Air- Sea Interactions And Ocean Dynamics In The Southwest Tropical Indian Ocean

The Southwest Tropical Indian Ocean (SWTIO) features a unique, seasonal upward lift of the thermocline, which is known as the Seychelles-Chagos Thermocline Ridge (SCTR; 55°E-65°E, 5°S-12°S). It is known that a high correlation exists between the depth of the thermocline and sea surface temperature (SST; a key ingredient for tropical cyclogenesis). With a particular focus on 2012/2013, this study reveals the dynamic properties of the SCTR that play an important role in the modulation of tropical cyclones in the SWTIO. Phenomena including Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) are also well correlated to cyclogenesis through changes in the thermocline of the SCTR. More tropical cyclones form over the SWTIO when the thermocline is deeper, which has a positive relation to the arrival of downwelling Rossby waves originating in the southeast tropical Indian Ocean due to the anomalous effects of IOD. In addition to influencing cyclogeneis over the SCTR region, remote processes such as IOD and ENSO are also the primary drivers of the SCTR interannual variability with respect to both ocean temperature and salinity. Thus, this study also explores how temperature and salinity with depth, as well as at the surface, in the SCTR change with the climatic events in a given year. Although ENSO is known to have a stronger impact on SST south of the SCTR (10°S-15°S), this study reveals the stronger impact of ENSO on sea surface salinity (SSS) in the SCTR

CIRENE Air-Sea Interactions in the Seychelles-Chagos Thermocline Ridge Region

A field experiment in the southwestern Indian Ocean provides new insights into ocean-atmosphere interactions in a key climatic region

Cirene : air-sea iInteractions in the Seychelles-Chagos thermocline ridge region

Author Posting. © American Meteorological Society, 2009. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 90 (2009): 1337-1350, doi:10.1175/2008BAMS2499.1.The Vasco—Cirene program ex-plores how strong air—sea inter-actions promoted by the shallow thermocline and high sea surface temperature in the Seychelles—Chagos thermocline ridge results in marked variability at synoptic, intraseasonal, and interannual time scales. The Cirene oceano-graphic cruise collected oceanic, atmospheric, and air—sea flux observations in this region in Jan-uary—February 2007. The contem-poraneous Vasco field experiment complemented these measure-ments with balloon deployments from the Seychelles. Cirene also contributed to the development of the Indian Ocean observing system via deployment of a moor-ing and 12 Argo profilers. Unusual conditions prevailed in the Indian Ocean during Janu-ary and February 2007, following the Indian Ocean dipole climate anomaly of late 2006. Cirene measurements show that the Seychelles—Chagos thermocline ridge had higher-than-usual heat content with subsurface anomalies up to 7°C. The ocean surface was warmer and fresher than average, and unusual eastward currents prevailed down to 800 m. These anomalous conditions had a major impact on tuna fishing in early 2007. Our dataset also sampled the genesis and maturation of Tropical Cyclone Dora, including high surface temperatures and a strong diurnal cycle before the cyclone, followed by a 1.5°C cool-ing over 10 days. Balloonborne instruments sampled the surface and boundary layer dynamics of Dora. We observed small-scale structures like dry-air layers in the atmosphere and diurnal warm layers in the near-surface ocean. The Cirene data will quantify the impact of these finescale features on the upper-ocean heat budget and atmospheric deep convection.CNES funded the Vasco part of the experiment; INSU funded the Cirene part. R/V Suroît is an Ifremer ship. The contributions from ODU, WHOI, and FOI (Sweden) are supported by the National Science Foundation under Grant Number 0525657. The participation of the University of Miami group was funded though NASA (NNG04HZ33C). PMEL participation was supported through NOAA’s Office of Climate Observation

