scholarly journals A simple coupled model of the wind-evaporation-SST feedback with a role for stability

2022 ◽  
pp. 1-33
Keyword(s):  
Surface Wind ◽  
Coupled Model ◽  
Coupled System ◽  
Surface Wind Speed ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Wes Feedback ◽  
The Tropics ◽  
Sst Gradient ◽  
Sst Anomalies

Abstract The wind-evaporation-SST (WES) feedback describes a coupled mechanism by which an anomalous meridional sea surface temperature (SST) gradient in the tropics evolves over time. As commonly posed, the (positive) WES feedback depends critically on the atmospheric response to SST anomalies being governed by a process akin to that argued by Lindzen and Nigam (1987), and omits an alternative process by which SST anomalies modulate surface wind speed through vertical momentum mixing as proposed by Wallace et al. (1989) and Hayes et al. (1989). A simple model is developed that captures the essential coupled dynamics of the WES feedback as commonly posed, while also allowing for momentum entrainment in response to evolving SST anomalies. The evolution of the coupled system depends strongly on which effects are enabled in the model. When both effects are accounted for in idealized cases near the equator, the initial anomalous meridional SST gradient grows over a time scale of a few months, but is damped within one year. The sign and magnitude of the WES feedback depend on latitude within the tropics and exhibit hemispheric asymmetry. When constrained by realistic profiles of prevailing zonal wind, the model predicts that the WES feedback near the equator is stronger during boreal winter, while the domain over which it is positive is broader during boreal summer, and that low-frequency climate variability can also modulate the strength and structure of the WES feedback. These insights may aid in the interpretation of coupled climate behavior in observations and more complex models.

Midlatitude Excitation of Tropical Variability in the Pacific: The Role of Thermodynamic Coupling and Seasonality*

2009 ◽  
Vol 22 (3) ◽  
pp. 518-534 ◽  
Author(s):  
Daniel J. Vimont ◽  
Michael Alexander ◽  
Abigail Fontaine
Keyword(s):  
Heat Flux ◽  
Temporal Structure ◽  
Surface Wind ◽  
Coupled Model ◽  
Tropical Pacific ◽  
Boreal Winter ◽  
Wes Feedback ◽  
Thermodynamic Coupling ◽  
Sst Anomalies ◽  
The Tropical Pacific

Abstract A set of ensemble model experiments using the National Center for Atmospheric Research Community Atmospheric Model version 3.0 (CAM3) is run to investigate the tropical Pacific response to midlatitude atmospheric variability associated with the atmospheric North Pacific Oscillation (NPO). Heat flux anomalies associated with the NPO are used to force a set of model simulations during boreal winter (when the NPO is most energetic), after which the forcing is switched off and the coupled model evolves on its own. Sea surface temperature (SST) and wind anomalies continue to amplify in the tropical Pacific after the imposed forcing has been shut off, indicating that coupled ocean–atmosphere interactions in the tropical Pacific alter the spatial and temporal structure of variability associated with midlatitude forcing. The tropical circulation evolves through feedbacks between the surface wind, evaporation, and SST (the WES feedback), as well as through changes in the shortwave radiative heat flux (caused by changes in convection). Sensitivity experiments are run to investigate how thermodynamic coupling and seasonality affect the tropical response to NPO-related forcing. Seasonality is found to affect the WES feedback through (i) altering the sensitivity of surface evaporation to changes in the low-level wind field and (ii) altering the structure and strength of the lower-level wind response to SST anomalies. Thermodynamic coupling causes an equatorward and westward development of SST anomalies and an associated equatorward shift in the lower-level zonal wind anomalies.

scholarly journals Impact of Scatterometer Surface Wind Data in the ECMWF Coupled Assimilation System

2016 ◽  
Vol 144 (3) ◽  
pp. 1203-1217 ◽  
Author(s):  
Patrick Laloyaux ◽  
Jean-Noël Thépaut ◽  
Dick Dee
Keyword(s):  
Surface Wind ◽  
Coupled Model ◽  
Coupled System ◽  
Surface Wind Speed ◽  
Wind Data ◽  
Strong Interactions ◽  
Ocean Temperature ◽  
Atmospheric Observations ◽  
The Impact ◽  
Assimilation System

