Subtropical South Indian Ocean Research Articles

Subsurface phytoplankton responses to ocean eddies can run counter to satellite-based inference from surface properties in subtropical gyres

Phytoplankton contribute approximately half of the Earth’s primary production, playing a crucial role in marine ecosystem functioning and global carbon cycling. However, our understanding of how ocean dynamics regulate the distribution of phytoplankton remains limited due to the scarcity of depth-resolved observations. Here, we compared the simultaneous measurements of two biogeochemical-Argo floats that tracked a cyclonic eddy (CE) and an anticyclonic eddy (AE) in the subtropical South Indian Ocean for a period of 3 months. The observations revealed a decoupling of subsurface phytoplankton biomass from surface chlorophyll. The CE exhibited a subsurface phytoplankton bloom initiated by the springtime shoaling of mixed layer to euphotic layer, accompanied by a counter-intuitive decrease in surface chlorophyll due to physiological adaptation. In comparison, the AE exhibited vertically homogeneous phytoplankton biomass, with a weak subsurface chlorophyll maximum layer, as the mixed layer remained at a deep level in the spring. Despite a higher concentration of surface chlorophyll in the AE than in the CE, subsurface and depth-integrated phytoplankton chlorophyll, biomass, and productivity computed with a bio-optical model are greater in the CE than in the AE. Statistics on global biogeochemical-Argo measurements over the past decade confirm the generality of this phenomenon in subtropical gyres. Our findings demonstrate that high concentrations of chlorophyll near the surface in these regions are not necessary indicative of high phytoplankton abundance at the subsurface layer. The results have important implications for inferring subsurface phytoplankton abundance and productivity from satellite measurements.

Quasi‐Decadal Temperature Variability in the Intermediate Layer of Subtropical South Indian Ocean During the Argo Period

AbstractIt has been reported that the subtropical South Indian Ocean (SIO) has been rapidly warming over the past two decades and can therefore be characterized as one of the major heat accumulators among the oceanic basins. However, this strong warming is not uniformly distributed in the vertical direction. In comparison to the decade‐long warming in the upper layer (0–300 m) in 2004–2013, the intermediate layer (300–1,000 m) displays a shorter warming during 2004–2009 and an intense cooling during 2010–2016. By decomposing temperature variations into heaving and spice components, and performing a heat budget analysis, we show that temperature variations in the intermediate layer during these two periods are primarily contributed by isopycnal migrations driven by local wind forcing. Local wind change in the subtropical SIO can be explained by the Indian Ocean Dipole and El Niño–Southern Oscillation during 2004–2016, while Southern Annular Mode (SAM) favors anomalous wind change in mid‐latitudes and the formation of basin‐wide wind change in the SIO. Additionally, wind forcing in the Subantarctic Mode Water (SAMW) formation region, which is closely linked to the SAM, modulates the anomalous spreading of SAMW into the interior of the subtropical SIO. This, therefore, leads to the SAMW intrusion being of secondary importance to the quasi‐decadal temperature variability. Our findings demonstrate the independence of wind‐driven temperature changes on the quasi‐decadal scale in the intermediate layer of the subtropical SIO under the overall warming background of SIO waters.

Rainfall onset and cessation over Mozambique and relationship with sea surface temperature and atmospheric patterns

AbstractMozambique is highly dependent on subsistence agriculture. Thus a climatology of rainfall onset and cessation is derived together with their interannual variability and trends using gridded daily data and station data. Mean onset is found to occur from early to mid‐November over southernmost regions, mid‐ to late November over central regions and late November to early December over the northern and coastal regions while cessation is from mid to late March over southern and late March to early April over most of central and northern Mozambique. The two gridded datasets showed differences (~30 days) in rainfall onset and cessation dates over southern Mozambique with standard deviations of 20–35 days. Timing of early onset over northern and central Mozambique appears associated with both zonal and meridional moisture convergence and tropical extratropical cloud bands, while timing of late onset is associated with southeast moisture fluxes from the subtropical South Indian Ocean and uplift over the landmass. Early (late) onset appears to be linked to La Niña (El Niño), particularly over central Mozambique. Rainfall cessation over most of Mozambique tends to be early (late) during El Niño (La Niña) summers. The relationship of ENSO with onset and cessation over large areas in Mozambique may help farmers and other user groups in planning their activities for the rainy season.

