Weakened Seasonality of the Ocean Surface Mixed Layer Depth in the Southern Indian Ocean During 1980–2019

Temporal and spatial variations in the ocean surface mixed layer are important for the climate and ecological systems. During 1980–2019, the Southern Indian Ocean (SIO) mixed layer depth (MLD) displays a basin‐wide shoaling trend that is absent in the other basins within 40°S–40°N. The SIO MLD shoaling is mostly prominent in austral winter with deep climatology MLD, substantially weakening the MLD seasonality. Moreover, the SIO MLD changes are primarily caused by a southward shift of the subtropical anticyclonic winds and hence ocean gyre, associated with a strengthening of the Southern Annular Mode, in recent decades for both winter and summer. However, the poleward‐shifted subtropical ocean circulation preferentially shoals the SIO MLD in winter when the meridional MLD gradient is sharp but not in summer when the gradient is flat. This highlights the distinct subtropical MLD response to meridional mitigation in winds due to different background oceanic conditions across seasons.


Introduction
The well-mixed surface ocean mixed layer (ML) regulates the vertical exchanges in heat, carbon, and oxygen of the upper layer with the atmosphere and the deeper layer, which is crucial for the physical, chemical, and biological processes in global oceans (Bourgeois et al., 2022;Breitburg et al., 2018;Cabré et al., 2015;Fu et al., 2016;Scannell et al., 2020;Wang et al., 2020).The ML depth (MLD) is one of the most important quantities of the ocean and is tightly coupled with ocean stratification (Gao et al., 2023;Sallée et al., 2021).Under global climate change, global ocean stratification increases during 1960-2018(G. Li et al., 2020) ) and is projected to strengthen under future warming scenarios (Capotondi et al., 2012;Long et al., 2020;R. Xia et al., 2021;X. Xia et al., 2021;Yamaguchi & Suga, 2019;Yeh et al., 2009).A more stratified upper ocean inhibits vertical mixing and tends to shoal the ML (Bindoff et al., 2019;Kwiatkowski et al., 2020;Somavilla et al., 2017).However, ocean stratification displays substantial variations across regions and vertical layers despite that it generally increases in regions with surface-intensified warming.It is necessary to investigate the associated regional MLD changes and underlying mechanisms.Under the seasonally evolved atmospheric conditions, the MLD displays pronounced seasonality that is generally deep in local winter and shallow in summer.Sallée et al. (2021) show that the summer ML deepens but with an increased density gradient across the ML base during 1970-2018.In the North Pacific, the MLD shoals the most in local summer and thus enhances its seasonality under future warming scenarios (C.Chen & Wang, 2015).Moreover, the Southern Hemisphere (SH) atmospheric circulation changes significantly under ozone depletion and increased greenhouse gases (Fogt & Marshall, 2020;Fyfe & Saenko, 2006;J.-R. Shi et al., 2020;Thompson et al., 2011), which can lead to substantial MLD changes in the SH.For instance, there is an increasing trend of summer MLD in the Atlantic and Indian Ocean sectors of the Antarctic between 2002 and 2011, which is dominated by wind changes (Panassa et al., 2018).
The Indian Ocean plays a substantial role in storing anthropogenic heat under global climate change (Levitus et al., 2012;W. Liu et al., 2016;Nieves et al., 2015).In recent decades, the Indian Ocean has experienced a basinwide increase trend in sea surface temperature (Roxy et al., 2014;Sharma et al., 2023;Zhang et al., 2019), which displays the most prominent warming in the south subtropics since the mid-1990s (J.Y. Li & Su, 2021).The ocean heat content increase in recent decades (Duan et al., 2023) also mainly occurs in the south Indian Ocean with deep ML (Z.Li et al., 2023), suggesting the unique features of the south Indian Ocean climate changes.However, the seasonal MLD changes of the south Indian Ocean in recent decades and underlying processes have not been systematically examined.One noteworthy aspect we focus on is the striking poleward shift of the SH circulation in recent decades (G.Chen et al., 2008;Shaw, 2019;Wu et al., 2021;Yang et al., 2020;Yin, 2005), which is supposed to displace the subtropical ocean gyre poleward and thus affect the MLD.
Therefore, the present study utilizes oceanic and atmospheric variables from observation and various reanalysis data sets to investigate the south Indian Ocean MLD changes after 1980 when observations are relatively sufficient.The results show that among global oceans within 40°S-40°N, only the Southern Indian Ocean (SIO, 55-110°E and 36-20°S) displays a basin-wide prominent MLD shoaling trend during 1980-2019 (Figure 1).The SIO shoaling mainly occurs in austral winter but is insignificant in summer, substantially reducing the MLD seasonality.The strengthening of the Southern Annular Mode (SAM) leads to a southward shift of the subtropical winds.The wind-driven ocean gyre adjustment dominates the distinct SIO MLD changes between winter and summer.

