| 09 Sep 2025
Drivers and Variability of Marine Heatwaves in the North Indian Ocean and their Impacts on South Asian Monsoon Rainfall
Abstract. Our planet is warming rapidly, and, with it, the frequency and intensity of marine heatwaves (MHWs) are increasing. While MHWs disrupt marine ecosystems, they also significantly influence regional climate systems, including the Asian monsoon. This study investigates the variability, drivers, and monsoon impacts of MHWs in the North Indian Ocean using detrended sea surface temperature anomalies from 1982 to 2024. An Empirical Orthogonal Function (EOF) analysis of MHW intensity reveals two leading modes. The first mode (PC1), explaining 22 % of the variance, shows widespread MHWs with stronger intensity in the Arabian Sea. It is associated with anomalously high pressure over the North Indian Ocean and low pressure in the Southern Hemisphere, which weakens monsoon winds, reduces evaporation and cloud cover, and increases shortwave radiation, thereby warming the upper ocean. The second mode (PC2), explaining 8 % of the variance, displays a dipole pattern, with MHWs in the Bay of Bengal and suppressed activity in the Arabian Sea during its positive phase, and the reverse during its negative phase. Large-scale climate modes modulate MHW development. El Niño combined with the transition phase of MISO (from break to active) triggers basin-wide MHWs (PC1), while La Niña during a similar MISO phase promotes PC2-like warming in the Bay of Bengal. These modes influence rainfall as well. PC1 and PC2+ are linked to wetter conditions in southern India and drier conditions in the north, while PC2- corresponds to widespread dryness. MHW termination can enhance rainfall through the revival of monsoon winds and heat release. These findings suggest potential feedback between MHWs and MISO, with implications for improved monsoon prediction under climate change.
Review of "Drivers and Variability of Marine Heatwaves in the North Indian Ocean and their Impacts on South Asian Monsoon Rainfall" by Joseph, Skliris et al
General comments:
This manuscript analyzes marine heat waves (MHWs) in the Indian Ocean, using SST anomalies to identify two dominant patterns: a basin-wide mode strongest in the Arabian Sea and an east-west dipole pattern. The authors discuss how these MHW patterns are modulated by large-scale climate phenomena like ENSO and the monsoon intraseasonal oscillation (MISO), and how the different MHW modes alter regional precipitation.
The goal of assessing how MHWs alter the Asian monsoon is worthwhile and interesting, and the method of identifying MHWs as anomalies seems a good approach for climate dynamics studies, compared to using an absolute definition that does not account for long-term warming trends. However, this manuscript essentially ignores the well-documented coupled air-sea interaction in the boreal summer intraseasonal oscillation that can explain most of the results presented in Figures 4-8. An existing set of theoretical, observational, and modeling studies published over the last decade or two provides substantial insight into the mechanisms by which air-sea interaction causes the SST, monsoon winds, and precipitating clouds to covary on time scales of weeks to months during boreal summer. For this reason, I have recommended rejection. Should the authors wish to further develop this work, I suggest they first undertake a thorough examination of the below papers and other works cited therein. A revised manuscript should also clearly identify how the methodology used here to explore anomalous MHWs is distinct from the large number of prior studies that have explored anomalous SST variations on intraseasonal time scales in the Indian Ocean.
Specific comments:
1) The spatial patterns of anomalies in SST, precipitation, wind, and surface fluxes shown in Figures 4, 5 and 7 essentially reproduce what is shown in Zhang et al 2018 (Role of North Indian Ocean Air–Sea Interaction in Summer Monsoon Intraseasonal Oscillation, DOI: 10.1175/JCLI-D-17-0691.1) and other prior work on intraseasonal SST variation in the Indian Ocean. For example, the SST/wind/precipitation patterns shown for PC1 in Fig 4 of this manuscript closely resemble the top row of Fig 4 in Zhang et al 2018, with the various surface flux quantities shown in Fig 5 of this manuscript looking very much like the top row of Fig 5 of Zhang et al 2018. The Zhang paper is only one in a fairly large series of papers documenting this sort of behavior on intraseasonal time scales, only a few of which I list below.
This strong overlap with the literature on boreal summer intraseasonal variability results from the choice of defining MHWs using low-passed (by a 31-day binomial filter) SST anomalies, essentially capturing the SST variations that are coupled to the boreal summer intraseasonal oscillation.
The authors acknowledge that their MHW patterns are associated with the MISO (which in turn has strong association with the BSISO, the boreal summer MJO, and general intraseasonal variability in the Indian Ocean), but the discussion in this manuscript is descriptive and often confuses association with causation. A much more thorough assessment of the coupled physics was presented in some of the past work, with Zhang et al 2018, for example, presenting a theoretical air-sea coupling model that demonstrates the period of the oscillation is expected to be proportional to the square root of the ocean mixed layer depth. Even the prior observational analyses have demonstrated that the precipitation and SST anomalies are in quadrature, enabling their coupling in northward propagation. That sort of important detail on the phase relationship was not demonstrated clearly here.
2) A separate issue is that I found the description of the methodology to be insufficient in many places, so that I was confused about what exactly was done. This began in a fairly minor way on line 90 when the authors described the initial processing of SST anomalies, stating that they used an 11-day moving window (presumably a moving average), which was "then smoothed using a 31-day binomial filter". I can probably figure out roughly what was done there, but the selection of MHW events from the PCs in section 4 was very confusing: in Section 2 they state that MHWs are identified when SSTs exceed the 90th percentile, but then do the authors really take an EOF of those MHWs and then select the 90th percentile of the first PC? In other words, they are taking the 90th percentile of the PCs obtained through an EOF analysis of events defined by taking the 90th percentile of SST? Throughout much of the discussion in Sections 4-6 I often lost track of whether the authors were discussing the patterns associated with the PCs or the 90th percentile of those PCs.
3) Much of the discussion of physical mechanisms claimed causation when only association was demonstrated. One brief example is on line 199-200 where the authors state that a weakened meridional pressure gradient causes a weakening of the zonal monsoon winds: this is just geostrophic balance, where neither the winds nor the pressure gradients are causative. The same statement is made on line 267. The authors are correct that some of the anomalies can be causal, such as reduced atmospheric convection and cloudiness leading to enhanced surface shortwave radiation and SST warming. However, phase relationships are important here, as I mentioned above, and the prior papers discussed above and cited below provide a more detailed and quantitative description of the air-sea coupling in these interactions
Technical comments:
I do not list technical corrections due to the major and significant nature of the issues described above.
References:
Kemball-Cook and Wang 2001, https://doi.org/10.1175/1520-0442(2001)014<2923:EWAASI>2.0.CO;2
Fu, X., Wang, B., & McCreary, J. P. (2003). Coupling between northward-propagating, intraseasonal oscillations and sea surface temperature in the Indian Ocean.
DeMott, C. A., Stan, C., & Randall, D. A. (2013). Northward propagation mechanisms of the boreal summer intraseasonal oscillation in the ERA-Interim and SP-CCSM. Journal of Climate, 26, 1973–1992
Fu, X., Wang, B., Waliser, D. E., & Tao, L. (2007). Impact of atmosphere–ocean coupling on the predictability of monsoon intraseasonal oscillations. Journal of the Atmospheric Sciences, 64, 157–174
Fu, X., & Wang, B. (2004). The boreal-summer intraseasonal oscillations simulated in a hybrid coupled atmosphere–ocean model. Monthly Weather Review, 132, 2628–2649