Crucial role of subsurface ocean variability in tropical cyclone genesis

Domain: earth_science
Published: 2025-01-26
Editorial outcome: minor_revision

Abstract
The upper ocean provides thermal energy to tropical cyclones. However, the impacts of the subsurface ocean on tropical cyclogenesis have been largely overlooked. Here, we show that the subsurface variabilities associated with the variation in the 26°C isothermal depth have pronounced impacts on tropical cyclogenesis over global oceans. The sea surface wind stress and its curl before tropical cyclogenesis are large enough to perturb the ocean interior down to more than one hundred meters due to entrainment and upwelling. The 26°C isothermal depth can fluctuate by tens of meters to significantly modify the upper ocean heat content. Consequently, sea surface temperature anomalies under nascent tropical cyclones are induced, and tropical cyclogenesis is modulated. Our results substantiate an unexpected relation between ocean interior variations and tropical cyclogenesis.

Introduction
Tropical cyclones (TCs), producing damaging winds, heavy rainfalls, storm surges, and inland floods, are among the deadliest natural disasters worldwide. During 1970–2019, TCs resulted in 779,324 fatalities and 1.4 trillion U.S. dollars in economic losses. As coastal cities continue to become more populated and developed, the need for TC risk assessment and management becomes increasingly critical. The locations and frequencies of TC genesis modulate the lifetime maximum intensities of TCs and exposure to hazards. Therefore, enhancing our understanding of TC genesis is crucial for reducing TC-related risks and devastation, which are expected to worsen with continued global warming.

TC genesis is determined by many environmental properties, such as the earth’s rotational effects, the atmospheric parameters of low-level relative vorticity, vertical wind shear, mid-level relative humidity, and atmospheric instability. Ocean is the energy source for TCs. The upper ocean heat content is also essential for TC genesis, which was recognized long ago. Based on observations, the “ocean thermal energy”, representing the ocean heat content above the depth of the 26°C isotherm (D26; Fig. 1a), was adopted as one of the six parameters in the first empirical model for TC genesis (i.e., the TC genesis potential index established in ref. 22). However, there has been disagreement on the influence of the upper ocean heat content on TC genesis, and numerical simulations have drawn different conclusions. The controversy has grown because satellite-observed sea surface temperatures (SSTs) do not capture the subsurface conditions. In contrast, the impacts of upper ocean heat content (or generally variabilities in the subsurface ocean) on TC intensity, rather than on TC genesis, have been well examined. SST was replaced with the inclusion of subsurface temperature information in the potential intensity index, yielding a better representation of TC intensity and improving its forecast. Nevertheless, SSTs are assumed to play a dominant role in TC genesis in most studies, which is different from the original idea.

The reason that the upper ocean heat content was largely overlooked in TC genesis may be the fallacious assumption that the “weak” winds during TC genesis could not induce pronounced variabilities in the ocean interior. Consequently, the likelihood of the subsurface ocean influence on TC genesis has generally been dismissed. Practically, the depth influenced by TC winds can serve as a proxy for the depth of cold water that is drawn up into the upper mixed layer and has impacts on ocean stratification, D26, and upper ocean heat content. In fact, in-situ subsurface mooring observations demonstrated that the immediate influence of Hurricane Frederic (1979) could extend from the surface down to as deep as 950 m, underscoring the importance of TC-subsurface ocean interactions. A moderate TC with a maximum wind speed of 20–40 m s−1 deepened the ocean mixed layer by 90 m and led to a strong Ekman upwelling velocity of 2 × 10−3 m s−1 (which was equivalent to a vertical displacement of 86 m in half a day). A recent study revealed the negative role of the subsurface ocean in TC genesis due to Ekman upwelling during El Niño, contrasting with the positive role of SSTs.

In this study, we show that variabilities in the subsurface ocean have detectable impacts on TC genesis by examining the genesis stages of 2032 global TCs over the past 25 years from 1998 to 2022. By definition, the maximum wind speed is less than 35 knots (18 m s−1; 1 knot ≈ 0.51 m s−1) during the pre-genesis stage. We show that this wind is strong enough to displace D26 and thus modify the upper ocean heat content. Consequently, SST variabilities due to TC’s entrainment mixing and the Ekman effect by wind stress curl beneath nascent TCs are prominent enough to influence TC genesis. Our findings highlight the role of the subsurface ocean in TC genesis. Considering the out-of-sync warming trends in the ocean surface and subsurface under climate change, it is essential to explore the impacts of the subsurface ocean on TC genesis and their evolution under future scenarios. These findings also demand additional attention to model accuracy in winds, coupled air-sea interactions, and particularly ocean model initialization to better represent variabilities in D26 and ocean heat content in a warming world.

Results
Significant disturbances in the ocean interior by pre-genesis TCs
According to the World Meteorological Organization, a TC is born when its intensity reaches 35 knots (18 m s−1) or, equivalently, T2.5 according to the Dvorak technique. A TC is considered in the pre-genesis stage (see Methods) when its intensity reaches 30 knots or T2.0 in the Dvorak technique. The first day of the pre-genesis stage is referred to as Day 0 and the negative (positive) days represent the days before (after) Day 0 hereafter. The locations and intensities of TCs are obtained from the best-track records (Methods). A total of 2032 pre-genesis TCs are identified from 1998 to 2022 (Fig. 1b).

Temperature profiles are obtained from the Global Ocean Reanalysis and Simulations (GLORYS) 12v1 product. D26 is used to quantify subsurface variabilities, specifically, the ocean heat content in the upper layer. The differences between the median D26 from Days −10 to −4 and D26 on Day 0 (denoted as ΔD26 hereafter; Methods) are shown in Fig. 2a. The positive ΔD26 is attributable to the vertical mixing (or so-called entrainment), which cools the surface water and warms the subsurface water. A negative ΔD26 is induced by Ekman upwelling, which cools both the surface and the subsurface water. It is also consistent with the results of ref. 42 that D26 can be both shallowed by TC-induced upwelling and deepened by TC-induced downwelling, which combines the effect of vertical mixing. A maximum D26 deepening of 53 m and a maximum D26 shoaling of 48 m are observed (Fig. 2a). The pre-genesis winds of 10% of the TCs induce at least a 6-m increase or 10-m decrease in D26 (Fig. 2b). The ratios of ΔD26 to the local standard deviation of the monthly D26 variabilities are shown in Fig. 2c. Thirty-two percent of pre-genesis TCs perturb D26 by more than 50% of monthly D26 variabilities (Fig. 2d). Specifically, 10% pre-genesis TCs-induced ΔD26 is larger than +50%, and 22% pre-genesis TCs-induced ΔD26 is smaller than −50%.

Dynamics of the deep penetration of wind forcing during pre-genesis of TCs
The wind speeds during the pre-genesis stage of a TC are much lower than those during its mature stage. Thus, it has traditionally been presumed that weak winds during pre-genesis cannot have significant impacts on the subsurface ocean. However, in nature, the wind speeds are already greater than 15 m s−1 during TC pre-genesis, which can be as much as Force 8 on the Beaufort scale. The associated sea state codes are usually 6–7, which means a probable wave height of approximately 6 m, and the maximum wave height can reach 7.5 m. Thus, the winds during TC pre-genesis are not weak at all. In addition, the wind stress curl is very efficient in driving the ocean via Ekman pumping and suction. The climatological near-surface wind stress curl is on the order of 10−7 kg m−2 s−2 (Supplementary Fig. 1a), and its standard deviation is on the order of 10−7 kg m−2 s−2 (Supplementary Fig. 1b). Such a wind stress curl leads to pronounced upwelling in the California Current system in the Pacific and off Northwest Africa in the Atlantic. In contrast, the wind stress curl during pre-genesis TCs is greater by one order or even two orders of magnitude (Supplementary Fig. 2), which is strong enough to drive surface-subsurface interactions and produce changes in SST.

Quantitatively, according to classical Ekman theory, the Ekman depth is given by D_E = 2A_z / f, where A_z is the vertical momentum diffusivity and f is the Coriolis parameter. There are few observations for A_z; hence, the Ekman depth is typically estimated by empirical formulas. The Ekman depths estimated with four widely applied empirical formulas (Supplementary Table 1) and D26 under each pre-genesis TC are shown in Fig. 3a. Before the TC genesis, D26 is mostly shallower than 121 m (black dots in Fig. 3a). In the deep tropics (5–10° latitude), the mean D26 is approximately 69 m. It increases towards the mid-latitudes (20–30° latitude), which is consistent with the spatial distribution of climatological D26. In the empirical formulas, D_E is inversely proportional to sin ϕ or |sin ϕ|, where ϕ is the latitude. Thus, the estimated D_E increases with the decrease in latitude. In the deep tropics, all estimated values of D_E are much deeper than those of D26. Beyond 10° of latitude around the equator, the estimated D_E are still deeper than D26. The ratios of D26 to D_E are mostly greater than 2 (Fig. 3b) regardless of the empirical formula used. Therefore, we show that the winds and the associated wind stress curl during TC pre-genesis, which were traditionally assumed to be weak, are in fact strong enough to significantly displace the local D26 in the ocean.

