Recommendation: Major revision

The manuscript addresses an important topic and introduces an interesting analytical framework. The VUA results are compelling and the paper is clearly written. However, stronger justification is needed for the PRE interpretation, particularly regarding the inferred nutrient pathway, and the use of satellite derived surface chlorophyll observations as a biological indicator. Addressing these issues would substantially strengthen the manuscript.

General comments:

The paper focuses on determining the physical mechanisms of the impact of Marine heatwaves (MHW) on chlorophyll-a in the South China Sea using a statistical method that quantifies causality and suited for complex interactions, instead of simpler linear approaches. The results show that in addition to heats tress, the chlorophyll-a is also forced by regional physical pathways rather a dominant independent driver. This approach could be applied to other marginal seas affected by MHWs. The paper is well-written, and the methods are sound overall. I recommend minor revision. My main concern with the study is about addressing why the Chl-a responses to MHW were only looked at seasonally (see line by line comments for details). It is also unclear why some large-scale driver (like the WNPSH) connects to the summer MHWs. These points need to be addressed. My specific comments are below:

Specific comments:

Line 74: Do you mean two representative ecosystem regions?

Line 57: Was SURF method applied before to understand MHW or marine extremes drivers? Was it applied more widely in oceanography studies. Please provide references.

Line 95: What is the method used to determine MLD, using a temperature or density threshold? What’s the threshold?

Line 125: Add the definition for each MHW metric: frequence, duration ….

Line 129: Change analysi to analysis.

Section 2.3.2: Add what function package and programming language does the SURD method could be implemented from

Figure 2. Add boxes to indicate both regions PRE and VUA

Section 3.2: Why the Chl-a responses to MHW were only looked at seasonally while the MHW are computed on an annual basis. It’d interesting to examine the Chl-a anomalies for the whole period and not just per season.

Line 212-233: From “Cumulative SST .. .” To move to Methods.

Line 347: Please give a brief definition for the western Northern Pacific subtropical high and a justification / rationale on why it was thought to impact the summer MHWs (for example, is this a commonly reported driver for these events in the region?)

Sections 3 and 4: These sections should be merged and renamed Results & Discussion as the authors are presenting results in section but also comparing and discussing them relative to previous findings. Additionally former subsection 4.3 is mainly about the limitation of the SURD method (which fits for a discussion)