RAMA : the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction

Author Posting. © American Meteorological Society, 2009. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 90 (2009):459-480, doi:10.1175/2008BAMS2608.1.The Indian Ocean is unique among the three tropical ocean basins in that it is blocked at 25°N by the Asian landmass. Seasonal heating and cooling of the land sets the stage for dramatic monsoon wind reversals, strong ocean–atmosphere interactions, and intense seasonal rains over the Indian subcontinent, Southeast Asia, East Africa, and Australia. Recurrence of these monsoon rains is critical to agricultural production that supports a third of the world's population. The Indian Ocean also remotely influences the evolution of El Niño–Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), North American weather, and hurricane activity. Despite its importance in the regional and global climate system though, the Indian Ocean is the most poorly observed and least well understood of the three tropical oceans. This article describes the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA), a new observational network designed to address outstanding scientific questions related to Indian Ocean variability and the monsoons. RAMA is a multinationally supported element of the Indian Ocean Observing System (IndOOS), a combination of complementary satellite and in situ measurement platforms for climate research and forecasting. The article discusses the scientific rationale, design criteria, and implementation of the array. Initial RAMA data are presented to illustrate how they contribute to improved documentation and understanding of phenomena in the region. Applications of the data for societal benefit are also described

Relative contributions of heat flux and wind stress on the spatiotemporal upper-ocean variability in the tropical Indian Ocean

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Yuan, X., Ummenhofer, C. C., Seo, H., & Su, Z. Relative contributions of heat flux and wind stress on the spatiotemporal upper-ocean variability in the tropical Indian Ocean. Environmental Research Letters, 15(8), (2020): 084047, doi:10.1088/1748-9326/ab9f7f.High-resolution ocean general circulation model (OGCM) simulations are employed to investigate interannual variability of the upper-ocean temperature in the tropical Indian Ocean (20°S–20°N). The seasonal cycle and interannual variability in the upper-ocean temperature in the tropical Indian Ocean in the forced ocean simulation are in good agreement with available observation and reanalysis products. Two further sensitivity OGCM simulations are used to separate the relative contributions of heat flux and wind stress. The comparison of the model simulations reveals the depth-dependent influences of heat flux and wind stress on the ocean temperature variability in the tropical Indian Ocean. Generally, heat flux dominates the temperature variability in the top 30 m, while wind stress contributes most strongly to the subsurface temperature variability below 30 m. This implies that a transition depth should exist at each location, where the dominant control of the ocean temperature variability switched from heat flux to wind stress. We define the depth of this transition point as the 'crossing depth' and make use of this concept to better understand the depth-dependent impacts of the heat flux and wind stress on the upper-ocean temperature variability in the tropical Indian Ocean. The crossing depth tends to be shallower in the southern tropical Indian Ocean (20°S-EQ), including the Seychelles-Chagos Thermocline Ridge (SCTR) and the eastern part of the Indian Ocean Dipole (IOD), suggesting the dominance of forcing due to wind stress and the resulting ocean dynamical processes in the temperature variability in those regions. The crossing depth also shows prominent seasonal variability in the southern tropical Indian Ocean. In the SCTR, the variability of the subsurface temperature forced by the wind stress dominates largely in boreal winter and spring, resulting in the shallow crossing depth in these seasons. In contrast, the intensified subsurface temperature variability with shallow crossing depth in the eastern part of the IOD is seen during boreal autumn. Overall, our results suggest that the two regions within the tropical Indian Ocean, the SCTR and the eastern part of the IOD, are the primary locations where the ocean dynamics due to wind-stress forcing control the upper-ocean temperature variability.This research was supported by a Research Fellowship by the Alexander von Humboldt Foundation to CCU. HS is grateful for support by ONR (N00014-17-1-2398) and NOAA (NA17OAR4310255)

Nutrient Input in the Chagos Archipelago - the Controlling Mechanisms of Chlorophyll-a Distribution