Abstract The European Centre for Medium-Range Weather Forecasts (ECMWF) has developed a coupled assimilation system that ingests simultaneously ocean and atmospheric observations in a coupled ocean–atmosphere model. Employing the coupled model constraint in the analysis implies that assimilation of an ocean observation has immediate impact on the atmospheric state estimate, and, conversely, assimilation of an atmospheric observation affects the ocean state. In this context, observing system experiments have been carried out withholding scatterometer surface wind data over the period September–November 2013. Impacts in the coupled assimilation system have been compared to the uncoupled approach used in ECMWF operations where atmospheric and ocean analyses are computed sequentially. The assimilation of scatterometer data has reduced the background surface wind root-mean-square error in the coupled and uncoupled assimilation systems by 3.7% and 2.5%, respectively. It has been found that the ocean temperature in the mixed layer is improved in the coupled system, while the impact is neutral in the uncoupled system. Further investigations have been conducted over a case of a tropical cyclone when strong interactions between atmospheric wind and ocean temperature occur. Cyclone Phailin in the Bay of Bengal has been selected since the conventional observing system has measured surface wind speed and ocean temperature at a high frequency. In this case study, the coupled assimilation system outperforms the uncoupled approach, being able to better use the scatterometer measurements to estimate the cold wake after the cyclone.

scholarly journals The Impact of Extratropical Atmospheric Variability on ENSO: Testing the Seasonal Footprinting Mechanism Using Coupled Model Experiments

2010 ◽  
Vol 23 (11) ◽  
pp. 2885-2901 ◽  
Author(s):  
Michael A. Alexander ◽  
Daniel J. Vimont ◽  
Ping Chang ◽  
James D. Scott
Keyword(s):  
Heat Flux ◽  
Coupled Model ◽  
Enso Event ◽  
Surface Heat ◽  
Kelvin Waves ◽  
Heat Fluxes ◽  
Atmospheric Variability ◽  
The Tropics ◽  
Sst Anomalies ◽  
The Tropical Pacific

Abstract Previous studies suggest that extratropical atmospheric variability influences the tropics via the seasonal footprinting mechanism (SFM), in which fluctuations in the North Pacific Oscillation (NPO) impact the ocean via surface heat fluxes during winter and the resulting springtime subtropical SST anomalies alter the atmosphere–ocean system over the tropics in the following summer, fall, and winter. Here, the authors test the SFM hypothesis by imposing NPO-related surface heat flux forcing in an atmospheric GCM coupled to a reduced gravity ocean model in the tropics and a slab ocean in the extratropics. The forcing is only imposed through the first winter, and then the model is free to evolve through the following winter. The evolution of the coupled model response to the forcing is consistent with the SFM hypothesis: the NPO-driven surface fluxes cause positive SST anomalies to form in the central and eastern subtropics during winter; these anomalies propagate toward the equator along with westerly wind anomalies during spring, reach the equator in summer, and then amplify, which leads to an ENSO event in the following winter. The anomalies reach the equator through a combination of thermodynamically coupled air–sea interactions, namely, the wind–evaporation–SST (WES) feedback and equatorial ocean dynamics. The initial off-equatorial anomaly propagates toward the equator through a relaxation of the climatological easterly winds south of the dominant SST anomalies, which leads to a reduction in upward latent heat flux. These westerly anomalies reach the equator during boreal summer, where they can excite downwelling equatorial Kelvin waves. The connection between off-equatorial variations and tropical ENSO-like conditions may also occur via the excitation of westward-propagating equatorial Rossby waves during spring, which reflect off of the western boundary as Kelvin waves, depressing the thermocline in the eastern Pacific during the following summer. NPO-related anomalies that form during the first winter in the tropical Pacific may also contribute to the development of an El Niño event in the following winter. The imposition of the NPO-related forcing caused warming in the ENSO region in ∼70% of the ensemble of 60 simulations; therefore, the response depends on the state of the tropical atmosphere–ocean system. For years where the control simulation was poised to develop into a neutral or negative ENSO event, the addition of the NPO heat fluxes tended to cause anomalous warming in the tropical Pacific in the following fall/winter; if the control was heading toward a warm ENSO event, the imposition of NPO forcing tends to reduce the amplitude of that event.

