OS23B-1262
Indian Ocean Rossby Wave Structure Examined Using Multi-Sensor Satellite Observations and HYCOM Simulations
Rossby waves are difficult to detect with in situ methods, however they have been well identified in satellite observations of sea surface height (SSH), sea surface temperature (SST), and ocean color observations of Chlorophyll-a (Chl-a), as well as HYbrid Coordinate Ocean Model (HYCOM) simulations of SSH, SST, and Sea Surface Salinity (SSS) in the Indian Ocean. While the surface structure of Rossby waves can be elucidated from comparisons of the signal in different sea surface parameters, models are needed to gain direct information about how these waves affect the ocean at depth. The effect of Rossby waves on SSH, SST, ocean color, and SSS are studied using satellite data and 1/12° global HYCOM simulations. HYCOM simulations are then used to study the baroclinic structure of Rossby waves in the Indian Ocean.
OS23B-1263
Indian Ocean Kelvin waves in the Indonesian Throughflow Exit Passages
Equatorial Kelvin waves generated by westerly wind anomalies over the central Indian Ocean propagate eastward to Indonesia, where they can enter the outflow passages of the Indonesian Throughflow (ITF) and affect transports of mass and heat. This has potential consequences for local thermohaline properties and may also be related to larger scale climate modes such as the Indian Ocean Dipole (IOD) and Madden-Julian Oscillation (MJO). Moorings were deployed in Lombok and Ombai Straits, two ITF exit passages, as part of the three-year International Nusantara STratification ANd Transport (INSTANT) program. These data provide the first comprehensive, full-depth, high-resolution measurements of velocity and temperature in the exit passages and have allowed us to thoroughly characterize the properties of Kelvin waves in the throughflow region. Here, we discuss the relationship between ITF Kelvin waves and the structure and timing of the wind forcing over the Indian Ocean. Specifically, we focus on the partitioning of Kelvin wave energy between the ITF outflow passages, mode-1 versus mode-2 vertical structure of the Kelvin waves, and the connection between wind forcing, MJO, IOD, and monsoon dynamics.
OS23B-1264
The Dynamics of Zonal Current Variations in the Central Equatorial Indian Ocean
This paper examines the depth integrated zonal momentum balance in the central equatorial Indian Ocean (0 °, 80.5 °E) for two periods in 1993-1994 and 2004-2006 using a combination of in-situ and satellite observations. One- to two-year record length mean balances are approximately linear and steady state. Monthly to seasonal time scale variations are also governed by linear dynamics, with imbalances between the pressure gradient force and surface wind stress leading to zonal mass transport variations along the equator. Interannual variations in zonal transports along the equator associated with Indian Ocean Dipole events in 1994 and 2006 are likewise consistent with linear dynamics. Implications of these results for understanding the role of Indian Ocean circulation in climate variability will be discussed.
OS23B-1265
Indian Ocean Intraseasonal Sea Surface Temperature Variability During Boreal Summer: Madden-Julian Oscillation Versus Submonthly Forcing and Processes
Intraseasonal sea-surface temperature (SST) variability in the Indian Ocean during boreal summer is investigated with a series of experiments using the HYbrid Coordinate Ocean Model (HYCOM). QuickSCAT winds and satellite observed outgoing longwave radiation (OLR) are used to identify the wind and convection patterns associated with atmospheric intraseasonal oscillations (ISOs). Effects of the Madden-Julian Oscillation (MJO; 30-90 days) and submonthly ISOs are separately examined. Similar to winter, MJO forcing dominates summertime SST variability, even though submonthly forcing is stronger. Wind plays a much larger role in altering SSTs than either shortwave fluxes or precipitation. Different from winter cases, the maximum summertime SST variability shifts to the Arabian Sea (AS) and the Bay of Bengal (BOB), when ISOs also shift to the northern hemisphere. In the BOB, surface heat fluxes due to changes in wind speed have a stronger influence on SST than upwelling and advection induced by wind stress, whereas in winter the effects of wind speed and stress are comparable. This difference arises from the barrier layer and thin surface mixed layer in the BOB, which reduces the effects of upwelling and amplifies the effects of surface heat fluxes. In the AS, surface heat fluxes and entrainment cooling caused by wind speed have a larger effect on MJO-scale SST than wind stress, while the two have comparable effects on submonthly SST. In the equatorial region, wind speed and stress are equally important.