Phytoplankton primary productivity is vital in supporting the marine trophic system, and an understanding of the nutrient sources facilitating this is critical for informing conservation management. Using a combination of in situ and remotely sensed data, we assess the contributions of different nutrient sources and corresponding productivity in a tropical island ecosystem free from anthropogenic nutrient pollution at varying temporal scales. Beyond the near-shore, annual mean normalised fluorescence line height (nFLH, a proxy for chlorophyll-a concentration) was 27% higher in shallow water (approx. 20m) than deep water (>5000m), demonstrating a chlorophyll-a enhancement in shallow waters. Linear regression revealed a significant (p=<0.001, N=852) relationship where increasing sea surface temperature (SST) was associated with decreasing nFLH. The same relationship was also found between SST anomaly and nFLH anomaly. These relationships were stronger in shallow water (R2=0.39 for SST and nFLH, and 0.18 for the anomaly) than deep water (R2=0.17 for SST and nFLH, and 0.10 for the anomaly), indicating shallower waters are more sensitive to strengthened stratification. Elevated SST is often associated with strengthened stratification, restricting entrainment from below the thermocline, and therefore restricting the movement of nutrients to the surface waters. In situ CTD measurements revealed a highly stratified environment, with a steep pycnocline. In the mouth of Egmont Atoll, the flood tide was, on average, cooler (by 0.07°C) and more saline (by 0.025 PSU) than the ebb tide (two-sampled t-test, p=<0.001). This represents a change in water properties towards those below the pycnocline, suggesting tidally driven mixing is entraining water from below the surface mixed layer. However, no significant difference was observed in nitrate concentration. Instead, nitrate concentration was associated with salinity. Decreases in salinity were associated with increases in nitrate, following precipitation with an approximately 2-day lag, indicating that precipitation-driven surface runoff is a key nutrient source in waters adjacent to land. Overall, results suggest that high-frequency, short-term variability in nitrate concentration is driven by precipitation, with longer term productivity variability governed by the strength of stratification, and consequential restriction of entrainment

Simulation of variability in the tropical Western Indian Ocean

Includes bibliographical references.The oceanic circulation and properties in the Tanzanian shelf region in the tropical western Indian Ocean have been studied in this thesis using a regional ocean model. The study investigated the influences of the Northeast Madagascar Current (NEMC) in the Tanzanian shelf waters at the annual cycle. Furthermore, the thesis examined the interannual variability of the sea surface temperature (SST) in the Tanzanian shelf region, and compares it with that offshore or with subsurface temperature. At the annual cycle, the westward-flowing NEMC advects relatively warm and fresh waters from the north of Madagascar towards the Tanzanian shelf region by interrupting the upwelled water from the Seychelles-Chagos ridge. At interannual timescales, the weakest interannual SST variations, which lie over the weak subsurface waters variations, occur in the coastal waters off Tanzania, where its variance is shared with waters to the north of Madagascar. Such SST variations are dominated by variability at about five year periods. The strongest interannual SST variations, which lie over the strongest subsurface temperature variations, occur offshore, being dominated by two periods, one at about 2.7 and the other near five years. The interannual variability of the region seems to be linked to El Niño- Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events, which induce changes in the thermocline and surface forcing in the region. Local surface heat flux exchanges driven by the anomalous shortwave radiation dominate the weakest interannual SST variability in the Tanzanian shelf region, with some contribution by the advection of heat anomalies from the NEMC. Further offshore, the strongest interannual variability of the SST is dominated by the thermocline variations induced by local Ekman pumping from local wind stress curl and by remote forcing from large-scale climate modes.

United States contributions to the Second International Indian Ocean Expedition (US IIOE-2)

From the Preface: The purpose of this document is to motivate and coordinate U.S. participation in the Second International Indian Ocean Expedition (IIOE-2) by outlining a core set of research priorities that will accelerate our understanding of geologic, oceanic, and atmospheric processes and their interactions in the Indian Ocean. These research priorities have been developed by the U.S. IIOE-2 Steering Committee based on the outcomes of an interdisciplinary Indian Ocean science workshop held at the Scripps Institution of Oceanography on September 11-13, 2017. The workshop was attended by 70 scientists with expertise spanning climate, atmospheric sciences, and multiple sub-disciplines of oceanography. Workshop participants were largely drawn from U.S. academic institutions and government agencies, with a few experts invited from India, China, and France to provide a broader perspective on international programs and activities and opportunities for collaboration. These research priorities also build upon the previously developed International IIOE-2 Science Plan and Implementation Strategy. Outcomes from the workshop are condensed into five scientific themes: Upwelling, inter-ocean exchanges, monsoon dynamics, inter-basin contrasts, marine geology and the deep ocean. Each theme is identified with priority questions that the U.S. research community would like to address and the measurements that need to be made in the Indian Ocean to address them.We thank the following organizations and programs for financial contributions, support and endorsement: the U.S. National Oceanic and Atmospheric Administration; the U.S. Ocean Carbon and Biogeochemistry program funded by the National Science Foundation and the National Aeronautics and Space Administration; the NASA Physical Oceanography Program; Scripps Institution of Oceanography; and the Indo-US Science and Technology Forum

Indonesian Throughflow Heat Transport, and Spreading within the Eastern Tropical Indian Ocean