scholarly journals Atmospheric Response to Oceanic Cold Eddies West of Luzon in the Northern South China Sea

Atmosphere ◽  
2019 ◽  
Vol 10 (5) ◽  
pp. 255
Author(s):  
Haoya Liu ◽  
Shumin Chen ◽  
Weibiao Li ◽  
Rong Fang ◽  
Zhuo Li ◽  
...  
Keyword(s):  
Wind Speed ◽  
South China Sea ◽  
South China ◽  
Northern South China Sea ◽  
Surface Wind ◽  
Surface Wind Speed ◽  
Turbulent Heat ◽  
Sst Front ◽  
China Sea ◽  
Sst Anomalies

Using the compositing method, two kinds of sea surface temperature (SST) anomalies associated with mesoscale ocean eddies and their effects on the atmosphere over the northern South China Sea were investigated. We focused on Luzon cold eddies (LCEs), which form during the winter monsoon and occur repeatedly to the west of Luzon Island, where a SST front exists. Using satellite and reanalysis data, 20 LCEs from 2000–2016 were classified into two types according to their impact on the atmosphere. One type consisted of cold SST anomalies within the eddy interior; subsequent turbulent heat flux and surface wind speed decreased over the cold core, presenting a monopole pattern. The second type comprised SST anomalies on either side of the eddy, which mostly propagated along the SST front. For this type of LCEs, cyclonic eddy currents acting on the SST front led to the SST anomalies. They produced a dipole, with surface wind deceleration and acceleration over negative and positive SST anomalies, respectively, on either side of the eddy’s flank. Dynamically, for both types of LCE, a vertical mixing mechanism appeared to be responsible for the wind anomalies. Moreover, anomalous vertical circulations developed over the LCEs that extended over the whole boundary layer and penetrated into the free atmosphere, leading to an anomalous convective rain rate. Quantitatively, the surface wind speed changed linearly with SST; atmospheric anomalies related to LCEs explained 5%–14% of the total daily variance.

Wind Speed, Surface Flux, and Convection Coupling from CYGNSS Data

2021 ◽  
Author(s):  
Eric Maloney ◽  
Hien Bui ◽  
Emily Riley Dellaripa ◽  
Bohar Singh
Keyword(s):  
Heat Flux ◽  
Wind Speed ◽  
Latent Heat ◽  
Latent Heat Flux ◽  
Surface Wind ◽  
Surface Flux ◽  
Surface Wind Speed ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Boreal Summer

<p>This study analyzes wind speed and surface latent heat flux anomalies from the Cyclone Global Navigation Satellite System (CYGNSS), aiming to understand the physical mechanisms regulating intraseasonal convection, particularly associated with the Madden-Julian oscillation (MJO). The importance of wind-driven surface flux variability for supporting east Pacific diurnal convective disturbances during boreal summer is also examined. An advantage of CYGNSS compared to other space-based datasets is that its surface wind speed retrievals have reduced attenuation by precipitation, thus providing improved information about the importance of wind-induced surface fluxes for the maintenance of convection. Consistent with previous studies from buoys, CYGNSS shows that enhanced MJO precipitation is associated with enhanced wind speeds, and that associated surface heat fluxes anomalies have a magnitude about 7%-12% of precipitation anomalies. Thus, latent heat flux anomalies are an important maintenance mechanism for MJO convection through the column moist static energy budget. A composite analysis during boreal summer over the eastern north Pacific also supports the idea that wind-induced surface flux is important for MJO maintenance there. We also show the surface fluxes help moisten the atmosphere in advance of diurnal convective disturbances that propagate offshore from the Colombian Coast during boreal summer, helping to sustain such convection.  </p>

Predictability of extreme events in UFS at subseasonal time scale

2021 ◽  
Author(s):  
Christiana Stan
Keyword(s):  
Extreme Events ◽  
Time Scale ◽  
Initial Conditions ◽  
Coupled Model ◽  
Forecast Skill ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Forecast System ◽  
Cold Events ◽  
Second Category

