ALBERT

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.

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.

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: Highlights: • We compare the simulated Arctic Ocean in 15 global ocean–sea ice models. • There is a large spread in temperature bias in the Arctic Ocean between the models. • Warm bias models have a strong temperature anomaly of inflow of Atlantic Water. • Dense outflows formed on Arctic shelves are not captured accurately in the models. In this paper we compare the simulated Arctic Ocean in 15 global ocean-sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE-II). Most of these models are the ocean and sea-ice components of the coupled climate models used in the Coupled Model Intercomparison Project Phase 5 (CMIP5) experiments. We mainly focus on the hydrography of the Arctic interior, the state of Atlantic Water layer and heat and volume transports at the gateways of the Davis Strait, the Bering Strait, the Fram Strait and the Barents Sea Opening. We found that there is a large spread in temperature in the Arctic Ocean between the models, and generally large differences compared to the observed temperature at intermediate depths. Warm bias models have a strong temperature anomaly of inflow of the Atlantic Water entering the Arctic Ocean through the Fram Strait. Another process that is not represented accurately in the CORE-II models is the formation of cold and dense water, originating on the eastern shelves. In the cold bias models, excessive cold water forms in the Barents Sea and spreads into the Arctic Ocean through the St. Anna Through. There is a large spread in the simulated mean heat and volume transports through the Fram Strait and the Barents Sea Opening. The models agree more on the decadal variability, to a large degree dictated by the common atmospheric forcing. We conclude that the CORE-II model study helps us to understand the crucial biases in the Arctic Ocean. The current coarse resolution state-of-the-art ocean models need to be improved in accurate representation of the Atlantic Water inflow into the Arctic and density currents coming from the shelves.

Description: Highlights: • Arctic sea ice extent and solid freshwater in 14 CORE-II models are inter-compared. • The models better represent the variability than the mean state. • The September ice extent trend is reasonably represented by the model ensemble mean. • The descending trend of ice thickness is underestimated compared to observations. • The models underestimate the reduction in solid freshwater content in recent years. Abstract: The Arctic Ocean simulated in fourteen global ocean-sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE II) is analyzed. The focus is on the Arctic sea ice extent, the solid freshwater (FW) sources and solid freshwater content (FWC). Available observations are used for model evaluation. The variability of sea ice extent and solid FW budget is more consistently reproduced than their mean state in the models. The descending trend of September sea ice extent is well simulated in terms of the model ensemble mean. Models overestimating sea ice thickness tend to underestimate the descending trend of September sea ice extent. The models underestimate the observed sea ice thinning trend by a factor of two. When averaged on decadal time scales, the variation of Arctic solid FWC is contributed by those of both sea ice production and sea ice transport, which are out of phase in time. The solid FWC decreased in the recent decades, caused mainly by the reduction in sea ice thickness. The models did not simulate the acceleration of sea ice thickness decline, leading to an underestimation of solid FWC trend after 2000. The common model behavior, including the tendency to underestimate the trend of sea ice thickness and March sea ice extent, remains to be improved.

Description: Highlights: • Arctic liquid freshwater budget simulated in 14 CORE-II models is studied. • The models better represent the temporal variability than the mean state. • Multi-model mean (MMM) FW fluxes compare well with observations. • MMM FWC shows an upward trend in the recent years, with an underestimated rate. • FW flux interannual variability is more consistent where volume flux determines it. Abstract: The Arctic Ocean simulated in 14 global ocean-sea ice models in the framework of the Coordinated Ocean-ice Reference Experiments, phase II (CORE-II) is analyzed in this study. The focus is on the Arctic liquid freshwater (FW) sources and freshwater content (FWC). The models agree on the interannual variability of liquid FW transport at the gateways where the ocean volume transport determines the FW transport variability. The variation of liquid FWC is induced by both the surface FW flux (associated with sea ice production) and lateral liquid FW transport, which are in phase when averaged on decadal time scales. The liquid FWC shows an increase starting from the mid-1990s, caused by the reduction of both sea ice formation and liquid FW export, with the former being more significant in most of the models. The mean state of the FW budget is less consistently simulated than the temporal variability. The model ensemble means of liquid FW transport through the Arctic gateways compare well with observations. On average, the models have too high mean FWC, weaker upward trends of FWC in the recent decade than the observation, and low consistency in the temporal variation of FWC spatial distribution, which needs to be further explored for the purpose of model development.

Description: The Agulhas is a convoluted and multifarious system [1]. It consists of a western boundary current, the Agulhas Current, which is arguably one of the most prominent current systems of the Southern Hemisphere (Fig.1). The Agulhas Current, roughly on par with its Northern Hemisphere counterpart, the Gulf Stream, carries vast amount of heat and salt towards the pole [2].

Description: Along the eastern boundaries of the eastern tropical Atlantic (ETA), intense oxygen minimum zones (OMZs) at intermediate depth result from the large remineralization of organic matter and the weak interior ventilation. The ETA is ventilated in the upper thermocline by the equatorial undercurrent (EUC). A part of the water transported by the EUC originate from the oxygen-rich subtropical regions, where subduction processes occur. The connection between subtropics and tropics is effectuated by wind driven shallow circulation cells, called subtropical-tropical cells (STC). To quantify the importance of the oxygen supply (in particular by the EUC and the STC) and the biological consumption, we perform simulations using an eddy resolving (1/10°) ocean model, embedded by a two-way nesting approach in a global coarse resolution model and coupled to a previously calibrated biogeochemical model that simulates phosphorus, oxygen, and nitrogen fluxes (MOPS-1.0). We perform two inter-decadal (1948 – 2007) experiments using two different atmospheric forcing data sets, CORE-II and JRA55-do. We show that the strength of the wind stress has a significant impact on the ocean circulation, the respiration rate and ultimately the oxygen levels