ALBERT

Description: Northern Europe and the UK experienced an exceptionally warm and wet winter in 2019/20, driven by an anomalously positive North Atlantic Oscillation (NAO). This positive NAO was well forecast by several seasonal forecast systems, suggesting that this winter the NAO was highly predictable at seasonal lead times. A very strong positive Indian Ocean dipole (IOD) event was also observed at the start of winter. Here we use composite analysis and model experiments, to show that the IOD was a key driver of the observed positive NAO. Using model experiments that perturb the Indian Ocean initial conditions, two teleconnection pathways of the IOD to the north Atlantic emerge: a tropospheric teleconnection pathway via a Rossby wave train travelling from the Indian Ocean over the Pacific and Atlantic, and a stratospheric teleconnection pathway via the Aleutian region and the stratospheric polar vortex. These pathways are similar to those for the El Niño Southern Oscillation link to the north Atlantic which are already well documented. The anomalies in the north Atlantic jet stream location and strength, and the associated precipitation anomalies over the UK and northern Europe, as simulated by the model IOD experiments, show remarkable agreement with those forecast and observed.

Description: There is a zonally oriented teleconnection pattern over the high-latitude Eurasian continent, which is maintained through baroclinic energy conversion. In this study, we investigate the unique features of the maintenance mechanism of this teleconnection. It is found that the baroclinic energy conversion is most efficient in both the mid-troposphere and the lower troposphere, and that the baroclinic energy conversion in the lower troposphere is comparable to that in the mid-troposphere. Further results indicate that the basic state plays a crucial role in the baroclinic energy conversion. For both the mid and lower troposphere, the atmospheric stability is low and the Coriolis parameter is large over high-latitude Eurasia, favoring strong baroclinic energy conversion. Particularly, in the lower troposphere, the atmospheric stability exhibits a clear land-sea contrast, favoring baroclinic energy conversion over the continents rather than the oceans. Furthermore, in the lower troposphere, the in-phase configuration of the meridional wind and temperature anomalies, which results from the strong meridional gradient of mean temperature around the north edge of the Eurasian continent, also significantly contributes to baroclinic energy conversion. This study highlights the role of the basic state of temperature rather than zonal wind in maintaining the high-latitude teleconnection through baroclinic energy conversion.

Description: A method using a linear shallow water model is presented for decomposing the temporal variability of the barotropic streamfunction in a high‐resolution ocean model. The method is based in the vertically‐averaged momentum equations and is applied to the time series of annual mean streamfunction from the model configuration VIKING20 for the northern North Atlantic. An important result is the role played by the nonlinear advection terms in VIKING20 for driving transport. The method is illustrated by examining how the Gulf Stream transport in the recirculation region responds to the winter North Atlantic Oscillation (NAO). While no statistically significant response is found in the year overlapping with the winter NAO index, there is a tendency for the Gulf Stream transport to increase as the NAO becomes more positive. This becomes significant in lead years 1 and 2 when the mean flow advection (MFA) and eddy momentum flux (EMF) contributions, associated with nonlinear momentum advection, dominate. Only after 2 years, does the potential energy (PE) term, associated with the density field, start to play a role and it is only after 5 years that the transport dependence on the NAO ceases to be significant. It is also shown that the PE contribution to the transport streamfunction has significant memory of up to 5 years in the Labrador and Irminger Seas. However, it is only around the northern rim of these seas that VIKING20 and the transport reconstruction exhibit similar memory. This is due to masking by the MFA and EMF contributions.

Description: Equatorial deep jets (EDJ) are vertically stacked, downward propagating zonal currents that alternate in direction with depth. In the tropical Atlantic, they have been shown to influence both surface conditions and tracer variability. Despite their importance, the EDJ are absent in most ocean models. Here we show that EDJ can be generated in an idealized ocean model when the model is driven only by the convergence of the meridional flux of intraseasonal zonal momentum diagnosed from a companion model run driven by steady wind forcing, corroborating the recent theory that intraseasonal momentum flux convergence maintains the EDJ. Additionally, the EDJ in our model nonlinearly generate mean zonal currents at intermediate depths that show similarities in structure to the observed circulation in the deep equatorial Atlantic, indicating their importance for simulating the tropical ocean mean state.

Description: Mesoscale eddies can be strengthened by the absorption of submesoscale eddies resulting from mixed-layer baroclinic instabilities. This is shown for mesoscale eddies in the Agulhas Current system by investigating the kinetic energy cascade with a spectral and a coarse-graining approach in two model simulations of the Agulhas region. One simulation resolves mixed-layer baroclinic instabilities and one does not. When mixed-layer baroclinic instabilities are included, the largest submesoscale near-surface fluxes occur in winter-time in regions of strong mesoscale activity for upscale as well as downscale directions. The forward cascade at the smallest resolved scales occurs mainly in frontogenetic regions in the upper 30 m of the water column. In the Agulhas ring path, the forward cascade changes to an inverse cascade at a typical scale of mixed-layer eddies (15 km). At the same scale, the largest sources of the upscale flux occur. After the winter, the maximum of the upscale flux shifts to larger scales. Depending on the region, the kinetic energy reaches the mesoscales in spring or early summer aligned with the maximum of mesoscale kinetic energy. This indicates the importance of submesoscale flows for the mesoscale seasonal cycle. A case study shows that the underlying process is the mesoscale absorption of mixed-layer eddies. When mixed-layer baroclinic instabilities are not included in the simulation, the open-ocean upscale cascade in the Agulhas ring path is almost absent. This contributes to a 20 %-reduction of surface kinetic energy at mesoscales larger than 100 km when submesoscale dynamics are not resolved by the model.