<p>The predictability of extreme events over the continental United States (CONUS) in the Unified Forecast System (UFS) Couple Model is studied at subseasonal time scale. The benchmark runs of UFS (GFSv15), a coupled model consisting of atmospheric component (FV3GFS) with 28 km resolution and ocean (MOM6) and sea ice (CICE5) components with global 0.25° resolution, for the period April 2011–December 2017 have been assessed. The model’s month-long forecasts initiated on the first and fifteenth of each month are used to examine the predictability of extreme events in precipitation and 2m temperature. The atmospheric and ice initial conditions are from CFSR data, and the ocean initial conditions are from 3Dvar CPC. The errors in the week 1–4 predictions and the corresponding spatial correlation between model and observation over CONUS are presented. The differences in the predictability of the extreme events between the boreal summer and winter are discussed. Two categories of extreme events are evaluated: 95<sup>th</sup> and 99<sup>th</sup> percentile, respectively. The forecast skill of extreme events in the 95<sup>th</sup> percentile is higher than the forecast skill of events in the second category. The forecast skill of warm and cold events in the 95<sup>th</sup> percentile shows seasonal dependence and is higher during the boreal winter.</p>

scholarly journals Tropical Atlantic SST Prediction with Coupled Ocean–Atmosphere GCMs

2006 ◽  
Vol 19 (23) ◽  
pp. 6047-6061 ◽  
Author(s):  
Timothy N. Stockdale ◽  
Magdalena A. Balmaseda ◽  
Arthur Vidard
Keyword(s):  
Initial Conditions ◽  
Coupled Model ◽  
Boreal Summer ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
Seasonal Forecasts ◽  
Coupled Models ◽  
Equatorial Ocean ◽  
Sst Anomalies

Abstract Variations in tropical Atlantic SST are an important factor in seasonal forecasts in the region and beyond. An analysis is given of the capabilities of the latest generation of coupled GCM seasonal forecast systems to predict tropical Atlantic SST anomalies. Skill above that of persistence is demonstrated in both the northern tropical and equatorial Atlantic, but not farther south. The inability of the coupled models to correctly represent the mean seasonal cycle is a major problem in attempts to forecast equatorial SST anomalies in the boreal summer. Even when forced with observed SST, atmosphere models have significant failings in this area. The quality of ocean initial conditions for coupled model forecasts is also a cause for concern, and the adequacy of the near-equatorial ocean observing system is in doubt. A multimodel approach improves forecast skill only modestly, and large errors remain in the southern tropical Atlantic. There is still much scope for improving forecasts of tropical Atlantic SST.

scholarly journals Estimating the Role of SST in Atmospheric Surface Wind Variability over the Tropical Atlantic and Pacific

2019 ◽  
Vol 32 (13) ◽  
pp. 3899-3915 ◽  
Author(s):  
Ingo Richter ◽  
Takeshi Doi
Keyword(s):  
Surface Wind ◽  
Internal Variability ◽  
Early Summer ◽  
Boreal Summer ◽  
Minor Role ◽  
Boreal Winter ◽  
Tropical Atlantic ◽  
Equatorial Atlantic ◽  
Wind Variability ◽  
A Minor

Abstract The influence of sea surface temperature (SST) on interannual surface wind variability in the tropical Atlantic and Pacific is estimated using sensitivity experiments with the SINTEX-F GCM and the ensemble spread in a nine-member control simulation. Two additional estimates are derived for both SINTEX-F and the ERA-Interim reanalysis using regression analysis and singular value decomposition. All methods yield quite consistent estimates of the fraction of surface wind variability that is determined by SST and therefore potentially predictable. In the equatorial Atlantic, analysis suggests that for the period 1982–2014 approximately 2/3 of surface zonal wind variability in boreal spring and early summer is potentially predictable, while 1/3 is due to noise. Of the predictable component, up to about 35% may be driven from outside the tropical Atlantic, suggesting an important role for remote forcing and a diminished one for local feedbacks. In the northern tropical Atlantic, only 30% of boreal winter variability is predictable, most of which is forced from the Pacific. This suggests a minor role for local coupled air–sea feedbacks. For the equatorial Pacific, the results suggest high predictability throughout the year, most of which is due to local SST, with the tropical Atlantic only playing a minor role in boreal summer. In the tropical Atlantic, atmospheric internal variability is strongly dependent on the presence of deep convection, which, in turn, is related to mean SST. A similar, but weaker, state dependence of internal variability is evident in the tropical Pacific.

