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Tommy G. Jensen

Abstract

The effect of reducing the barotropic gravity wave speed in a layered ocean model in order to gain computational speed is explored. In theory the error in the propagation of baroclinic gravity waves typically is less than 3% for a reduction of the external gravity speed by one order of magnitude. This is confirmed in a numerical experiment. For baroclinic Rossby waves, the phase speed error is even less. The barotropic response is limited to the reduced radius of deformation. The method, which we will refer to as gravity wave retardation, is therefore applicable only for oceanic flows where the barotropic mode is of minor importance. It is demonstrated that the method gives very good results for the baroclinic flow of an equatorial jet, spinup of a midlatitude ocean and flow over a midoceanic ridge. The method can be considered as an alternative to multilayer reduced gravity models, and has the advantage that bottom topography can be included.

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Tommy G. Jensen

Abstract

Composites of Florida State University winds (1970–99) for four different climate scenarios are used to force an Indian Ocean model. In addition to the mean climatology, the cases include La Niña, El Niño, and the Indian Ocean dipole (IOD). The differences in upper-ocean water mass exchanges between the Arabian Sea and the Bay of Bengal are investigated and show that, during El Niño and IOD years, the average clockwise Indian Ocean circulation is intensified, while it is weakened during La Niña years. As a consequence, high-salinity water export from the Arabian Sea into the Bay of Bengal is enhanced during El Niño and IOD years, while transport of low-salinity waters from the Bay of Bengal into the Arabian Sea is enhanced during La Niña years. This provides a venue for interannual salinity variations in the northern Indian Ocean.

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Tommy G. Jensen
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Tommy G. Jensen

Abstract

The gravity wave retardation (GWR) method is a simple technique that allows layer models to include bottom topography. Here the method is applied, and its accuracy is evaluated, for monthly climatological wind forcing in an Indian Ocean model with realistic bottom topography. This is an extension of previous studies where the GWR method was applied to idealized wind forcing in oceans with idealized basin geometry. Comparison to a model integration with a flat bottom demonstrates that GWR integrations with speedup factors of up to 16 indeed capture the influence of the bottom relief and have less error in the deep volume transports. For a speedup factor of that magnitude, a GWR integration is also found to have less error than a reduced gravity model simulation. It is concluded that integrations using the GWR method give remarkably good results for the upper-layer circulation as well as the deep flow with a speedup factor of up to 8.

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Adam V. Rydbeck and Tommy G. Jensen

Abstract

A theory for intraseasonal atmosphere–ocean–atmosphere feedback is supported whereby oceanic equatorial Rossby waves are partly forced in the eastern Indian Ocean by the Madden–Julian oscillation (MJO), reemerge in the western Indian Ocean ~70 days later, and force large-scale convergence in the atmospheric boundary layer that precedes MJO deep convection. Downwelling equatorial Rossby waves permit high sea surface temperature (SST) and enhance meridional and zonal SST gradients that generate convergent circulations in the atmospheric boundary layer. The magnitude of the SST and SST gradient increases are 0.25°C and 1.5°C Mm−1 (1 megameter is equal to 1000 km), respectively. The atmospheric circulations driven by the SST gradient are estimated to be responsible for up to 45% of the intraseasonal boundary layer convergence observed in the western Indian Ocean. The SST-induced boundary layer convergence maximizes 3–4 days prior to the convective maximum and is hypothesized to serve as a trigger for MJO deep convection. Boundary layer convergence is shown to further augment deep convection by locally increasing boundary layer moisture. Warm SST anomalies facilitated by downwelling equatorial Rossby waves are also associated with increased surface latent heat fluxes that occur after MJO convective onset. Finally, generation of the most robust downwelling equatorial Rossby waves in the western Indian Ocean is shown to have a distinct seasonal distribution.

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Prasad G. Thoppil, Alan J. Wallcraft, and Tommy G. Jensen

Abstract

Along-track Argo observations in the northern Arabian Sea during 2017–19 showed by far the most contrasting winter convective mixing; 2017–18 was characterized by less intense convective mixing resulting in a mixed layer depth of 110 m, while 2018–19 experienced strong and prolonged convective mixing with the mixed layer deepening to 150 m. The response of the mixed layer to contrasting atmospheric forcing and the associated formation of Arabian Sea High Salinity Water (ASHSW) in the northeastern Arabian Sea are studied using a combination of Argo float observations, gridded observations, a data assimilative general circulation model, and a series of 1D model simulations. The 1D model experiments show that the response of winter mixed layer to atmospheric forcing is not only influenced by winter surface buoyancy loss, but also by a preconditioned response to freshwater fluxes and associated buoyancy gain by the ocean during the summer that is preceding the following winter. A shallower and short-lived winter mixed layer occurred during 2017–18 following the exceptionally high precipitation over evaporation during the summer monsoon in 2017. The precipitation-induced salinity stratification (a salinity anomaly of −0.7 psu) during summer inhibited convective mixing in the following winter, resulting in a shallow winter mixed layer (103 m). Combined with weak buoyancy loss due to weaker surface heat loss in the northeastern Arabian Sea, this caused an early termination of the convective mixing (26 February 2018). In contrast, the winter convective mixing during 2018–19 was deeper (143 m) and long-lived. The 2018 summer, by comparison, was characterized by normal or below normal precipitation which generated a weakly stratified ocean preconditioned to winter mixing. This combined with colder and drier air from the landmass to the north with low specific humidity led to strong buoyancy loss, and resulted in prolonged winter convective mixing through 25 March 2019.

