the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jun 2025
The multi-year negative Indian Ocean Dipole of 2021–2022
Abstract. The years 2021 and 2022 witnessed negative Indian Ocean Dipole (nIOD) conditions, with the 2022 event being the strongest on record. The dipole mode index was negative since the summer of 2021 and remained negative until early winter 2022, an unprecedented duration of 19 months. This makes it the first such occurrence of a multi-year nIOD. It co-existed with a triple-dip La Niña event during 2020–2022. In this study, we explore the dynamics behind the occurrence of this multi-year nIOD event. The tropical Indian Ocean (TIO) witnessed predominant westerly wind anomalies starting in the summer of 2021 and lasting till the end of 2022, with a record number and duration of westerly wind bursts (WWBs). The anomalous westerlies were supported by the background La Niña state and anomalous convection over the eastern TIO associated with tropical intra-seasonal oscillations. Occurrences of WWBs outside their preferred climatological months and strong westerly wind anomalies modulated the intensity of the zonal currents and the Wyrtki jets in the TIO. The associated heat and mass transfer caused the depression of the thermocline in the eastern TIO, resulting in the sustenance of nIOD conditions. Anomalous westerly wind activity in the TIO during the spring of 2022 served as a bridge between the two nIOD events and sustained it for a record duration. This multi-year nIOD event thus prevented the Indian summer monsoon rainfall from being in large excess, as the monsoon conducive modulation of the Walker circulation was counteracted by the anomalous subsidence over India by the nIOD-modulated regional Hadley circulation.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-2303', Anonymous Referee #1, 10 Jul 2025
- AC1: 'Reply on RC1', Ankur Srivastava, 27 Jul 2025
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RC2: 'Comment on egusphere-2025-2303', Anonymous Referee #2, 05 Aug 2025
The manuscript is solid study of the different contributions to the multi-year IOD event of 2021-2022 and a good timeline of how the 2021-22 event came to be. However, the manuscript's stated goal is to explore how unusual the characteristics of the 2021-22 IOD event are. Here, the manuscript fails to explore in greater detail the uniqueness of the 2012-22 period or its similarities to other multiyear IOD events in the record. Part of this is the record length. For the detailed part of the study, many of the datasets used are not long enough to get produce a detailed exploration of the extended IOD event and its relationship with the unusual multi-year La Niña event in the Pacific. However, even without all the detail, some of the reanalysis products used do have extended records that would allow for comparisons of other extended IOD events and the potential for co-occurrence of extended La Niña events. In addition to looking at canonical definitions of La Niña, I would recommend considering using the relative Niño Index (van Oldenborgh et al 2021), for a better understanding of ENSO state and climate change and La Niña in relationship to the rest of the tropics. Further, an expanded exploration of long lasting La Niña and IOD events would also provide more information on the importance of multi-year events for Indian Summer Monsoon Rainfall and how anomalous summer rainfall was during this event compared with other extended IOD or ENSO events as discussed in section 3.4.
Sources:
Van Oldenborgh GJ, Hendon H, Stockdale T, L’Heureux M, De Perez EC, Singh R, Van Aalst M. Defining El Niño indices in a warming climate. Environmental research letters. 2021 Mar 11;16(4):044003
Citation: https://doi.org/10.5194/egusphere-2025-2303-RC2 - AC2: 'Reply on RC2', Ankur Srivastava, 09 Aug 2025
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- 1
Main Comment #1
In this paper, the authors emphasize the role of westerly wind anomalies, particularly those in January 2021 and the frequent, long-lived westerly wind bursts (WWBs) from 2021-2022, in sustaining the multi-year nIOD event of 2021-2022. However, based on Figure 2, it seems that other nIOD events also exhibit periods of strong or frequent westerly wind activity before or during the events.
a) Are the WWBs during 2021-2022 significantly stronger, longer-lasting, or more frequent than those in other nIOD events? A more quantitative comparison on winds between the 2021-2022 event and other nIOD events would help support the claim that the WWB occurrence in 2021-2022 is unusual.
b) How do we understand nIOD events that developed following strong easterly winds, as seen in late 1997, for example.
c) This also relates to point (a). In line 130, the statement “each nIOD event has a unique evolution of zonal winds” suggests that wind patterns prior to nIOD events vary significantly. Does this imply that every nIOD event has a distinct formation mechanism? I think it would be helpful to clarify whether the 2021-2022 nIOD is a rare case due to specific conditions like long-lasting westerly wind bursts and Wyrtki Jets, or that nIOD events in general do not follow a common formation mechanism.
Main Comment #2
Lines 178–234 describe the proposed mechanisms and processes in detail, but some parts would benefit from clearer explanation or stronger supporting evidence.
For example, in lines 184-186, the statement that “this unusual occurrence of the WJ in January 2021 was due to the formation of a low-pressure system off the coast of Sri Lanka. This system led to widespread extreme rainfall over parts of Southern India” is interesting, but there is little evidence presented for the low-pressure system and its link to the WJ and rainfall.
Similarly, in lines 207-208, the claim that “the weakening of climatological south-easterlies along the Java-Sumatra coast during boreal summer reduced the evaporative cooling and maintained the positive SST anomaly” seems a bit speculative without enough supporting analysis. I would recommend either providing additional evidence or softening the language.
Minor Comments
Lines 24-29:
a) This sentence seems to imply that negative IOD events correspond to drier conditions in Indonesia and Australia. However, these regions typically experience *increased* rainfall during negative IOD events. I suggest revising this part for clarity.
b) The impacts of IOD events on Sri Lanka, South China, and Brazil are not clearly described. It would be helpful to be more specific about the impacts on these regions.
Lines 31-33:
a) Are the anomalies computed relative to a climatology? Please specify.
b) How is the standard deviation calculated for standardizing the DMI? Which baseline period is used for this?
c) Include references describing the method for computing the DMI.
Lines 37-39: Please specify what is meant by “never been associated with other nIOD events”
Line 58: 2021 was a normal monsoon year (99% of LPA)
Lines 60-61:
a) How about the modulation of the Walker Circulation?
b) Include references that discuss the role of the local Hadley Circulation and/or Walker Circulation.
Lines 72-73: Clarify what specific “anomalous changes” are being referred to here.
Line 74: The question introduced here feels abrupt and disconnected from the following paragraph. It may not be necessary in my view, however, if you choose to keep it, consider adding how the previous studies have examined this question and/or if it will be addressed in this work.
Figure 1:
a) I assume the different colors for NIOD, PIOD, and Neutral are based on standard deviation thresholds. Please clarify the criteria used.
b) What do the orange bars in Figure 1b represent?
Lines 75-77: Phrases like “the strongest recorded to date” and “the first occurrence of a multi-year nIOD” are quite strong. Please be specific, for example, clarify the time period (e.g., since when) these statements refer to.
Line 127: zonal-mean?
Lines 158-161: This paragraph describes what is shown in Figures 4 and 5, and may be more appropriate for the figure captions. Consider moving the detailed descriptions (e.g., event numbers) to the captions and using the main text to focus more on interpretation or analysis.
Line 169: The abbreviation “AVISO” is not defined before. Is this the same SLA product available from the Copernicus Marine Service website?
Line 173: The abbreviation “OSCAR” is not defined before.
Figure 10: Include the units for both the vectors and shadings.