scholarly journals Understanding the Mid-Holocene Climate

2006 ◽  
Vol 19 (12) ◽  
pp. 2801-2817 ◽  
Author(s):  
Sang-Ik Shin ◽  
Prashant D. Sardeshmukh ◽  
Robert S. Webb ◽  
Robert J. Oglesby ◽  
Joseph J. Barsugli
Keyword(s):  
North America ◽  
Mixed Layer ◽  
North Africa ◽  
General Circulation ◽  
Circulation Model ◽  
Tropical Pacific ◽  
La Niña ◽  
La Nina ◽  
Mixed Layer Ocean ◽  
The Tropical Pacific

Abstract Paleoclimatic evidence suggests that during the mid-Holocene epoch (about 6000 yr ago) North America and North Africa were significantly drier and wetter, respectively, than at present. Modeling efforts to attribute these differences to changes in orbital parameters and greenhouse gas (GHG) levels have had limited success, especially over North America. In this study, the importance of a possibly cooler tropical Pacific Ocean during the epoch (akin to a permanent La Niña–like perturbation to the present climate) in causing these differences is emphasized. Systematic sets of atmospheric general circulation model experiments, with prescribed sea surface temperatures (SSTs) in the tropical Pacific basin and an interactive mixed layer ocean elsewhere, are performed. Given the inadequacies of current fully coupled climate models in simulating the tropical Pacific climate, this intermediate coupling model configuration is argued to be more suitable for quantifying the contributions of the altered orbital forcing, GHG levels, and tropical Pacific SST conditions to the different mid-Holocene climates. The simulated responses in this configuration are in fact generally more consistent with the available evidence from paleovegetation and sedimentary records. Coupling to the mixed layer ocean enhances the wind–evaporation–SST feedback over the tropical Atlantic Ocean. The net response to the orbital changes is to shift the North Atlantic intertropical convergence zone (ITCZ) northward, and make North Africa wetter. The response to the reduced GHG levels opposes, but does not eliminate, these changes. The northward-shifted ITCZ also blocks the moisture supply from the Gulf of Mexico into North America. This drying tendency is greatly amplified by the local response to La Niña–like conditions in the tropical Pacific. Consistent with the paleoclimatic evidence, the simulated North American drying is also most pronounced in the growing (spring) season.

scholarly journals Dipole Structure of Mixed Layer Salinity in Response to El Niño‐La Niña Asymmetry in the Tropical Pacific

2019 ◽  
Vol 46 (21) ◽  
pp. 12165-12172 ◽  
Author(s):  
Cong Guan ◽  
Shijian Hu ◽  
Michael J. McPhaden ◽  
Fan Wang ◽  
Shan Gao ◽  
...  
Keyword(s):  
Mixed Layer ◽  
El Niño ◽  
El Nino ◽  
Tropical Pacific ◽  
La Niña ◽  
Dipole Structure ◽  
La Nina ◽  
The Tropical Pacific

scholarly journals The Summer Asia-North America Teleconnection and its Modulation by ENSO in Community Atmosphere Model, Version 5 (CAM5)

2021 ◽  
Author(s):  
Kelsey Malloy ◽  
Ben P. Kirtman
Keyword(s):  
North America ◽  
General Circulation ◽  
East Asian Monsoon ◽  
Wave Train ◽  
Circulation Model ◽  
La Niña ◽  
Destructive Interference ◽  
Seasonal Forecasts ◽  
Model Version ◽  
La Nina

Abstract Seasonal forecasts of summer continental United States (CONUS) rainfall have relatively low skill, partly due to a lack of consensus about its sources of predictability. The East Asian monsoon (EAM) can excite a cross-Pacific Rossby wave train, also known as the Asia-North America (ANA) teleconnection. In this study, we analyze the ANA teleconnection in observations and model simulations from the Community Atmospheric Model, version 5 (CAM5), comparing experiments with prescribed climatological SSTs and prescribed observed SSTs. Observations indicate a statistically significant relationship between a strong EAM and increased probability of positive precipitation anomalies over the U.S. west coast and the Plains-Midwest. The ANA teleconnection and CONUS rainfall patterns are improved in the CAM5 experiment with prescribed observed SSTs, suggesting that SST variability is necessary to simulate this teleconnection over CONUS. We find distinct ANA patterns between ENSO phases, with the La Niña-related patterns in CAM5_obsSST disagreeing with observations. Using linear steady-state quasi-geostrophic theory, we conclude that incorrect EAM forcing location greatly contributed to CAM5 biases, and jet stream disparities explained the ENSO-related biases. Finally, we compared EAM forcing experiments with different mean states using a simple dry nonlinear atmospheric general circulation model. Overall, the ANA pattern over CONUS and its modulation by ENSO forcing are well described by dry dynamics on seasonal-to-interannual timescales, including the constructive (destructive) interference between El Niño (La Niña) modulation and the ANA patterns over CONUS.

scholarly journals North American Droughts of the Last Millennium from a Gridded Network of Tree-Ring Data

