Description: Ocean circulation models do not generally exhibit equatorial deep jets (EDJs), even though EDJs are a recognised feature of the observed ocean circulation along the equator and they are thought to be important for tracer transport along the equator and even equatorial climate. EDJs are nevertheless found in nonlinear primitive equation models with idealised box geometry. Here we analyse several such model runs. We note that the variability of the zonal velocity in the model is dominated by the gravest linear equatorial basin mode for a wide range of baroclinic vertical normal modes and that the EDJs in the model are dominated by energy contained in vertical modes between 10 and 20. The emergence of the EDJs is shown to involve the linear superposition of several such neighbouring basin modes. Furthermore, the phase of these basin modes is set at the start of the model run and, in the case of the reference experiment, the same basin modes can be found in a companion experiment in which the amplitude of the forcing has been reduced by a factor of 1000. We also argue that following the spin-up, energy must be transferred between different vertical modes. This is because the model simulations are dominated by downward phase propagation following the spin-up whereas our reconstructions imply episodes of upward and downward propagation. The transfer of energy between the vertical modes is associated with a decadal modulation of the EDJs.

Description: Recent evidence from mooring data in the equatorial Atlantic reveals that semi-annual and longer time scale ocean current variability is close to being resonant with equatorial basin modes. Here we show that intraseasonal variability, with time scales of 10's of days, provides the energy to maintain these resonant basin modes against dissipation. The mechanism is analogous to that by which storm systems in the atmosphere act to maintain the atmospheric jet stream. We demonstrate the mechanism using an idealised model set-up that exhibits equatorial deep jets. The results are supported by direct analysis of available mooring data from the equatorial Atlantic Ocean covering a depth range of several thousand meters. The analysis of the mooring data suggests that the same mechanism also helps maintain the seasonal variability.

Description: An ocean circulation model is run using two different idealized equatorial basin configurations under steady wind forcing. Both model versions produce bands of vertically alternating zonal flow at depth, similar to observed Equatorial Deep Jets (EDJs) and with a time scale corresponding to that of the gravest equatorial basin mode for the dominant baroclinic vertical normal mode. Both model runs show evidence for enhanced variability in the surface signature of the North Equatorial Counter Current (NECC) with the same time scale. We also find the same link between the observed NECC and the EDJs in the Atlantic by comparing the signature of the EDJ in moored zonal velocity data at 23° W on the equator with the signature of the NECC in geostrophic velocities from altimeter data. We argue that the presence of a peak in variability in the NECC associated with the EDJ basin mode period is evidence that the influenceatthis time scale is upward, from the EDJ to the NECC

Description: The Equatorial Deep Jets are a series of stacked zonal jets below the Equatorial Undercurrent with a vertical scale of several hundred metres and a time scale of many years (4.5 years in the Atlantic Ocean). They are an ubiquitous feature of the equatorial ocean system, but to this day realistic domain ocean models fail to reproduce their basic features. This thesis begins by describing a set of idealised ocean model simulations that shed some light on why ocean models have such difficulties with the deep jets. The idealised model solutions are able to reproduce the basin mode like variability that can be found in observations. A sufficient vertical resolution is seen to be necessary for the evolution of deep jets in the model, and in contrast to previous studies the introduction of a realistic coastline does not eliminate the deep jet variability. But another feature, the prominent upward energy propagation of the deep jets found in observations, cannot be reproduced in a simulation with realistic bottom topography, albeit even though in this simulation the deep jets have the vertical scale and corresponding time scale of theoretical basin modes. In the next part of the thesis we explore a surface influence of the equatorial deep jets. The North Equatorial Countercurrent (NECC) exhibits a modulation in its meandering with the same time scale as the deep jet period. This was shown by comparing satellite data of the NECC with moored observations of the deep jets. The surface influence was then confirmed in two idealised model simulations with different basin widths, where the shorter basin width leads to a shorter basin mode period. We also diagnosed the vertical energy flux associated with the deep jets, the first time this has been done in a model, to give a reasoning for the deep jets’ influence on the NECC. Following that is a chapter about the emergence of deep jets in the idealised model simulation. The spin-up of these deep jets can be explained by the linear superposition of equatorial basin modes, where the time at which the model starts to exhibit downward phase propagation corresponds to the beat period of two superposed basin modes. The basin modes are excited at the start of the model run as Rossby and Kelvin waves at the eastern and western boundaries of the model and appear to be amplified by the non-linearity in the solution.

Description: The sea surface temperature (SST) in the eastern tropical Atlantic exhibits pronounced variability on interannual time scales being associated with wind and rainfall anomalies within the tropical Atlantic region. It has been proposed that the interannual variability of SST is partly driven by the variability of the deep equatorial zonal circulation, the so-called equatorial deep jets (EDJs). The EDJs may be described as a superposition of quasi-resonant equatorial basin modes and the direction of vertical phase propagation implies that their energy is propagating towards the surface. Furthermore, recent findings revealed that the EDJs in turn are maintained by intra-seasonal waves that are generated by the barotropic and baroclinic instability of the near-surface circulation. This talk will present the relevant mechanisms that are involved in the conversion of energy from one type of variability to another, i.e. from chaotic intra-seasonal surface variability via deep interannual zonal variability to interannual surface climate variability, with a special focus on the maintenance of the EDJs by intra-seasonal waves. Since EDJs, a key component of the mechanism discussed above, are not well represented in state-of-the-art Ocean General Circulation Models, preliminary findings on the sensitivity of the EDJs to model parameters and configuration are presented.

Description: The variability of the zonal circulation along the equator in the Atlantic Ocean is dominated by the seasonal cycle and the presence of the equatorial deep jets (EDJs). The seasonal cycle is externally driven by surface wind variability, however the mechanism which generates and maintains the EDJs against dissipation is not fully understood yet. Additionally, intra-seasonal stochastic variability, the tropical instability waves (TIWs), is generated in the upper ocean by both baroclinic and barotropic instability. The intra-seasonal energy at the equator reaches to depths of about 2000 m. We argue that the intra-seasonal variability gets distorted by the presence of the lower frequency zonal velocity variability. This causes a systematic convergence of intra-seasonal momentum flux such that the seasonal cycle and the EDJs are maintained against dissipation. The presence of this mechanism is demonstrated from two OGCM simulations and moored observations at 23W in the equatorial Atlantic.