Mechanisms of Northern Tropical Atlantic Variability and Response to CO2 Doubling

2007 ◽  
Vol 20 (11) ◽  
pp. 2691-2705 ◽  
Author(s):  
Wim-Paul Breugem ◽  
Wilco Hazeleger ◽  
Reindert J. Haarsma
Keyword(s):  
Moisture Transport ◽  
Seasonal Cycle ◽  
General Circulation ◽  
Circulation Model ◽  
Atmospheric Co2 ◽  
Boreal Winter ◽  
Tropical Atlantic ◽  
Wes Feedback ◽  
Meridional Mode ◽  
Northern Tropical Atlantic

Abstract A model study has been made of the mechanisms of the meridional mode in the northern tropical Atlantic (NTA) and the response to a doubling of atmospheric CO2. The numerical model consists of an atmospheric general circulation model (GCM) coupled to a passive mixed layer model for the ocean. Results from two simulations are shown: a control run with present-day atmospheric CO2 and a run with a doubled CO2 concentration. The results from the control run show that the wind–evaporation–SST (WES) feedback is confined to the deep NTA. Furthermore, the temporal evolution of the meridional mode is phase locked with the seasonal cycle of the climatological intertropical convergence zone (CITCZ). The WES feedback is positive in boreal winter and spring when the CITCZ is located close to the equator but negative in summer and fall when the CITCZ shifts toward the north of the deep NTA. Similarly, the damping of the SST anomalies in the deep NTA by moisture-induced evaporation anomalies is much stronger in summer and fall than in winter and spring, related to a change in anomalous moisture transport. The results from the double-CO2 run show a substantial northward shift of the CITCZ in boreal winter and spring but little change in summer and fall. The change in the CITCZ can be explained by strong warming at the high northern latitudes in combination with a seasonally dependent WES feedback with accompanying changes in moisture transport in the deep NTA. The latter indicates that the change in the CITCZ is subject to phase locking with the seasonal cycle of the CITCZ itself. The meridional mode in the double-CO2 run weakens by 10%–20%. This originates from the weakening of the positive WES feedback in the deep NTA, which in turn is attributed to the northward shift of the CITCZ; because in the double-CO2 run the CITCZ stays south of the deep NTA for a shorter time period, the positive WES feedback in the deep NTA acts less long, and damping by moisture-induced evaporation anomalies starts earlier than in the control run.

scholarly journals Interannual Variations in Summer Precipitation in Southwest China: Anomalies in Moisture Transport and the Role of the Tropical Atlantic

2020 ◽  
Vol 33 (14) ◽  
pp. 5993-6007 ◽  
Author(s):  
Chaoxia Yuan ◽  
Mengzhou Yang
Keyword(s):  
Pacific Ocean ◽  
North Pacific ◽  
Moisture Transport ◽  
General Circulation ◽  
Western North Pacific ◽  
North Pacific Ocean ◽  
Circulation Model ◽  
Summer Precipitation ◽  
Tropical Atlantic ◽  
Anomalous Anticyclone

AbstractUsing a Lagrangian trajectory model, contributions of moisture from the Indian Ocean (IO), the South China Sea (SCS), the adjacent land region (LD), and the Pacific Ocean (PO) to interannual summer precipitation variations in southwestern China (SWC) are investigated. Results show that, on average, the IO, SCS, LD, and PO contribute 48.8%, 21.1%, 23.6%, and 3.7% of the total moisture release in SWC, respectively. In summers with the above-normal precipitation, moisture release from the IO and SCS increases significantly by 41.4% and 15.1%, respectively. In summers with below-normal precipitation, moisture release from the IO and SCS decreases significantly by 44.2% and 24.6%, respectively. In addition, the moisture anomalies from the four source regions together explain 86.5% of the total interannual variances of SWC summer precipitation, and the IO and SCS only can explain 75.7%. Variations in moisture transport from the IO, SCS, and LD to SWC are not independent of one another and are commonly influenced by the anomalous anticyclone in the western North Pacific Ocean, which enhances the moisture transport from the IO and SCS by the anomalous southwesterlies over its northwestern quadrant but reduces that from the LD east of SWC by the anomalous westerlies along its northern edge. Anomalous warming in the tropical Atlantic Ocean can modify the Walker circulation, induce anomalous descending motion over the central tropical Pacific, and excite the anomalous anticyclone in the western North Pacific as the classic Matsuno–Gill response. The observed impacts of the tropical Atlantic warming on the anomalous anticyclone and summer precipitation in SWC can be well reproduced in an atmospheric general circulation model.

