Factors Causing Stratocumulus to Deviate from Subtropical High Variability on Seasonal to Interannual Timescales

Ding, Hairu; Stevens, Bjorn; Schmidt, Hauke

doi:https://doi.org/10.5194/egusphere-2025-876

Preprints

https://doi.org/10.5194/egusphere-2025-876

Preprints

28 Feb 2025

| 28 Feb 2025

Factors Causing Stratocumulus to Deviate from Subtropical High Variability on Seasonal to Interannual Timescales

Hairu Ding, Bjorn Stevens, and Hauke Schmidt

Abstract. Stratocumulus (Sc) covers the eastern flanks of maritime subtropical high pressure systems and exerts an influence on the global energy budget comparable to CO₂. Previous studies have identified the temperature difference between 700 hPa and the surface as the primary driver of Sc variability. However, the mechanistic linkages between subtropical highs and this critical temperature gradient, which defines lower tropospheric stability, remain unresolved. While subsidence modulates temperatures at 700 hPa and wind-driven cooling affects surface temperatures, the observed decoupling between subtropical high intensity and Sc fraction on seasonal to interannual timescales lacks a mechanical explanation. This study uses reanalysis data to test two hypothesized pathways linking the strength of the subtropical highs to the lower tropospheric stability. Results demonstrate that neither pathway dominates, as correlations between Sc-regime temperatures and subtropical high dynamics exhibit strong regional and temporal dependencies, indicating that correlation does not apply causation. Additionally, Sc-regime conditions do not systematically align with subtropical high variability, highlighting the need for further investigation into the dynamical processes governing temperatures in the lower troposphere.

Received: 25 Feb 2025 – Discussion started: 28 Feb 2025

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Hairu Ding, Bjorn Stevens, and Hauke Schmidt

Status: closed

RC1:
'Comment on egusphere-2025-876', Anonymous Referee #1, 24 Mar 2025

Please see attached a pdf file.

Citation: https://doi.org/10.5194/egusphere-2025-876-RC1
- AC1: 'Reply on RC1', Hairu Ding, 24 May 2025
  
  Dear reviewer,
  Thank you so much for your comments. Please find my responses in the attached PDF.
  
  Citation: https://doi.org/10.5194/egusphere-2025-876-AC1
RC2:
'Comment on egusphere-2025-876', Anonymous Referee #2, 31 Mar 2025

Review of "Factors causing stratocumulus to deviate

from subtropical high variability on seasonal to

interannual timescales"

by H. Ding, B. Stevens, and H. Schmidt
[A] Overall recommendation
This study presents statistical analyses of observational

data for maritime stratocumulus cloud fraction and

atmospheric conditions. The study discusses whether the

cloud fraction is controlled by the atmospheric

conditions, focusing on the strength of the subtropical

high pressure system.
The issue addressed in the paper is important.

Stratocumulus cloud plays a major role in the global

energy budget, and therefore, its changes can lead to

sizable impact on the global climate. This motivates

research on the factors controlling the stratocumulus

cloud.
This study forwards understanding of the issue by

rejecting two hypothesized processes by which the

subtropical highs may affect the stratocumulus cloud.

The authors also illustrate that correlation between

the stratocumulus regime temperatures and the

subtropical high dynamics exhibit strong regional and

temporal dependencies, which is a valuable piece of

information for those interested in the stratocumulus

cloud variation.
There are, however, some minor points to mention

regarding the presentation or interpretation of the

results as listed below. I would therefore recommend

accepting the manuscript for publication after minor

revision.

[B] Minor Comments
(1) P.3 L.65 "This paper uses the second version of

the ATSR-AATSR"
Readers may be interested in the characteristics of

this data set when compared with other observations

such as ISCCP or MODIS. If the authors could comment

on this, that will be helpful.
(2) P.5 L.109 "... it is fixed to be 500m"
It would be helpful to readers if the authors

explain why the LCL is fixed to be 500m, rather

than being estimated.
(3) P.5 Figure 2 caption "low cloud fraction (kappa)

from 2003 to 2014"
This appears inconsistent with the statement in

L.70, that low cloud fraction data for the period

from Jan 2003 to Dec 2011 is analyzed.
(4) P.6 L.143 "C_D=0.0015 the drag coefficient"
Readers might wonder why the drag coefficient is

assumed to be a constant value of 0.0015.

