Underestimated responses of Walker circulation to ENSO-related SST anomaly in atmospheric and coupled models

AbstractThe Pacific Walker circulation (WC) is a major component of the global climate system. It connects the Pacific sea surface temperature (SST) variability to the climate variabilities from the other ocean basins to the mid- and high latitudes. Previous studies indicated that the ENSO-related atmospheric feedback, in particular, the surface wind response is largely underestimated in AMIP and CMIP models. In this study, we further investigate the responses in the WC stream function and the sea level pressure (SLP) to the ENSO-related SST variability by comparing the responses in 45 AMIP and 63 CMIP models and six reanalysis datasets. We reveal a diversity in the performances of simulated SLP and WC between different models. While the SLP responses to the El Niño-related SST variability are well simulated in most of the atmospheric and coupled models, the WC stream function responses are largely underestimated in most of these models. The WC responses in the AMIP5/6 models capture ~ 75% of those in the reanalysis, whereas the CMIP5/6 models capture ~ 58% of the responses. Further analysis indicates that these underestimated circulation responses could be partially attributed to the biases in the precipitation scheme in both the atmospheric and coupled models, as well as the biases in the simulated ENSO-related SST patterns in the coupled models. One should pay special attention to these biases when studying the WC or the tropical atmosphere–ocean interactions using numerical models.

Download Full-text

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

Geoscience Letters ◽

10.1186/s40562-021-00180-0 ◽

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.

Download Full-text

The Influence of Pacific SST Variability on the Precipitation over Southern Africa

Journal of Climate ◽

10.1175/2785.1 ◽

2003 ◽

Vol 16 (14) ◽

pp. 2408-2418 ◽

Cited By ~ 32

Author(s):

Vasubandhu Misra

Keyword(s):

Pacific Ocean ◽

Southern Africa ◽

Walker Circulation ◽

Temporal Variations ◽

Dominant Mode ◽

Precipitation Anomaly ◽

Sst Variability ◽

The Pacific ◽

The Pacific Ocean ◽

Experimental Runs

Abstract This study is an analysis of AGCM model results to understand the dynamics of the response of precipitation over southern Africa (SA) to anomalies in the sea surface temperature (SST) over the Pacific Ocean. The pattern of interannual precipitation anomaly over SA and its temporal variations are quite similar in both the ensemble mean of the control (where AGCM is forced with observed SSTs in all ocean basins) and experimental runs (where AGCM is forced with seasonally varying climatological SST over the Pacific Ocean). However, the amplitude of the variability is found to be relatively reduced in the experimental runs. This is shown to be a result of the modulation of the Walker circulation by the variability of Pacific Ocean SST. The regional teleconnection pattern between the dominant mode of SA precipitation variability and SST anomalies over the eastern Indian Ocean is also influenced by the variations in Pacific SST. The nature of the teleconnection between SA precipitation and eastern Indian SST is apparent only when the Pacific SST variability is excluded. This is corroborated from observations as well.

Download Full-text

Assessing the Climate Impacts of the Observed Atlantic Multidecadal Variability Using the GFDL CM2.1 and NCAR CESM1 Global Coupled Models

Journal of Climate ◽

10.1175/jcli-d-16-0127.1 ◽

2017 ◽

Vol 30 (8) ◽

pp. 2785-2810 ◽

Cited By ~ 78

Author(s):

Yohan Ruprich-Robert ◽

Rym Msadek ◽

Frederic Castruccio ◽

Stephen Yeager ◽

Tom Delworth ◽

...