2007 ◽  
Vol 20 (7) ◽  
pp. 1353-1376 ◽  
Author(s):  
Celine Herweijer ◽  
Richard Seager ◽  
Edward R. Cook ◽  
Julien Emile-Geay
Keyword(s):  
North America ◽  
Tree Ring ◽  
North American ◽  
Tropical Pacific ◽  
La Niña ◽  
Last Millennium ◽  
Spatial Mode ◽  
Drought Variability ◽  
La Nina ◽  
The Tropical Pacific

Abstract Drought is the most economically expensive recurring natural disaster to strike North America in modern times. Recently available gridded drought reconstructions have been developed for most of North America from a network of drought-sensitive tree-ring chronologies, many of which span the last 1000 yr. These reconstructions enable the authors to put the famous droughts of the instrumental record (i.e., the 1930s Dust Bowl and the 1950s Southwest droughts) into the context of 1000 yr of natural drought variability on the continent. We can now, with this remarkable new record, examine the severity, persistence, spatial signatures, and frequencies of drought variability over the past milllennium, and how these have changed with time. The gridded drought reconstructions reveal the existence of successive “megadroughts,” unprecedented in persistence (20–40 yr), yet similar in year-to-year severity and spatial distribution to the major droughts experienced in today’s North America. These megadroughts occurred during a 400-yr-long period in the early to middle second millennium a.d., with a climate varying as today’s, but around a drier mean. The implication is that the mechanism forcing persistent drought in the West and the Plains in the instrumental era is analagous to that underlying the megadroughts of the medieval period. The leading spatial mode of drought variability in the recontructions resembles the North American ENSO pattern: widespread drought across the United States, centered on the Southwest, with a hint of the opposite phase in the Pacific Northwest. Recently, climate models forced by the observed history of tropical Pacific SSTs have been able to successfully simulate all of the major North American droughts of the last 150 yr. In each case, cool “La Niña–like” conditions in the tropical Pacific are consistent with North American drought. With ENSO showing a pronounced signal in the gridded drought recontructions of the last millennium, both in terms of its link to the leading spatial mode, and the leading time scales of drought variability (revealed by multitaper spectral analysis and wavelet analysis), it is postulated that, as for the modern day, the medieval megadroughts were forced by protracted La Niña–like tropical Pacific SSTs. Further evidence for this comes from the global hydroclimatic “footprint” of the medieval era revealed by existing paleoclimatic archives from the tropical Pacific and ENSO-sensitive tropical and extratropical land regions. In general, this global pattern matches that observed for modern-day persistent North American drought, whereby a La Niña–like tropical Pacific is accompanied by hemispheric, and in the midlatitudes, zonal, symmetry of hydroclimatic anomalies.

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 Influence of ENSO on North American subseasonal surface air temperature variability

2021 ◽  
Vol 2 (2) ◽  
pp. 395-412
Author(s):  
Patrick Martineau ◽  
Hisashi Nakamura ◽  
Yu Kosaka
Keyword(s):  
North America ◽  
Air Temperature ◽  
North American ◽  
Surface Air Temperature ◽  
Tropical Pacific ◽  
La Niña ◽  
Western North America ◽  
The North ◽  
Subseasonal Variability ◽  
La Nina

Abstract. The wintertime influence of tropical Pacific sea surface temperature (SST) variability on subseasonal variability is revisited by identifying the dominant mode of covariability between 10–60 d band-pass-filtered surface air temperature (SAT) variability over the North American continent and winter-mean SST over the tropical Pacific. We find that the El Niño–Southern Oscillation (ENSO) explains a dominant fraction of the year-to-year changes in subseasonal SAT variability that are covarying with SST and thus likely more predictable. In agreement with previous studies, we find a tendency for La Niña conditions to enhance the subseasonal SAT variability over western North America. This modulation of subseasonal variability is achieved through interactions between subseasonal eddies and La Niña-related changes in the winter-mean circulation. Specifically, eastward-propagating quasi-stationary eddies over the North Pacific are more efficient in extracting energy from the mean flow through the baroclinic conversion during La Niña. Structural changes of these eddies are crucial to enhance the efficiency of the energy conversion via amplified downgradient heat fluxes that energize subseasonal eddy thermal anomalies. The enhanced likelihood of cold extremes over western North America is associated with both an increased subseasonal SAT variability and the cold winter-mean response to La Niña.

scholarly journals Atmospheric Energetics over the Tropical Pacific during the ENSO Cycle

2017 ◽  
Vol 30 (10) ◽  
pp. 3635-3654 ◽  
Author(s):  
Di Dong ◽  
Jianping Li ◽  
Lidou Huyan ◽  
Jiaqing Xue
Keyword(s):  
El Nino ◽  
Tropical Pacific ◽  
La Niña ◽  
Western Pacific ◽  
Eastern Pacific ◽  
Enso Cycle ◽  
Upper Troposphere ◽  
La Nina ◽  
Central Eastern ◽  
The Tropical Pacific