Seasonal and Interannual Variability of the Subtropical South Indian Ocean Sea Surface Salinity Maximum

AbstractThe sea surface salinity (SSS) maximum of the South Indian Ocean (the SISSS‐max) is a high‐salinity feature centered at 30°S, 90°E, near the center of the South Indian subtropical gyre. It is located poleward of a region of strong evaporation and weak precipitation. Using several different satellites and in situ data sets, we track changes in this feature since the early 2000s. The centroid of the SISSS‐max moves seasonally north and south, furthest north in late winter and farthest south in late summer. Interannually, the SISSS‐max has moved on a northeast‐southwest path about 1,500 km in length. The size and maximum SSS of the feature vary in tandem with this motion. It gets larger (smaller) and saltier (fresher) as it moves to the northeast (southwest) closer to (further from) the area of strongest surface freshwater flux. The area of the SISSS‐max almost doubles from its smallest to largest extent. It was maximum in area in 2006, decreased steadily until it reached a minimum in 2013, and then increased again. The seasonal variability of the SISSS‐max is controlled by the changes that occur on its poleward, or southern, side, whereas interannual variability is controlled by changes on its equatorward side. The variations in the SISSS‐max are a complex dance between changes in evaporation, precipitation, wind forcing, gyre‐scale ocean circulation, and downward Ekman pumping. Its motion correlated with SSS changes throughout the South Indian Ocean and may be an indicator of changes in the basin's subtropical circulation.

Variations in the Southern Ocean carbonate production, preservation, and hydrography for the past 41, 500 years: Evidence from coccolith and CaCO3 records

Changes in ocean alkalinity affect atmospheric pCO2 (i.e., higher alkalinity lowers atmospheric pCO2). Ocean alkalinity is partly determined by sedimentary burial of carbonates, which is primarily controlled by carbonate flux and the degree of deep ocean carbonate saturation. In this study, we investigate the factors determining the coccolith burial in subantarctic sediments and the surface ocean changes in the subtropical South Indian Ocean. The downcore coccolith records from the subantarctic region (SK200/22a) of the Indian sector of the Southern Ocean display low coccolith concentration during the glacial period. A possible explanation for this is, 1) the low glacial production of coccolithophores due to the competition from diatoms and 2) dilution by biogenic silica in the glacial sediments. Additionally, reduced carbonate burial owing to the low carbonate saturation of the deep-water accounts for the decline in glacial coccolith concentration. This also explains the low coccolith dissolution index and enrichment of the large dissolution-resistant coccolith species, Coccolithus pelagicus subsp. braarudii in the glacial sediments. The low carbonate saturation is attributed to, 1) the replacement of carbonate saturated, North Atlantic Deep Waters by the undersaturated southern sourced water masses and 2) increased storage of dissolved CO2 in the deep glacial Southern Ocean. Our study suggests that changes in coccolith production and the deep ocean carbonate saturation determine their burial in subantarctic sediments for the last 41,500 years. Other than these changes, the study region also records the changes in the Agulhas Return Current via variation in the proportion of tropical-subtropical coccolith assemblage.