Feedback of subsurface variabilities onto TC genesis
Subsurface ocean perturbations (e.g., D26 variations) due to wind stress and wind stress curl during TC pre-genesis can significantly modify ocean temperatures, including local SSTs, which are essential for consequent TC growth and intensification. Similar to the definition of ΔD26, the differences between temperatures on Day 0 and the undisturbed temperatures (i.e., the median temperatures from Days −10 to −4) denote the changes during TC pre-genesis (Methods). The pre-genesis TCs modify the ocean potential temperature profiles down to approximately 200 m. Even at greater depths, their influence is still noticeable, though weaker (Fig. 4a and Supplementary Fig. 3). Moreover, TC-induced temperature changes reach deeper in the context of deep climatological D26 (Supplementary Fig. 4). The mixed layer is characterized by nearly uniform temperature, salinity, and density. The mixed layer depth is defined as the depth where the temperature is cooler than the temperature at 10 m depth by 0.2 °C, following ref. 49. The ocean temperature at the mixed layer depth determines the heat exchanges between the subsurface ocean and the upper ocean mixed layer. Therefore, their changes during TC pre-genesis (Fig. 4b) can have a direct impact on the ocean heat content in the upper ocean. As a result, SSTs are modified significantly (Fig. 4c). The statistically significant dependence of SST variabilities on D26 changes is shown in Fig. 5a. An increase in D26 leads to ocean warming below the surface due to Ekman pumping, while a decrease in D26 reduces ocean temperatures due to the upwelling of cool waters, and wind stresses are strong enough to entrain cold water into the mixed layer from the ocean interior even during TC pre-genesis. Meanwhile, in addition to the changes in D26, the propensity for SST changes is also subject to the background state of D26 (colour shading in Fig. 1b). When the local climatological D26 is shallow, the changes of SSTs are more sensitive to changes in D26 (Supplementary Fig. 5a). When the climatological D26 is deep, the warm water layer in the upper ocean is thick, and SSTs are not easily perturbed. Thus, the SSTs are typically less sensitive to D26 changes during the pre-genesis (Supplementary Fig. 5b). Therefore, both local D26 changes and climatological D26 are important to SST changes.

For mature TCs, sea surface cooling is known to have a significant negative feedback effect on TC intensification. A similar conclusion is found for pre-genesis TCs. As shown in Fig. 5b, there is a statistically significant positive correlation between SST changes and the 90th percentile of the TC intensity change rate from Day 0 to Day 3 (a proxy for quantifying TC genesis). The changes in SSTs during TC pre-genesis are determined using daily satellite data, Argo data, and hourly drifter data (see Methods). The sensitivity of TC genesis to SST changes is 8 knots day−1 °C−1 (Methods). Therefore, the SST changes due to D26 variations during TC pre-genesis have important influences on the increase in TC intensity. Furthermore, the role of D26 in TC genesis is also supported by the direct relationship between the changes in D26 and the rate of TC intensity change (Fig. 5c).

Methods
TC data
The 6-hourly TC best-track datasets are the revised Hurricane Database (HURDAT2) managed by the U.S. National Hurricane Center (NHC) for the North Atlantic, Northeast and North Central Pacific and the best-track archive provided by the Joint Typhoon Warning Center (JTWC) for the other basins. Both NHC and JTWC report TC intensities in terms of the 1-minute mean wind speeds.

We use the H*Wind analyses produced by the Hurricane Research Division of the U.S. National Oceanic and Atmospheric Administration (NOAA) to derive the surface wind stress curl for pre-genesis TCs. The H*Wind software produces a gridded analysis by interpolating and smoothing all available wind speed observations. H*Wind analyses provide more than 1500 snapshots of the TC surface wind field, mainly from 1995 to 2013, over the North Atlantic and North Pacific. The horizontal resolution of these snapshots is 6 km.

SST data
We use the daily Remote Sensing Systems (REMSS) SST data for 25 years from 1998 to 2022. The spatial resolution is 0.25° × 0.25°. The product is generated by microwave radiometer measurements and an optimum interpolation method, which is designed to depict the SSTs at a depth of approximately 1 m without the diurnal warming. Thermal infrared SST measurements have high spatial resolution (~9 km) but only work in cloud-free atmospheres. Passive microwave SST measurements have relatively low spatial resolution (~25 km) but are not affected by clouds or other atmospheric effects. Due to the dense cirrus clouds covering TCs, microwave SSTs have been widely used in TC-related studies. In December 1997, the Tropical Rainfall Measuring Mission Microwave Imager took measurements at 10.7 GHz, which provided the first access to high-quality microwave SST data. Hourly SST data are obtained from surface drifting buoys deployed by the Global Drifter Program (GDP) funded by NOAA. Ocean temperatures (including SSTs) are adopted from Argo profiles maintained by the international Argo program. Argo profiles are identified and downloaded using an R package called argoFloats. We compare the SSTs from drifter, Argo and microwave (Supplementary Fig. 9) and D26 from Argo and GLORYS12v1 (Supplementary Fig. 10), and they are highly consistent.

Daily ocean temperature data
Daily and global ocean environmental variables are obtained from the GLORYS12v1 product. The product is eddy-resolving with a horizontal resolution of 1/12°. There are 50 vertical levels. The GLORYS12v1 output is largely based on the current real-time global forecasting Copernicus Marine Environmental Monitoring Service (CMEMS) system. GLORYS12v1 is generated by the Nucleus for European Modelling of the Ocean (NEMO) ocean model, which is forced at the surface by the European Center for Medium-Range Weather Forecasts (ECMWF) ERA-Interim and then the ERA5 reanalysis for recent years. In addition, GLORYS12v1 jointly assimilates along-track altimeter data, satellite-based SST, and in-situ temperature and salinity vertical profiles.

Monthly ocean temperature data
Ocean Reanalysis System 5 (ORAS5) reanalysis monthly ocean temperatures with a horizontal resolution of 1/4° are obtained from ECMWF. The EN4.2.2 analysis monthly ocean temperatures with a 1° horizontal resolution are obtained from the Met Office Hadley Centre. The Institute of Atmospheric Physics version 4 (IAPv4) global ocean temperature gridded product at 1° horizontal resolution are obtained from the IAP.

Daily near-surface wind data
Daily near-surface winds are obtained from NCEP-National Center for Atmospheric Research (NCAR) Reanalysis 1 with a horizontal resolution of 2.5° × 2.5°.

Definition of pre-genesis TCs
The Dvorak technique based on cloud patterns in satellite imagery is the primary method and “gold standard” for estimating TC intensities. The Dvorak technique quantifies the TC intensity on a scale of 1–8 (at 0.5 intervals) called T-Numbers. For example, T2.0 corresponds to 30 knots, and T2.5 corresponds to 35 knots. The Dvorak technique yields estimates that are internally consistent but biased depending on the intensity. However, Dvorak technique-based estimates have almost zero bias and are as accurate as in-situ or airborne observations when the TC intensity is no more than 35 knots. Thirty-five knots are typically used as the threshold for TC genesis. The closest to and lower than the 35 knot threshold is 30 knots in the 6-hourly best-track datasets. Thus, the records that reach 30 knots for the first time are defined as pre-genesis TCs. Applying the criteria above, we identify 2209 pre-genesis TCs from 1998 to 2022. We use 2032 pre-genesis TCs after removing 177 cases since their genesis locations are less than 25 km away from land.

Quantile regression and bootstrapping
Quantile regression models are used to estimate the percentiles of the dependent variables, conditional on the values of the explanatory variables. Unlike linear regression models, estimating the standard errors of the coefficients or the p-values of a quantile regression model is difficult mathematically. Bootstrapping provides a good estimate of the standard error of any statistic by approximating the shape of the sampling distribution. In this study, bootstrapping with 10,000 replicates is used to estimate the p-values of quantile regressions.