scholarly journals Sensitivity of the tropical climate to an interhemispheric thermal gradient: the role of tropical ocean dynamics

2018 ◽  
Vol 9 (1) ◽  
pp. 285-297 ◽  
Author(s):  
Stefanie Talento ◽  
Marcelo Barreiro
Keyword(s):  
Seasonal Cycle ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Ocean Dynamics ◽  
Thermocline Depth ◽  
Thermal Forcing ◽  
Tropical Ocean ◽  
Slab Ocean

Abstract. This study aims to determine the role of the tropical ocean dynamics in the response of the climate to extratropical thermal forcing. We analyse and compare the outcomes of coupling an atmospheric general circulation model (AGCM) with two ocean models of different complexity. In the first configuration the AGCM is coupled with a slab ocean model while in the second a reduced gravity ocean (RGO) model is additionally coupled in the tropical region. We find that the imposition of extratropical thermal forcing (warming in the Northern Hemisphere and cooling in the Southern Hemisphere with zero global mean) produces, in terms of annual means, a weaker response when the RGO is coupled, thus indicating that the tropical ocean dynamics oppose the incoming remote signal. On the other hand, while the slab ocean coupling does not produce significant changes to the equatorial Pacific sea surface temperature (SST) seasonal cycle, the RGO configuration generates strong warming in the central-eastern basin from April to August balanced by cooling during the rest of the year, strengthening the seasonal cycle in the eastern portion of the basin. We hypothesize that such changes are possible via the dynamical effect that zonal wind stress has on the thermocline depth. We also find that the imposed extratropical pattern affects El Niño–Southern Oscillation, weakening its amplitude and low-frequency behaviour.

scholarly journals Sensitivity of the tropical climate to an interhemispheric thermal gradient: the role of tropical ocean dynamics

2017 ◽  
Author(s):  
Stefanie Talento ◽  
Marcelo Barreiro
Keyword(s):  
Seasonal Cycle ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Ocean Dynamics ◽  
Thermocline Depth ◽  
Thermal Forcing ◽  
Tropical Ocean ◽  
Slab Ocean

Abstract. This study aims to determine the role of the tropical ocean dynamics in the response of the climate to an extratropical thermal forcing. We analyse and compare the outcomes of coupling an atmospheric general circulation model (AGCM) with two ocean models of different complexity. In the first configuration the AGCM is coupled with a slab ocean model while in the second a Reduced Gravity Ocean (RGO) model is additionally coupled in the tropical region. We find that the imposition of an extratropical thermal forcing (warming in the Northern Hemisphere and cooling in the Southern Hemisphere with zero global mean) produces, in terms of annual means, a weaker response when the RGO is coupled, thus indicating that the tropical ocean dynamics opposes the incoming remote signal. On the other hand, while the slab ocean coupling does not produce significant changes to the equatorial Pacific sea surface temperature (SST) seasonal cycle, the RGO configuration generates a strong warming in the centre-east of the basin from April to August balanced by a cooling during the rest of the year, strengthening the seasonal cycle in the eastern portion of the basin. We hypothesize that such changes are possible via the dynamical effect that zonal wind stress has on the thermocline depth. We also find that the imposed extratropical pattern affects El Niño Southern Oscillation, weakening its amplitude and low-frequency behaviour.

scholarly journals Coupled Ocean–Atmosphere Response to Seasonal Modulation of Ocean Color: Impact on Interannual Climate Simulations in the Tropical Pacific

2007 ◽  
Vol 20 (2) ◽  
pp. 353-374 ◽  
Author(s):  
J. Ballabrera-Poy ◽  
R. Murtugudde ◽  
R-H. Zhang ◽  
A. J. Busalacchi
Keyword(s):  
Solar Radiation ◽  
Interannual Variability ◽  
Seasonal Cycle ◽  
General Circulation ◽  
Ocean Color ◽  
Circulation Model ◽  
Enso Events ◽  
Ocean Atmosphere ◽  
Atmosphere Model ◽  
The Impact