Decadal Cooling Events in the South Indian Ocean During the Argo Era

AbstractDespite the rapid warming in the south Indian Ocean (SIO) during the recent two decades, the upper 300 m of the tropical southwest Indian Ocean (SWIO) experienced significant cooling during 2005–2011. The decreasing rate of the observed temperature reached −0.50 ± 0.1°C/decade. Heat budget analysis reveals that the cooling trend over SWIO results from the competing effect between local air‐sea heat flux and horizontal heat advection. Moreover, all the three observational datasets indicate that in the subtropical SIO, a prolonged cooling event occurred beneath the thermocline during 2010–2016. Although there exist some discrepancies between the observations and estimating the circulation and climate of the ocean (ECCO) reanalysis before 2010, the observations and ECCO reanalysis agree in terms of the post–2010 temperature variability in the intermediate layer of the subtropical region, probably because of the better constraint of the ECCO assimilation product by the increased sampling of Argo floats in the deeper ocean. The heat budget analysis indicates that during the period 2010–2016, the decadal cooling in the 400–1,000 m layer is primarily modulated by vertical ocean advection, and the horizontal ocean advection makes a secondary contribution. This study expands our knowledge of the decadal temperature variability and heat balance in the SIO during the Argo era.

Maintenance Mechanisms of the Wintertime Subtropical High over the South Indian Ocean

Abstract Climatologically the surface Mascarene high over the subtropical south Indian Ocean (SIO) shifts westward toward austral winter, and its strength as a planetary-wave component maximizes in late austral winter, unlike its counterpart over other subtropical oceans. The present study investigates the maintenance mechanisms for the wintertime Mascarene high with a linear atmospheric dynamical model (LBM) and an atmospheric general circulation model (AGCM). The LBM experiments reveal the importance of cross-equatorial tropical influences. Deep convection associated with the Asian summer monsoon acts not only to shift the Mascarene high westward as its direct influence but also to enhance midtropospheric subsidence and equatorward surface winds over the central and western portions of the subtropical SIO. The associated near-surface cold advection and subsidence promote (suppress) the formation of low-level (deep convective) clouds. The resultant enhanced radiative cooling and reduced deep condensation heating both reinforce the equatorward portion of the surface high. The LBM experiments also reveal that seasonally enhanced storm-track activity over the SIO is important for maintaining the poleward portion of the Mascarene high through eddy heat and vorticity fluxes. The AGCM experiments demonstrate that the Agulhas Current system and the associated sea surface temperature (SST) front reinforce the high by energizing the storm-track activity. The present study thus proposes that both the Asian summer monsoon and the enhanced storm-track activity maintained by the Agulhas SST front externally modulate the positively coupled system between the wintertime Mascarene high and low-level clouds to realize its unique seasonality.

Indian Ocean Subtropical Underwater and the Interannual Variability in Its Annual Subduction Rate Associated with the Southern Annular Mode

Abstract In this study, the Indian Ocean Subtropical Underwater (IOSTUW) was investigated as a subsurface salinity maximum using Argo floats (2000–20) for the first time. It has mean salinity, potential temperature, and potential density values of 35.54 ± 0.29 psu, 17.91° ± 1.66°C, and 25.56 ± 0.35 kg m−3, respectively, and mainly extends between 10° and 30°S along the isopycnal surface in the subtropical south Indian Ocean. The annual subduction rate of the IOSTUW during the period of 2004–19 was investigated based on a gridded Argo dataset. The results revealed a mean value of 4.39 Sv (1 Sv ≡ 106 m3 s−1) with an interannual variability that is closely related to the southern annular mode (SAM). The variation in the annual subduction rate of the IOSTUW is dominated by the lateral induction term, which largely depends on the winter mixed layer depth (MLD) in the sea surface salinity (SSS) maximum region. The anomalies of winter MLD are primarily determined by SAM-related air–sea heat flux and zonal wind anomalies through modulation of the buoyancy. As a result, the annual subduction rate of the IOSTUW generally increased when the SAM index showed negative anomalies and decreased when the SAM index showed positive anomalies. Exceptional cases occurred when the wind anomaly within the SSS maximum region was weak or was dominated by its meridional component. Significance Statement The Indian Ocean Subtropical Underwater (IOSTUW) is characterized as a subsurface salinity maximum in the subtropical south Indian Ocean, and it has potential contribution to regional sea level change and interoceanic salinity exchange. The purpose of this work is to give a more precise description of the IOSTUW and to better understand the formation process of this water mass. Our results shine new light on hydrological characteristics of the IOSTUW based on an observational dataset collected during the past two decades. In addition, the formation of the IOSTUW was found to be closely related to the leading mode of variability in the atmospheric circulation of the Southern Hemisphere though air–sea heat flux and surface wind anomaly.