TCs in the HighResMIP models
Based on 6-hourly model outputs, a tracking method named “TRACK” is used to identify simulated TCs in the hierarchy of HighResMIP models (Supplementary Table 2). No tuning of the detection parameters is used in the process of tracking (see ref. 61 for details). Here, we use a hierarchy of HighResMIP models based on the Hadley Centre Global Environment Model in the Global Coupled configuration 3.1 (HadGEM3-GC3.1).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Tropical cyclone is a strong disastrous synoptic phenomenon, and the impact factors for the tropical cyclone (TC) genesis are the foucs of attention of this field. This work shows that the subsurface variabilities associated with the variation of 26°C isothermal depth are important for TC genesis over the global oceans. The cooling of ocean temperature can reach to hundreads of meters by sea surface wind stress curl before tropical cyclogenesis, and the Ekman suction effect modifies the 26°C isothermal depth as well as the upper ocean heat content, and then the TC genesis. The logistics of this work are smooth and reasonable, the results are interesting and I only have a bit comments:\n1. The authors show the changes of D26 (Fig. 2) and temperature anomalies (Fig. 4) due to pre-genesis TC. In my view, it is consistent with the results of Zhang (2023) that D26 can be both shallowed by TC-induced upwelling and deepened by TC-induced downwelling, which composite with the effect of vertical mixing. Fig. 4a shows the cold anomaly that reached 800 m, as the effect of upwelling is greater than downwelling near TC track.\nReference: Zhang H 2023. Modulation of Upper Ocean Vertical Temperature Structure and Heat Content by a Fast-Moving Tropical Cyclone. Journal of Physical Oceanography, 53: 493-508.\n2. For the vertical variations of isotherms by TC-induced upwelling/downwelling, they are constitue of the Ekman pumping effect due wind stress curl (the net effect) and the near-inertial puming due to the relaxation of the water to the TC wind forcing (the oscillation part). However, Ekman pumping may be dominate for the pre-genesis TCs as the oscillation part mainly be evident in the TC relaxation stage. As this study mainly studies the Ekman puming part, such discussions merit to be added in discussion. What is more, the SST change is greater during slow-moving TC may because there is more mixing-upwelling interaction and entranment of cold water into mixed layer.\n3. The authors give the relationship between the change of D26 and change of SST (Fig. 5a), as well as the relationship between SST cooling and TC intensity change. As the manuscript emphasizes the role of subsurface temperature on TC, it may better to also give a direct relationship between change of D26 and rate of intensity change?\n4. Some sentences can be rewritten to make the flow better. For example, \"thus, feedbak from ... was assumed to be unlikely\" in page 4, \"When the local climatological ... with increasing D26\" in Page 8, \"the sea surface cooling under ... D26 changes\" in Page 8.", "reviewer_2": "This study introduces the hypothesis that subsurface ocean variability, more specifically the variability in the depth of the 26C isotherm (D26), plays an important role in tropical cyclone (TC) genesis. This is a new and interesting idea that is worth exploring further. While the idea has been developed in the right direction, significant work is still required to make this study publishable. There are some major science issues that need to be addressed to make the study robust. Because the main hypothesis is that a deeper D26 (which prevents significant sea surface cooling) facilitates TC genesis, one would expect to see a larger number of TC genesis cases over the western side of the ocean basins where climatological D26 attains peak values. But this is not case, as the study does not show a clear pattern of increase in the number of cases of TC genesis as we move in the westward direction over each ocean basin.\nAlso, the study needs an analysis of the dependence of the number of cases of TC genesis as a function of sea surface cooling or D26. The key factor that could increase the number of these cases is a reduction in sea surface cooling. The latter process is not just affected by D26, as strong salinity stratification also plays a role over the eastern side of both the tropical Atlantic Ocean and Pacific Ocean. Without considering this salinity stratification influence, we cannot explain TC genesis over these regions where D26 is shallow.\nSee below for more details on other issues.\nMajor Comments\nP5. “The positive ΔD26 is attributable to the “heat pump”38, 39, which deepens the isothermal layers and pumps heat from the atmosphere to the ocean interior.” This statement seems to imply that the heat pump is driven by Ekman pumping. However, heat is a flux driven by irreversible diabatic turbulent mixing that entrains warm OML water into the thermocline. Downwelling of isotherms (a process that cause positive values in ΔD26), due to horizontal current convergence, is an adiabatic process that redistributes mass (and temperature), and it has nothing to do with a ‘heat pump’. Change in horizontal position of ocean features can also lead to positive values in ΔD26; ocean features can move tens of kilometers over the 7-day interval of analysis (days -10 to -4) as to affect the values in ΔD26.\nP6-7. Concepts of linear Ekman theory (steady state; no background currents) should not be applied to incipient TCs, which are highly time dependent. Moreover, wind-driven upwelling can be highly non-linear over the areas of TC genesis analyzed in this study (Fig. 1), due to the presence of western boundary currents and mesoscale oceanic eddies that modify the curl of the wind stress. The climatological structure (steady state) of the curl of the wind stress should not be applied to incipient TCs either.\nP7. “The statistically significant dependence of SST variabilities on D26 changes is shown in Fig. 5a.” Based on this figure, the linear dependence of SST variability on D26 change seems to be weak. It looks like if outlier values were removed, the regression line would be essentially flat—no dependence on D26. Moreover, for any D26 change there is large spread in the SST anomaly (the latter can be either positive or negative). SST change is not just governed by the vertical structure of temperature over the upper ocean (D26). How about salinity stratification over the eastern tropical Atlantic Ocean and eastern tropical Pacific Ocean?\nOther Comments\nP3. Reference #21. Leipper and Volgenau (1972) introduced the concept of upper ocean heat content relative to the 26C isotherm, or tropical cyclone heat potential.\nP4. “The reason that the upper ocean heat content was largely overlooked for TC genesis may be the fallacious assumption that the “weak” winds during TC genesis could not induce pronounced variabilities in the ocean interior; thus, feedback from the subsurface ocean to TC genesis was assumed to be unlikely.” Is there a reference for this statement? Note that it is not just the high levels in OHC that can reduce sea surface cooling; how about strongly stratified warm water?\nP4. “In fact, subsurface mooring observations demonstrated that the immediate influence of Hurricane Frederic (1979) reached a depth of 950 m.” Why should we care about responses at these deeper levels? We do not see cold water from these depth levels being entrained or upwelled into the ocean mixed layer (OML). Should not the attention sbe focused on waters warmer than 26C, the threshold typically assumed for TC genesis and maintenance?\nP4. “Consequently, SST variabilities due to the Ekman effect are prominent beneath nascent TCs, and subsequent TC genesis is influenced.” It’s not just Ekman-like upwelling. How about instantaneous wind stirring or erosion?\nP4. “These findings also demand additional attention to model accuracy in winds and coupled air-sea interactions in a warming world.” How about accuracy in ocean model initialization, to better represent variability in D26 (topic of this study) and OHC?\nP5. “A negative ΔD26 is induced by “cold suction” due to Ekman upwelling38, 40, which brings cool water up to the ocean mixed layer and typically cools SSTs.” How about the contribution to ΔD26 from turbulent vertical entrainment of cooler thermocline water into the OML, and from change in horizontal position of ocean features?\nP5. “The ratios of ΔD26 to the local standard deviation of the monthly D26 variabilities are shown in Fig. 2c.” More needs to be said about what is the purpose of obtaining this ratio. It is not clear how the “standard deviation of the monthly D26 variabilities” was estimated.\nP6; and Extended Data (ED) Fig. 1. Nothing was said about what is the purpose of discussing the climatological near-surface relative vorticity of winds. Is there a value in this vorticity that can facilitate TC genesis? Do you see a link of this parameter to D26 or SST?\nP7. “The infant TCs modify the ocean potential temperature profiles down to approximately 800 m (Fig. 4a).” How do we know whether this temperature change is related or not to variability in background ocean features? Ocean features can move over the 7-day interval of analysis (days -10 to -4) as to affect the error bars in D26. Given the deepening in D26 in the westward direction in all ocean basins, should we expect a reduction in error bars in the westward direction? Showing just one mean profile prevents evaluating this critical variability in D26.