Additional clarification would be helpful.
(5) P.7 Table 1 caption "The dominant contribution

to EIS is denoted by a bold font"
It would be helpful to readers if the authors explain

in more detail what the dominant contribution means.

Does it mean that magnitude of the correlation or the

standard deviation at one pressure level is larger

than the magnitude at the other level?
(6) P.7 L.158 "explain most of the variability in EIS"
Readers might wonder if this statement is supported by

Table 1, because magnitude of the correlation is

smaller at 700hPa than at 1000hPa at NP and SI on

interannual timescale.

Additional clarification would be helpful.
(7) P.7 L.171 "this correlation does not imply causation"
Here the authors argue that the correlation does not

imply causation. Does this argument apply to both

seasonal cycle and interannual variability, or does it

apply to interannual variability only?

Additional clarification would be helpful.
(8) P.8 Figure 3
I suggest that the authors give definition of the primes

for EIS, Q_c, LHF_c, and W_E.
(9) P.8 L.187 "a clear relationship between Q_c and

omega_700,c"
It would be helpful if the authors write the pressure

level at which the Q_c is evaluated. Is it 700hPa?
(10) P.10 L.204 "Upwelling areas are restricted to the

coastal regions where the wind-stress curl is large"
In Figure 7(b), there appears to be no upwelling along

the equator in the Pacific and the Atlantic. Readers

might wonder how the SST cold tongue is maintained.

Additional clarification would be helpful.
(11) P.10 L.207 "variations in near-surface geopotential

gradients are not the primary driver of changes in

theta_1000"
It may be a good idea to show scatter plots for

variations in geopotential gradients and variations

in theta_1000, for both seasonal and interannual

timescales, so that readers can better follow the

argument.

[C] Typos
(1) P.6 L.133 "geopolitical"
geopotential?
(2) P.7 L.164 "SP"
SI?
(3) P.8 L.187 "theta_700"
omega_700?
(4) P.9 Figure 4 caption "omega_700(left)"
omega_700(top)?
(5) P.9 Figure 4 caption "Q(right)"
Q(bottom)?
(6) P.9 Figure 4 caption "Stratocumulus regions"
Subtropical high areas?
(7) P.12 Figure 8 caption "omega_700(top)"
omega_700(left)?
(8) P.12 Figure 8 caption "d(phi)/dx(bottom)"
d(phi)/dx(right)?

Citation: https://doi.org/10.5194/egusphere-2025-876-RC2
- AC2: 'Reply on RC2', Hairu Ding, 24 May 2025
  
  Dear reviewer,
  Thank you so much for your overall recommendation and comments. Please find my responses in the attached PDF.
  
  Citation: https://doi.org/10.5194/egusphere-2025-876-AC2

Status: closed

RC1:
'Comment on egusphere-2025-876', Anonymous Referee #1, 24 Mar 2025

Please see attached a pdf file.

Citation: https://doi.org/10.5194/egusphere-2025-876-RC1
- AC1: 'Reply on RC1', Hairu Ding, 24 May 2025
  
  Dear reviewer,
  Thank you so much for your comments. Please find my responses in the attached PDF.
  
  Citation: https://doi.org/10.5194/egusphere-2025-876-AC1
RC2:
'Comment on egusphere-2025-876', Anonymous Referee #2, 31 Mar 2025

Review of "Factors causing stratocumulus to deviate

from subtropical high variability on seasonal to

interannual timescales"

by H. Ding, B. Stevens, and H. Schmidt
[A] Overall recommendation
This study presents statistical analyses of observational

data for maritime stratocumulus cloud fraction and

atmospheric conditions. The study discusses whether the

cloud fraction is controlled by the atmospheric

conditions, focusing on the strength of the subtropical

high pressure system.
The issue addressed in the paper is important.

Stratocumulus cloud plays a major role in the global

energy budget, and therefore, its changes can lead to

sizable impact on the global climate. This motivates

research on the factors controlling the stratocumulus

cloud.
This study forwards understanding of the issue by

rejecting two hypothesized processes by which the

subtropical highs may affect the stratocumulus cloud.

