Modulation of Westerly Wind Bursts by Sea Surface Temperature: A Semistochastic Feedback for ENSO

2007 ◽  
Vol 64 (9) ◽  
pp. 3281-3295 ◽  
Author(s):  
Geoffrey Gebbie ◽  
Ian Eisenman ◽  
Andrew Wittenberg ◽  
Eli Tziperman
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Warm Pool ◽  
Detailed Knowledge ◽  
Westerly Wind ◽  
Stochastic Representation ◽  
Wind Bursts ◽  
Westerly Wind Bursts

Abstract Westerly wind bursts (WWBs) in the equatorial Pacific are known to play a significant role in the development of El Niño events. They have typically been treated as a purely stochastic external forcing of ENSO. Recent observations, however, show that WWB characteristics depend upon the large-scale SST field. The consequences of such a WWB modulation by SST are examined using an ocean general circulation model coupled to a statistical atmosphere model (i.e., a hybrid coupled model). An explicit WWB component is added to the model with guidance from a 23-yr observational record. The WWB parameterization scheme is constructed such that the likelihood of WWB occurrence increases as the western Pacific warm pool extends: a “semistochastic” formulation, which has both deterministic and stochastic elements. The location of the WWBs is parameterized to migrate with the edge of the warm pool. It is found that modulation of WWBs by SST strongly affects the characteristics of ENSO. In particular, coupled feedbacks between SST and WWBs may be sufficient to transfer the system from a damped regime to one with self-sustained oscillations. Modulated WWBs also play a role in the irregular timing of warm episodes and the asymmetry in the size of warm and cold events in this ENSO model. Parameterizing the modulation of WWBs by an increase of the linear air–sea coupling coefficient seems to miss important dynamical processes, and a purely stochastic representation of WWBs elicits only a weak ocean response. Based upon this evidence, it is proposed that WWBs may need to be treated as an internal part of the coupled ENSO system, and that the detailed knowledge of wind burst dynamics may be necessary to explain the characteristics of ENSO.

scholarly journals Essential Ingredients to the Dynamics of Westerly Wind Bursts

2019 ◽  
Vol 32 (17) ◽  
pp. 5549-5565 ◽  
Author(s):  
Minmin Fu ◽  
Eli Tziperman
Keyword(s):  
General Circulation ◽  
Circulation Model ◽  
Basic Mechanism ◽  
Surface Heat ◽  
Convective Heating ◽  
Short Time Scale ◽  
Westerly Wind ◽  
Atmospheric Phenomena ◽  
Wind Bursts ◽  
Westerly Wind Bursts

Abstract Westerly wind bursts (WWBs) are brief, anomalously westerly winds in the tropical Pacific that play a role in the dynamics of ENSO through their forcing of ocean Kelvin waves. They have been associated with atmospheric phenomena such as tropical cyclones, the MJO, and convectively coupled Rossby waves, yet their basic mechanism is not yet well understood. We study WWBs using an aquaplanet general circulation model, and find that eastward-propagating convective heating plays a key role in the generation of model WWBs, consistent with previous studies. Furthermore, wind-induced surface heat exchange (WISHE) acts on a short time scale of about two days to dramatically amplify the model WWB winds near the peak of the event. On the other hand, it is found that radiation feedbacks (i.e., changes in the net radiative anomalies accompanying westerly wind bursts) are not essential for the development of WWBs, and act as a weak negative feedback on WWBs and their associated convection. Similarly, sensible surface heat flux anomalies are not found to have an effect on the development of model WWBs.

scholarly journals Predictability of SST-Modulated Westerly Wind Bursts

2009 ◽  
Vol 22 (14) ◽  
pp. 3894-3909 ◽  
Author(s):  
Geoffrey Gebbie ◽  
Eli Tziperman
Keyword(s):  
Large Scale ◽  
Nonlinear Function ◽  
Warm Pool ◽  
Ensemble Prediction ◽  
Westerly Wind ◽  
Enso Dynamics ◽  
Pacific Warm Pool ◽  
Ensemble Mean ◽  
Wind Bursts ◽  
Westerly Wind Bursts