\nP8. “However, the regression of SST changes against D26 changes for fast-moving TCs (> 6 m s-1) is slightly greater than that for slow-moving TCs (< 3 m s-1).” Do you have any hypothesis of why is so? Should we care about the marginal difference in slope in the regression analysis for the three groups of storm’s speed? It looks like if we were to conduct PDFs for the three cases, the distribution and data spread could be similar.\nP14. Caption of Fig. 2: Consider deleting “Significant”. The caption should describe the data sources for the analysis.\nP15. There is an inconsistency on how the ratio is defined. The label over the y-axis in Fig. 3b defines a ratio of Ekman depth to D26, but the caption defines a ratio of D26 to Ekman depth. Based on the magnitude of this ratio in Fig. 3b, the latter definition seems to be right.\nP16. Based on the hypothesis of this study, should not we expect a clear pattern in changes in OML temperature or SST (Fig. 4) in relation to climatological D26 (Fig. 1a)? That is, warmer OMLs/SSTs toward the west?\nP17. Fig. 5. Would not be more helpful to investigate the relationship between the rate of intensity change and SST change (supported by an analysis of the link between pre-storm SSTs, sea surface cooling, and D26)?\nP25. Figure caption. Need to clarify that sigma level 995 corresponds to the atmospheric side.\nP26. Need to describe how the TC translations speeds were estimated. Using the same x-axis and y-axis in the three panels would be helpful.\nP30. It would be interesting to look at the PDFs. Based on the hypothesis of this study, we would expect the distributions to be skewed toward higher values of D26; a normal distribution would not support the hypothesis. If we remove the outliers in Extended Data Fig. 6, it would be hard to see any linear increase in cyclogenesis number in relation to D26.\nP31. What is the purpose of this figure (D26 trend; ED Fig. 7)?\nP32. A similar analysis for D26, between Argo and Reanalysis profiles, would be helpful."}, "author_response": "Response to Reviewers\nWe greatly appreciate the insightful comments, and the very specific and valuable suggestions\nprovided by the two reviewers. The manuscript is revised accordingly, and we hope that the\nrevisions are satisfactory to the reviewers.\nReviewer #1 (Remarks to the Author):\nTropical cyclone is a strong disastrous synoptic phenomenon, and the impact factors for\nthe tropical cyclone (TC) genesis are the focus of attention of this field. This work shows that\nthe subsurface variabilities associated with the variation of 26°C isotherm depth are\nimportant for TC genesis over the global oceans. The cooling of ocean temperature can reach\nto hundreds of meters by sea surface wind stress curl before tropical cyclogenesis, and the\nEkman suction effect modifies the 26°C isothermal depth as well as the upper ocean heat\ncontent, and then the TC genesis. The logistics of this work are smooth and reasonable, the\nresults are interesting and I only have a bit comments:\nResponse: We thank the reviewer for the positive and insightful comments on our original\nmanuscript. We have taken the reviewer's specific comments into careful consideration and have\naddressed each of them in detail in the following paragraphs and in our revised manuscript.\n1. The authors show the changes of D26 (Fig. 2) and temperature anomalies (Fig. 4) due\nto pre-genesis TC. In my view, it is consistent with the results of Zhang (2023) that D26 can\nbe both shallowed by TC-induced upwelling and deepened by TC-induced downwelling,\nwhich composite with the effect of vertical mixing. Fig. 4a shows the cold anomaly that\nreached 800 m, as the effect of upwelling is greater than downwelling near TC track.\nReference: Zhang H 2023. Modulation of Upper Ocean Vertical Temperature Structure\nand Heat Content by a Fast-Moving Tropical Cyclone. Journal of Physical Oceanography,\n53: 493-508.\nResponse: We agree with the reviewer that Zhang (2023) reported consistent processes,\nwhich support our results. We thank the reviewer for providing the useful information. Zhang\n(2023) is cited in our revised manuscript as Ref.42 on Lines 97–99.\n2. For the vertical variations of isotherms by TC-induced upwelling/downwelling, they\nare constituted of the Ekman pumping effect due to wind stress curl (the net effect) and the\nnear-inertial pumping due to the relaxation of the water to the TC wind forcing (the\noscillation part). However, Ekman pumping may dominate for the pre-genesis TCs as the\noscillation part is mainly evident in the TC relaxation stage. As this study mainly studies the\nEkman pumping part, such discussions merit being added in discussion. What is more, the\nSST change is greater during slow-moving TCs, maybe because there is more mixing-\nupwelling interaction and entrainment of cold water into the mixed layer.\nResponse: We thank the reviewer for the insightful discussions on the physical mechanism\nhow TCs affect the ocean. In this study, we highlight the Ekman pumping because we focus on the\nocean’s response in the forced stage (before and on Day 0). In the relaxation stage (after Day 0),\nthe TC-induced near-inertial current corresponds to upwelling and downwelling, which are evident\nin the observations by buoys and moorings and also in numerical simulations (Zhang et al., 2016;\nZhang et al., 2018). We have added these into the discussion section of our revised manuscript to\nshow the full picture.\nAdditionally, we acknowledge the reviewer's point that the greater SST change during slow-\nmoving TCs may be attributed to strengthened mixing-upwelling interactions and the entrainment\nof cold water into the mixed layer. We have incorporated these points into our discussion to provide\na more comprehensive understanding of the mechanisms at play.\nReferences:\nZhang, H., Chen, D., Zhou, L., Liu, X., Ding, T., & Zhou, B. (2016). Upper ocean response\nto Typhoon Kalmaegi (2014). Journal of Geophysical Research: Oceans, 121(8), 6520–6535.\nhttps://doi.org/10.1002/2016jc012064\nZhang, H., Wu, R., Chen, D., Liu, X., He, H., Tang, Y., … & Zhang, W. (2018). Net\nmodulation of upper ocean thermal structure by Typhoon Kalmaegi (2014). Journal of Geophysical\nResearch: Oceans, 123(10), 7154–7171. https://doi.org/10.1029/2018jc014119\n3. The authors give the relationship between the change of D26 and change of SST (Fig.\n5a), as well as the relationship between SST cooling and TC intensity change. As the\nmanuscript emphasizes the role of subsurface temperature on TC, it may be better to also\ngive a direct relationship between change of D26 and rate of intensity change?\nResponse: The relationship between D26 anomalies and TC intensity changes is shown\nbelow in Fig. R1. Figure R1 has been included in our revised manuscript as Fig. 5c.\nFig. R1 | Relationship between the change of D26 and the rate of intensity change. D26 shallowing\nindicates that D26 decreases due to pre-genesis TCs.\n4. Some sentences can be rewritten to make the flow better. For example, \"thus,\nfeedback from ... was assumed to be unlikely\" in page 4, \"When the local climatological ...\nwith increasing D26\" in Page 8, \"the sea surface cooling under ... D26 changes\" in Page 8.\nResponse: We thank the reviewer for the suggestion to improve the flow of our original\nmanuscript. We have improved these sentences for clarity and better flow:\nOn Page 4, \"thus, feedback from the subsurface ocean to TC genesis was assumed to be\nunlikely.\" has been rewritten to \"consequently, the likelihood of the subsurface ocean influencing\nTC genesis was dismissed.\"\nOn Page 8, \"When the local climatological D26 is shallow, the SSTs are highly sensitive to\nchanges in D26, i.e., the SSTs more readily increase with increasing D26\" has been rewritten to \"\nWhen the local climatological D26 is shallow, the changes of SSTs are highly sensitive to changes\nin D26.\"\nOn Page 8, \"The sea surface cooling under different translation speeds of pre-genesis TCs\nshows different sensitivities to D26 changes\" has been removed in our revised manuscript due to\nlarge uncertainties of the analyses on TC translation speeds.\nReviewer #2 (Remarks to the Author):\nThis study introduces the hypothesis that subsurface ocean variability, more specifically\nthe variability in the depth of the 26C isotherm (D26), plays an important role in tropical\ncyclone (TC) genesis. This is a new and interesting idea that is worth exploring further. While\nthe idea has been developed in the right direction, significant work is still required to make\nthis study publishable. There are some major science issues that need to be addressed to\nmake the study robust. Because the main hypothesis is that a deeper D26 (which prevents\nsignificant sea surface cooling) facilitates TC genesis, one would expect to see a larger\nnumber of TC genesis cases over the western side of the ocean basins where climatological\nD26 attains peak values. But this is not case, as the study does not show a clear pattern of\nincrease in the number of cases of TC genesis as we move in the westward direction over each\nocean basin.