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Yalin Fan, W. Erick Rogers, and Tommy G. Jensen

Abstract

The possibility of teleconnections between Southern Ocean swells and sea surface temperature (SST) anomalies on interannual time scales in the eastern Pacific Niño-3 region and southeastern Indian Ocean is investigated using numerical wave models. Two alternative parameterizations for swell dissipation are used. It is found that swell dissipation in the models is not directly correlated with large interannual variations such as El Niño–Southern Oscillation (ENSO) or the Indian Ocean dipole (IOD). However, using one of the two swell dissipation parameterizations, a correlation is found between observed SST anomalies and the modification of turbulent kinetic energy flux (TKEF) by Southern Ocean swells due to the damping of short wind waves: modeled reduction of TKEF is opposite in phase to the SST anomalies in the Niño-3 region, indicating a potential positive feedback. The modeled bimonthly averaged TKEF reduction in the southeastern Indian Ocean is also well correlated with the IOD mode.

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Patrick T. Haertel, David A. Randall, and Tommy G. Jensen

Abstract

A Lagrangian numerical model is used to simulate upwelling in an idealized large lake. This simulation is carried out to test the model's potential for simulating lake and ocean circulations.

The model is based on the slippery sack (SS) method that was recently developed by the authors. It represents the lake as a pile of conforming sacks. The motions of the sacks are determined using Newtonian dynamics. The model uses gravity wave retardation to allow for long time steps and has pseudo-Eulerian vertical mixing.

The lake is exposed to northerly winds for 29 h. Upwelling develops in the eastern edge of the basin, and after the winds shut off, upwelling fronts propagate around the lake. This case was previously simulated using a height- and sigma-coordinate ocean model. The SS model produces circulations that are similar to those produced by the other models, but the SS simulation exhibits less mixing.

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Toshiaki Shinoda, Tommy G. Jensen, Maria Flatau, and Sue Chen

Abstract

Simulation of surface wind and upper-ocean variability associated with the Madden–Julian oscillation (MJO) by a regional coupled model, the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS), is evaluated by the comparison with in situ and satellite observations. COAMPS is configured for the tropical Indian Ocean domain with the horizontal resolution of 27 km for the atmospheric component and ⅛° for the ocean component. A high-resolution nested grid (9 km) for the atmospheric component is used for the central Indian Ocean. While observational data are assimilated into the atmospheric component, no data are assimilated into the ocean component. The model was integrated during 1 March–30 April 2009 when an active episode of large-scale convection associated with the MJO passed eastward across the Indian Ocean. During this MJO event, strong surface westerly winds (~8 m s−1) were observed in the central equatorial Indian Ocean, and they generated a strong eastward jet (~1 m s−1) on the equator. COAMPS can realistically simulate these surface wind and upper-ocean variations. The sensitivity of upper-ocean variability to the atmospheric model resolution is examined by the COAMPS experiment without the high-resolution nested grid. The equatorial jet generated in this experiment is about 20% weaker than that in the first experiment, which significantly influences upper-ocean salinity and temperature. The large diurnal warming of SST during the suppressed phase of the MJO is also adequately simulated by the model. Weak winds during this period are mostly responsible for the large SST diurnal variation based on the comparison with the spatial variation of surface forcing fields.

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Adam V. Rydbeck, Tommy G. Jensen, and Matthew R. Igel

Abstract

The atmospheric response to sea surface temperature (SST) variations forced by oceanic downwelling equatorial Rossby waves is investigated using an idealized convection-resolving model. Downwelling equatorial Rossby waves sharpen SST gradients in the western Indian Ocean. Changes in SST cause the atmosphere to hydrostatically adjust, subsequently modulating the low-level wind field. In an idealized cloud model, surface wind speeds, surface moisture fluxes, and low-level precipitable water maximize near regions of strongest SST gradients, not necessarily in regions of warmest SST. Simulations utilizing the steepened SST gradient representative of periods with oceanic downwelling equatorial Rossby waves show enhanced patterns of surface convergence and precipitation that are linked to a strengthened zonally overturning circulation. During these conditions, convection is highly organized, clustering near the maximum SST gradient and ascending branch of the SST-induced overturning circulation. When the SST gradient is reduced, as occurs during periods of weak or absent oceanic equatorial Rossby waves, convection is much less organized and total rainfall is decreased. This demonstrates the previously observed upscale organization of convection and rainfall associated with oceanic downwelling equatorial Rossby waves in the western Indian Ocean. These results suggest that the enhancement of surface fluxes that results from a steepening of the SST gradient is the leading mechanism by which oceanic equatorial Rossby waves prime the atmospheric boundary layer for rapid convective development.

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