Data and Methods
The present study utilizes monthly-mean atmospheric variables from the National Centers for Environmental Prediction-Department of Energy reanalysis (NCEP I and II, Kalnay et al., 1996), the fifth generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis (ERA5, Hersbach et al., 2020), and surface wind from the Cross-Calibrated Multi-Platform (CCMP v2, 1988(CCMP v2, -1980) ) (Atlas et al., 2011).The station-based SAM index is estimated by the zonal pressure difference between the latitudes of 40°S and 65°S and provided by Marshall (2003).The ocean temperature and salinity from the Estimating the Circulation and Climate of the Ocean (ECCO2 v4r4, 1992-2017) (Forget et al., 2015), the Institute of Atmospheric Physics (IAP, 1940(IAP, -2019) ) (Cheng et al., 2017), the ECMWF Ocean Reanalysis System 4 (ORAS4, 1958-2017) (Balmaseda et al., 2013), and the Ocean General Circulation Model for the Earth Simulator (OFES, 1950(OFES, -2019) ) (JAM-STEC, 2009) are also used.Given that the reanalysis data sets mainly provide outputs up to 2019, we mainly focus on the changes during 1980-2019.
The MLD variable is a direct output in BOA Argo and deemed observation, which is provided by H. Li et al. (2017) based on the objective maximum angle method (Chu & Fan, 2011).For reanalysis data sets, the MLD is calculated as the depth at which potential density exceeds the surface level (usually 5 m) density at a criterion of 0.125 kg/m³.The reanalysis data sets generally perform well in reproducing the magnitude and spatial pattern of MLD in BOA Argo (Figure S1 in Supporting Information S1).The upper ocean stratification is estimated as the squared buoyancy frequency calculated by the vertical gradient of the density N 2 = ρ g ∂ρ ∂z , where ρ is seawater potential density, g = 9.8 m s 2 is the acceleration of gravity, and z is the vertical depth.

The MLD Changes During Recent Decades
Figure 1a shows the linear trends of the annual-mean MLD since 1980, which displays large regional variations within 40°S-40°N.There are significant increasing trends concentrating in the equatorial western Pacific and broad but scattered decreasing trends appearing in the Pacific and Atlantic Oceans.Indeed, significant ML shoaling (magenta dots in Figure 1a) only displays a basin-wide feature in the Indian Ocean south of 20°S.Furthermore, the south Indian Ocean MLD changes display remarkable seasonal differences (Figures 1b and 1c).In austral winter (June-November), the MLD prominently decreases over the SIO (black box in Figures 1b and  1c).The winter decreasing trend is robust across data sets and larger than 15 m (above 15% of the 1980-1999 mean, Figure S2 in Supporting Information S1) during 1980-2019.In contrast, the MLD changes are weak in austral summer (December-May).The area-mean results also reveal that the SIO MLD mainly decreases in winter (colored bars in Figure 1d) when the climatology ML is deep (black line in Figure 1d) and displays insignificant shoaling or even increasing trend in summer when the climatology ML is shallow.As a result, the SIO area-mean MLD displays a significant weakening trend in the magnitude of its seasonality during 1980-2019 (black and red lines in Figure 1d), at a percentage of around 20% relative to the 1980-1999 climatology.
The temporal evolution of the SIO area-mean MLD further reveals that the significant winter MLD shoaling mainly occurs after the mid-1990s (Figure 1e).In contrast, the summer MLD displays a relatively weak shoaling trend before the mid-1990s and gradually recovers after 2000, leading to an overall insignificant trend during the whole analyzed period 1980-2019 (Figure 1f).As a result, the SIO MLD seasonality, estimated as the winter minus summer MLD, significantly weakens after the mid-1990s.It is interesting that the MLD seasonality slightly recovers after 2010 due to a slight shoaling trend in the summer MLD.The results are similar if we utilize a criterion of 0.05 kg/m 3 to calculate the MLD (Figure S3 in Supporting Information S1).The distinct spatial and temporal variations in the winter and summer MLD highlight the seasonal differences in the detailed processes controlling the MLD changes.

