Reconciling opposing Walker circulation trends in observations and model projections

Chung, Eunhyea; Timmermann, Axel; Soden, Brian J.; Ha, Kyung‐Ja; Shi, Lei; John, Viju O.

doi:10.1038/s41558-019-0446-4

Cited by 114 publications

(112 citation statements)

References 53 publications

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“…signal) and internal variability (i.e. noise) because the same radiative forcing scenario is applied to the ensemble members with a slightly different initial condition in atmospheric state (Chung et al 2019). We assume that the standard deviation across the precipitation trends of ensemble members can be regarded as the internal variability because the internal atmospheric variability contributes substantially to the spread of the precipitation trends of ensemble members (Deser et al 2012).…”

Section: Methodsmentioning

confidence: 99%

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Major factors of global and regional monsoon rainfall changes: natural versus anthropogenic forcing

Kim

Chung

et al. 2020

Environ. Res. Lett.

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A number of studies have investigated the mechanisms that determine changes in precipitation, including how a wet region gets wetter. However, not all monsoon areas get wetter-there is a need to understand the major factors behind changes in regional monsoon precipitation, in terms of external forcing and internal variabilities in the last six decades by a combination of different observed datasets and model runs. We have found that time of emergence of anthropogenic signals is robustly detected in the northern African monsoon before the 1990s with the use of the CESM Large Ensemble Project. From CMIP5 model runs and three reanalysis datasets, the results found are that the change in rainfall over African monsoon (AFM) is mainly due to anthropogenic forcing and that over Asian-Australian monsoon (AAM) is affected by internal variability. Moreover, the cause of American monsoon (AMM) rainfall change cannot be known due to a discrepancy among observed datasets. Here we show that the asymmetry between Northern Hemisphere (NH) and Southern Hemisphere (SH) parts by green-house gas (GHG) is detected over the AFM and AAM regions. However, the land monsoon rainfall in the northern AMM is decreased by a combination of GHG and aerosol forcing. In general, the aerosol forcing causes a decreasing rainfall over the monsoon regions. In future projection, the land rainfall over the AAM and AMM are expected to increase and decrease in the future from most models' results. The asymmetry between an increase in NH and a decrease in SH is dominant in the future from most models' future simulation results, which is well shown over the AFM and AAM. This study suggests that the physical process of GHG and aerosol effects in rainfall should be explored in the context of regional aspects.

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Section: Methodsmentioning

confidence: 99%

“…According to the previous studies (Huang et al 2013, Hua et al 2016, Chung et al 2019, the strength and extension of the Walker circulation have been responsible for the changes in rainfall.…”

Section: Importance Of Natural Variabilities and The Walker Circulationmentioning

confidence: 99%

Major factors of global and regional monsoon rainfall changes: natural versus anthropogenic forcing

Kim

Chung

et al. 2020

Environ. Res. Lett.

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“…Recent multidecadal enhancement of global wind forcing and ocean circulation was likely contributed by the phase change of interdecadal variability in the Pacific Ocean (31,36). A recent study based on multiple independent observational records, including satellite measurements, indicates that there is a common strengthening trend of the Pacific Walker circulation over recent decades and that internal variability plays a dominant role in the strengthening trend of the Pacific Walker circulation (37). During the recent negative PDO phase, wind work and wind speed strengthen over the tropical oceans and the Southern Ocean compared to during the positive PDO phase ( fig.…”

Section: Pdo Influence Versus the Long-term Global Warming Trendmentioning

confidence: 99%

“…S8). Further, although CMIP5 models project a weakening of the Pacific Walker Circulation (36,37), the global mean sea surface wind shows an increasing trend that is significant at the 99% confidence level in both the historical and Representative Concentration Pathway (RCP) 8.5 runs of the CMIP5 models ( fig. S9).…”

Section: Pdo Influence Versus the Long-term Global Warming Trendmentioning

confidence: 99%

Deep-reaching acceleration of global mean ocean circulation over the past two decades

et al. 2020

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Ocean circulation redistributes Earth’s energy and water masses and influences global climate. Under historical greenhouse warming, regional ocean currents show diverse tendencies, but whether there is an emerging trend of the global mean ocean circulation system is not yet clear. Here, we show a statistically significant increasing trend in the globally integrated oceanic kinetic energy since the early 1990s, indicating a substantial acceleration of global mean ocean circulation. The increasing trend in kinetic energy is particularly prominent in the global tropical oceans, reaching depths of thousands of meters. The deep-reaching acceleration of the ocean circulation is mainly induced by a planetary intensification of surface winds since the early 1990s. Although possibly influenced by wind changes associated with the onset of a negative Pacific decadal oscillation since the late 1990s, the recent acceleration is far larger than that associated with natural variability, suggesting that it is principally part of a long-term trend.

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“…However, observations from recent decades point to a strengthening. Apart from data issues, this strengthening might have been due to internal variability, which is a dominant factor controlling the strength of the Pacific Walker circulation (Chung et al 2019). The Walker circulation is closely linked to El Niño, the climate mode with largest global influence, which again shows substantial variability in its variance on multidecadal timescales (Wittenberg 2009).…”

Section: Circulation Related To Atmospheric or Coupled Variabilitymentioning

confidence: 99%

Causes of climate change over the historical record

Hegerl

Brönnimann

Cowan

et al. 2019

Environ. Res. Lett.

128

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This review addresses the causes of observed climate variations across the industrial period, from 1750 to present. It focuses on long-term changes, both in response to external forcing and to climate variability in the ocean and atmosphere. A synthesis of results from attribution studies based on palaeoclimatic reconstructions covering the recent few centuries to the 20th century, and instrumental data shows how greenhouse gases began to cause warming since the beginning of industrialization, causing trends that are attributable to greenhouse gases by 1900 in proxy-based temperature reconstructions. Their influence increased over time, dominating recent trends. However, other forcings have caused substantial deviations from this emerging greenhouse warming trend: volcanic eruptions have caused strong cooling following a period of unusually heavy activity, such as in the early 19th century; or warming during periods of low activity, such as in the early-to-mid 20th century. Anthropogenic aerosol forcing most likely masked some global greenhouse warming over the 20th century, especially since the accelerated increase in sulphate aerosol emissions starting around 1950. Based on modelling and attribution studies, aerosol forcing has also influenced regional temperatures, caused long-term changes in monsoons and imprinted on Atlantic variability. Multi-decadal variations in atmospheric modes can also cause longterm climate variability, as apparent for the example of the North Atlantic Oscillation, and have influenced Atlantic ocean variability. Long-term precipitation changes are more difficult to attribute to external forcing due to spatial sparseness of data and noisiness of precipitation changes, but the observed pattern of precipitation response to warming from station data supports climate model simulated changes and with it, predictions. The long-term warming has also led to significant differences in daily variability as, for example, visible in long European station data. Extreme events over the historical record provide valuable samples of possible extreme events and their mechanisms.

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Reconciling opposing Walker circulation trends in observations and model projections

Cited by 114 publications

References 53 publications

Major factors of global and regional monsoon rainfall changes: natural versus anthropogenic forcing

Major factors of global and regional monsoon rainfall changes: natural versus anthropogenic forcing

Deep-reaching acceleration of global mean ocean circulation over the past two decades

Causes of climate change over the historical record

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