Description: Highlights: • The pivot point for sea level shifted to the west of the Nino4 region in the 2000s. • This enabled the thermocline feedback to increase strongly in the Central Pacific. • The resulting increase in CP events maintains the pivot point to the west, a positive feedback mechanism. Monthly mean sea level variations computed using a linear, reduced-gravity, multi-mode model are combined with satellite measurements to explore why Central Pacific (CP) ENSO events occur more frequently since 2000s. The pivot point for sea level (and hence thermocline) variations has shifted westward in response to an increase in zonal wind stress variance in the western equatorial Pacific. As a result, the Nino4 region is increasingly to the east of the pivot point enabling the thermocline feedback to operate there, strengthening the Bjerknes feedback mechanism in the Nino4 region and leading to an increase in the occurrence of CP events. The increased variance of wind stress in the western Pacific is, in turn, caused by the resulting increase in the frequency of CP events. These arguments imply a positive feedback in which CP events are self-maintaining and suggest that they may be part of the natural variability of the climate system and could occur without the need for changes in external forcing.

Description: An ensemble of idealized experiments with the simplified general circulation model PUMA is used to analyze the response to reduced surface friction, that is a strengthening of the eddy-driven jet, a weakening of the Eulerian mean overturning, and a suppression of baroclinic instability. The suppression of baroclinic instability is caused by an effect called the barotropic governor by which increased horizontal shear restricts the ability of baroclinic disturbances to convert available potential energy into kinetic energy. This governor effect ensures that the residual circulation and Eliassen–Palm flux (EP flux) divergence are largely invariant to the surface friction parameter despite the connection between surface friction, the Eulerian mean overturning, and the eddy-momentum flux. The suppression of instability leads to an increase in persistence measured by the period of peak variance on synoptic time-scales and a strengthened signal-to-noise ratio on seasonal time-scales. These findings suggest that the signal-to-noise paradox seen in the context of seasonal prediction can be caused by excess mechanical damping in atmospheric prediction systems inhibiting the barotropic governor effect.

Description: Equatorial deep jets (EDJ) are zonal currents along the equator in all three ocean basins that alternate in direction with depth and time. In the Atlantic below the thermocline, they are the dominant variability on interannual timescales. Observations of equatorial deep jets are available but scarce, given the EDJs’ location at depth, their small vertical scale and their long periodicity of several years. In the last few years, Argo floats have added a significant amount of measurements at intermediate depth. In this study we therefore revise estimates of the EDJ scales based on Argo float data. Mostly, we use velocity data at 1000 m depth calculated from float displacement, which yield robust estimates of the Atlantic EDJ period (4.6 years), amplitude distribution, phase distribution, zonal wavelength (146.7°), and meridional structure. We also show that the equatorial amplitude of the EDJs’ first meridional mode Rossby wave component (9.8 cm s −1 ) is larger than that of their Kelvin wave component (2.8 cm s −1 ). Additionally, we present a new estimation of the EDJs’ vertical structure throughout the Atlantic basin, based on an equatorial geostrophic velocity reconstruction from hydrographic Argo float measurements from depths between 400 and 2000 m. Our new estimates from Argo float data provide the first basin-wide assessment of the Atlantic EDJ scales, as well as having smaller uncertainties than estimates from earlier studies.

Description: The Greenland high (GL-high) coincides with a local center of action of the summer North Atlantic Oscillation and is known to have significant influence on Greenland ice sheet melting and summer Arctic sea ice. However, the mechanism behind the influence on regional Arctic sea ice is not yet clear. In this study, using reanalysis datasets and satellite observations, the influence of the GL-high in early summer on Arctic sea ice variability, and the mechanism behind it, are investigated. In response to an intensified GL-high, sea ice over the Beaufort Sea shows significant decline in both concentration and thickness from June through September. This decline in sea ice is primarily due to thermodynamic and mechanical redistribution processes. Firstly, the intensified GL-high increases subsidence over the Canadian Basin, leading to an increase in surface air temperature by adiabatic heating, and a substantial decrease in cloud cover and thus increased downward shortwave radiation. Secondly, the intensified GL-high increases easterly wind frequency and wind speed over the Beaufort Sea, pushing sea ice over the Canadian Basin away from the coastlines. Both processes contribute to an increase in open water areas, amplifying ice–albedo feedback and leading to sea ice decline. The mechanism identified here differs from previous studies that focused on northward moisture and heat transport and the associated increase in downward longwave radiation over the Arctic. The impact of the GL-high on the regional sea ice (also Arctic sea ice extent) can persist from June into fall, providing an important source for seasonal prediction of Arctic sea ice. The GL-high has an upward trend and reached a record high in 2012 that coincided with a record minimum summer Arctic sea ice extent, and has strong implications for summer Arctic sea ice changes.