Abstract Westerly wind bursts (WWBs), a significant player in ENSO dynamics, are modeled using an observationally motivated statistical approach that relates the characteristics of WWBs to the large-scale sea surface temperature. Although the WWB wind stress at a given location may be a nonlinear function of SST, the characteristics of WWBs are well described as a linear function of SST. Over 50% of the interannual variance in the WWB likelihood, zonal location, duration, and fetch is explained by changes in SST. The model captures what is seen in a 17-yr record of satellite-derived winds: the eastward migration and increased occurrence of wind bursts as the western Pacific warm pool extends. The WWB model shows significant skill in predicting the interannual variability of the characteristics of WWBs, while the prediction skill of the WWB seasonal cycle is limited by the record length of available data. The novel formulation of the WWB model can be implemented in a stochastic or deterministic mode, where the deterministic mode predicts the ensemble-mean WWB characteristics. Therefore, the WWB model is especially appropriate for ensemble prediction experiments with existing ENSO models that are not capable of simulating realistic WWBs on their own. Should only the slowly varying component of WWBs be important for ENSO prediction, this WWB model allows a shortcut to directly compute the slowly varying ensemble-mean wind field without performing many realizations.

scholarly journals Westerly Wind Bursts: ENSO’s Tail Rather than the Dog?

2005 ◽  
Vol 18 (24) ◽  
pp. 5224-5238 ◽  
Author(s):  
Ian Eisenman ◽  
Lisan Yu ◽  
Eli Tziperman
Keyword(s):  
Large Scale ◽  
Warm Pool ◽  
Equatorial Pacific ◽  
Short Time Scale ◽  
Westerly Wind ◽  
Stochastic Forcing ◽  
Enso Events ◽  
Ocean Atmosphere ◽  
Wind Bursts ◽  
Westerly Wind Bursts

Abstract Westerly wind bursts (WWBs) in the equatorial Pacific occur during the development of most El Niño events and are believed to be a major factor in ENSO’s dynamics. Because of their short time scale, WWBs are normally considered part of a stochastic forcing of ENSO, completely external to the interannual ENSO variability. Recent observational studies, however, suggest that the occurrence and characteristics of WWBs may depend to some extent on the state of ENSO components, implying that WWBs, which force ENSO, are modulated by ENSO itself. Satellite and in situ observations are used here to show that WWBs are significantly more likely to occur when the warm pool is extended eastward. Based on these observations, WWBs are added to an intermediate complexity coupled ocean–atmosphere ENSO model. The representation of WWBs is idealized such that their occurrence is modulated by the warm pool extent. The resulting model run is compared with a run in which the WWBs are stochastically applied. The modulation of WWBs by ENSO results in an enhancement of the slow frequency component of the WWBs. This causes the amplitude of ENSO events forced by modulated WWBs to be twice as large as the amplitude of ENSO events forced by stochastic WWBs with the same amplitude and average frequency. Based on this result, it is suggested that the modulation of WWBs by the equatorial Pacific SST is a critical element of ENSO’s dynamics, and that WWBs should not be regarded as purely stochastic forcing. In the paradigm proposed here, WWBs are still an important aspect of ENSO’s dynamics, but they are treated as being partially stochastic and partially affected by the large-scale ENSO dynamics, rather than being completely external to ENSO. It is further shown that WWB modulation by the large-scale equatorial SST field is roughly equivalent to an increase in the ocean–atmosphere coupling strength, making the coupled equatorial Pacific effectively self-sustained.

scholarly journals Towards a regional ocean forecasting system for the IBI (Iberia-Biscay-Ireland area): developments and improvements within the ECOOP project framework

Ocean Science ◽  
2012 ◽  
Vol 8 (2) ◽  
pp. 143-159 ◽  
Author(s):  
S. Cailleau ◽  
J. Chanut ◽  
J.-M. Lellouche ◽  
B. Levier ◽  
C. Maraldi ◽  
...  
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Vertical Mixing ◽  
Circulation Model ◽  
Storm Surges ◽  
Ocean General Circulation Model ◽  
Tidal Mixing ◽  
Basin Scale ◽  
Forecasting System ◽  
Ocean Forecasting

Abstract. The regional ocean operational system remains a key element in downscaling from large scale (global or basin scale) systems to coastal ones. It enables the transition between systems in which the resolution and the resolved physics are quite different. Indeed, coastal applications need a system to predict local high frequency events (inferior to the day) such as storm surges, while deep sea applications need a system to predict large scale lower frequency ocean features. In the framework of the ECOOP project, a regional system for the Iberia-Biscay-Ireland area has been upgraded from an existing V0 version to a V2. This paper focuses on the improvements from the V1 system, for which the physics are close to a large scale basin system, to the V2 for which the physics are more adapted to shelf and coastal issues. Strong developments such as higher regional physics resolution in the NEMO Ocean General Circulation Model for tides, non linear free surface and adapted vertical mixing schemes among others have been implemented in the V2 version. Thus, regional thermal fronts due to tidal mixing now appear in the latest version solution and are quite well positioned. Moreover, simulation of the stratification in shelf areas is also improved in the V2.

scholarly journals Can Lagrangian Tracking Simulate Tracer Spreading in a High-Resolution Ocean General Circulation Model?