The authors also illustrate that correlation between

the stratocumulus regime temperatures and the

subtropical high dynamics exhibit strong regional and

temporal dependencies, which is a valuable piece of

information for those interested in the stratocumulus

cloud variation.
There are, however, some minor points to mention

regarding the presentation or interpretation of the

results as listed below. I would therefore recommend

accepting the manuscript for publication after minor

revision.

[B] Minor Comments
(1) P.3 L.65 "This paper uses the second version of

the ATSR-AATSR"
Readers may be interested in the characteristics of

this data set when compared with other observations

such as ISCCP or MODIS. If the authors could comment

on this, that will be helpful.
(2) P.5 L.109 "... it is fixed to be 500m"
It would be helpful to readers if the authors

explain why the LCL is fixed to be 500m, rather

than being estimated.
(3) P.5 Figure 2 caption "low cloud fraction (kappa)

from 2003 to 2014"
This appears inconsistent with the statement in

L.70, that low cloud fraction data for the period

from Jan 2003 to Dec 2011 is analyzed.
(4) P.6 L.143 "C_D=0.0015 the drag coefficient"
Readers might wonder why the drag coefficient is

assumed to be a constant value of 0.0015.

Additional clarification would be helpful.
(5) P.7 Table 1 caption "The dominant contribution

to EIS is denoted by a bold font"
It would be helpful to readers if the authors explain

in more detail what the dominant contribution means.

Does it mean that magnitude of the correlation or the

standard deviation at one pressure level is larger

than the magnitude at the other level?
(6) P.7 L.158 "explain most of the variability in EIS"
Readers might wonder if this statement is supported by

Table 1, because magnitude of the correlation is

smaller at 700hPa than at 1000hPa at NP and SI on

interannual timescale.

Additional clarification would be helpful.
(7) P.7 L.171 "this correlation does not imply causation"
Here the authors argue that the correlation does not

imply causation. Does this argument apply to both

seasonal cycle and interannual variability, or does it

apply to interannual variability only?

Additional clarification would be helpful.
(8) P.8 Figure 3
I suggest that the authors give definition of the primes

for EIS, Q_c, LHF_c, and W_E.
(9) P.8 L.187 "a clear relationship between Q_c and

omega_700,c"
It would be helpful if the authors write the pressure

level at which the Q_c is evaluated. Is it 700hPa?
(10) P.10 L.204 "Upwelling areas are restricted to the

coastal regions where the wind-stress curl is large"
In Figure 7(b), there appears to be no upwelling along

the equator in the Pacific and the Atlantic. Readers

might wonder how the SST cold tongue is maintained.

Additional clarification would be helpful.
(11) P.10 L.207 "variations in near-surface geopotential

gradients are not the primary driver of changes in

theta_1000"
It may be a good idea to show scatter plots for

variations in geopotential gradients and variations

in theta_1000, for both seasonal and interannual

timescales, so that readers can better follow the

argument.

[C] Typos
(1) P.6 L.133 "geopolitical"
geopotential?
(2) P.7 L.164 "SP"
SI?
(3) P.8 L.187 "theta_700"
omega_700?
(4) P.9 Figure 4 caption "omega_700(left)"
omega_700(top)?
(5) P.9 Figure 4 caption "Q(right)"
Q(bottom)?
(6) P.9 Figure 4 caption "Stratocumulus regions"
Subtropical high areas?
(7) P.12 Figure 8 caption "omega_700(top)"
omega_700(left)?
(8) P.12 Figure 8 caption "d(phi)/dx(bottom)"
d(phi)/dx(right)?

Citation: https://doi.org/10.5194/egusphere-2025-876-RC2
- AC2: 'Reply on RC2', Hairu Ding, 24 May 2025
  
  Dear reviewer,
  Thank you so much for your overall recommendation and comments. Please find my responses in the attached PDF.
  
  Citation: https://doi.org/10.5194/egusphere-2025-876-AC2

Hairu Ding, Bjorn Stevens, and Hauke Schmidt

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Short summary

This study examines the physical link between subtropical highs and stratocumulus variability. Using reanalysis data, we test two proposed pathways—one at the surface and one in the free troposphere—but find that neither is a dominant mechanism for stratocumulus variability on seasonal and interannual timescales. These results challenge the assumed influence of subtropical highs on stratocumulus and highlight the need for further research into lower tropospheric stability dynamics.


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