scholarly journals Megadroughts in Southwestern North America in ECHO-G Millennial Simulations and Their Comparison to Proxy Drought Reconstructions*

2013 ◽  
Vol 26 (19) ◽  
pp. 7635-7649 ◽  
Author(s):  
Sloan Coats ◽  
Jason E. Smerdon ◽  
Richard Seager ◽  
Benjamin I. Cook ◽  
J. F. González-Rouco
Keyword(s):  
North American ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Tropical Pacific ◽  
Drought Severity ◽  
The North ◽  
Hydroclimate Variability ◽  
And Control ◽  
The Tropical Pacific

Abstract Simulated hydroclimate variability in millennium-length forced transient and control simulations from the ECHAM and the global Hamburg Ocean Primitive Equation (ECHO-G) coupled atmosphere–ocean general circulation model (AOGCM) is analyzed and compared to 1000 years of reconstructed Palmer drought severity index (PDSI) variability from the North American Drought Atlas (NADA). The ability of the model to simulate megadroughts in the North American southwest is evaluated. (NASW: 25°–42.5°N, 125°–105°W). Megadroughts in the ECHO-G AOGCM are found to be similar in duration and magnitude to those estimated from the NADA. The droughts in the forced simulation are not, however, temporally synchronous with those in the paleoclimate record, nor are there significant differences between the drought features simulated in the forced and control runs. These results indicate that model-simulated megadroughts can result from internal variability of the modeled climate system rather than as a response to changes in exogenous forcings. Although the ECHO-G AOGCM is capable of simulating megadroughts through persistent La Niña–like conditions in the tropical Pacific, other mechanisms can produce similarly extreme NASW moisture anomalies in the model. In particular, the lack of low-frequency coherence between NASW soil moisture and simulated modes of climate variability like the El Niño–Southern Oscillation, Pacific decadal oscillation, and Atlantic multidecadal oscillation during identified drought periods suggests that stochastic atmospheric variability can contribute significantly to the occurrence of simulated megadroughts in the NASW. These findings indicate that either an expanded paradigm is needed to understand multidecadal hydroclimate variability in the NASW or AOGCMs may incorrectly simulate the strength and/or dynamics of the connection between NASW hydroclimate variability and the tropical Pacific.

scholarly journals Are Simulated Megadroughts in the North American Southwest Forced?*

2014 ◽  
Vol 28 (1) ◽  
pp. 124-142 ◽  
Author(s):  
Sloan Coats ◽  
Jason E. Smerdon ◽  
Benjamin I. Cook ◽  
Richard Seager
Keyword(s):  
North American ◽  
General Circulation ◽  
Southern Oscillation ◽  
Tropical Pacific ◽  
American Southwest ◽  
Climate System Model ◽  
Multidecadal Variability ◽  
The North ◽  
North American Southwest ◽  
The Tropical Pacific

Abstract Multidecadal drought periods in the North American Southwest (25°–42.5°N, 125°–105°W), so-called megadroughts, are a prominent feature of the paleoclimate record over the last millennium (LM). Six forced transient simulations of the LM along with corresponding historical (1850–2005) and 500-yr preindustrial control runs from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are analyzed to determine if atmosphere–ocean general circulation models (AOGCMs) are able to simulate droughts that are similar in persistence and severity to the megadroughts in the proxy-derived North American Drought Atlas. Megadroughts are found in each of the AOGCM simulations of the LM, although there are intermodel differences in the number, persistence, and severity of these features. Despite these differences, a common feature of the simulated megadroughts is that they are not forced by changes in the exogenous forcing conditions. Furthermore, only the Community Climate System Model (CCSM), version 4, simulation contains megadroughts that are consistently forced by cooler conditions in the tropical Pacific Ocean. These La Niña–like mean states are not accompanied by changes to the interannual variability of the El Niño–Southern Oscillation system and result from internal multidecadal variability of the tropical Pacific mean state, of which the CCSM has the largest magnitude of the analyzed simulations. Critically, the CCSM is also found to have a realistic teleconnection between the tropical Pacific and North America that is stationary on multidecadal time scales. Generally, models with some combination of a realistic and stationary teleconnection and large multidecadal variability in the tropical Pacific are found to have the highest incidence of megadroughts driven by the tropical Pacific boundary conditions.