Abstract The ability to use remotely sensed ocean color data to parameterize biogenic heating in a coupled ocean–atmosphere model is investigated. The model used is a hybrid coupled model recently developed at the Earth System Science Interdisciplinary Center (ESSIC) by coupling an ocean general circulation model with a statistical atmosphere model for wind stress anomalies. The impact of the seasonal cycle of water turbidity on the annual mean, seasonal cycle, and interannual variability of the coupled system is investigated using three simulations differing in the parameterization of the vertical attenuation of downwelling solar radiation: (i) a control simulation using a constant 17-m attenuation depth, (ii) a simulation with the spatially varying annual mean of the satellite-derived attenuation depth, and (iii) a simulation accounting for the seasonal cycle of the attenuation depth. The results indicate that a more realistic attenuation of solar radiation slightly reduces the cold bias of the model. While a realistic attenuation of solar radiation hardly affects the annual mean and the seasonal cycle due to anomaly coupling, it significantly affects the interannual variability, especially when the seasonal cycle of the attenuation depth is used. The seasonal cycle of the attenuation depth interacts with the low-frequency equatorial dynamics to enhance warm and cold anomalies, which are further amplified via positive air–sea feedbacks. These results also indicate that interannual variability of the attenuation depths is required to capture the asymmetric biological feedbacks during cold and warm ENSO events.

scholarly journals Dominant Interannual Covariations of the East Asian–Australian Land Precipitation during Boreal Winter

2019 ◽  
Vol 32 (11) ◽  
pp. 3279-3296 ◽  
Author(s):  
Lin Liu ◽  
Jianping Guo ◽  
Wen Chen ◽  
Renguang Wu ◽  
Lin Wang ◽  
...  
Keyword(s):  
Indian Ocean ◽  
General Circulation ◽  
Southern Oscillation ◽  
East Asian ◽  
Circulation Model ◽  
Boreal Winter ◽  
Dipole Structure ◽  
Second Mode ◽  
Precipitation Anomalies ◽  
Sst Anomalies

AbstractThe present study applies the empirical orthogonal function (EOF) method to investigate the interannual covariations of East Asian–Australian land precipitation (EAALP) during boreal winter based on observational and reanalysis datasets. The first mode of EAALP variations is characterized by opposite-sign anomalies between East Asia (EA) and Australia (AUS). The second mode features an anomaly pattern over EA similar to the first mode, but with a southwest–northeast dipole structure over AUS. El Niño–Southern Oscillation (ENSO) is found to be a primary factor in modulating the interannual variations of land precipitation over EA and western AUS. By comparison, the Indian Ocean subtropical dipole mode (IOSD) plays an important role in the formation of precipitation anomalies over northeastern AUS, mainly through a zonal vertical circulation spanning from the southern Indian Ocean (SIO) to northern AUS. In addition, the ENSO-independent cold sea surface temperature (SST) anomalies in the western North Pacific (WNP) impact the formation of the second mode. Using the atmospheric general circulation model ECHAM5, three 40-yr numerical simulation experiments differing in specified SST forcings verify the impacts of the IOSD and WNP SST anomalies. Further composite analyses indicate that the dominant patterns of EAALP variability are largely determined by the out-of-phase and in-phase combinations of ENSO and IOSD. These results suggest that in addition to ENSO, IOSD should be considered as another crucial factor influencing the EAALP variability during the boreal winter, which has large implications for improved prediction of EAALP land precipitation on the interannual time scale.

scholarly journals Hindcasts of Tropical Atlantic SST Gradient and South American Precipitation: The Influences of the ENSO Forcing and the Atlantic Preconditioning

2009 ◽  
Vol 22 (9) ◽  
pp. 2405-2421 ◽  
Author(s):  
Huei-Ping Huang ◽  
Andrew W. Robertson ◽  
Yochanan Kushnir ◽  
Shiling Peng
Keyword(s):  
South America ◽  
General Circulation ◽  
Southern Oscillation ◽  
Boreal Winter ◽  
South American ◽  
Primary Role ◽  
Tropical Atlantic ◽  
Boreal Spring ◽  
Sst Anomaly ◽  
Sst Gradient