\nResponse: We thank the reviewer for the recognition of our main idea. We also thank the\nreviewer for providing insightful and detailed comments to make our study more robust.\nThe reviewer is right that deep D26 prior to TCs facilitates TC genesis, which is not\ncounterintuitive but has been overlooked for a long time until now. Moreover, it is essential to\nconsider the changes of D26 in the studies of TC genesis because the changes of D26 will affect\nthe surface changes due to TCs. However, though D26 (or subsurface factor) is a contributor (as\nhighlighted in this research), it is not the sole contributor, since other factors (including vertical\nwind shear, mid-level relative humidity, and low-level wind vorticity) are also involved, that is\nwhy sometimes it is harder to see a simple linear relationship. The complexity stems from the fact\nthat there are many contributors to TC genesis and genesis is the joint consequence of all factors,\nrather than any single factor. In fact, not only D26, a direct relationship between TC genesis with\nother well-known contributors may not be directly seen as well (e.g., with vertical wind shear), as\nexplain further below.\nIn our study, we highlight the role of the subsurface ocean in TC genesis. However, it does\nnot deny the effects of other environmental factors (such as vertical wind shear) on TC genesis.\nMoreover, all these environmental factors including the subsurface ocean are regarded as necessary\nconditions instead of sufficient conditions. Thus, a deeper D26 alone does not guarantee more TCs.\nIt is clearly hard to explain the TC genesis using just one parameter and that is why we need TC\ngenesis potential index as a combination of many parameters. Consequently, we do not and should\nnot expect that the TC genesis pattern only follows the D26 pattern. Therefore, it is not a surprise\nthat there are differences between Fig. 1a and Fig. 1b, which is the reason why the reviewer asked\nthe question. In existing studies, the role of the subsurface ocean in TC genesis was generally\nneglected. We want to emphasise that we should pay more attention to the subsurface ocean in\nfuture studies focusing on TC genesis, but we should also avoid exaggerating the role of the\nsubsurface ocean.\nIn addition, we check the spatial distribution of vertical wind shear (Fig. R2), which is a well-\nknown critical factor for TC genesis. According to the above argument, since no single variable\ncan determine the TC genesis pattern, one can also see clear discrepancy between the distributions\nof vertical wind shear (colour shades in Fig. R2) and TC genesis (dots in Fig. R2).\nFinally, though D26 alone is not expected to explain every aspect of TC genesis. However,\nfor the zonal distribution of TC genesis, a consistence is indeed observed between the zonal\ndistributions of TC genesis numbers and D26 over the western North Pacific (Fig. R3).\nWe have addressed these points in our revised manuscript on Lines 192–194.\nFig. R2 | Climatological vertical wind shear and positions of pre-genesis TCs. Color shades denote\nthe climatological vertical wind shear from 1980 to 2022, which is defined as a change in wind\nvelocity between 850 hPa and 200 hPa. Positions of pre-genesis TCs are denoted with dots. They\nare the same as shown in Fig. 1b in our original manuscript. The positions where SSTs are observed\nby microwave satellites are denoted with black dots; the positions observed by the drifters are\ndenoted with purple dots; and those observed by the Argo floats are denoted with red dots.\nFig. R3 | Zonal distributions of TC genesis numbers and D26 over the western North Pacific. a,\nThe TC genesis cases are counted for every 5° longitude from 130°E to 180° during 1980–2022.\nHere we only consider the cases during the peak typhoon season (July to October). b, The\nmeridional mean D26 is calculated over the range of 5°N–20°N during 1980–2022. Also, we only\naverage the D26 during the peak typhoon season.\nAlso, the study needs an analysis of the dependence of the number of cases of TC genesis\nas a function of sea surface cooling or D26. The key factor that could increase the number of\nthese cases is a reduction in sea surface cooling. The latter process is not just affected by D26,\nas strong salinity stratification also plays a role over the eastern side of both the tropical\nAtlantic Ocean and Pacific Ocean. Without considering this salinity stratification influence,\nwe cannot explain TC genesis over these regions where D26 is shallow.\nResponse: We agree with the reviewer that salinity also matters for the interaction between\nTCs and the ocean in addition to temperature. Please see the revised Fig. 5b, c in the main text.\nSome studies have already demonstrated the significant impacts of salinity on TC intensification\n(e.g., Balaguru et al., 2012; Balaguru et al., 2016; Balaguru et al., 2020). We have conducted the\nstratification analyses as suggested by the reviewer in Fig. R4. In Fig. R4, we conducted 4 analyses,\ni.e., total stratification, temperature contribution only, salinity contribution only, and ratio between\ntemperature and salinity contributions). From the results, particularly in Fig. R4c, d, one can see\nthat as reviewer has pointed out, salinity’s contribution to stratification over the eastern North\nPacific is evident (i.e., ~ 30%, as detailed below). This salinity’s positive contribution is consistent\nwith the TC genesis over the Eastern North Pacific, since D26 over this region is shallow (Fig. 1b\nin main text). Thank you for the good suggestion.\nAlso, as Balaguru et al. (2012) have pointed out, in certain circumstances, salinity’s impact\non cooling suppression can be as much as 0.3°C. Thus, as in Fig. 5b of main text, salinity can also\npositively contribute to TC genesis via cooling suppression. We have also addressed these points\nin our revised manuscript on Lines 196–201.\nDetails of our analysis is explained below. Firstly, temperature and salinity have influences\non the upper ocean stratification, i.e., on D26. Therefore, changes in D26 are attributable to\nchanges in both temperature and salinity. We posit that a focus on D26 in fact includes\nconsiderations on salinity influence.\nQuantitatively, ocean stratification is represented by the square of Brunt-Väisälä frequency\n(N), which is defined as\n1𝜕𝜌 𝜕𝑇 𝜕𝑆\n𝑁2 = 𝑔(− ) = 𝑔(𝛼 −𝛽 )⋯(𝑅1),\n𝜌𝜕𝑧 𝜕𝑧 𝜕𝑧\nwhere g is the gravity acceleration; 𝜌 is the seawater density; 𝛼 is the thermal expansion\ncoefficient; β is the saline contraction coefficient. The term 𝑔𝛼𝜕𝑇/𝜕𝑧 denotes the contribution of\ntemperature gradient to ocean stratification and the term −𝑔𝛽𝜕𝑇/𝜕𝑧 denotes the contribution of\nsalinity gradient to ocean stratification. N2 is relatively strong in the eastern part of Pacific and\nAtlantic Oceans (Fig. R4a). Temperature and salinity stratifications are shown in Fig. R4b and Fig.\nR4c, respectively. The ratios of the latter to the former are shown in Fig. R4d.\nThe overall impacts on N2 from salinity are generally smaller than from temperature.\nNevertheless, over certain regions (e.g., eastern Pacific), salinity can contribute to ~ 30% of the\nimpacts from temperature, showing its non-deniable role. Nevertheless, the salinity effects are\nconsistent with previous findings that dynamic potential intensity (Balaguru et al., 2015) is mostly\ncontrolled by the temperature gradient (Fig 2a, b in Balaguru et al., 2016).\nIn our revised manuscript, more explicit discussion on salinity impacts is added, e.g., on Lines\n196–201.\nReferences:\nBalaguru, K., Chang, P., Saravanan, R., Leung, L. R., Xu, Z., Li, M., & Hsieh, J. S. (2012).\nOcean barrier layers’ effect on tropical cyclone intensification. Proceedings of the National\nAcademy of Sciences, 109(36), 14343–14347. https://doi.org/10.1073/pnas.1201364109\nBalaguru, K., Foltz, G. R., Leung, L. R., & Emanuel, K. (2016). Global warming-induced\nupper-ocean freshening and the intensification of super typhoons. Nature Communications, 7(1).\nhttps://doi.org/10.1038/ncomms13670\nBalaguru, K., Foltz, G. R., Leung, L. R., Kaplan, J., Xu, W., Reul, N., & Chapron, B. (2020).\nPronounced impact of salinity on rapidly intensifying tropical cyclones. Bulletin of the American\nMeteorological Society, 101(9), E1497–E1511. https://doi.org/10.1175/BAMS-D-19-0303.1\nBalaguru, K., Foltz, G. R., Leung, L. R., Asaro, E. D., Emanuel, K. A., Liu, H., & Zedler, S.\nE. (2015). Dynamic potential intensity: An improved representation of the ocean's impact on\ntropical cyclones. Geophysical Research Letters, 42(16), 6739–6746.\nhttps://doi.org/10.1002/2015GL064822\nFig. R4 | Climatological ocean stratification and its dependence on temperature and salinity. a,\n𝜕𝑇 𝜕𝑇\nClimatological N2; b, temperature stratification (𝑔𝛼 ); c, salinity stratification (−𝑔𝛽 ); d, the\n𝜕𝑧 𝜕𝑧\nratio of c to b. All variables are calculated from 1 m to 50 m depth and averaged between 1980\nand 2022.\nSee below for more details on other issues.