The Upper Layer Stratification Changes
To understand the seasonal differences in the SIO MLD changes, we first examine the vertical profiles of the zonal-mean (55-110°E) changes in potential density, buoyancy frequency, and potential temperature in winter and summer (Figure 2).The shoaling of the winter MLD (black dashed and green solid lines) is associated with robust surface-intensified density decrease (Figure 2a).Correspondingly, the stratification strengthens across the bottom of the ML in winter, which is robust south of 25°S (Figure 2b).In comparison, the summer density decrease is nearly vertically uniform within the ML at almost all latitudes and is maximal below the ML north of 25°S, leaving uncertain strengthening or even weakening in the ML stratification (Figures 2d and 2e).Furthermore, the seasonal differences in the upper layer density decrease are tightly associated with the different vertical warming structures between the two seasons.There is robust surface-enhanced warming south of 25°S in winter (Figure 2c) but relatively weak and nearly vertically uniform ML warming in summer (Figure 2f).Therefore, the vertical warming structures largely shoal the winter MLD but suppress the summer MLD shoaling.Moreover, the salinity changes substantially differ in their sign and magnitude across data sets, that is, large uncertainty, at most latitudes (Figure S4a in Supporting Information S1).Only north of 25°S, the decrease in the upper 100 m salinity is relatively robust and tends to shoal the winter MLD (Figure S4a in Supporting Information S1) but exerts negligible summer MLD changes (Figure S4b in Supporting Information S1) due to vertical uniform structure within the ML.The prominent winter MLD shoaling between 36 and 25°S is associated with a robust increase in winter buoyancy frequency (Figure 2c) around the bottom of the ML.Overall, the upper layer buoyancy frequency changes are dominated by the temperature changes (Figures S4c and S4d in Supporting Information S1) while the salinity changes (Figures S4e and S4f in Supporting Information S1) display a secondary or opposite role and are highly uncertain.Indeed, the temporal evolution of the upper SIO area-mean is highly consistent among reanalysis data sets for temperature changes (Figure S5 in Supporting Information S1) but displays large discrepancies across data sets for salinity changes (Figure S6 in Supporting Information S1), especially before 2000.This result further supports the argument that the seasonal differences in the upper ocean warming structure dominate the distinct SIO stratification changes and concomitant buoyance frequency and MLD changes between the winter and summer, especially south of 25°S.
The zonal-mean sections of the climatology density and temperature (contours in Figure 2) indicate that the SIO warming is possibly associated with a southward shift of the subtropical ocean gyre and hence warm low-latitude water mass, leading to warm advection into the SIO (Duan et al., 2023;J.-R. Shi et al., 2023;Yang et al., 2020).Indeed, if we displace the zonal-mean ML base during 1980-1999 (black dashed line) southward by 1°latitude, the shifted ML base (red dashed lines in Figure 2) is nearly identical to the ML base during 2000-2019 (green solid line), especially south of 30°S in winter.Therefore, it is natural to hypothesize that the winter MLD shoaling is mainly a dynamical response of the southward displaced subtropical gyre.

