Contributions of downstream baroclinic development to strong Southern Hemisphere cut‐off lows

Pinheiro, Henri Rossi; Hodges, Kevin I.; Gan, Manoel Alonso; Ferreira, Sérgio Henrique; Andrade, Kelen Martins

doi:10.1002/qj.4201

Cited by 11 publications

(24 citation statements)

References 98 publications

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“…8 above. This is quite different from what was found for COLs in the South African domain (Ndarana et al 2021b) and for strong COLs in the Southern Hemisphere (Pinheiro et al, 2021) A more detailed comparison of these two downstream development regions is presented in the next section.…”

Section: Baroclinic Conversion and Upper-level Downstream Developmentmentioning

confidence: 57%

“…Based on Orlanski and Sheldon (1995) for baroclinic waves in general and for the COL case in the South Atlantic Ocean/southern African sector (Ndarana et al, 2021b) and Southern Hemisphere in general (Pinheiro et al, 2021)…”

Section: Review Of Flow Processes and Downstream Development In Baroc...mentioning

confidence: 99%

“…Unlike previous studies on energetics of baroclinic waves (e.g. Orlanski and Katzfey, 1991;Orlanski and Sheldon, 1995;Lackmann et al, 1999;McLay and Martin, 2002;Decker and Martin, 2005;Danielson et al, 2006;Harr and Dea, 2009;Piva et al, 2010;Gan and Piva, 2013;Ndarana et al, 2021b;Pinheiro et al, 2021) who integrated the diagnostics vertically, we consider vertical variations of the processes associated with ridging high eddy energy exchange. The reason for this is that not only does the flow at the surface differ during the evolution of a ridge, there are also variations between types of ridges as shown in Ndarana et al (2022).…”

Section: Vertical Profiles Of the Energetics Diagnosticsmentioning

confidence: 99%

“…There are many instances when a ridging high occurs with an ordinary trough aloft. Whilst these complex relations still need to be quantified; the fact that COLs exhibit particular downstream development characteristics (Ndarana et al 2021b;Pinheiro et al 2021) means that the ridging highs associated with them may be characterised by the same type or structure of the evolution of the eddy kinetic energy. However, it is still an open question whether for the general case, that is, the case when a general trough is associated with the ridging high would behave as the COL or not.…”

Section: Introductionmentioning

confidence: 99%

“…[Adapted fromOrlanski and Sheldon (1995). It also draws from the findings ofNdarana et al (2021) andPinheiro et al (2021) for COLs in the South Atlantic Ocean/southern African sector and Southern Hemisphere, respectively. ]…”

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confidence: 99%

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Downstream development associated with two types of ridging South Atlantic Ocean anticyclones over South Africa

Ndarana

Lekoloane

Rammopo

et al. 2022

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Abstract. There are at least two types of ridging South Atlantic Ocean high pressure systems in the South African domain. Type-N events occur north of 40° S and Type-S occur south of this latitude line. This study shows that there is no evidence of surface downstream development in terms of the evolution of eddy kinetic energy and associated ageostrophic geopotential fluxes for both types of ridging high events. Rather, for these systems downstream development is an upper level process. The baroclinic waves associated with the ridging develop from baroclinic instability, by converting eddy available potential energy to eddy kinetic energy. The bulk of the conversion is located at the upstream end of the waves. The downstream trough, which is the part of the wave that influences upward motion over South Africa, develops from the transport of eddy kinetic energy across the trough axis by means of ageostrophic geopotential fluxes. These fluxes are stronger for Type-S events. The absence of downstream development at the surface and the presence of it aloft demonstrates that there are differences in the underlying dynamics in the evolutions of these systems in the vertical. The evolution of eddy kinetic energy associated with baroclinic waves that occur during the ridging events is different from what has been observed for cut-off low pressure systems in the South African domain.

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Section: Baroclinic Conversion and Upper-level Downstream Developmentmentioning

confidence: 57%

Section: Review Of Flow Processes and Downstream Development In Baroc...mentioning

confidence: 99%

Section: Vertical Profiles Of the Energetics Diagnosticsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

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Downstream development associated with two types of ridging South Atlantic Ocean anticyclones over South Africa

Ndarana

Lekoloane

Rammopo

et al. 2022

Preprint

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Are Cut-off Lows simulated better in CMIP6 compared to CMIP5?

et al. 2022

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This is the first study to show the global Cut-off Low (COL) activity in 46 models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) and Phase 6 (CMIP6). The COL historical simulations for the period 1979–2005 obtained from the CMIP5 and CMIP6 models and their ensembles are compared with the ERA5 reanalysis using an objective feature-tracking algorithm. The results show that the CMIP6 models simulate the spatial distribution of COLs more realistically than the CMIP5 models. Some improvements include reduced equatorward bias and underestimation over regions of high COL density. Reduced biases in CMIP6 are mainly attributed to the improved representation of the zonal wind due to the poleward shift of the subtropical jet streams. The CMIP5 models systematically underestimate the COL intensity as measured by the T42 vorticity at 250 hPa. In CMIP6, the intensity is still underestimated in summer, but overestimated in winter in part due to increased westerlies. The overestimation is enhanced by the finer spatial resolution models that identify more of the strong systems compared to coarser resolution models. Other aspects of COLs such as their temporal and lifetime distributions are modestly improved in CMIP6 compared to CMIP5. Finally, the predictive skill of climate models is evaluated using five variables and the Taylor diagram. We find that 15 out of the 20 (75%) best coupled models belong to CMIP6, and this highlights the overall improvement compared to its predecessor CMIP5. Despite this, the use of the multi-model ensemble average seems to be better in simulating COLs than individual models.

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Downstream development during ridging South Atlantic Ocean anticyclones

et al. 2023

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Ridging South Atlantic high pressure systems (ridging highs) are often accompanied by cut-off low (COL) pressure systems aloft, but may also occur without them, in which case a linear baroclinic wave would be observed propagating across the South African domain in the upper troposphere. Using 41 years of ERA-5 reanalysis data, this study documents differences between the characteristics of the prevailing dynamical processes and associated local eddy kinetic energy generation, its downstream transfer and dissipation during these two scenarios. The study shows that when COLs are present then baroclinic conversion is strong and it is confined east of the Greenwich Meridian, whereas it is located downstream of South Africa and it is much weaker, when ridging occurs without COLs. The differences in strength and locations of the baroclinic conversion are associated with the differing jet streak configurations between the two scenarios; which lead to Rossby wave breaking and the absence thereof when there are COLs and when ridging occurs without COLs, respectively. The presence of breaking during COLs leads to trans-ridge downstream development that facilitates energy transfer from the midlatitudes into the South African domain. When there are no COLs present, the trans-trough downstream development is stronger than it is across the upstream ridge. Barotropic conversion from eddy kinetic energy to mean kinetic energy occurs in the South African domain during COLs, but occurs much further downstream when there are no COLs during ridging highs. The difference in the characteristics identified in this study can be traced back to the differences in the potential vorticity anomaly structures, which are largely due to whether the waves break or do not during the evolution of ridging events in the South African domain.

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Contributions of downstream baroclinic development to strong Southern Hemisphere cut‐off lows

Cited by 11 publications

References 98 publications

Downstream development associated with two types of ridging South Atlantic Ocean anticyclones over South Africa

Downstream development associated with two types of ridging South Atlantic Ocean anticyclones over South Africa

Are Cut-off Lows simulated better in CMIP6 compared to CMIP5?

Downstream development during ridging South Atlantic Ocean anticyclones

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