scholarly journals Impacts of Interhemispheric Asymmetric Thermal Forcing on Tropical Pacific Climate: Surface Air–Sea Coupling and Subduction

2013 ◽  
Vol 26 (2) ◽  
pp. 575-582 ◽  
Author(s):  
Chun Li ◽  
Lixin Wu ◽  
Shang-Ping Xie
Keyword(s):  
General Circulation ◽  
Cold Water ◽  
Circulation Model ◽  
South Pacific ◽  
Feedback Mechanism ◽  
Tropical Pacific ◽  
Second Stage ◽  
The North ◽  
Mean Circulation ◽  
The Tropical Pacific

Abstract Paleoclimate observations and modeling studies suggest that extratropical climate change affects the tropical Pacific. A global coupled general circulation model is used to investigate the equatorial Pacific response to extratropical surface heat flux forcing that is downward (upward) poleward of 40°N (S). The equatorial response consists of two distinct stages: the zonal sea surface temperature (SST) gradient strengthens for the first two to three decades and then weakens afterward. In the first stage, fast surface air–sea coupling feedback mechanism communicates the extratropical warming (cooling) from the North (South) Pacific toward the equator. The second stage is characterized by a basinwide shoaling of the tropical Pacific thermocline as the subtropical cell (STC) advects cold water from the South Pacific along the thermocline. This preference of Southern Hemisphere anomalies is due to the meridional asymmetry in the mean circulation: the interior pathway for STC is open south but partially blocked north of the equator. Paleoclimate implications are discussed.

Optimal Tropical Sea Surface Temperature Forcing of North American Drought

2010 ◽  
Vol 23 (14) ◽  
pp. 3907-3917 ◽  
Author(s):  
Sang-Ik Shin ◽  
Prashant D. Sardeshmukh ◽  
Robert S. Webb
Keyword(s):  
Time Series ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North American ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Sea Surface ◽  
Drought Severity ◽  
Sst Anomaly

Abstract The optimal anomalous sea surface temperature (SST) pattern for forcing North American drought is identified through atmospheric general circulation model integrations in which the response of the Palmer drought severity index (PDSI) is determined for each of 43 prescribed localized SST anomaly “patches” in a regular array over the tropical oceans. The robustness and relevance of the optimal pattern are established through the consistency of results obtained using two different models, and also by the good correspondence of the projection time series of historical tropical SST anomaly fields on the optimal pattern with the time series of the simulated PDSI in separate model integrations with prescribed time-varying observed global SST fields for 1920–2005. It is noteworthy that this optimal drought forcing pattern differs markedly in the Pacific Ocean from the dominant SST pattern associated with El Niño–Southern Oscillation (ENSO), and also shows a large sensitivity of North American drought to Indian and Atlantic Ocean SSTs.

scholarly journals Subtropical Forcing of Tropical Pacific Climate and Decadal ENSO Modulation

2008 ◽  
Vol 21 (18) ◽  
pp. 4691-4709 ◽  
Author(s):  
Daniela Matei ◽  
Noel Keenlyside ◽  
Mojib Latif ◽  
Johann Jungclaus
Keyword(s):  
North Pacific ◽  
General Circulation ◽  
Decadal Variability ◽  
Southern Oscillation ◽  
Circulation Model ◽  
South Pacific ◽  
Tropical Pacific ◽  
Ocean Dynamics ◽  
The North ◽  
The North Pacific