Eastward Shift of Interannual Climate Variability in the South Indian Ocean since 1950

Abstract The subtropical Indian Ocean dipole (SIOD) and Ningaloo Niño are the two dominant modes of interannual climate variability in the subtropical south Indian Ocean. Observations show that the SIOD has been weakening in the recent decades, while Ningaloo Niño has been strengthening. In this study, we investigate the causes for such changes by analyzing climate model experiments using the NCAR Community Earth System Model, version 1 (CESM1). Ensemble-mean results from CESM1 large-ensemble (CESM1-LE) show that the external forcing causes negligible changes in the amplitudes of the SIOD and Ningaloo Niño, suggesting a dominant role of internal climate variability. Meanwhile, results from CESM1 pacemaker experiments reveal that the observed changes in the two climate modes cannot be attributed to the effect of sea surface temperature anomalies (SSTA) in either the eastern tropical Pacific Ocean or tropical Indian Ocean. By further comparing different ensemble members from the CESM1-LE, we find that a warm pool dipole mode of decadal variability, with opposite SSTA in the southeast Indian Ocean and the western-central tropical Pacific Ocean plays an important role in driving the observed changes in the SIOD and Ningaloo Niño. These changes in the two climate modes have considerable impacts on precipitation and sea level variabilities in the south Indian Ocean region.

Structure and maintenance mechanisms of the Mascarene High in austral winter

AbstractThe Mascarene High (MH), is a key component of the Asian‐Africa‐Australia monsoon system in austral winter (JJA), spanning over the South Indian Ocean (15°–35°S, 15°–110°E). Its three‐dimensional structures and maintenance mechanisms are examined in this study. It is a low‐level subtropical high dominating the southern Africa and South Indian Ocean, characterized by a north‐westward tilt with height, which is attributed to its spatially inhomogeneous thermal structure. Large‐scale subsidence characterizes the main body of the MH, with the stronger subsidence to the east than to the west. Diagnosis using the complete form of the vertical vorticity tendency equation shows that the anticyclonic structure of the MH, which can be described by the distribution of meridional wind, is maintained mainly by the vertical gradient of diabatic heating, change in static stability, and friction dissipation. In particular, a combination of sensible heating and longwave radiative cooling results in a vertical decreasing gradient of diabatic heating in the lower troposphere. It generates the stronger southerlies over the subtropical South Indian Ocean than over the southern Africa. Meanwhile, over the South Indian Ocean, the increasing static stability as a result of the downward transport of a more stable atmosphere partly offsets the effect of the vertical gradient of diabatic heating, and southerlies still prevail there. Over the southern Africa, topographic friction dissipation induces northerlies, balancing the effect of the vertical gradient of diabatic heating with a stronger magnitude, and northerlies prevail.

Relationships between NDVI, river discharge and climate in the Okavango River Basin region

AbstractThe Okavango River Basin (ORB) is a highly sensitive and biodiverse region in southern Africa whose climate, vegetation and river discharge characteristics are not well understood. This study investigated relationships between rainfall, temperature, Normalized Difference Vegetation Index (NDVI) and river discharge over the region as well as their trends and interannual variability. It is found that spatial patterns of NDVI are closely related to those of rainfall, but less so with temperature at monthly and seasonal time scales. The relationships between NDVI and rainfall/temperature differ north of 18.9°S where rainfall is higher than to its south. Typically, there are lags of 1–2‐months between NDVI and either rainfall or temperature. Also, there are large areas across the region that show significant warming trends in all seasons as well as wetting (mainly in the north). This increasing trend in surface temperature may act to worsen the impacts of extreme events such as severe drought and fire in the region. There is also pronounced interannual variability with significant correlations found with El Niño‐Southern Oscillation (ENSO), the subtropical South Indian Ocean Dipole and the Botswana High for rainfall and temperature, and for NDVI with ENSO. For the Southern Annular Mode, significant correlations were found with rainfall and temperature only in December and April. On longer time scales, focus was also placed on the wet 2006–2013 period relative to much drier 1999–2005 epoch for October–December. The wetter conditions during 2006–2013 appear related to La Niña Modoki conditions and warmer sea surface temperature near Angola as well as regional circulation differences.