2019 ◽  
Vol 49 (5) ◽  
pp. 1141-1157 ◽  
Author(s):  
Patrick Wagner ◽  
Siren Rühs ◽  
Franziska U. Schwarzkopf ◽  
Inga Monika Koszalka ◽  
Arne Biastoch
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Circulation Model ◽  
Eddy Diffusivity ◽  
Ocean General Circulation Model ◽  
Horizontal Resolution ◽  
Ocean Model ◽  
Time Step ◽  
Lagrangian Particles ◽  
Cape Basin

AbstractTo model tracer spreading in the ocean, Lagrangian simulations in an offline framework are a practical and efficient alternative to solving the advective–diffusive tracer equations online. Differences in both approaches raise the question of whether both methods are comparable. Lagrangian simulations usually use model output averaged in time, and trajectories are not subject to parameterized subgrid diffusion, which is included in the advection–diffusion equations of ocean models. Previous studies focused on diffusivity estimates in idealized models but could show that both methods yield similar results as long as the deformations-scale dynamics are resolved and a sufficient amount of Lagrangian particles is used. This study compares spreading of an Eulerian tracer simulated online and a cloud of Lagrangian particles simulated offline with velocities from the same ocean model. We use a global, eddy-resolving ocean model featuring 1/20° horizontal resolution in the Agulhas region around South Africa. Tracer and particles were released at one time step in the Cape Basin and below the mixed layer and integrated for 3 years. Large-scale diagnostics, like mean pathways of floats and tracer, are almost identical and 1D horizontal distributions show no significant differences. Differences in vertical distributions, seen in a reduced vertical spreading and downward displacement of particles, are due to the combined effect of unresolved subdaily variability of the vertical velocities and the spatial variation of vertical diffusivity. This, in turn, has a small impact on the horizontal spreading behavior. The estimates of eddy diffusivity from particles and tracer yield comparable results of about 4000 m2 s−1 in the Cape Basin.

scholarly journals The GPU version of LASG/IAP Climate System Ocean Model version 3 (LICOM3) under the heterogeneous-compute interface for portability (HIP) framework and its large-scale application

2021 ◽  
Vol 14 (5) ◽  
pp. 2781-2799
Author(s):  
Pengfei Wang ◽  
Jinrong Jiang ◽  
Pengfei Lin ◽  
Mengrong Ding ◽  
Junlin Wei ◽  
...  
Keyword(s):  
General Circulation ◽  
Large Scale ◽  
Vertical Direction ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Ocean Model ◽  
Climate System ◽  
Global Ocean ◽  
Processing Unit ◽  
Model Version

Abstract. A high-resolution (1/20∘) global ocean general circulation model with graphics processing unit (GPU) code implementations is developed based on the LASG/IAP Climate System Ocean Model version 3 (LICOM3) under a heterogeneous-compute interface for portability (HIP) framework. The dynamic core and physics package of LICOM3 are both ported to the GPU, and three-dimensional parallelization (also partitioned in the vertical direction) is applied. The HIP version of LICOM3 (LICOM3-HIP) is 42 times faster than the same number of CPU cores when 384 AMD GPUs and CPU cores are used. LICOM3-HIP has excellent scalability; it can still obtain a speedup of more than 4 on 9216 GPUs compared to 384 GPUs. In this phase, we successfully performed a test of 1/20∘ LICOM3-HIP using 6550 nodes and 26 200 GPUs, and on a large scale, the model's speed was increased to approximately 2.72 simulated years per day (SYPD). By putting almost all the computation processes inside GPUs, the time cost of data transfer between CPUs and GPUs was reduced, resulting in high performance. Simultaneously, a 14-year spin-up integration following phase 2 of the Ocean Model Intercomparison Project (OMIP-2) protocol of surface forcing was performed, and preliminary results were evaluated. We found that the model results had little difference from the CPU version. Further comparison with observations and lower-resolution LICOM3 results suggests that the 1/20∘ LICOM3-HIP can reproduce the observations and produce many smaller-scale activities, such as submesoscale eddies and frontal-scale structures.

