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Extreme turbulent heat flux (THF) event is a key linkage between the atmosphere and ocean. Based on ERA5 reanalysis data, this study investigates the synoptic and interannual variability of extreme THF events in the Southern Indian Ocean during austral winter (May–September) from 1979 to 2018. On synoptic timescale, the extreme THF events account for approximately 36% of total THF, with the duration of 20% within the total analysis period. The extreme THF events are induced by strong sea surface temperature (SST) gradient in the oceanic front and synoptic eastward Rossby wave train with equivalent barotropic structure, associated with advecting relatively dry and cold air over warm SST at the edge of eddies. On interannual time scales, the occurrences of extreme THF events have a significant negative correlation with Southern Annular Mode (SAM). During the negative phase of SAM, a negative pressure anomaly is located in the Southern Indian Ocean, corresponding to a large cyclonic circulation anomaly. As a result, surface wind speeds and the differences of temperature and humidity between the atmosphere and ocean are strengthened over the warm flank of the oceanic front, leading to the onset of extreme THF event. Additionally, the accumulated effect of extreme THF events can not only deepen the oceanic mixed layer locally, but also have a downstream impact possibly through oceanic advection effect. Plain Language Summary: Turbulent heat flux plays an important role in air‐sea interaction, influencing the convection of both ocean and atmosphere. The extreme turbulent heat flux events are frequent over the oceanic front at midlatitudes. This study describes the synoptic and interannual characteristics of extreme turbulent heat flux events in the Southern Indian Ocean, over the warm flank of Agulhas Return Current front. The causes of extreme turbulent heat flux events are associated with climatological sea surface temperature (SST) front and atmospheric transient eddies on synoptic time scale, while the Southern Annular Mode can modulate its occurrence on interannual time scale. These strong events have great feedback on oceanic mixed layer. Key Points: The extreme turbulent heat flux (THF) events in the Southern Indian Ocean are related to the climatological sea surface temperature (SST) front and atmospheric transient eddiesSouthern Annular Mode can modulate the occurrence of extreme THF events on interannual time scalesThe accumulated effect of extreme THF events can deepen the oceanic mixed layer locally on interannual time scales [ABSTRACT FROM AUTHOR]

This study identifies a new triggering mechanism of the Indian Ocean Dipole (IOD) from the Southern Hemisphere. This mechanism is independent from the El Niño‐Southern Oscillation (ENSO) and tends to induce the IOD before its canonical peak season. The joint effects of this mechanism and ENSO may explain different lifetimes and strengths of the IOD. During its positive phase, development of sea surface temperature cold anomalies commences in the southern Indian Ocean, accompanied by an anomalous subtropical high system and anomalous southeasterly winds. The eastward movement of these anomalies enhances the monsoon off Sumatra‐Java during May–August, leading to an early positive IOD onset. The pressure variability in the subtropical area is related with the Southern Annular Mode, suggesting a teleconnection between high‐latitude and midlatitude climate that can further affect the tropics. To include the subtropical signals may help model prediction of the IOD event. Plain Language Summary: An Indian Ocean Dipole (IOD) at its positive phase featuring anomalously high and low sea surface temperature (SST) in the west and east equatorial Indian Ocean, respectively, shifts atmosphere convection westward, causing severe floods and droughts in surrounding west and east Indian Ocean‐rim regions. Known triggering mechanisms, such as the external El Niño‐Southern Oscillation (ENSO), cannot explain development or intensity of many IOD events. Here we find a novel triggering mechanism of the IOD from the Southern Hemisphere, in which the subtropical high pressure and wind anomalies forcing cool SST anomalies evolve to trigger onset of an IOD event via enhancing the monsoon off Sumatra‐Java. This Southern Hemisphere Mechanism can operate independently from the ENSO and commences earlier than the ENSO forcing, thus providing explanation of different IOD characteristics and a longer prediction lead time. Key Points: A triggering mechanism of the IOD originated from the Southern Hemisphere is identifiedThe Southern Hemisphere Mechanism is independent from ENSO and associated with subtropical and high‐latitude climatic variabilityThe joint effects of such mechanism and ENSO produce different characteristics of the IOD [ABSTRACT FROM AUTHOR]