Keyword(s):

North Atlantic ◽

Climate Models ◽

Walker Circulation ◽

Climate Impacts ◽

Boreal Summer ◽

Boreal Winter ◽

Atlantic Multidecadal Variability ◽

Coupled Models ◽

Multidecadal Variability ◽

The Pacific

The climate impacts of the observed Atlantic multidecadal variability (AMV) are investigated using the GFDL CM2.1 and the NCAR CESM1 coupled climate models. The model North Atlantic sea surface temperatures are restored to fixed anomalies corresponding to an estimate of the internally driven component of the observed AMV. Both models show that during boreal summer the AMV alters the Walker circulation and generates precipitation anomalies over the whole tropical belt. A warm phase of the AMV yields reduced precipitation over the western United States, drier conditions over the Mediterranean basin, and wetter conditions over northern Europe. During boreal winter, the AMV modulates by a factor of about 2 the frequency of occurrence of El Niño and La Niña events. This response is associated with anomalies over the Pacific that project onto the interdecadal Pacific oscillation pattern (i.e., Pacific decadal oscillation–like anomalies in the Northern Hemisphere and a symmetrical pattern in the Southern Hemisphere). This winter response is a lagged adjustment of the Pacific Ocean to the AMV forcing in summer. Most of the simulated global-scale impacts are driven by the tropical part of the AMV, except for the winter North Atlantic Oscillation–like response over the North Atlantic–European region, which is driven by both the subpolar and tropical parts of the AMV. The teleconnections between the Pacific and Atlantic basins alter the direct North Atlantic local response to the AMV, which highlights the importance of using a global coupled framework to investigate the climate impacts of the AMV. The similarity of the two model responses gives confidence that impacts described in this paper are robust.

Download Full-text

Structure of the Pacific Walker Circulation Depicted by the Reanalysis and CMIP6

Atmosphere ◽

10.3390/atmos12091219 ◽

2021 ◽

Vol 12 (9) ◽

pp. 1219

Author(s):

Emmanuel Olaoluwa Eresanya ◽

Yuping Guan

Keyword(s):

Large Scale ◽

Structural Changes ◽

Numerical Models ◽

Walker Circulation ◽

Future Climate Change ◽

Western Boundary ◽

Extreme Caution ◽

The Pacific ◽

Mean State

The Pacific Walker circulation (PWC) is one of the most important components of large-scale tropical atmospheric circulations. The PWC and its influences have been studied extensively by numerical models and reanalysis. The newly released ERA5 and NCEP2 are the most widely used reanalysis datasets and serve as benchmarks for evaluation of model simulations. If the results of these datasets differ significantly, this could lead to a bias in projected long-term climate knowledge. For better understanding of future climate change, it is necessary to evaluate PWC reanalysis productions. As a result, we compared the PWC structures between the ERA5 and NCEP2 datasets from month to seasonal time scales. We used the zonal mass streamfunction (ZMS) over the equatorial Pacific to indicate the strength of the PWC. The PWC’s average monthly or seasonal cycle peaks around July. From February to June, the NCEP2 shows a higher PWC intensity, whereas the ERA5 shows greater intensity from July to December. The circulation center in the NCEP2 is generally stronger and wider than in the ERA5. The ERA5, however, revealed that the PWC’s west edge (zero line of ZMS over the western Pacific) had moved 10 degrees westward in comparison to the NCEP2. In addition, we compared the PWC mean state in the reanalysis and CMIP6 models; the mean state vertical structures of the tropical PWC in the CMIP6 multi-model ensemble (MME) are similar to those of the reanalyses in structure but weaker and wider than in the two reanalysis datasets. The PWC is broader in CMIP6, and the western boundary is 7 and 17 degrees farther west than in the ERA5 and NCEP2, respectively. This study suggests that, when using reanalysis datasets to evaluate PWC structural changes in intensity and western edge, extreme caution should be exercised.

Download Full-text

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

Journal of Climate ◽

10.1175/jcli-d-17-0151.1 ◽

2017 ◽

Vol 31 (1) ◽

pp. 81-97 ◽

Cited By ~ 9

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.