Decadal Variability in Salinity of the Indian Ocean Subtropical Underwater During the Argo Period

AbstractSalinity of the Indian Ocean Subtropical Underwater (IOSTUW) showed robust decadal variability during the Argo period, with a freshening trend between 2005 and 2013 and a salinification trend between 2014 and 2019. The salinity variability of the IOSTUW originated from both the sea surface salinity maximum region in the subtropical South Indian Ocean and the southeastern tropical Indian Ocean. The mixed layer salinity variabilities in these two regions are dominated by horizontal advection, which is mainly influenced by anomalous local surface currents and salinity anomalies from the western tropical Pacific and Maritime Continent. Both of these two factors have a close relationship with the El Niño‐Southern Oscillation (ENSO) signal. As a result, the mixed layer salinity in those two regions changed dramatically during the strong and long‐lasting ENSO events and thus led to the decadal variability in salinity of the IOSTUW.

Moisture sources associated with heavy rainfall over the Limpopo River Basin, southern Africa

Moisture sources and pathways over the Limpopo River Basin (LRB) in southern Africa were identified using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model applied to NCEP II (2.5° × 2.5°) reanalysis data for 1981–2016. The 10-day air parcel backward trajectories were produced for the extended wet season (October–April) as well as the early and late summer. Analysis of a 36-year climatology of air parcel trajectories indicated seven moisture source regions for the LRB; namely, local continental, tropical southeast Atlantic Ocean, midlatitude South Atlantic Ocean, tropical northwest Indian Ocean, tropical southwest Indian Ocean, subtropical southwest Indian Ocean, and the Agulhas Current. The results have shown that important differences in moisture source regions and pathways exist between early (October–December) and late (January–April) summers, with the tropical northwestern Indian Ocean and the northern Agulhas Current sources more prominent during JFMA than OND. On interannual time scales, there are notable differences in moisture source regions between anomalously wet and dry summers, with the South Indian Ocean moisture contribution and transport over the LRB smaller during dry summers than wet summers. Changes in specific humidity for trajectory linked to heavy daily rainfall events (defined from CHIRPS data) showed that subtropical South Indian Ocean source is more extensive for the heavy rainfall than for the moderate case. Generally, moisture source regions and transport pathways for LRB tend to be influenced by both the regional summer season circulation and the synoptic setting.

Eddy‐Induced Meridional Salt Flux and Its Impacts on the Sea Surface Salinity Maxima in the Southern Subtropical Oceans

AbstractEnergetic eddy activities are observed in the southern subtropical oceans. Among the three ocean basins, eddies are most active in the subtropical south Indian Ocean, where they induce a northward salt flux in the north and a southward salt flux in the south, resulting in a divergence of salt in the central part of the basin. Further analysis of satellite‐sea surface salinity observations shows that eddy‐induced meridional salt flux takes more than half of the salt gain from the excess of evaporation over precipitation out of the sea surface salinity maximum region in the south Indian Ocean, while this ratio falls below 19% in the South Pacific and 12% in the South Atlantic. The result may partially explain why salinity of the sea surface salinity maximum in the south Indian Ocean is the lowest among the three southern subtropical oceans.