Abstract Hindcast experiments for the tropical Atlantic sea surface temperature (SST) gradient G1, defined as tropical North Atlantic SST anomaly minus tropical South Atlantic SST anomaly, are performed using an atmospheric general circulation model coupled to a mixed layer ocean over the Atlantic to quantify the contributions of the El Niño–Southern Oscillation (ENSO) forcing and the preconditioning in the Atlantic to G1 in boreal spring. The results confirm previous observational analyses that, in the years with a persistent ENSO SST anomaly from boreal winter to spring, the ENSO forcing plays a primary role in determining the tendency of G1 from winter to spring and the sign of G1 in late spring. In the hindcasts, the initial perturbations in Atlantic SST in boreal winter are found to generally persist beyond a season, leaving a secondary but nonnegligible contribution to the predicted Atlantic SST gradient in spring. For 1993/94, a neutral year with a large preexisting G1 in winter, the hindcast using the information of Atlantic preconditioning alone is found to reproduce the observed G1 in spring. The seasonal predictability in precipitation over South America is examined in the hindcast experiments. For the recent events that can be validated with high-quality observations, the hindcasts produced dryness in boreal spring 1983, wetness in spring 1996, and wetness in spring 1994 over northern Brazil that are qualitatively consistent with observations. An inclusion of the Atlantic preconditioning is found to help the prediction of South American rainfall in boreal spring. For the ENSO years, discrepancies remain between the hindcast and observed precipitation anomalies over northern and equatorial South America, an error that is partially attributed to the biased atmospheric response to ENSO forcing in the model. The hindcast of the 1993/94 neutral year does not suffer this error. It constitutes an intriguing example of useful seasonal forecast of G1 and South American rainfall anomalies without ENSO.

scholarly journals Modelling the wintertime response to upper tropospheric and lower stratospheric ozone anomalies over the North Atlantic and Europe

2003 ◽  
Vol 21 (10) ◽  
pp. 2107-2118 ◽  
Author(s):  
I. Kirchner ◽  
D. Peters
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Initial Conditions ◽  
Stratospheric Ozone ◽  
Circulation Model ◽  
Sensitivity Study ◽  
Boreal Winter ◽  
Radiative Processes ◽  
The North ◽  
The North Atlantic

Abstract. During boreal winter months, mean longitude-dependent ozone changes in the upper troposphere and lower stratosphere are mainly caused by different ozone transport by planetary waves. The response to radiative perturbation induced by these ozone changes near the tropopause on the circulation is unclear. This response is investigated with the ECHAM4 general circulation model in a sensitivity study. In the simulation two different mean January realizations of the ozone field are implemented in ECHAM4. Both ozone fields are estimated on the basis of the observed mean January planetary wave structure of the 1980s. The first field represents a 14-year average (reference, 1979–1992) and the second one represents the mean ozone field change (anomaly, 1988–92) in boreal extra-tropics during the end of the 1980s. The model runs were carried out pairwise, with identical initial conditions for both ozone fields. Five statistically independent experiments were performed, forced with the observed sea surface temperatures for the period 1988 to 1992. The results support the hypothesis that the zonally asymmetric ozone changes of the 80s triggered a systematic alteration of the circulation over the North Atlantic – European region. It is suggested that this feedback process is important for the understanding of the decadal coupling between troposphere and stratosphere, as well as between subtropics and extra-tropics in winter.Key words. Meteorology and atmospheric dynamics (general circulation; radiative processes; synoptic-scale meteorology)

scholarly journals An objective tropical Atlantic sea surface temperature gradient index for studies of south Amazon dry-season climate variability and change

2008 ◽  
Vol 363 (1498) ◽  
pp. 1761-1766 ◽  
Author(s):  
Peter Good ◽  
Jason A Lowe ◽  
Mat Collins ◽  
Wilfran Moufouma-Okia
Keyword(s):  
Climate Change ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
General Circulation ◽  
Southern Oscillation ◽  
Circulation Model ◽  
Dry Season ◽  
Sea Surface ◽  
Tropical Atlantic ◽  
Sst Gradient

Future changes in meridional sea surface temperature (SST) gradients in the tropical Atlantic could influence Amazon dry-season precipitation by shifting the patterns of moisture convergence and vertical motion. Unlike for the El Niño-Southern Oscillation, there are no standard indices for quantifying this gradient. Here we describe a method for identifying the SST gradient that is most closely associated with June–August precipitation over the south Amazon. We use an ensemble of atmospheric general circulation model (AGCM) integrations forced by observed SST from 1949 to 2005. A large number of tropical Atlantic SST gradient indices are generated randomly and temporal correlations are examined between these indices and June–August precipitation averaged over the Amazon Basin south of the equator. The indices correlating most strongly with June–August southern Amazon precipitation form a cluster of near-meridional orientation centred near the equator. The location of the southern component of the gradient is particularly well defined in a region off the Brazilian tropical coast, consistent with known physical mechanisms. The chosen index appears to capture much of the Atlantic SST influence on simulated southern Amazon dry-season precipitation, and is significantly correlated with observed southern Amazon precipitation. We examine the index in 36 different coupled atmosphere–ocean model projections of climate change under a simple compound 1% increase in CO 2 . Within the large spread of responses, we find a relationship between the projected trend in the index and the Amazon dry-season precipitation trends. Furthermore, the magnitude of the trend relationship is consistent with the inter-annual variability relationship found in the AGCM simulations. This suggests that the index would be of use in quantifying uncertainties in climate change in the region.