Download Full-text

Antarctic Climate History and Global Climate Changes

10.1093/oxfordhb/9780190699420.013.18 ◽

2017 ◽

Author(s):

Pontus Lurcock ◽

Fabio Florindo

Keyword(s):

Ocean Circulation ◽

Climate Changes ◽

Numerical Models ◽

Global Climate ◽

Sediment Cores ◽

Ice Sheets ◽

Global Ocean ◽

Climate History ◽

Global Climate Changes ◽

Planetary Albedo

Antarctic climate changes have been reconstructed from ice and sediment cores and numerical models (which also predict future changes). Major ice sheets first appeared 34 million years ago (Ma) and fluctuated throughout the Oligocene, with an overall cooling trend. Ice volume more than doubled at the Oligocene-Miocene boundary. Fluctuating Miocene temperatures peaked at 17–14 Ma, followed by dramatic cooling. Cooling continued through the Pliocene and Pleistocene, with another major glacial expansion at 3–2 Ma. Several interacting drivers control Antarctic climate. On timescales of 10,000–100,000 years, insolation varies with orbital cycles, causing periodic climate variations. Opening of Southern Ocean gateways produced a circumpolar current that thermally isolated Antarctica. Declining atmospheric CO2 triggered Cenozoic glaciation. Antarctic glaciations affect global climate by lowering sea level, intensifying atmospheric circulation, and increasing planetary albedo. Ice sheets interact with ocean water, forming water masses that play a key role in global ocean circulation.

Download Full-text

The evolving response of mesopelagic fishes to declining midwater oxygen concentrations in the southern and central California Current

ICES Journal of Marine Science ◽

10.1093/icesjms/fsy154 ◽

2018 ◽

Vol 76 (3) ◽

pp. 626-638 ◽

Cited By ~ 2

Author(s):

J Anthony Koslow ◽

Pete Davison ◽

Erica Ferrer ◽

S Patricia A Jiménez Rosenberg ◽

Gerardo Aceves-Medina ◽

...

Keyword(s):

Baja California ◽

Southern Oscillation ◽

Global Climate ◽

California Current ◽

Deep Ocean ◽

Near Surface ◽

Oxygen Minimum Zones ◽

The North ◽

The Pacific ◽

Oxygen Concentrations

Abstract Declining oxygen concentrations in the deep ocean, particularly in areas with pronounced oxygen minimum zones (OMZs), are a growing global concern related to global climate change. Its potential impacts on marine life remain poorly understood. A previous study suggested that the abundance of a diverse suite of mesopelagic fishes off southern California was closely linked to trends in midwater oxygen concentration. This study expands the spatial and temporal scale of that analysis to examine how mesopelagic fishes are responding to declining oxygen levels in the California Current (CC) off central, southern, and Baja California. Several warm-water mesopelagic species, apparently adapted to the shallower, more intense OMZ off Baja California, are shown to be increasing despite declining midwater oxygen concentrations and becoming increasingly dominant, initially off Baja California and subsequently in the CC region to the north. Their increased abundance is associated with warming near-surface ocean temperature, the warm phase of the Pacific Decadal oscillation and Multivariate El Niño-Southern Oscillation Index, and the increased flux of Pacific Equatorial Water into the southern CC.

Download Full-text

Spurious North Tropical Atlantic precursors to El Niño

Nature Communications ◽

10.1038/s41467-021-23411-6 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Wenjun Zhang ◽

Feng Jiang ◽

Malte F. Stuecker ◽

Fei-Fei Jin ◽

Axel Timmermann

Keyword(s):

El Niño ◽

Cross Correlation ◽

El Nino ◽

Southern Oscillation ◽

Global Climate ◽

Tropical Atlantic ◽

The North ◽

Sst Variability ◽

Primary Driver ◽

North Tropical Atlantic

AbstractThe El Niño-Southern Oscillation (ENSO), the primary driver of year-to-year global climate variability, is known to influence the North Tropical Atlantic (NTA) sea surface temperature (SST), especially during boreal spring season. Focusing on statistical lead-lag relationships, previous studies have proposed that interannual NTA SST variability can also feed back on ENSO in a predictable manner. However, these studies did not properly account for ENSO’s autocorrelation and the fact that the SST in the Atlantic and Pacific, as well as their interaction are seasonally modulated. This can lead to misinterpretations of causality and the spurious identification of Atlantic precursors for ENSO. Revisiting this issue under consideration of seasonality, time-varying ENSO frequency, and greenhouse warming, we demonstrate that the cross-correlation characteristics between NTA SST and ENSO, are consistent with a one-way Pacific to Atlantic forcing, even though the interpretation of lead-lag relationships may suggest otherwise.