Subtropical sea surface salinity maxima in the South Indian Ocean

Subtropical sea surface salinity (SSS) maximum is formed in the subtropical South Indian Ocean (SIO) by excessive evaporation over precipitation and serves as the primary salt source of the SIO. Space-borne SSS measurements by Aquarius satellite during September 2011–May 2015 detect three disconnected SSS maximum regions (>35.6) in the eastern (105°E–115°E, 38°S–28°S), central (60°E–100°E, 35°S–25°S), and western (25°E–10°E, 38°S–20°S) parts of the subtropical SIO, respectively. Such structure is however not seen in gridded Argo data. Analysis of Argo profile data confirms the existence of the eastern maximum patch and also reveals SSS overestimations of Aquarius near the western and eastern boundaries. Although subjected to large uncertainties, a mixed-layer budget analysis is employed to explain the seasonal cycle of SSS. The eastern and central regions reach the highest salinity in February–March and lowest salinity in August–September, which can be well explained by surface freshwater forcing (SFF) term. SFF is however not controlled by evaporation (E) or precipitation (P). Instead, the large seasonal undulations of mixed layer depth (MLD) is the key factor. The shallow (deep) MLD in austral summer (winter) amplifies (attenuates) the forcing effect of local positive E–P and causes SSS rising (decreasing). Ocean dynamics also play a role. Particularly, activity of mesoscale eddies is a critical factor regulating SSS variability in the eastern and western regions.

A classification of synoptic weather patterns linked to extreme rainfall over the Limpopo River Basin in southern Africa

In the last few decades, the Limpopo River Basin (LRB) has experienced a number of extreme rainfall events which were responsible for considerable socio-economic and environmental impacts. Most of the population here is poor and dependent on rain-fed agriculture. In order to better understand these events over the LRB, CHIRPS, 0.05° gridded rainfall data are used to identify the daily extreme events, analyse their interannual variability and examine relationships with large scale climate modes over the 1981–2016 period. Analysis of the top 20 events suggests a pattern with rainfall generally decreasing from the eastern to western parts of the basin. Typically, the highest rainfall amounts occur over the regions where there are steep topographical gradients between the mountainous regions of northeastern South Africa and the Mozambican floodplains. Almost half of the top 200 extreme events are associated with tropical extra-tropical cloud bands (48%), with tropical low-pressure systems (28%), Mesoscale Convective Systems (14%), and cut-off lows (10%) in the mid-upper atmosphere, also making sizeable contributions. The monthly distribution of the events showed that most of the events occurred during the late summer months (January–March) when tropical lows and cloud bands are more common. On interannual time-scales, most of the summers with above average number of events coincide with La Nina conditions and, to lesser extent, a positive subtropical South Indian Ocean Dipole.

Quasi-biennial Rossby and Kelvin waves in the South Indian Ocean: Tropical and subtropical modes and the Indian Ocean Dipole

The quasi-biennial (QB) band (1.2 yr ≤T < 3 yr) accounts for large part of the interannual variability in the South Indian Ocean (SIO). Despite that, it did not receive much attention to date, especially in the subtropical region. In the present work, we investigate the QB variability in sea surface height, the relationship between the tropics and the subtropics in this period band and the influence of the Indian Ocean Dipole (IOD). We use 24 years of altimeter-derived sea surface height anomalies and the MERRA-2 reanalysis winds. We find QB signals propagating westward in the tropical and subtropical SIO and eastward along the Equator; they are very likely to be expressions of Rossby and Kelvin-like waves in the QB frequency. A Complex Empirical Orthogonal Function (CEOF) analysis reveals two distinct reconstructed modes in terms of spatial distribution, which we refer as the tropical and the subtropical modes. The tropical mode explains about half of the QB variance and is given by the leading CEOF; the subtropical mode explains the QB variance in the subtropics and is formed by the superposition of the second to fifth CEOFs. In the subtropics, we show that the vertical structure of the background eastward-flowing South Indian Countercurrent affects the QB signal propagation. The tropical mode contains clear IOD spatial fingerprints, its temporal amplitudes increase in IOD years and has a high correlation with the Dipole Mode Index. Most importantly, the QB tropical waves that emerge in IOD years also appear in non-IOD years albeit with weaker amplitudes. An intriguing finding is the QB eastward propagation along the Equator, resembling a Kelvin wave. This eastward equatorial QB wave also has the highest amplitude in IOD-years, and the Maldives island chain at 73° E seems to disturb its propagation.