scholarly journals Enhancing Dynamical Seasonal Predictions through Objective Regionalization

2017 ◽  
Vol 56 (5) ◽  
pp. 1431-1442 ◽  
Author(s):  
Saleh Satti ◽  
Benjamin F. Zaitchik ◽  
Hamada S. Badr ◽  
Tsegaye Tadesse
Keyword(s):  
East Africa ◽  
General Circulation ◽  
Large Scale ◽  
Rainfall Variability ◽  
Circulation Model ◽  
Grid Cell ◽  
Ocean General Circulation Model ◽  
Seasonal Forecasts ◽  
Interannual Rainfall Variability ◽  
Local Grid

AbstractImproving seasonal forecasts in East Africa has great implications for food security and water resources planning in the region. Dynamically based seasonal forecast systems have much to contribute to this effort, as they have demonstrated ability to represent and, to some extent, predict large-scale atmospheric dynamics that drive interannual rainfall variability in East Africa. However, these global models often exhibit spatial biases in their placement of rainfall and rainfall anomalies within the region, which limits their direct applicability to forecast-based decision-making. This paper introduces a method that uses objective climate regionalization to improve the utility of dynamically based forecast-system predictions for East Africa. By breaking up the study area into regions that are homogenous in interannual precipitation variability, it is shown that models sometimes capture drivers of variability but misplace precipitation anomalies. These errors are evident in the pattern of homogenous regions in forecast systems relative to observation, indicating that forecasts can more meaningfully be applied at the scale of the analogous homogeneous climate region than as a direct forecast of the local grid cell. This regionalization approach was tested during the July–September (JAS) rain months, and results show an improvement in the predictions from version 4.5 of the Max Plank Institute for Meteorology’s atmosphere–ocean general circulation model (ECHAM4.5) for applicable areas of East Africa for the two test cases presented.

scholarly journals Westerly Wind Bursts and Their Relationship with Intraseasonal Variations and ENSO. Part II: Energetics over the Western and Central Pacific

2007 ◽  
Vol 135 (10) ◽  
pp. 3346-3361 ◽  
Author(s):  
Ayako Seiki ◽  
Yukari N. Takayabu
Keyword(s):  
El Niño ◽  
Large Scale ◽  
El Nino ◽  
Southern Oscillation ◽  
The Philippines ◽  
Westerly Wind ◽  
Central Pacific ◽  
Upper Troposphere ◽  
Wind Bursts ◽  
Westerly Wind Bursts

Abstract The mechanism of synoptic-scale eddy development in the generation of westerly wind bursts (WWBs) over the western–central Pacific, and their relationship with the El Niño–Southern Oscillation (ENSO) and the Madden–Julian oscillation (MJO), were examined. In the WWB occurrences, barotropic structures of equatorial eddy westerlies with cyclonic disturbances were found from the surface to the upper troposphere. The dominant contributions to substantial eddy kinetic energy (EKE) were the barotropic energy conversion (KmKe) in the lower and middle tropospheres and the conversion from eddy available potential energy (PeKe) in the upper troposphere. Low-frequency environmental westerlies centered near the equator preceded strong zonal convergence and meridional shear, resulting in the substantial KmKe. The activation of synoptic convection also contributed to an increase in EKE through PeKe. These energies were redistributed to the lower-equatorial troposphere through energy flux convergence (GKe). These results showed that environmental fields contribute to the EKE increase near the equator and are important factors in WWB occurrences. Next, eddy growth was compared under different phases of MJO and ENSO. The MJO westerly phases of strong MJO events were classified into two groups, in terms of ENSO phases. Higher EKE values were found over the equatorial central Pacific in the WWB–ENSO correlated (pre–El Niño) periods. The energetics during these periods comported with those of the WWB generations. In the uncorrelated periods, the enhancement of eddy disturbances occurred far from the equator near the Philippines, where the activities of the easterly wave disturbances are well known. It is noteworthy that the enhanced region of the disturbances in the pre–El Niño periods coincided with the vicinity of large-scale MJO convection. It is suggested that coincidence corresponds with an enhancement of the internal disturbances embedded in the MJO, which is found only when the environmental conditions are favorable in association with ENSO.

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