scholarly journals Influence of the Seasonal Cycle on the Termination of El Niño Events in a Coupled General Circulation Model

2006 ◽  
Vol 19 (9) ◽  
pp. 1850-1868 ◽  
Author(s):  
Matthieu Lengaigne ◽  
Jean-Philippe Boulanger ◽  
Christophe Menkes ◽  
Hilary Spencer
Keyword(s):  
El Niño ◽  
General Circulation ◽  
El Nino ◽  
Circulation Model ◽  
Coupled System ◽  
Equatorial Pacific ◽  
Boreal Winter ◽  
Coupled General Circulation Model ◽  
El Niño Events ◽  
El Nino Events

Abstract In this study, the mechanisms leading to the El Niño peak and demise are explored through a coupled general circulation model ensemble approach evaluated against observations. The results here suggest that the timing of the peak and demise for intense El Niño events is highly predictable as the evolution of the coupled system is strongly driven by a southward shift of the intense equatorial Pacific westerly anomalies during boreal winter. In fact, this systematic late-year shift drives an intense eastern Pacific thermocline shallowing, constraining a rapid El Niño demise in the following months. This wind shift results from a southward displacement in winter of the central Pacific warmest SSTs in response to the seasonal evolution of solar insolation. In contrast, the intensity of this seasonal feedback mechanism and its impact on the coupled system are significantly weaker in moderate El Niño events, resulting in a less pronounced thermocline shallowing. This shallowing transfers the coupled system into an unstable state in spring but is not sufficient to systematically constrain the equatorial Pacific evolution toward a rapid El Niño termination. However, for some moderate events, the occurrence of intense easterly wind anomalies in the eastern Pacific during that period initiate a rapid surge of cold SSTs leading to La Niña conditions. In other cases, weaker trade winds combined with a slightly deeper thermocline allow the coupled system to maintain a broad warm phase evolving through the entire spring and summer and a delayed El Niño demise, an evolution that is similar to the prolonged 1986/87 El Niño event. La Niña events also show a similar tendency to peak in boreal winter, with characteristics and mechanisms mainly symmetric to those described for moderate El Niño cases.

The Role of the Central Asian Mountains on the Midwinter Suppression of North Pacific Storminess

2010 ◽  
Vol 67 (11) ◽  
pp. 3706-3720 ◽  
Author(s):  
Hyo-Seok Park ◽  
John C. H. Chiang ◽  
Seok-Woo Son
Keyword(s):  
Energy Conversion ◽  
North Pacific ◽  
General Circulation ◽  
Wave Packets ◽  
Circulation Model ◽  
Boreal Winter ◽  
Stationary Waves ◽  
Central Asian ◽  
Baroclinic Energy Conversion

Abstract The role of the central Asian mountains on North Pacific storminess is examined using an atmospheric general circulation model by varying the height and the areas of the mountains. A series of model integrations show that the presence of the central Asian mountains suppresses the North Pacific storminess by 20%–30% during boreal winter. Their impact on storminess is found to be small during other seasons. The mountains amplify stationary waves and effectively weaken the high-frequency transient eddy kinetic energy in boreal winter. Two main causes of the reduced storminess are diagnosed. First, the decrease in storminess appears to be associated with a weakening of downstream eddy development. The mountains disorganize the zonal coherency of wave packets and refract them more equatorward. As the zonal traveling distance of wave packets gets substantially shorter, downstream eddy development gets weaker. Second, the central Asian mountains suppress the global baroclinic energy conversion. The decreased baroclinic energy conversion, particularly over the eastern Eurasian continent, decreases the number of eddy disturbances entering into the western North Pacific. The “barotropic governor” does not appear to explain the reduced storminess in our model simulations.

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