Underlying Mechanisms of the SIO MLD Changes
Given that in the subtropics, the overlying atmospheric circulation can significantly influence ocean gyre and, consequently, the MLD (Wu et al., 2021;Yamagami & Tozuka, 2015;Yang et al., 2020).We further investigate the changes in the subtropical ocean gyre and their connection with the SH atmospheric circulation.Figure 3a shows that there is a prominent poleward shift of the subtropical ocean gyre as indicated by the changes in σ26.5 isopycnal depth (Figure 3a), consistent with Yang et al. (2020).The southward shift of the subtropical gyre is also evident in summer (Figure 3b).We thus estimate the influence of a 1°latitude southward displacement of the subtropical gyre on MLD changes in both seasons by calculating the MLD y •1°lat, where MLD y is the meridional gradient of the MLD at each grid.Consistent with the hypothesis proposed based on Figure 2, the poleward shift of the subtropical gyre would profoundly decrease the winter MLD, it respectively explains 102.3% and 54.4% of the SIO MLD shoaling south and north of 30°S (Figures 4c and 4d).It is also interesting to note the winter MLD shoaling (Figure 1b) is larger in the eastern than the western part of the selected domain.This pattern is also consistent with the MLD change pattern caused by the poleward shift of the gyre (Figure 3c) as the meridional shift is more evident in the east than the west (Figure 4a).The σ26.5 isopycnal depth also shows that there is an eastward shift in subtropical gyre.However, the zonal shift in the gyre causes limited MLD changes (not shown) as the MLD gradient is much weaker in the zonal than the meridional direction.The results confirm the dominant role of the ocean dynamical adjustment on the SIO MLD shoaling, especially south of 30°S, through the "slumping" of the meridional MLD gradient (Johnson & Lyman, 2022).Moreover, the ocean-atmosphere coupling processes may play an additional role as there are differences in the spatial pattern and magnitude of the gyre shift-induced MLD changes (shading in Figure 4c) and the original MLD changes (contours in Figure 4c) north of 30°S.However, the meridional shift of the ocean gyre exerts minor effects on the summer MLD changes.This is because the meridional slope of the ML base is sharp in winter (i.e., large MLD y ) but is negligible in summer as the slope is flat, as indicated by the green contours in Figures 1b and 1c and black dashed lines in Figure 2.
In climatology, winter seawater is converged into the SIO primarily from the deep ML in the Southern Ocean (Figure S1 in Supporting Information S1) due to westerlies (vectors in Figures 3a and 3d), and the loss of surface heat and freshwater into the atmosphere (Figure S7 in Supporting Information S1) all favor a relatively deep MLD in the SIO.In recent decades, there are prominent anomalous easterly winds over the SIO in winter and summer (Figures 4a and 4b), which strengthen the easterly trade winds but weaken the westerlies.This indicates a significant southward shift of the subtropical anticyclonic winds.The anomalous easterly wind would drive southward Ekman transport and hence a poleward displacement of the subtropical ocean gyre in both seasons.However, the southward-shifted ocean gyre would preferentially reduce the winter ML mass converging from the Southern Ocean into the SIO due to large meridional MLD gradient, thus leaving significant shoaling effects on the winter MLD.Furthermore, the station-based SAM index from Marshall (2003) displays an overall strengthening trend during 1980-2019 in both winter and summer, indicating a general enhancement and southward mitigation of the circumpolar westerlies (Figure 3b).Correspondingly, the overall increasing trends of the SIO area-mean surface zonal winds are evident in both seasons during 1980-2019.Indeed, the temporal evolution of the SIO zonal winds highly mimics the trajectory of the SAM index.In winter, the strengthening of the SAM index mainly occurs after the mid-1990s, associated with a significant enhancement of the SIO zonal winds after the mid-1990s.In contrast, the summer surface zonal winds first rapidly strengthen before the mid-1990s but display an insignificant trend after the mid-1990s, largely following the temporal evolution of the summer SAM index.The results reveal the coherent long-term changes of the subtropical and mid-latitude atmospheric circulations in the SH associated with the strengthened SAM, which is the primary driver of the SIO MLD seasonality change.
The MLD changes are influenced by mechanical (wind) stirring and surface buoyancy forcing in addition to the ocean dynamics (Cushman-Roisin, 1981;Halpern, 1974;Q. Liu & Lu, 2016;Ushijima & Yoshikawa, 2020).In recent decades, the wind stirring, estimated by the scalar wind speed (Figure 4a), generally increases north of 35°S while the surface net heat flux and freshwater flux both decrease over the SIO (Figure S7 in Supporting Information S1), all deepening the SIO MLD instead of shoaling.Only south of 35°S, the decreased wind speed (Figure S7d in Supporting Information S1) and increased surface freshwater flux (Figure S7f in Supporting Information S1) seem to play a role but they still cannot explain the broad MLD shoaling trend in the SIO.This further confirms the dominant role of the wind-driven poleward shift in subtropical ocean gyre on the SIO MLD shoaling.Moreover, the vertical profiles of changes in potential density, temperature, and salinity due to 1°l atitude southward displacement of the subtropical gyre (Figure S8 in Supporting Information S1) are to some extent similar to those of the changes between 2000-2019and 1980-1999 (Figure 2 and Figure S4 in Supporting Information S1), but with a larger magnitude and different structures to some extent.This suggests that the oceanatmosphere coupling processes triggered by the gyre shift, associated with the surface heat and freshwater changes, may also modulate the upper layer changes.Therefore, the distinct seasonal upper layer warming structure and resultant winter MLD shoaling in the SIO is a combined effect of the wind-driven ocean dynamics and the ocean-atmosphere coupling, while the wind changes display a dominant role.