OS23B-1266
Seasonal mixed layer heat balance in the southwestern tropical Indian Ocean
Measurements from a long-term moored buoy are used in combination with satellite, in situ, and atmospheric reanalysis data sets to analyze the seasonal mixed layer heat balance in the southwestern tropical Indian Ocean. This region is characterized by a shallow thermocline, annual mean upwelling, and a strong seasonal cycle of SST. It is found that surface heat fluxes and horizontal heat advection contribute significantly to the seasonal cycle of mixed layer heat storage. Vertical turbulent mixing, estimated as a residual in the heat balance, also undergoes a strong seasonal cycle, reaching a maximum in boreal summer, when surface wind and buoyancy forcing are strongest.
OS23B-1267
Contribution of Indian Ocean SST to Regional Rainfall Variability: Mechanisms and Implications for Forecasting
The potential impact of Indian Ocean sea surface temperature (SST) anomalies in modulating low- to mid-
latitude precipitation around the Indian Ocean-rim countries is examined in a series of atmospheric general
circulation model simulations. Two sets of integrations of opposite sign forced with a seasonally evolving
pattern in Indian Ocean SST with characteristics of both the tropical and subtropical Indian Ocean dipoles are
shown to induce precipitation changes around the adjacent land masses. In additional experiments, the
relative importance of the various tropical and subtropical Indian Ocean SST poles, both individually and in
combination, to regional precipitation changes is quantified.
A mechanism explaining the modification in the rainfall is proposed, by which the SST anomalies induce a
reorganization of the large-scale atmospheric circulation across the Indian Ocean basin. The pattern of
large-scale circulation changes over the tropical Indian Ocean and adjacent land masses is consistent with
an anomalous
strengthening of the Walker cell. A reduction (increase) in sea level pressure over the western (eastern) half
of the Indian Ocean and converging (diverging) wind anomalies over East Africa (the Indonesian
Archipelago) lead to moisture
convergence (divergence) and increased (reduced) convective activity over the region. In the simulations,
enhancement of the East African rainy season is predominantly driven by the local warm SST anomalies in
the western equatorial
Indian Ocean, while the eastern cold pole of the tropical Indian Ocean dipole is of lesser importance. Over
the mid-latitudes, the SST anomalies give rise to changes in
the thermal properties of the atmosphere, meridional thickness gradient, subtropical jet, thermal wind, and
baroclinicity. This leads to shifts in the precipitation over western and southern regions of Australia.
http://www.maths.unsw.edu.au/~ matthew
OS23B-1268
Respective Forcing Role of the Indian Ocean and Western Pacific Warming on the Northern Hemisphere Atmospheric Circulation
The impact of the Indo-Pacific warming from the mid-XXth century onwards is investigated through coordinated experiments using 5 atmospheric global circulation models within the EU-DYNAMITE project. Sea Surface Temperature seasonal trends diagnosed from observations are prescribed in the models, the oceanic forcing domain either covering the entire Indo-Pacific warmpool (hereafter IP experiments) or being restricted to the limited geographical Indian Ocean (hereafter IO experiments). In winter, IP warming leads to North Atlantic atmospheric changes projecting onto the positive phase North Atlantic Oscillation in all the models. Even if the mean reponses of each model are close enough, the associated mechanisms appear to be different. Some models favor an hemispheric route via the North Pacific through the propagation of Rossby waves along the upper-level jet wave guide. Others suggest a more indirect path via the tropical Atlantic. The difference between models reponses is attributed to the discrepencies between their mean climatological states and in particular the respective weight of their climatological transients/stationary waves activity. In summer, IP warming leads to large-scale northern hemisphere warming with maximum loadings over Europe, projecting onto the so-called Blocking pattern. In the IO experiments, double amplitude responses are found in winter in all the models. Comparing to observational changes, the strength of the response is unrealistic suggesting a critical oversensitivity of the models to the western Pacific oceanic conditions. The IO-Northern Atlantic connection is sustained in all models via the overly dominant Pacific route. By contrast, summer IO responses are significantly reduced in all the models and cannot explain the observed trends. Such coordinated experiments suggest that the mean climatological states of the models are crucial to understand the individual responses to a similar oceanic forcing. IP and IO comparisons point out the extreme sensibility of the models responses to the change in the tropical forcing domain. This suggests to be cautious and precise enough in what is commonly referred to as "Indian Ocean" and related remote impacts.