Abstract The atmospheric perturbation potential energy (PPE) over the tropical Pacific is calculated and analyzed in a composite ENSO cycle. The PPE over the tropical Pacific troposphere increases during El Niño and decreases during La Niña, displaying two centers symmetrical about the equator and delaying the central–eastern Pacific SST anomaly by two months. Generated from atmospheric diabatic heating, the smaller part of PPE in the lower troposphere varies synchronously with the central–eastern Pacific SST through sensible heating, while the larger part of PPE lies in the mid- and upper troposphere and lags the central–eastern Pacific SST about one season because of latent heat release. As the tropical Pacific PPE peaks during the boreal late winter in an El Niño event, two anticyclones form in the upper troposphere as a result of the Gill model response. More PPE is converted to atmospheric kinetic energy (KE) above the central–western Pacific, but less over the eastern Pacific, leading to intensified Hadley circulations over the central–western Pacific and weakened Hadley circulations over the eastern Pacific. The strengthened Hadley circulations cause surface easterly wind bursts through KE convergence in the western equatorial Pacific, which may trigger a La Niña event. The reverse situation occurs during La Niña. Thus, the response of the Hadley circulations in the central–western Pacific provides a negative feedback during the ENSO cycle.

scholarly journals Mechanisms Involved in the Amplification of the 11-yr Solar Cycle Signal in the Tropical Pacific Ocean

2012 ◽  
Vol 25 (14) ◽  
pp. 5102-5118 ◽  
Author(s):  
Stergios Misios ◽  
Hauke Schmidt
Keyword(s):  
Solar Cycle ◽  
Pacific Ocean ◽  
Deep Convection ◽  
Tropical Pacific ◽  
La Niña ◽  
Tropical Pacific Ocean ◽  
La Nina ◽  
Ocean Coupling ◽  
Indian And Atlantic Oceans ◽  
The Tropical Pacific

Abstract It is debated whether the response of the tropical Pacific Ocean to the 11-yr solar cycle forcing resembles a La Niña– or El Niño–like signal. To address this issue, ensemble simulations employing an atmospheric general circulation model with and without ocean coupling are conducted. The coupled simulations show no evidence for a La Niña–like cooling in solar maxima. Instead, the tropical sea surface temperature rises almost in phase with the 11-yr solar cycle. A basinwide warming of about 0.1 K is simulated in the tropical Pacific, whereas the warming in the tropical Indian and Atlantic Oceans is weaker. In the western Pacific, the region of deep convection shifts to the east, thus reducing the surface easterlies. This shift is independent of the ocean coupling because deep convection moves to the east in the uncoupled simulations too. The reduced surface easterlies cool the subsurface but warm the surface due to the reduction of heat transport divergence. The latter mechanism operates together with water vapor feedback, resulting in a stronger tropical Pacific warming relative to the warming over the tropical Indian and Atlantic Oceans. These results suggest that the atmospheric response to the 11-yr solar cycle drives the tropical Pacific response, which is amplified by atmosphere–ocean feedbacks operating on decadal time scales. Based on the coupled simulations, it is concluded that the tropical Pacific Ocean should warm when the sun is more active.

scholarly journals Role of Nonlinear Atmospheric Response to SST on the Asymmetric Transition Process of ENSO

2009 ◽  
Vol 22 (1) ◽  
pp. 177-192 ◽  
Author(s):  
Masamichi Ohba ◽  
Hiroaki Ueda
Keyword(s):  
Observational Data ◽  
Zonal Wind ◽  
General Circulation ◽  
Circulation Model ◽  
La Niña ◽  
Atmospheric Response ◽  
Cold Event ◽  
Transition Processes ◽  
La Nina ◽  
Sst Anomalies

Abstract Physical processes that are responsible for the asymmetric transition processes between El Niño and La Niña events are investigated by using observational data and physical models to examine the nonlinear atmospheric response to SST. The air–sea coupled system of ENSO is able to remain in a weak, cold event for up to 2 yr, while the system of a relatively warm event turns into a cold phase. Through analysis of the oceanic observational data, it is found that there is a strong difference in thermocline variations in relation to surface zonal wind anomalies in the equatorial Pacific (EP) during the mature-to-decaying phase of ENSO. The atmospheric response for the warm phase of ENSO causes a rapid reduction of the EP westerlies in boreal winter, which play a role in hastening the following ENSO transition through the generation of upwelling oceanic Kelvin waves. However, the anomalous EP easterlies in the cold phase persist to the subsequent spring, which tends to counteract the turnabout from the cold to warm phase of ENSO. A suite of idealized atmospheric general circulation model (AGCM) experiments are performed by imposing two different ENSO-related SST anomalies, which have equal amplitudes but opposite signs. The nonlinear climate response in the AGCM is found at the mature-to-decaying phase of ENSO that closely resembles the observations, including a zonal and meridional shift in the equatorial positions of the atmospheric wind. By using a simple ocean model, it is determined that the asymmetric responses of the equatorial zonal wind result in different recovery times of the thermocline in the eastern Pacific. Thus, the differences in transition processes between the warm and cold ENSO event are fundamentally due to the nonlinear atmospheric response to SST, which originates from the distribution of climatological SST and its seasonal changes. By including the asymmetric wind responses the intermediate air–sea coupled model herein demonstrates that the essential elements of the redevelopment of La Niña arise from the nonlinear atmospheric response to SST anomalies.

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