Conclusions and Discussion
We have investigated the long-term changes in the Indian Ocean MLD during 1980-2019 and show that the MLD displays a significant basin-wide shoaling trend in the SIO.The SIO MLD shoaling mostly occurs in winter when the climatology ML is deep and is negligible in summer when the climatology ML is shallow.The distinct seasonal MLD changes lead to a prominent weakening (around 20%) in the seasonality of the SIO MLD during 1980-2019, which is mostly prominent after the mid-1990s.We show that the weakened SIO MLD seasonality is primarily associated with the opposite vertical warming structures between winter and summer.The shoaled winter MLD generally accompanies with strengthened stratification around the ML bottom due to surfaceintensified warming structure, which primarily results from the poleward shift of the subtropical gyre and hence warm low-latitude water mass.Specifically, in recent decades, the subtropical anticyclonic winds shift southward following the strengthened SAM, leading to prominent anomalous easterly winds in the SIO in both winter and summer.The wind changes displace the subtropical ocean gyre poleward and would preferentially and largely suppress the deep winter ML mass converging from the Southern Ocean into the SIO.As a result, the SIO MLD shoaling is most prominent in winter when the meridional MLD gradient is sharp, which causes the MLD to be vulnerable for the meridional mitigation of the ocean gyre.The ocean-atmosphere coupling processes may additionally contribute to changes in the upper layer conditions, but they may also likely relate to the wind changes in recent decades.
The results highlight that despite both under southward shifted winds, the subtropical MLD responses would be distinct between winter and summer due to different background oceanic conditions.Interestingly, the MLD shoaling is not evident in the Pacific and Atlantic sectors north of 40°S despite that the strengthened SAMinduced shifts in the subtropical ocean gyre and surface zonal winds are evident globally (Figure S9 in Supporting Information S1).This is because north of 40°S, the meridional gradient of the winter climatology MLD is relatively weak in the Pacific and Atlantic sectors (contours in Figure S9c in Supporting Information S1).South of 40°S in the Pacific sectors, the poleward shifted gyre-induced winter MLD shoaling is also prominent over specific regions with large MLD gradient.However, the corresponding winter and summer MLD changes in the Southern Ocean involve more complicated processes and display large uncertainty in the magnitude across data sets (Figure S10 in Supporting Information S1).
The seasonality changes in the SIO MLD may be important for understanding the long-term changes in regional climate (Behera & Yamagata, 2001;Sharma et al., 2023;Zhao et al., 2023) and marine heatwaves (Amaya et al., 2021;Elzahaby et al., 2022;Saranya et al., 2022;J. Shi et al., 2022) in recent decades.For example, Yamagami and Tozuka (2015) show that the interdecadal MLD shoaling in the south Indian Ocean can amplify the warming effect from shortwave radiation, and thus even a weak atmospheric forcing may trigger the south subtropical Indian Ocean dipole (SIOD), which is an important climate mode that substantially influences the South African and Australian climate.It is important to examine the influence of the MLD changes on the longterm evolution of the SIOD under the current global climate and future warming scenarios.It is worth noting that the SAM index displays distinct trends before and after the mid-1990s for both winter and summer, which has not received wide attention and is possibly related to seasonal differences in effects of stratospheric ozone depletion (Gillett & Thompson, 2003;Thompson & Solomon, 2002;Thompson et al., 2011;Turner et al., 2009), greenhouse gas increase (Fogt & Marshall, 2020;Hartmann et al., 2000;Marshall et al., 2004), and Antarctic temperature changes (Kwok & Comiso, 2002;Schneider et al., 2004;Turner et al., 2019;van den Broeke & van Lipzig, 2003).The SIO is the transition zone of the easterly trade winds and westerlies and is important in the formation, subduction, and propagation of the water mass.The long-term changes in the mode water and intermediate water associated with the MLD shoaling and SAM change between different seasons (Downes et al., 2017;Jing et al., 2023;Wu et al., 2021;R. Xia et al., 2021;X. Xia et al., 2021;Xu et al., 2021) are also worth of further investigation.