ABSTRACT The study reports on existence of a dipole pattern in maximum significant wave height ( MSWH) as well in mean significant wave height over the Southern Indian Ocean ( SIO) based on analysis using 24 years of satellite observations. Analysis using quality checked daily altimeter data from nine satellite missions for entire Indian Ocean region revealed the spatio-temporal variability of observed dipole phenomena. Interestingly, the second empirical orthogonal function ( EOF) mode ( EOF2) in monthly and monthly anomaly MSWH data clearly shows the prevalence of dipole over the SIO having a period of 6 months for monthly dataset Principal Component ( PC) leading to a 3 months oscillation. The EOF of monthly anomaly dataset analysis also exhibited a similar dipole pattern for the EOF2 mode having a period of 2.1 years. In addition, the study also considered seasonal EOF analysis for four austral seasons December-January-February ( DJF), March-April-May ( MAM), June-July-August ( JJA), and September-October-November ( SON). The EOF2 for all seasons exhibits a similar dipole pattern in SIO basin, except for the DJF months. Based on seasonal EOF analysis, the DJF and JJA EOF2s have similar dipole pattern that reverses its polarity because of wind variability for the MAM and SON EOF2s justifying 3 months dipole oscillation obtained for monthly datasets. The Radon transform is used to investigate the orientation of this dipole, and for the first time this method is used in the field of climatology to study orientation and the degree of polarity of a dipole pattern. Furthermore, the study investigated possible correlation of dipole with various proven climate indices establishing the fact that Southern Annular Mode Index ( SAMI) showed a significant correlation. The other seasonal datasets also correlated well with SAMI, except DJF months that do not show the dipole pattern, and it had a good correlation with Nino3 index representing the El Nino Southern Oscillation Index. [ABSTRACT FROM AUTHOR]

In 2010 eastern Australia received its highest springtime (September-November) rainfall since 1900. Based on historical relationships with sea surface temperatures (SST) and other climate indices, this record rainfall in 2010 was shown to be largely commensurate with the occurrence of a very strong La Niña event and an extreme positive excursion of the SAM. The pattern and magnitude of the tropical SST anomalies in austral spring 2010 were diagnosed to be nearly perfect to produce high rainfall across eastern Australia. Key aspects of this SST pattern were the strong cold anomaly in the central equatorial Pacific, and the strong warm anomalies in the eastern Indian Ocean and the far western Pacific to the north of Australia. Although the recent upward trend in SSTs in the western Pacific/eastern Indian Ocean warm pool accounted for about 50 % of the SST anomaly surrounding northern Australia in 2010, the contribution by the warming trend in these SSTs to the Australian rainfall anomaly in 2010 was assessed to be relatively modest. The strong positive swing in SAM was estimated to have accounted for upwards of 40 % of the regional anomaly along the central east coastal region and about 10 % of the area mean anomaly across eastern Australia. This contribution by the SAM suggests that a significant portion of the rainfall in 2010 may not have been seasonally predictable. However, predictability arising from the promotion of high SAM by the extreme La Nina event can not be ruled out. [ABSTRACT FROM AUTHOR]

Abstract: The decadal variability of the fugacity of carbon dioxide (fCO2) at the sea surface is analyzed for the first time in the south-western Indian Ocean and corresponding Antarctic sector. This study is based on seasonal cruises (MINERVE and OISO) conducted onboard the R.S.S. Marion-Dufresne during the period 1991–2007. Based on shipboard observations the average annual rate of the atmospheric CO2 was 1.72ppm/yr, almost equal to the annual growth rate derived from high-quality measurements recorded at monitoring stations in the Southern Hemisphere. An evaluation based on oceanic observations in the Southern Indian Ocean (>20°S), indicates that oceanic fCO2 increased at a rate of 2.11 (±0.07)μatm/yr for the period 1991–2007, i.e. about 0.4μatm/yr faster than in the atmosphere. In order to investigate the processes that explain the oceanic fCO2 variations (and the potential reduction of the ocean carbon sink), the decadal variability is analyzed in detail in four regions (20–35°S, 35–40°S, 40–42°S and 50–55°S) for austral summer (December–March) and winter (June–August). During austral summer, the fCO2 increase is similar in the four regions (between +2.2 and +2.4μatm/yr). For austral winter the growth rate is lower north of 40°S (+1.5 to +1.7μatm/yr) than at higher latitudes (+2.2μatm/yr). Because these regions experienced different warming or cooling, the evolution of temperature normalized fCO2 (fCO2 norm) has also been investigated. In the southern subtropical region (35–40°S), warming occurred in winter, leading to a small change of fCO2 norm (+0.6μatm/yr). In this region, anthropogenic CO2 uptake must be compensated by a reduction of dissolved inorganic carbon (DIC) in surface waters. At latitudes >40°S, the observed cooling during winter leads to a rapid increase of fCO2 norm (+3.6 to +4.7μatm/yr), suggesting that the gradual import of DIC in surface water occurs in addition to anthropogenic CO2. The contrasting variations observed north and south of 40°S are likely related to the high index state of the Southern Annular Mode (SAM) during the 1990s. The increase of the westerlies at latitudes >40°S could have enhanced the vertical import of CO2-enriched deep waters in high-latitude surface layers, whereas the decrease of the wind speed north of 40°S would have reduced vertical mixing. Although this analysis is limited to a relatively short period, 1991–2007, this is the first time that a link between the SAM and the decadal reduction of the Southern Ocean carbon sink is suggested from in-situ ocean carbon dioxide observations. This offers an encouraging result in the perspective of model validation and understanding of the future evolution of the ocean carbon sink and its coupling with climate change. [Copyright &y& Elsevier]