\nMajor Comments\nP5. “The positive ΔD26 is attributable to the “heat pump”38, 39, which deepens the\nisothermal layers and pumps heat from the atmosphere to the ocean interior.” This statement\nseems to imply that the heat pump is driven by Ekman pumping. However, heat is a flux\ndriven by irreversible diabatic turbulent mixing that entrains warm OML water into the\nthermocline. Downwelling of isotherms (a process that cause positive values in ΔD26), due to\nhorizontal current convergence, is an adiabatic process that redistributes mass (and\ntemperature), and it has nothing to do with a ‘heat pump’. Change in horizontal position of\nocean features can also lead to positive values in ΔD26; ocean features can move tens of\nkilometers over the 7-day interval of analysis (days -10 to -4) as to affect the values in ΔD26.\nResponse: We thank the reviewer for the helpful comment. However, we did not imply that\nthe “heat pump” is achieved by Ekman pumping. Here, “heat pump” refers to the hypothesis\nproposed by Emanuel in 2001. “Heat pump” is achieved by the downward turbulent mixing of\nrelatively warm water into colder water in the subsurface ocean. In our revised manuscript, we\nhave replaced “heat pump” with “vertical mixing (or so-called entrainment)” for clarity.\nBecause our ΔD26 is defined as the difference between D26 of Day 0 minus D26 of Day – 4\nto –10 at a local region, the background states (i.e., whether there are ocean features or not) are\nmostly cancelled in ΔD26. However, this does not negate the general understanding that warm\nocean features (e.g. warm eddies) deepen the background D26, thus can positively contribute to\ngenesis due to cooling suppression. Certainly, it also does not negate the possibility, as the reviewer\npointed out, that if during the Day –10 to Day 0 period, there is an ocean feature (e.g. eddy) passing\nby, D26 or ΔD26 can be affected. Nevertheless, not all ocean basins have pronounced eddy\noccurrences (see Lin et al., 2005; Lin et al., 2008). Therefore, ocean eddy does not always pass\nwithin the Day –10 to Day 0 window or affect ΔD26. Thank you for pointing this out.\nReferences:\nEmanuel, K. (2001). Contribution of tropical cyclones to meridional heat transport by the\noceans. Journal of Geophysical Research: Atmospheres, 106(D14), 14771–14781.\nhttps://doi.org/10.1029/2000JD900641\nLin, I. I., Wu, C. C., Emanuel, K. A., Lee, I. H., Wu, C. R., & Pun, I. F. (2005). The interaction\nof Supertyphoon Maemi (2003) with a warm ocean eddy. Monthly Weather Review, 133(9), 2635–\n2649. https://doi.org/10.1175/MWR3005.1\nLin, I. I., Wu, C. C., Pun, I. F., & Ko, D. S. (2008). Upper-ocean thermal structure and the\nwestern North Pacific category 5 typhoons. Part I: Ocean features and the category 5 typhoons’\nintensification. Monthly Weather Review, 136(9), 3288–3306.\nhttps://doi.org/10.1175/2008MWR2277.1\nP6-7. Concepts of linear Ekman theory (steady state; no background currents) should\nnot be applied to incipient TCs, which are highly time dependent. Moreover, wind-driven\nupwelling can be highly non-linear over the areas of TC genesis analyzed in this study (Fig.\n1), due to the presence of western boundary currents and mesoscale oceanic eddies that\nmodify the curl of the wind stress. The climatological structure (steady state) of the curl of\nthe wind stress should not be applied to incipient TCs either.\nResponse: We thank the reviewer for raising this critical issue. We agree with the reviewer\nthat the classical Ekman theory is derived with ideal assumptions including a steady state, no\nbackground currents, and linearity.\nDespite the idealized assumptions of the classical Ekman theory, it is widely used to\nunderstand the wind-driven upper-ocean currents, including TC-driven ocean dynamics (e.g.,\nZhang et al., 2021; Wang et al., 2022). The upwelling induced by TCs has been well observed (e.g.,\nLin et al., 2003; Hu and Wang, 2016), which is consistent with Ekman processes forced by cyclonic\nwinds of TCs. In addition, one day should be enough to complete the oceanic adjustment by\nCoriolis acceleration (Talley et al., 2011) and thus Ekman theory is justifiable if the oceanic\nresponse lasts for more than one day. Our results demonstrate that the impacts of incipient TCs\nextend to four days before their arrival (not shown), which is consistent with Lloyd and Vecchi\n(2011).\nTo the best of our knowledge, there have been no exact time-dependent and nonlinear Ekman\nsolutions for TCs. Nevertheless, Lewis and Belcher (2004) extended the classic Ekman theory to\nthe time-dependent conditions, and the Ekman layer depth remains quantitatively the same as the\nclassical one. In addition, Pedlosky (2008) derived an analytical expression for the nonlinear\nEkman layer depth as,\n2𝐴\n𝐷∗ = 𝑧 ⋯(𝑅2),\n𝐸 √ 2 𝜕𝜏 𝜕𝜏 1 𝜕𝑢 𝜕𝑣\n𝑓− ( 𝑥 − 𝑦 )− ( 𝑔 − 𝑔 )\n𝜌𝑓𝐷 𝜕𝑦 𝜕𝑥 2 𝜕𝑦 𝜕𝑥\n𝐸\nwhere 𝐷∗ is the nonlinear Ekman layer depth; 𝐷 = √2𝐴 /𝑓 is the nondimensional value of linear\nE E 𝑧\nand classical Ekman layer depth; 𝜏 and 𝜏 are zonal and meridional wind stresses, respectively;\n𝑥 𝑦\n𝑢 and 𝑣 are zonal and meridional geostrophic currents, respectively; 𝜌 is the seawater density;\n𝑔 𝑔\n𝑓 is the Coriolis parameter; 𝐴 is the vertical momentum diffusivity. The ratio of 𝐷∗ to D is,\n𝑧 𝐸 E\n𝐷∗ 𝑓\n𝐸\n= ⋯(𝑅3).\n𝐷 √ 2 𝜕𝜏 𝜕𝜏 1 𝜕𝑢 𝜕𝑣\n𝐸 𝑓− ( 𝑥 − 𝑦 )− ( 𝑔 − 𝑔 )\n𝜌𝑓𝐷 𝜕𝑦 𝜕𝑥 2 𝜕𝑦 𝜕𝑥\n𝐸\nIn Eq. (R3), D applies the estimates used in the main text, the wind stresses (τ and τ ) and\nE x y\ngeostrophic currents (u and v ) are adopted for each TC genesis case. As a result, for all TC genesis\ng g\ncases, the ratio (𝐷∗/𝐷 ) is over 88%.\n𝐸 𝐸\nOverall, we agree with the reviewer that there is a caveat in applying the steady and linear\nclassical Ekman theory to TCs. However, using a time-dependent and nonlinear Ekman theory\nonly lead to quantitative differences, and it is not likely to change our main conclusions. In our\nrevised manuscript, the caveat about the classical theory is clarified and the estimates with time-\ndependent and nonlinear theories are included. Finally, global TC basins span a much larger area\nthan the western boundary current and eddy regions. Since western boundary currents and eddies\nare not present everywhere, their influence is confined to specific regions. Nevertheless, we\nbelieve this effect is plausible, though it may occur only under certain conditions and at specific\ntimes. We have also addressed these points in our revised manuscript on Lines 644–650.\nReferences:\nJaimes, B., & Shay, L. K. (2015). Enhanced wind-driven downwelling flow in warm oceanic\neddy features during the intensification of Tropical Cyclone Isaac (2012): Observations and theory.\nJournal of Physical Oceanography, 45(6), 1667–1689. https://doi.org/10.1175/JPO-D-14-0176.1\nZhang, H., He, H., Zhang, W., & Tian, D. (2021). Upper ocean response to tropical cyclones:\na review. Geoscience Letters, 8(1). https://doi.org/10.1186/s40562-020-00170-8\nWang, G., Wu, L., Mei, W., & Xie, S. (2022). Ocean currents show global intensification of\nweak tropical cyclones. Nature, 611(7936), 496–500. https://doi.org/10.1038/s41586-022-05326-\nevidence for enhanced ocean primary production triggered by tropical cyclone. Geophysical\nResearch Letters, 30(13). https://doi.org/10.1029/2003gl017141\nHu, J. and Wang, X. (2016). Progress on upwelling studies in the China seas. Reviews of\nGeophysics, 54(3), 653–673. https://doi.org/10.1002/2015rg000505\nTalley, L. D., G. L. Pickard, W. J. Emery, and J. H. Swift, 2011: Chapter 7 - Dynamical\nProcesses for Descriptive Ocean Circulation. Descriptive Physical Oceanography (Sixth Edition),\nL. D. Talley, G. L. Pickard, W. J. Emery, and J. H. Swift, Eds., Academic Press, 187–221.\nLloyd, I. D. and Vecchi, G. A. (2011). Observational evidence for oceanic controls on\nhurricane intensity. Journal of Climate, 24(4), 1138–1153. https://doi.org/10.1175/2010jcli3763.1\nLewis, D. M., & Belcher, S. E. (2004). Time-dependent, coupled, Ekman boundary layer\nsolutions incorporating Stokes drift. Dynamics of atmospheres and oceans, 37(4), 313–351.\nhttps://doi.org/10.1016/j.dynatmoce.2003.11.001\nPedlosky, J. (2008). On the weakly nonlinear Ekman layer: Thickness and flux. Journal of\nphysical oceanography, 38(6), 1334–1339. https://doi.org/10.1175/2007JPO3830.1\nFlierl, G., & McGillicuddy, D. J. (2002). Mesoscale and submesoscale physical-biological\ninteractions. The Sea, 12, 113–185.\nP7. “The statistically significant dependence of SST variabilities on D26 changes is\nshown in Fig. 5a.” Based on this figure, the linear dependence of SST variability on D26\nchange seems to be weak. It looks like if outlier values were removed, the regression line\nwould be essentially flat—no dependence on D26. Moreover, for any D26 change there is\nlarge spread in the SST anomaly (the latter can be either positive or negative). SST change\nis not just governed by the vertical structure of temperature over the upper ocean (D26).\nHow about salinity stratification over the eastern tropical Atlantic Ocean and eastern\ntropical Pacific Ocean?\nResponse: We verify the robustness of that relationship by limiting the range of D26 change.\nAs shown in Fig. R5, narrowing the range of D26 change does not violate the dependence of SST\nchange on D26 change. We have replaced Fig. 5a with Fig. R5 in our revised manuscript.\nAlthough Fig. 5a or Fig. R5 shows a strong correlation between D26 change and SST change,\nD26 change alone cannot fully explain SST change. As discussed above, ocean stratification also\nmodulates SST change and both temperature and salinity contribute to stratification, please see our\nearlier responses on salinity above.