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.

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 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 Thermodynamic feedback processes in a single-column sea-ice-ocean model

1997 ◽  
Vol 25 ◽  
pp. 327-332 ◽  
Author(s):  
Marika M. Holland ◽  
Julie L. Schramm ◽  
Judith A. Curry
Keyword(s):  
Heat Flux ◽  
Sea Ice ◽  
Surface Heat Flux ◽  
General Circulation ◽  
Accurate Determination ◽  
Circulation Model ◽  
Surface Heat ◽  
Physical Processes ◽  
Feedback Mechanisms ◽  
Albedo Feedback

Due to large uncertainties in many of the parameters used to model sea ice, it is possible that models with significantly different physical processes can be tuned to obtain realistic present-day simulations. However, in studies of climate change, it is the response of the model it various perturbations that is important, in studies response can be significantly different in sea-ice models that include or exclude various physical feedback mechanisms. Because simplifications in sea-ice physics are necessary for general circulation model experiments, it is important to assess which physical processes are essential for the accurate determination of the sensitivity of the ice pack to climate perturbations. We have attempted to address these issues using a new coupled ice-thickness distribution ocean mixed-layer model. The sensitivity of the model to surface heat-flux perturbations is examined and the importance of the ice ocean and ice-albedo feedback mechanisms in determining this sensitivity is analyzed. We find that the ice ocean and ice-albedo feedback processes are not mutually exclusive, and that they both significantly alter the model response to surface heat flux perturbations.

Westerly wind bursts in the tropical Pacific

Weather ◽  
1998 ◽  
Vol 53 (9) ◽  
pp. 282-284 ◽  
Author(s):  
Sarah Verbickas
Keyword(s):  
Tropical Pacific ◽  
Westerly Wind ◽  
Wind Bursts ◽  
Westerly Wind Bursts ◽  
The Tropical Pacific

scholarly journals Diagnosing Natural Variability of North Atlantic Water Masses in HadCM3

2005 ◽  
Vol 18 (12) ◽  
pp. 1925-1941 ◽  
Author(s):  
Keith Haines ◽  
Chris Old
Keyword(s):  
Mixed Layer ◽  
General Circulation ◽  
Circulation Model ◽  
Surface Heat ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Mode Water ◽  
Surface Heat Fluxes ◽  
Mode Waters ◽  
Winter Mixed Layer

Abstract A study of thermally driven water mass transformations over 100 yr in the ocean component of the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3) is presented. The processes of surface-forced transformations, subduction and mixing, both above and below the winter mixed layer base, are quantified. Subtropical Mode Waters are formed by surface heat fluxes and subducted at more or less the same rate. However, Labrador Seawater and Nordic Seawater classes (the other main subduction classes) are primarily formed by mixing within the mixed layer with very little formation directly from surface heat fluxes. The Subpolar Mode Water classes are dominated by net obduction of water back into the mixed layer from below. Subtropical Mode Water (18°C) variability shows a cycle of formation by surface fluxes, subduction ∼2 yr later, followed by mixing with warmer waters below the winter mixed layer base during the next 3 yr, and finally obduction back into the mixed layer at 21°C, ∼5 yr after the original formation. Surface transformation of Subpolar Mode Waters, ∼12°C, are led by surface transformations of warmer waters by up to 5 yr as water is transferred from the subtropical gyre. They are also led by obduction variability from below the mixed layer, by ∼2 yr. The variability of obduction in Subpolar Mode Waters also appears to be preceded, by 3–5 yr, by variability in subduction of Labrador Sea Waters at ∼6°C. This supports a mechanism in which southward-propagating Labrador seawater anomalies below the subpolar gyre can influence the upper water circulation and obduction into the mixed layer.

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