The Indonesian Throughflow (ITF) is the only low latitude connector between the Pacific and Indian Oceans affecting upper ocean stratification and regional climate. Here we focus on the Indian Ocean side of this connection, first identifying changes within the primary throughflow pathway within the Indonesian Seas, then following the throughflow as it moves within the eastern tropical Indian Ocean. Moored velocity measurements and an ENSO varying temperature profile developed from all available observations within the Makassar Strait are used to determine the southward heat flux anomaly (HFa) within this primary pathway of the ITF. Variability in the velocity profile is more important than that of the temperature profile for determining changes in the total heat flux with the former accounting for 72% of the variance in HFa and the latter 28%. As the upper layer (0-300 m) is the site of the largest volume transports and also the largest transport variability, upper layer HFa is far more dominant than the lower (320-740 m) in influencing the total depth integrated HFa. Upper ocean heat content anomaly (0-300 m; HCa) in the eastern tropical Indian Ocean calculated from gridded Argo datasets is well correlated with Makassar HFa at interannual timescales (r = 0.8). The lag between the two is 2.5 years, indicating that this is consistent with an advective signal. From the Indo-Australian basin ITF waters flow either into the South Equatorial Current (SEC) to the west or the Leeuwin Current (LC) to the south. Gridded Argo data is used to track upper ocean heat content changes from the immediate outflow area into these two currents. The heat content anomaly timeseries in the region closest to the Indonesian Seas is well correlated with that at the easternmost section of the SEC with r = 0.8 at a 5 month lag. A notable exception occurs during 2011 when a positive heat content anomaly in the ITF outflow region is not later reflected in the SEC region, but rather expressed as an HCa increase the LC region. When compared to a previous HCa increase in the ITF outflow region during 2009, GODAS reanalysis shows that the velocity within the SEC was stronger eastward and the LC stronger southward during 2011. The Ningaloo Niño of 2011 was characterized by a low pressure anomaly off the west Australian Coast, which induced anomalous cyclonic circulation seen in NCEP/NCAR reanalysis winds at 1000 HPa. The positive zonal wind anomalies over the SEC and the reduction of southerly winds over the LC influenced these changes in current velocity. During the Ningaloo Niño of 2000 a similar pattern in atmospheric and oceanic circulation was identified. These results confirm the importance of the Ningaloo Niño in influencing the pathways of the ITF out of the Indo-Australian basin. Additionally, over the Argo time period, volume transport via the LC and SEC pathways appears anti-correlated, with increases in SEC outflow coupled with decreases in LC outflow. As the SEC is the major pathway for the ITF within the Indian Ocean, we examine the propagation of these low salinity waters within the SEC thermocline. Using gridded Argo data, we examine the salinity along the 24σ surface as a proxy for ITF propagation, and the depth of the 20°C isotherm (d20) to determine how changes in the thermocline depth may affect the flow. The d20 was correlated with the salinity (r=-0.5) in the region of the Seychelles Chagos Thermocline Ridge (SCTR), indicating that this region of upwelling, and the geostrophic currents that form around it, play a role in the westward propagation of the ITF. When examining the seasonal cycle, the effect of the SCTR is apparent as low salinity contours within the western portion of the basin show the furthest westward propagation during austral winter, when the SCTR is strong and most longitudinally expansive. On interannual timescales two years, 2010/11 and 2016/17, show anomalously high salinity in the SEC thermocline indicative of a reduction of ITF westward propagation. During late 2010 and 2016 anomalously strong upwelling regions are present at about 80°E and 10°S, out of the normal season for strong upwelling at this location. GODAS reanalysis velocity at 105 m shows cyclonic circulation developed around these upwelling centers, disrupting the normal zonal pathway of the SEC and reducing the amount of ITF able to propagate into the central Indian. As seen in the 34.8 salinity contour, both 2011 and 2017 show a reduction of 20 degrees of longitude of ITF westward propagation when compared to climatology. These upwelling regions were caused by both regional winds conducive to Ekman upwelling at that location, in addition to the absence of the annual westward propagating downwelling Rossby wave. This wave was absent during both late 2010 and 2016 due to positive zonal wind anomalies in the south east tropical Indian Ocean caused by a simultaneous occurrence of La Niña and a negative IOD