Download Full-text

Intercomparison of Mesoscale Model Simulations of the Daytime Valley Wind System

Monthly Weather Review ◽

10.1175/2010mwr3523.1 ◽

2011 ◽

Vol 139 (5) ◽

pp. 1389-1409 ◽

Cited By ~ 42

Author(s):

Juerg Schmidli ◽

Brian Billings ◽

Fotini K. Chow ◽

Stephan F. J. de Wekker ◽

James Doyle ◽

...

Keyword(s):

Land Surface ◽

Mesoscale Model ◽

Numerical Models ◽

Surface Wind ◽

Sensible Heat Flux ◽

Surface Wind Speed ◽

Time Shift ◽

Wind System ◽

Valley Wind ◽

The Mean

Three-dimensional simulations of the daytime thermally induced valley wind system for an idealized valley–plain configuration, obtained from nine nonhydrostatic mesoscale models, are compared with special emphasis on the evolution of the along-valley wind. The models use the same initial and lateral boundary conditions, and standard parameterizations for turbulence, radiation, and land surface processes. The evolution of the mean along-valley wind (averaged over the valley cross section) is similar for all models, except for a time shift between individual models of up to 2 h and slight differences in the speed of the evolution. The analysis suggests that these differences are primarily due to differences in the simulated surface energy balance such as the dependence of the sensible heat flux on surface wind speed. Additional sensitivity experiments indicate that the evolution of the mean along-valley flow is largely independent of the choice of the dynamical core and of the turbulence parameterization scheme. The latter does, however, have a significant influence on the vertical structure of the boundary layer and of the along-valley wind. Thus, this ideal case may be useful for testing and evaluation of mesoscale numerical models with respect to land surface–atmosphere interactions and turbulence parameterizations.

Download Full-text

The Linear Response of ENSO to the Madden–Julian Oscillation

Journal of Climate ◽

10.1175/jcli3408.1 ◽

2005 ◽

Vol 18 (13) ◽

pp. 2441-2459 ◽

Cited By ~ 73

Author(s):

J. Zavala-Garay ◽

C. Zhang ◽

A. M. Moore ◽

R. Kleeman

Keyword(s):

Large Fraction ◽

Surface Wind ◽

Low Frequency ◽

Coupled Model ◽

Cumulative Effect ◽

Enso Event ◽

Kelvin Waves ◽

Madden Julian Oscillation ◽

Coupled Models ◽

Stochastic Forcing

Abstract The possibility that the tropical Pacific coupled system linearly amplifies perturbations produced by the Madden–Julian oscillation (MJO) is explored. This requires an estimate of the low-frequency tail of the MJO. Using 23 yr of NCEP–NCAR reanalyses of surface wind and Reynolds SST, we show that the spatial structure that dominates the intraseasonal band (i.e., the MJO) also dominates the low-frequency band once the anomalies directly related to ENSO have been removed. This low-frequency contribution of the intraseasonal variability is not included in most ENSO coupled models used to date. Its effect in a coupled model of intermediate complexity has, therefore, been studied. It is found that this “MJO forcing” (τMJO) can explain a large fraction of the interannual variability in an asymptotically stable version of the model. This interaction is achieved via linear dynamics. That is, it is the cumulative effect of individual events that maintains ENSOs in this model. The largest coupled wind anomalies are initiated after a sequence of several downwelling Kelvin waves of the same sign have been forced by τMJO. The cumulative effect of the forced Kelvin waves is to persist the (small) SST anomalies in the eastern Pacific just enough for the coupled ocean–atmosphere dynamics to amplify the anomalies into a mature ENSO event. Even though τMJO explains just a small fraction of the energy contained in the stress not associated with ENSO, a large fraction of the modeled ENSO variability is excited by this forcing. The characteristics that make τMJO an optimal stochastic forcing for the model are discussed. The large zonal extent is an important factor that differentiates the MJO from other sources of stochastic forcing.

Download Full-text