\nFig. R5 | Relationship between SST change and D26 change. The blue line is the same as the blue\nline in Fig. 5a in our original manuscript. The yellow line is for the linear regression with the range\nof D26 change from −25 m to 25 m. The red line is for the linear regression with the range of D26\nchange from −10 m to 10 m. The 95% points are enclosed by the purple dashed ellipse.\nOther Comments\nP3. Reference #21. Leipper and Volgenau (1972) introduced the concept of upper ocean\nheat content relative to the 26C isotherm, or tropical cyclone heat potential.\nResponse: We have included the work in our revised manuscript to acknowledge the\nfoundational contributions to this area of research.\nReference:\nLeipper, D. F., & Volgenau, D. (1972). Hurricane heat potential of the Gulf of Mexico. Journal\nof Physical Oceanography, 2(3), 218–224. https://doi.org/10.1175/1520-\n0485(1972)002<0218:HHPOTG>2.0.CO;2\nP4. “The reason that the upper ocean heat content was largely overlooked for TC genesis\nmay be the fallacious assumption that the “weak” winds during TC genesis could not induce\npronounced variabilities in the ocean interior; thus, feedback from the subsurface ocean to\nTC genesis was assumed to be unlikely.” Is there a reference for this statement? Note that it\nis not just the high levels in OHC that can reduce sea surface cooling; how about strongly\nstratified warm water?\nResponse: We thank the reviewer for his/her interest in the statement. There is no reference\nfor this statement. The statement partially comes from the discussions with some TC experts,\nincluding Philip Klotzbach (CSU) and Kerry Emanuel (MIT). We have acknowledged the\ncontributions from Philip Klotzbach and Kerry Emanuel in our revised manuscript.\nWe fully agree with the reviewer that strong stratification can also affect surface cooling.\nMore discussion on the influence of stratification and salinity is added in our revised manuscript.\nP4. “In fact, subsurface mooring observations demonstrated that the immediate influence of\nHurricane Frederic (1979) reached a depth of 950 m.” Why should we care about responses at\nthese deeper levels? We do not see cold water from these depth levels being entrained or\nupwelled into the ocean mixed layer (OML). Should not the attention be focused on waters\nwarmer than 26C, the threshold typically assumed for TC genesis and maintenance?\nResponse: This sentence is based on in-situ observational evidence, which demonstrates that\nTC’s impact can reach as deep as 950 m. It does not mean we care about 950 m, it simply highlights\nthat TC can interact from surface down to even 950 m, as evidenced by in-situ observation. The\ninteractions between TC and subsurface ocean are usually unconsciously overlooked by many\natmospheric scientists, thus we would like to highlight this by providing solid observational\nevidence. The depth influenced by TC winds serves as a proxy for the depth of cold water that may\nbe drawn up into the upper mixed layer, potentially impacting ocean stratification, D26, and UOHC.\nThis is clarified in our revised manuscript on Lines 62–65.\nP4. “Consequently, SST variabilities due to the Ekman effect are prominent beneath\nnascent TCs, and subsequent TC genesis is influenced.” It’s not just Ekman-like upwelling.\nHow about instantaneous wind stirring or erosion?\nResponse: We thank the reviewer for the helpful comment. We have replaced “heat pump”\nwith “vertical mixing (or so-called entrainment)” in our revised manuscript. Indeed, it is the\ninfluence from both entrainment mixing and upwelling, but not only from upwelling alone. Sorry\nfor not making this clear in our original manuscript and we have revised this accordingly.\nP4. “These findings also demand additional attention to model accuracy in winds and\ncoupled air-sea interactions in a warming world.” How about accuracy in ocean model\ninitialization, to better represent variability in D26 (topic of this study) and OHC?\nResponse: We thank the reviewer for pointing out the implications of our findings. A previous\nstudy has demonstrated the critical importance of correctly initializing vertical temperature and\nsalinity profiles to accurately simulate the magnitude and pattern of sea surface temperature\ncooling through a realistic free-running HYCOM simulation of the ocean response to Hurricane\nIvan. Inaccurate initialization of vertical temperature and salinity profiles degrades the ocean\nmixed layer evolution and SST cooling in the ocean model (Halliwell et al., 2008). We have\nmodified the original sentence to “These findings also demand additional attention to model\naccuracy in winds, coupled air-sea interactions, and particularly ocean model initialization to better\nrepresent variabilities in D26 and ocean heat content in a warming world.” in our revised\nmanuscript.\nReference:\nHalliwell, G. R., Shay, L. K., Jacob, S. D., Smedstad, O. M., & Uhlhorn, E. W. (2008).\nImproving ocean model initialization for coupled tropical cyclone forecast models using GODAE\nnowcasts. Monthly weather review, 136(7), 2576–2591. https://doi.org/10.1175/2007MWR2154.1\nP5. “A negative ΔD26 is induced by “cold suction” due to Ekman upwelling38, 40, which\nbrings cool water up to the ocean mixed layer and typically cools SSTs.” How about the\ncontribution to ΔD26 from turbulent vertical entrainment of cooler thermocline water into\nthe OML, and from change in horizontal position of ocean features?\nResponse: The reviewer is right that vertical entrainment is one of the mechanisms via which\nTCs affect the subsurface ocean. We have replaced “heat pump” with “vertical mixing (or so-called\nentrainment)” in our revised manuscript. Sorry about the confusion in our terminology. Please see\nearly response also.\nP5. “The ratios of ΔD26 to the local standard deviation of the monthly D26 variabilities\nare shown in Fig. 2c.” More needs to be said about what is the purpose of obtaining this ratio.\nIt is not clear how the “standard deviation of the monthly D26 variabilities” was estimated.\nResponse: The purpose of the ratios shown in Fig. 2c is to show the significance of ΔD26\nduring the TC pre-genesis stage. The standard deviation (STD) of monthly D26 is used as the\nreference to quantify the significance. For example, a ratio of 0.5 indicates that D26 change due to\npre-genesis TC winds is up to half of D26 seasonal change.\nHere is the way to obtain the “standard deviation of the monthly D26 variabilities”. Firstly,\nD26 is derived from monthly GLORYS12v1 product. Secondly, the interannual and decadal\nvariabilities are removed by applying a 1-year high-pass filter to the derived D26. Finally, for each\ngrid, the standard deviation of the monthly D26 variabilities is obtained by calculating the standard\ndeviation of the monthly D26 (from 1993 to 2022). We have included the details in our revised\nmanuscript.\nP6; and Extended Data (ED) Fig. 1. Nothing was said about what is the purpose of\ndiscussing the climatological near-surface relative vorticity of winds. Is there a value in this\nvorticity that can facilitate TC genesis? Do you see a link of this parameter to D26 or SST?\nResponse: The purpose of the Extended Data Fig. 1 was to provide the background of global\nnear-surface winds and pave the way to compare the winds of pre-genesis TCs and the normal\nconditions. The reviewer is correct that this figure has no close relation with the main story.\nTherefore, it is deleted in our revised manuscript for clarity.\nP7. “The infant TCs modify the ocean potential temperature profiles down to\napproximately 800 m (Fig. 4a).” How do we know whether this temperature change is related\nor not to variability in background ocean features? Ocean features can move over the 7-day\ninterval of analysis (days -10 to -4) as to affect the error bars in D26. Given the deepening in\nD26 in the westward direction in all ocean basins, should we expect a reduction in error bars\nin the westward direction? Showing just one mean profile prevents evaluating this critical\nvariability in D26.\nResponse: Following the reviewer’s suggestion, the standard error of D26 changes in the\nwestern North Pacific with respect to longitude is shown in Fig. R6. The contrast of the standard\nerror of D26 changes from the east to the west is clear, which supports the reviewer’s expectation\nof “a reduction in error bars in the westward direction”.\nIn our revised manuscript, more vertical profiles of temperature changes (Fig. R7) during pre-\nTC genesis are provided, in the context of different climatological D26. Being consistent with the\nreviewer’s expectation, TC-induced temperature changes reach deeper in the western direction,\nwhich is related to deeper climatological D26 in the west. Meanwhile, despite the background D26,\nin all longitudes, pre-genesis TC winds can force the ocean interior to deeper depths than the local\nD26, which implies that the pre-genesis TC winds can cause water exchanges from the surface to\nthe deeper ocean and influence the upper ocean heat content. We have included Fig. R7 as\nSupplementary Fig. 4 in our revised manuscript.\nFig. R6 | The standard error of D26 change over the western North Pacific. At each longitude, D26\nchanges caused by TCs at all latitudes are taken into account in the mean standard error.