Ten years of data of biogenic aerosol (methane sulfonic acid, MSA, and non-sea salt sulfate, nssSO 4 ) collected at Concordia Station in the East Antarctic plateau (75° 06' S, 123° 20' E) are interpreted as a function of the Southern Annular Mode (SAM), Chlorophyll-a concentration (Chl-a; a proxy for phytoplankton biomass), sea ice extent and area. It is possible to draw three different scenarios that link these parameters in early, middle, and late summer. In early summer, the biogenic aerosol is significantly correlated to sea ice retreats through the phytoplankton biomass increases. Chl-a shows a significant correlation with nssSO 2- in the finest fraction (< 1 μm). In contrast, only Chl-a in West Pacific and Indian Ocean sectors correlates with MSA in the coarse fraction. The transport routes towards the inner Antarctic plateau and aerosol formation processes could explain the different correlation patterns of the two compounds both resulting from the DMS oxidation. In mid-summer, Chl-a concentrations are at the maximum and are not related to sea ice melting. Due to the complexity of transport processes of air masses towards the Antarctic plateau, the MSA concentrations are low and not related to Chl-a concentration. In late summer, MSA and nssSO 4 2- in the finest fraction (< 1 μm). In contrast, only Chl-a in West Pacific and Indian Ocean sectors correlates with MSA in the coarse fraction. The transport routes towards the inner Antarctic plateau and aerosol formation processes could explain the different correlation patterns of the two compounds both resulting from the DMS oxidation. In mid-summer, Chl-a concentrations are at the maximum and are not related to sea ice melting. Due to the complexity of transport processes of air masses towards the Antarctic plateau, the MSA concentrations are low and not related to Chl-a concentration. In late summer, MSA and nssSO 4 2- present the highest concentrations in their submicrometric aerosol fraction, and both are significantly correlated with Chl-a but not with the sea ice. In early and mid-summer, the enhanced efficiency of transport processes from all the surrounding oceanic sectors with air masses traveling at low elevation can explain the highest concentrations of nssSO 4 2- and especially MSA. Finally, considering the entire time series, MSA shows significant year-to-year variability. This variability is significantly correlated with SAM but with a different time lag in early (0-month lag) and late summer (4-months lag). This correlation likely occurs through the effect of the SAM on phytoplankton blooms., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

While Arctic sea ice has been decreasing in recent decades that is largely due to anthropogenic forcing, the extent of Antarctic sea ice showed a positive trend during 1979-2015, followed by an abrupt decrease. The shortness of the satellite record limits our ability to quantify the possible contribution of anthropogenic forcing and internal variability to the observed Antarctic sea ice variability. In this study, ice core and fast ice records with annual resolution from six sites are used to reconstruct the annual-resolved northernmost latitude of sea ice edge (NLSIE) for different sectors of the Southern Ocean, including the Weddell Sea (WS), Bellingshausen Sea (BS), Amundsen Sea (AS), Ross Sea (RS), and the Indian and western Pacific Ocean (IndWPac). The linear trends of the NLSIE are analyzed for each sector for the past 100-200 years and found to be -0.08°, -0.17°, +0.07°, +0.02°, and -0.03° per decade (≥95% confidence level) for the WS, BS, AS, RS, and IndWPac, respectively. For the entire Antarctic, our composite NLSIE shows a decreasing trend (-0.03° per decade, 99% confidence level) during the 20th century, with a rapid decline in the mid-1950s. It was not until the early 1980s that the observed increasing trend occurred. A comparison with major climate indices shows that the long-term linear trends in all five sectors are largely dominated by the changes in the Southern Annular Mode (SAM). The multi-decadal variability in WS, BS, and AS is dominated by the Interdecadal Pacific Oscillation, whereas that in the IndWPac and RS is dominated by the SAM., Competing Interests: Conflict of interest The authors declare that they have no conflict of interest., (Copyright © 2021 Science China Press. Published by Elsevier B.V. All rights reserved.)