Abstract The relative impact of the subtropical North and South Pacific Oceans on the tropical Pacific climate mean state and variability is estimated using an ocean–atmosphere–sea ice coupled general circulation model. Tailored experiments are performed in which the model is forced by idealized sea surface temperature anomalies (SSTAs) in the subtropics of both hemispheres. The main results of this study suggest that subtropical South Pacific climate variations play a dominant role in tropical Pacific decadal variability and in the decadal modulation of El Niño–Southern Oscillation (ENSO). In response to a 2°C warming in the subtropical South Pacific, the equatorial Pacific SST increases by about 0.6°C, approximately 65% larger than the change in the North Pacific experiment. The subtropics affect equatorial SST mainly through atmosphere–mixed layer interactions in the South Pacific experiments; the response is mostly accomplished within a decade. The “oceanic tunnel” dominates in the North Pacific experiments; the response takes at least 100 yr to be accomplished. Similar sensitivity experiments conducted with the stand-alone atmosphere model showed that both air–sea interactions and ocean dynamics are crucial in shaping the tropical climate response. The statistics of ENSO exhibit significant changes in amplitude and frequency in response to a warming/cooling of the subtropical South Pacific: a 2°C warming (cooling) of subtropical South Pacific SST reduces (increases) the interannual standard deviation by about 30% (20%) and shortens (lengthens) the ENSO period. The simulated changes in the equatorial zonal SST gradient are the main contributor to the modulation of ENSO variability. The simulated intensification (weakening) of the annual cycle in response to an enhanced warming (cooling) in subtropical South Pacific partly explains the shifts in frequency, but may also lead to a weaker (stronger) ENSO. The subtropical North Pacific thermal forcing did not change the statistical properties of ENSO as strongly.

scholarly journals Atlantic multidecadal oscillation drives interdecadal Pacific variability via tropical atmospheric bridge

2021 ◽  
pp. 1-46
Author(s):  
Xiaohe An ◽  
Bo Wu ◽  
Tianjun Zhou ◽  
Bo Liu
Keyword(s):  
North Pacific ◽  
General Circulation ◽  
Circulation Model ◽  
Atlantic Multidecadal Oscillation ◽  
Tropical Pacific ◽  
Convective Heating ◽  
The North ◽  
The North Pacific ◽  
Sst Anomalies ◽  
The Tropical Pacific

AbstractInterdecadal Pacific Oscillation (IPO) and Atlantic Multidecadal Oscillation (AMO), two leading modes of decadal climate variability, are not independent. It was proposed that ENSO-like sea surface temperature (SST) variations play a central role in the Pacific responses to the AMO forcing. However, observational analyses indicate that the AMO-related SST anomalies in the tropical Pacific are far weaker than those in the extratropical North Pacific. Here, we show that SST in the North Pacific is tied to the AMO forcing by convective heating associated with precipitation over the tropical Pacific, instead of by SST there, based on an ensemble of pacemaker experiments with North Atlantic SST restored to the observation in a coupled general circulation model. The AMO modulates precipitation over the equatorial and tropical southwestern Pacific through exciting an anomalous zonal circulation and an interhemispheric asymmetry of net moist static energy input into the atmosphere. The convective heating associated with the precipitation anomalies drive SST variations in the North Pacific through a teleconnection, but remarkably weaken the ENSO-like SST anomalies through a thermocline damping effect. This study has implications that the IPO is a combined mode generated by both AMO forcing and local air-sea interactions, but the IPO-related global-warming acceleration/slowdown is independent of the AMO.

Successive Modulation of ENSO to the Future Greenhouse Warming

2008 ◽  
Vol 21 (1) ◽  
pp. 3-21 ◽  
Author(s):  
Soon-Il An ◽  
Jong-Seong Kug ◽  
Yoo-Geun Ham ◽  
In-Sik Kang
Keyword(s):  
General Circulation ◽  
Southern Oscillation ◽  
General Circulation Models ◽  
Tropical Pacific ◽  
Greenhouse Warming ◽  
Delayed Response ◽  
Subsurface Temperature ◽  
Climate System Model ◽  
The Mean ◽  
The Tropical Pacific