SIDDIES Corridor: A Major East‐West Pathway of Long‐Lived Surface and Subsurface Eddies Crossing the Subtropical South Indian Ocean

AbstractSouth Indian Ocean eddies (SIDDIES), originating from a high evaporation region in the eastern Indian Ocean, are investigated by tracking individual eddies from satellite data and co‐located Argo floats. A subsurface‐eddy identification method, based on its steric dynamic height anomaly, is devised to assign Argo profiles to surface eddies (surfSIDDIES) or subsurface eddies (subSIDDIES). These westward‐propagating, long‐lived features (&gt;3 months) prevail over a preferential latitudinal band, forming a permanent structure linking the eastern to the western Indian Ocean, that we call the 'SIDDIES Corridor’. Key features have been revealed in the mean thermohaline vertical structure of these eddies. Anticyclonic SIDDIES are characterized by positive subsurface salinity anomalies, with subSIDDIES not exhibiting negative surface anomalies, as opposed to surfSIDDIES. Cyclonic subSIDDIES also occur, but their related salinity anomalies are weaker. SubSIDDIES exhibit two cores of different temperature polarities in their surface and subsurface levels. Cyclonic subSIDDIES have their cores at around 150‐200 m depth along the 25.4‐25.8 kg m−3 potential density layer with anticyclonic subSIDDIES having their cores at 250‐300 m along the 26‐26.4 kg m−3 density layer. The SIDDIES corridor acts as a zonal pathway for both eddy‐types to advect water masses and biogeochemical properties across the basin. This study provides a new insight on heat/salt fluxes, showing that 58% (32%) of the total heat eddy‐flux is ascribed to cyclonic (anticyclonic) subSIDDIES, respectively, in the eastern South Indian Ocean. Anticyclonic subSIDDIES have also been found to be the sole high‐saline water eddy‐conveyor toward the western Indian Ocean.

Observational insights into chlorophyll distributions of subtropical South Indian Ocean eddies

AbstractThe South Indian Ocean subtropical gyre has been described as a unique environment where anticyclonic ocean eddies highlight enhanced surface chlorophyll in winter. The processes responsible for this chlorophyll increase in anticyclones have remained elusive, primarily because previous studies investigating this unusual behavior were mostly based on satellite data, which only views the ocean surface. Here we present in situ data from an oceanographic voyage focusing on the mesoscale variability of biogeochemical variables across the subtropical gyre. During this voyage an autonomous biogeochemical profiling float transected an anticyclonic eddy, recording its physical and biological state over a period of 6 weeks. We show that several processes might be responsible for the eddy/chlorophyll relationship, including horizontal advection of productive waters and deeper convective mixing in anticyclonic eddies. While a deep chlorophyll maximum is present in the subtropical Indian Ocean outside anticyclonic eddies, mixing reaches deeper in anticyclonic eddy cores, resulting in increased surface chlorophyll due to the stirring of the deep chlorophyll maximum and possibly resulting in new production from nitrate injection below the deep chlorophyll maximum.

On the use of composite analyses to form physical hypotheses: An example from heat wave - SST associations.

This paper highlights some caveats in using composite analyses to form physical hypotheses on the associations between environmental variables. This is illustrated using a specific example, namely the apparent links between heat waves (HWs) and sea surface temperatures (SSTs). In this case study, a composite analysis is performed to show the large-scale and regional SST conditions observed during summer HWs in Perth, southwest Australia. Composite results initially point to the importance of the subtropical South Indian Ocean, where physically coherent SST dipole anomalies appear to form a necessary condition for HWs to develop across southwest Australia. However, sensitivity tests based on pattern correlation analyses indicate that the vast majority of days when the identified SST pattern appears are overwhelmingly not associated with observed HWs, which suggests that this is definitely not a sufficient condition for HW development. Very similar findings are obtained from the analyses of 15 coupled climate model simulations. The results presented here have pertinent implications and applications for other climate case studies, and highlight the importance of applying comprehensive statistical approaches before making physical inferences on apparent climate associations.