OS23B-1269
Indian Weather Events inducing Climate Changes in the Tropics
We study the link between sudden wind and rain events over the Indian ocean using QuikSCAT and TRMM satellite data and an Indian Ocean-Atmosphere-Land model. Comparing the results forced by daily- or monthly- averaged values, we find that the sudden events significantly affect the ocean from bi-weekly to annual changes, with consequences on the Pacific warm pool and beyond. By classifying TRMM data into weather regimes we find that the northward propagation of the Indian ocean signals up to the Bengladesh is linked to the eastward propagation of the MJOs around the Planet.
OS23B-1270
Validation of a Hybrid Coordinate Ocean Model for the Indian Ocean
Towards the goal of developing a forecasting system for the Indian Ocean, a Hybrid Coordinate Ocean Model (HYCOM) is set up for the Indian Ocean Region. The present work focuses on a detailed study of the model's capability to simulate the major surface and subsurface variables realistically. The model results are validated against in situ and satellite data. The model gives good comparisons with observed data, especially for the surface currents and the temperatures of the surface waters. A detailed comparison of results from the daily data of the model with the ARGO float data for the years from 2002 to 2004 showed that the model successfully simulates the subsurface temperature and salinity patterns as well. However the model does produce too diffuse thermocline in the northern parts of the domain, thus retaining warm waters in the subsurface regions. Possible reasons for these subsurface warm water simulations are being investigated now. The mini cold pool off the southern tip of India is clearly simulated by the model and a detailed study of this phenomenon is being done with HYCOM.
OS23B-1271
Seasonal and interannual variations of subsurface zonal currents in the eastern equatorial Indian Ocean
Variations of subsurface zonal currents in the eastern equatorial Indian Ocean are investigated by examining 6 years' data (December 2000 – November 2006) from acoustic Doppler current profiler (ADCP) mooring at 0S, 90E. The analysis indicates the presence of robust subsurface eastward currents during both boreal winter and summer. These subsurface eastward currents are generated by eastward pressure gradient associated with the equatorial wave dynamics. During boreal winter, the generation of eastward pressure gradient is caused primarily by upwelling equatorial Kelvin waves excited by prevailing easterly winds. On the other hand, the downwelling Rossby waves generated by the reflection of the spring downwelling Kelvin waves in the eastern boundary as well as the upwelling equatorial Kelvin waves triggered by easterlies create an oceanic state that favours the generation of the eastward pressure gradient during boreal summer. In addition, these subsurface eastward currents reveal a distinct seasonal asymmetry. The maximum eastward speed of 63 cm/s is observed in April and secondary maximum of 49 cm/s is seen in October. The zonal transport per unit width within the depth where these subsurface eastward currents are defined exhibits similar variations: reaching maximum eastward transport of 35 m2/s in April and secondary maximum of 29 m2/s in October. Moreover, the subsurface eastward current during boreal summer undergoes significant interannual variations: it was absent in 2003, but it was anomalously strong during 2006.
OS23B-1272
Eddy Generation by a Steady Poleward Outflow
The energetic eddy field coincident with the South Equatorial Current (SEC) in the eastern Indian Ocean has been variously attributed to baroclinic instability of the SEC, barotropic instability of the SEC and shedding of eddies by the branch of the Indonesian Throughflow (ITF) entering the basin through Timor Passage. We present an additional mechanism by demonstrating that in an idealized numerical model, a steady poleward outflow (meant to simulate the Lombok Strait branch of the ITF) can generate an eddy field with spatial and temporal patterns that bear a remarkable resemblance to observations of sea surface height varibility in the region. A simple conceptual model will be presented which links the nonlinear, eddy-generating dynamics to linear dynamics, thus making possible the prediction of eddy amplitudes and periodicity for a wide range of outflow latitudes and volume fluxes.