Abstract The multidecadal modulation of the El Niño–Southern Oscillation (ENSO) due to greenhouse warming has been analyzed herein by means of diagnostics of Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) coupled general circulation models (CGCMs) and the eigenanalysis of a simplified version of an intermediate ENSO model. The response of the global-mean troposphere temperature to increasing greenhouse gases is more likely linear, while the amplitude and period of ENSO fluctuates in a multidecadal time scale. The climate system model outputs suggest that the multidecadal modulation of ENSO is related to the delayed response of the subsurface temperature in the tropical Pacific compared to the response time of the sea surface temperature (SST), which would lead a modulation of the vertical temperature gradient. Furthermore, an eigenanalysis considering only two parameters, the changes in the zonal contrast of the mean background SST and the changes in the vertical contrast between the mean surface and subsurface temperatures in the tropical Pacific, exhibits a good agreement with the CGCM outputs in terms of the multidecadal modulations of the ENSO amplitude and period. In particular, the change in the vertical contrast, that is, change in difference between the subsurface temperature and SST, turns out to be more influential on the ENSO modulation than changes in the mean SST itself.

Tropical Atmospheric Variability Forced by Oceanic Internal Variability

2007 ◽  
Vol 20 (4) ◽  
pp. 765-771 ◽  
Author(s):  
Markus Jochum ◽  
Clara Deser ◽  
Adam Phillips
Keyword(s):  
General Circulation ◽  
Climate Models ◽  
Rainfall Variability ◽  
Circulation Model ◽  
Internal Variability ◽  
Tropical Pacific ◽  
Atmospheric Variability ◽  
Instability Waves ◽  
Tropical Instability Waves ◽  
The Tropical Pacific

Abstract Atmospheric general circulation model experiments are conducted to quantify the contribution of internal oceanic variability in the form of tropical instability waves (TIWs) to interannual wind and rainfall variability in the tropical Pacific. It is found that in the tropical Pacific, along the equator, and near 25°N and 25°S, TIWs force a significant increase in wind and rainfall variability from interseasonal to interannual time scales. Because of the stochastic nature of TIWs, this means that climate models that do not take them into account will underestimate the strength and number of extreme events and may overestimate forecast capability.

scholarly journals Links between Tropical Pacific SST and Cholera Incidence in Bangladesh: Role of the Eastern and Central Tropical Pacific

2008 ◽  
Vol 21 (18) ◽  
pp. 4647-4663 ◽  
Author(s):  
Benjamin A. Cash ◽  
Xavier Rodó ◽  
James L. Kinter
Keyword(s):  
El Niño ◽  
General Circulation ◽  
Physical Mechanism ◽  
El Nino ◽  
Southern Oscillation ◽  
Regional Climate ◽  
Circulation Model ◽  
Tropical Pacific ◽  
Bacterial Disease ◽  
Monsoon Circulation

Abstract Recent studies arising from both statistical analysis and dynamical disease models indicate that there is a link between incidence of cholera, a paradigmatic waterborne bacterial disease (WBD) endemic to Bangladesh, and the El Niño–Southern Oscillation (ENSO). However, a physical mechanism explaining this relationship has not yet been established. A regionally coupled, or “pacemaker,” configuration of the Center for Ocean–Land–Atmosphere Studies atmospheric general circulation model is used to investigate links between sea surface temperature in the central and eastern tropical Pacific and the regional climate of Bangladesh. It is found that enhanced precipitation tends to follow winter El Niño events in both the model and observations, providing a plausible physical mechanism by which ENSO could influence cholera in Bangladesh. The enhanced precipitation in the model arises from a modification of the summer monsoon circulation over India and Bangladesh. Westerly wind anomalies over land to the west of Bangladesh lead to increased convergence in the zonal wind field and hence increased moisture convergence and rainfall. This change in circulation results from the tropics-wide warming in the model following a winter El Niño event. These results suggest that improved forecasting of cholera incidence may be possible through the use of climate predictions.

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