scholarly journals Evaluation and Projection of Near-Surface Wind Speed over China Based on CMIP6 Models

Atmosphere ◽  
2021 ◽  
Vol 12 (8) ◽  
pp. 1062
Author(s):  
Hao Deng ◽  
Wei Hua ◽  
Guangzhou Fan
Keyword(s):  
Wind Speed ◽  
Surface Wind ◽  
Early Period ◽  
Coupled Model ◽  
Surface Wind Speed ◽  
Dust Storms ◽  
Historical Period ◽  
Near Surface ◽  
Wind Energy Development ◽  
Winter Time

The characteristics of near-surface wind speed (NWS) are important to the study of dust storms, evapotranspiration, heavy rainfall, air pollution, and wind energy development. This study evaluated the performance of 30 models of the Coupled Model Intercomparison Project Phase 6 (CMIP6) through comparison with observational NWS data acquired in China during a historical period (1975–2014), and projected future changes in NWS under three scenarios (SSP1-2.6, SSP2-4.5, and SSP5-8.5) based on an optimal multi-model ensemble. Results showed that most models reproduced the spatial pattern of NWS for all seasons and the annual mean, although the models generally overestimated NWS magnitude. All models tended to underestimate the trends of decline of NWS for all seasons and the annual mean. On the basis of a comprehensive ranking index, the KIOST-ESM, CNRM-ESM2-1, HadGEM3-GC31-LL, CMCC-CM2-SR5, and KACE-1-0-G models were ranked as the five best-performing models. In the projections of future change, nationally averaged NWS for all months was weaker than in the historical period, and the trends decreased markedly under all the different scenarios except the winter time series under SSP2-4.5. Additionally, the projected NWS over most regions of China weakened in both the early period (2021–2060) and the later period (2061–2100).

scholarly journals Spatial and temporal variability of surface chlorophyll-a in the Gulf of Tomini, Sulawesi, Indonesia

2018 ◽  
Vol 19 (3) ◽  
pp. 793-801 ◽  
Author(s):  
QURNIA WULAN SARI ◽  
EKO SISWANTO ◽  
DEDI SETIABUDIDAYA ◽  
INDRA YUSTIAN ◽  
ISKHAQ ISKANDAR
Keyword(s):  
Chlorophyll A ◽  
El Niño ◽  
Temporal Variability ◽  
El Nino ◽  
Southern Oscillation ◽  
Surface Wind ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Spatial And Temporal Variability ◽  
Chl A

Sari QW, Siswanto E, Setiabudidaya D, Yustian I, Iskandar I. 2018. Spatial and temporal variability of surface chlorophyll-a in the Gulf of Tomini, Sulawesi, Indonesia. Biodiversitas 19: 793-801. The Gulf of Tomini (GoT) is mostly influenced by seasonal and interannual events. So, the immensive aim of this study is to explore spatial and temporal variations of chlorophyll-a (chl-a) and oceanographic parameters in the GoT under the influences of monsoonal winds, El Niño Southern Oscillation (ENSO), and Indian Ocean Dipole (IOD). The data were collected from the satellite imaging of chl-a and sea and surface temperature (SST) as well as surface wind from the reanalysis data for a period of January 2003 to December 2015. Monthly variations of the chl-a and SST in the GoT reveal chl-a bloom in the center part to the mouth of the GoT during the southeast monsoon season (boreal summer). The chl-a concentrations were relatively higher (>0.1 mg m-3) and distributed throughout most of the areas near the Maluku Sea. The SST in the middle of the GoT was relatively lower than that near the Maluku Sea (the eastern part of the GoT). On the other hand, during the northwest monsoon (boreal winter), the chl-a concentration decreased (<0.1 mg m-3). During this season, the SST was relatively higher (28-29 °C) than that during the boreal summer (27-26 °C) and distributed uniformly. Meanwhile, on interannual timescale, the ENSO and IOD play important role in regulating chl-a distribution in the GoT. High surface chl-a concentration was observed during El Niño and/or positive IOD events. Enhanced surface chl-a concentration during El Niño and/or positive IOD events was associated with the upward Ekman pumping induced by the southeasterly wind anomalies. The situation was reversed during the Niña and/or negative IOD events.

Sign in / Sign up

Export Citation Format

Share Document