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The seasonality of the Southern Indian Ocean surface mixed layer (ML) depth prominently weakens during 1980-2019 • The weakened seasonality mainly results from a pronounced winter ML shoaling • The southward shift of the subtropical ocean gyre driven by the strengthened Southern Annular Mode dominates the ML shoaling Supporting Information: Supporting Information may be found in the online version of this article.

Figure 1 .
Figure 1.Multi-data set (ECCO, IAP, ORAS4, and OFES) mean linear trend of ocean surface mixed layer depth (MLD, m) after 1980 for (a) annual-mean of global oceans within 45°S-45°N, (b) austral winter half year (June-November mean), and (c) austral summer half year (December-May mean) of the south Indian Ocean.The dots indicate that the trend is significant at 95% confidence levels and the signal-to-noise ratio (SNR), defined as the mean value divided by the inter-data set standard deviation, is larger than 1, and the green contours indicate climatology (1980-1999 mean) MLD.(d) The Southern Indian Ocean (SIO, 55-110°E, 36-20°S) area-mean MLD trend (colored bar) at each calendar month, with the black line for the climatology MLD (1980-1999 mean) and red line for the climatology MLD plus the MLD trend (changed MLD).(e, f) The SIO area-mean MLD changes referenced to the last 10-year mean in each data set in winter and summer, respectively, and (g) is their difference, with the 9-year running mean being applied.

Figure 2 .
Figure 2. Multi-data set mean and zonal-mean (55-110°E) climatology (contours, black for 1980-1999 and magenta for 2000-2019) and changes (2000-2019 minus 1980-1999 mean, color shading) in potential density (∆Density, kg/m 3 ), buoyancy frequency (∆N 2 ), and potential temperature (∆Temp, K) for (a-c) austral winter and (d-f) austral summer.The black dashed and green solid lines respectively indicate the position of the mixed layer (ML) base for 1980-1999 mean and 2000-2019 mean, while the red solid lines indicate the position of the ML base if we shift the 1980-1999 mean ML base 1°latitude southward.Gray dots indicate large uncertainty, with less than 3 out of 4 data sets displaying the same sign of changes or the SNR is smaller than 1.

Figure 4 .
Figure 4. (a, b) Changes in surface scalar wind speed (sfcWind, shading, m/s) and wind vectors (m/s), (c, d) the station-based Southern Annular Mode index from Marshall (2003) and SIO area-mean surface zonal winds ( u10 m, m/s, positive for easterly winds) for winter and summer.