\nFig. R7 | Composite changes in ocean temperature profiles due to pre-genesis TCs in the western\nand the eastern North Pacific. Red (blue) dots and lines are for the western (eastern) North Pacific\nwith the composited mean D26 of 66 (49) m. The western North Pacific is within 0°–40°N and\n100°E–180°; the eastern Pacific is within 0°–40°N and 180°–100°W.\nP8. “However, the regression of SST changes against D26 changes for fast-moving TCs\n(> 6 m s-1) is slightly greater than that for slow-moving TCs (< 3 m s-1).” Do you have any\nhypothesis of why is so? Should we care about the marginal difference in slope in the\nregression analysis for the three groups of storm’s speed? It looks like if we were to conduct\nPDFs for the three cases, the distribution and data spread could be similar.\nResponse: The reviewer is right. The difference between the two slopes is not statistically\nsignificant. Thus, we have excluded the part in our revised manuscript. However, we expect that\nthe dependence of SST-D26 relation on TC’s translation speed is an interesting topic, which\ndeserve a dedicated study in the future. Thank you.\nP14. Caption of Fig. 2: Consider deleting “Significant”. The caption should describe the\ndata sources for the analysis.\nResponse: “Significant” has been deleted and data source (i.e., GLORYS12v1 daily product)\nhas been added in our revised manuscript.\nP15. There is an inconsistency on how the ratio is defined. The label over the y-axis in\nFig. 3b defines a ratio of Ekman depth to D26, but the caption defines a ratio of D26 to\nEkman depth. Based on the magnitude of this ratio in Fig. 3b, the latter definition seems to\nbe right.\nResponse: It should be the ratio of Ekman depth to D26. We have corrected it in our revised\nmanuscript.\nP16. Based on the hypothesis of this study, should not we expect a clear pattern in\nchanges in OML temperature or SST (Fig. 4) in relation to climatological D26 (Fig. 1a)? That\nis, warmer OMLs/SSTs toward the west?\nResponse: The reviewer is correct that the changes in OML temperature or SST partly depend\non climatological D26. However, the changes in SST also partly depend on D26 changes which\nare induced by TCs (including the pre-genesis TCs). Figure 5a in the manuscript shows a strong\npositive correlation between D26 changes and SST changes. Such correlations are subject to the\nbackground D26, i.e., SST changes are less (more) sensitive to D26 changes under a deep (shallow)\nclimatological D26 (Extended Data Fig. 4 in our original manuscript or Supplementary Fig. 5 in\nour revised manuscript). Therefore, both D26 changes and climatological D26 are important to\nSST changes. As a result, there is usually a nice match between changes in SST and climatological\nD26, but it is not always the case. More explanations are added in our revised manuscript, in order\nto avoid any misleading expectations.\nP17. Fig. 5. Would not be more helpful to investigate the relationship between the rate\nof intensity change and SST change (supported by an analysis of the link between pre-storm\nSSTs, sea surface cooling, and D26)?\nResponse: The relationship between SST change and D26 change is shown in Fig. 5a, and\nthe relationship between the rate of intensity change and sea surface cooling is shown in Fig. 5b.\nThe direct relationship between the change of D26 and the rate of intensity change is shown in Fig.\nR1 (also Fig. 5c in our revised manuscript). As shown in Fig. R8, with the increase of pre-SST, the\npre-SST tends to be cooled more. A significant and positive relationship between pre-D26 and pre-\nSST is observed (Fig. R9), which is mainly due to strong constraints of deep D26 on SST.\nFig. R8 | Relationship between pre-SST and change of SST.\nFig. R9 | Relationship between pre-D26 and pre-SST.\nP25. Figure caption. Need to clarify that sigma level 995 corresponds to the atmospheric\nside.\nResponse: We have modified “at sigma level 995” to “at sigma level 995 in atmosphere” in\nour revised manuscript.\nP26. Need to describe how the TC translations speeds were estimated. Using the same\nx-axis and y-axis in the three panels would be helpful.\nResponse: The TC translation speeds were estimated by the forward difference, i.e.,\n𝑑𝑖𝑠𝑡(𝑃 ,𝑃)\n𝑖+1 𝑖\n𝑡𝑠 = ⋯(𝑅4),\nΔ𝑡\nwhere ts is TC translation speed; dist is the function to calculate the geodesic distance between\ntwo points; P is the position of TCs 6 hours after pre-genesis (the first time to reach 30 knots);\ni+1\nP is the position of pre-genesis; Δt is the time interval between P and P, i.e., 6 hours. The\ni i+1 i\nforward difference, instead of backward difference or central difference, was used to avoid\ninvolving a missing record of 6 hours before pre-genesis.\nDue to large uncertainties of the analyses on TC translation speeds, the related parts have\nbeen removed in our revised manuscript.\nP30. It would be interesting to look at the PDFs. Based on the hypothesis of this study,\nwe would expect the distributions to be skewed toward higher values of D26; a normal\ndistribution would not support the hypothesis. If we remove the outliers in Extended Data\nFig. 6, it would be hard to see any linear increase in cyclogenesis number in relation to D26.\nResponse: We thank the reviewer for the insightful comments. We repeat the linear regression\nafter removing the first two groups and the last two groups (possible outliers) from Extended Data\nFig. 6. The new regressions are superimposed with red lines (Fig. R10). The red lines are almost\nthe same as the original regression lines in blue, supporting the robustness of our results. In our\nrevised manuscript, the original Extended Data Fig. 6 is replaced with Fig. R10 as Supplementary\nFig. 6.\nWe also check the PDF of TC genesis numbers with respect to D26. As shown in Fig. R11, in\nboth hemispheres, shallow D26 tends to support small TC genesis numbers, since shallow warm\nwater can limit the energy available to nourish TC genesis. In contrast, large TC genesis numbers\nskew to deep D26 due to plenty of ocean thermal energy to TC genesis. Fig. R11 is added as\nSupplementary Fig. 7 in our revised manuscript.\nFig. R10 | Dependence of tropical cyclogenesis on D26 in HighResMIP models. The data are\ndivided into groups with a D26 bin width of 2 m. Each group is shown by a box-plot. The blue\nlines with grey shades denote the linear regressions using all data. The red lines with grey shades\ndenote the linear regressions with the removal of the first two groups and the last two groups.\nFig. R11 | Probability density functions of D26 under various TC genesis conditions in\nHighResMIP models. a, The annual mean D26 from July to September over the tropics (0°–20°N)\nin the Northern Hemisphere is divided into three groups based on annual TC genesis number. The\nD26 probability functions of low (10–30), medium (30–50) and high (50–70) TC genesis number\ngroup are shown by blue, yellow and red, respectively. b, Same as a but from January to March\nover the tropics (20°S–0°) in the Southern Hemisphere.\nP31. What is the purpose of this figure (D26 trend; ED Fig. 7)?\nResponse: This figure indicates that there may be a bias in TC genesis projection due to the\nbias in D26 in the future scenarios. TC genesis projection is of global socioeconomic importance.\nCurrently, the main tool of TC projection is climate models, such as the outputs of CMIP6 and\nHighResMIP. In this study, we have highlighted the role of D26 in TC genesis. However, the\nensemble of 10 HighResMIP models could not reproduce the observed trend of D26 from 1980 to\n2022. The bias might persist in the future scenarios, which potentially leads to a bias in TC genesis\nprojection. We have elaborated the part in our revised manuscript.\nP32. A similar analysis for D26, between Argo and Reanalysis profiles, would be helpful.\nResponse: We thank the reviewer for the insightful comment. We have compared D26\nbetween Argo and GLORYS12v1 reanalysis. As shown in Fig. R12, there is a high consistency\nbetween Argo and GLORYS12v1 reanalysis for both median D26 from Days −10 to −4 and D26\non Day 0. We have included the comparison for D26 as Supplementary Fig. 10 in our revised\nmanuscript.\nIn-situ observations provide strong evidence to support our findings, further demonstrating\ntheir robustness and reliability. In addition, we have included Fig. R13, which is similar to Fig. 4a\nbut derived from Argo float data, as Supplementary Fig. 3 in our revised manuscript.\nFig. R12 | Comparisons of D26 from different sources. a, Comparisons of median D26 from Days\n−10 to −4 between GLORYS12v1 data and Argo data. b, Same as a but for D26 on Day 0. The\ncorrelation coefficients are shown in the upper left corners of each panel.\nFig. R13 | Changes in ocean temperature profiles due to pre-genesis TCs based on the Argo float\ndata. The black points denote the mean ocean temperature changes. Blue bars denote the\ncorresponding standard errors. Horizontal dotted lines denote the composite mean D26 (60 m)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have already revised the manuscript and given the responses in detail, I have no more questions.", "reviewer_2": "I am satisfied with the responses to my early comments. I have no further comments. Congratulations to the authors for this\ninteresting study."}, "author_response": ""}]

Decision letter