the eastward shift of atmospheric convection from the Indonesian maritime continent to the central tropical Pacific. Our results cannot establish whether the observed changes are due to natural variability or anthropogenic global warming, but they do show that the observed slowdown in the Walker Circulation is presumably driven by oceanic rather than atmospheric processes.
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Regional patterns of tropical Indo-Pacific climate change: evidence of the Walker Circulation weakening
Journal of Climate, 2012Co-Authors: Hiroki Tokinaga, Shang-ping Xie, Axel Timmermann, Shayne Mcgregor, Tomomichi Ogata, Hisayuki Kubota, Yuko M. OkumuraAbstract:AbstractRegional patterns of tropical Indo-Pacific climate change are investigated over the last six decades based on a synthesis of in situ observations and ocean model simulations, with a focus on physical consistency among sea surface temperature (SST), cloud, sea level pressure (SLP), surface wind, and subsurface ocean temperature. A newly developed bias-corrected surface wind dataset displays westerly trends over the western tropical Pacific and easterly trends over the tropical Indian Ocean, indicative of a slowdown of the Walker Circulation. This pattern of wind change is consistent with that of observed SLP change showing positive trends over the Maritime Continent and negative trends over the central equatorial Pacific. Suppressed moisture convergence over the Maritime Continent is largely due to surface wind changes, contributing to observed decreases in marine cloudiness and land precipitation there.Furthermore, observed ocean mixed layer temperatures indicate a reduction in zonal contrast in t...
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ocean atmosphere interaction in the making of the Walker Circulation and equatorial cold tongue
Journal of Climate, 1998Co-Authors: Shang-ping XieAbstract:Abstract The climate over the equatorial Pacific displays a pronounced asymmetry in the zonal direction that is characterized by the Walker Circulation in the atmosphere and the cold tongue in the ocean. An intermediate coupled ocean–atmosphere model is used to investigate the driving force and the ocean–atmosphere interaction mechanism for the generation of the zonal asymmetry. In the far eastern Pacific, the upwelling at the equator is weak because zonal winds are blocked by the Andes. The off-equatorial upwelling induced by southerly cross- equatorial winds is thus crucial for cooling the eastern Pacific. A realistic cold tongue appears in the coupled model only when this southerly wind forcing is included. The southerly winds cause the sea surface temperature to fall in the east, enhancing the zonal heat contrast and hence intensifying easterly winds across the basin. These anomalous easterlies induce more equatorial upwelling and raise the thermocline in the east, amplifying the initial cooling by th...
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Ocean–Atmosphere Interaction in the Making of the Walker Circulation and Equatorial Cold Tongue
Journal of Climate, 1998Co-Authors: Shang-ping XieAbstract:Abstract The climate over the equatorial Pacific displays a pronounced asymmetry in the zonal direction that is characterized by the Walker Circulation in the atmosphere and the cold tongue in the ocean. An intermediate coupled ocean–atmosphere model is used to investigate the driving force and the ocean–atmosphere interaction mechanism for the generation of the zonal asymmetry. In the far eastern Pacific, the upwelling at the equator is weak because zonal winds are blocked by the Andes. The off-equatorial upwelling induced by southerly cross- equatorial winds is thus crucial for cooling the eastern Pacific. A realistic cold tongue appears in the coupled model only when this southerly wind forcing is included. The southerly winds cause the sea surface temperature to fall in the east, enhancing the zonal heat contrast and hence intensifying easterly winds across the basin. These anomalous easterlies induce more equatorial upwelling and raise the thermocline in the east, amplifying the initial cooling by th...
Venkatachalam Ramaswamy - One of the best experts on this subject based on the ideXlab platform.
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A Radiative–Convective Equilibrium Perspective of Weakening of the Tropical Walker Circulation in Response to Global Warming
Journal of Climate, 2013Co-Authors: Xianglei Huang, Hui Wen Chuang, Andrew E. Dessler, Xiuhong Chen, K. Minschwaner, Yi Ming, Venkatachalam RamaswamyAbstract:AbstractBoth observational analysis and GCM simulations indicate that the tropical Walker Circulation is becoming weaker and may continue to weaken as a consequence of climate change. Here, the authors use a conceptual radiative–convective equilibrium (RCE) framework to interpret the weakening of the Walker Circulation as simulated by the GFDL coupled GCM. Based on the modeled lapse rate and clear-sky cooling rate profiles, the RCE framework can directly compute the change of vertical velocity in the descending branch of the Walker Circulation, which agrees with the counterpart simulated by the GFDL model. The results show that the vertical structure of clear-sky radiative cooling rate QR will change in response to the increased water vapor as the globe warms. The authors explain why the change of QR is positive in the uppermost part of the troposphere (
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a radiative convective equilibrium perspective of weakening of the tropical Walker Circulation in response to global warming
Journal of Climate, 2013Co-Authors: Xianglei Huang, Hui Wen Chuang, Andrew E. Dessler, Xiuhong Chen, K. Minschwaner, Yi Ming, Venkatachalam RamaswamyAbstract:AbstractBoth observational analysis and GCM simulations indicate that the tropical Walker Circulation is becoming weaker and may continue to weaken as a consequence of climate change. Here, the authors use a conceptual radiative–convective equilibrium (RCE) framework to interpret the weakening of the Walker Circulation as simulated by the GFDL coupled GCM. Based on the modeled lapse rate and clear-sky cooling rate profiles, the RCE framework can directly compute the change of vertical velocity in the descending branch of the Walker Circulation, which agrees with the counterpart simulated by the GFDL model. The results show that the vertical structure of clear-sky radiative cooling rate QR will change in response to the increased water vapor as the globe warms. The authors explain why the change of QR is positive in the uppermost part of the troposphere (<300 hPa) and is negative for the rest of the troposphere. As a result, both the change of clear-sky cooling rate and the change of tropospheric lapse rat...
