Evaluating the eastward propagation of the MJO in CMIP5 and CMIP6 models based on a variety of diagnostics

Given the climatic importance of the Madden-Julian Oscillation (MJO), this study evaluates the capability of CMIP6 models in simulating MJO eastward propagation in comparison with their CMIP5 counterparts. To understand the representation of MJO simulation in models, a set of diagnostics in respect of MJO-associated dynamic and thermodynamic structures are applied, including large-scale zonal circulation, vertical structures of diabatic heating and equivalent potential temperature, moisture convergence at planetary boundary layer (PBL), and the east-west asymmetry of moisture tendency relative to the MJO convection. The simulated propagation of the MJO in CMIP6 models shows an overall improvement on realistic speed and longer distance, which displays robust linear regression relationship against above-mentioned dynamic and thermodynamic structures. The improved MJO propagation in CMIP6 largely benefits from better representation of pre-moistening processes that is primarily contributed by improved PBL moisture convergence. In addition, the convergence of moisture and meridional advection of moisture prior to the MJO convection are enhanced in CMIP6, while the zonal advection of moisture is as weak as that in CMIP5. The increased convergence of moisture is a result of enhanced lower-tropospheric moisture and divergence, and the enhanced meridional advection of moisture can be caused by sharpened meridional gradient of mean low-tropospheric moisture in the western Pacific. Further examinations on lower-tropospheric moisture budget reveals that the enhanced zonal asymmetry of the moisture tendency in CMIP6 is driven by the drying process to the west of the MJO convection, which is accredited to the negative vertical and zonal advections of moisture.

Rapid dynamical evolution of ITCZ events over the east Pacific

The latitudinal location of the east Pacific Ocean intertropical convergence zone (ITCZ) changes on time scales of days to weeks during boreal spring. This study focuses on tropical near-surface dynamics in the days leading up to the two most frequent types of ITCZ events, nITCZ (Northern Hemisphere) and dITCZ (double). There is a rapid, daily evolution of dynamical features on top of a slower, weekly evolution that occurs leading up to and after nITCZ and dITCZ events. Zonally-elongated bands of anomalous cross-equatorial flow and off-equatorial convergence rapidly intensify and peak one day before or the day of these ITCZ events, followed one or two days later by a peak in near-equatorial zonal wind anomalies. In addition, there is a wide region north of the southeast Pacific subtropical high where anomalous northwesterlies strengthen prior to nITCZ events and southeasterlies strengthen before dITCZ events. Anomalous zonal and meridional near-surface momentum budgets reveal that the terms associated with Ekman balance are of first-order importance preceding nITCZ events, but that the meridional momentum advective terms are just as important before dITCZ events. Variations in cross-equatorial flow are promoted by the meridional pressure gradient force (PGF) prior to nITCZ events and the meridional advection of meridional momentum in addition to the meridional PGF before dITCZ events. Meanwhile, variations in near-equatorial easterlies are driven by the zonal PGF and the Coriolis force preceding nITCZ events and the zonal PGF, the Coriolis force, and the meridional advection of zonal momentum before dITCZ events.

Decadal Variability of the Upper-Ocean Salinity in the Southeast Indian Ocean: Role of Local Ocean–Atmosphere Dynamics

Ocean salinity plays a crucial role in the upper-ocean stratification and local marine ecosystem. This study reveals that ocean salinity presents notable decadal variability in upper 200 m over the southeast Indian Ocean (SEIO). Previous studies linked this salinity variability with precipitation anomalies over the Indo-Pacific region modulated by the tropical Pacific decadal variability. Here we conduct a quantitative salinity budget analysis and show that, in contrast, oceanic advection, especially the anomalous meridional advection, plays a dominant role in modulating the SEIO salinity on the decadal time scale. The anomalous meridional advection is mainly associated with a zonal dipole pattern of sea level anomaly (SLA) in the south Indian Ocean (SIO). Specifically, positive and negative SLAs in the east and west of the SIO correspond to anomalous southward oceanic current, which transports much fresher seawater from the warm pool into the SEIO and thereby decreases the local upper-ocean salinity, and vice versa. Further investigation reveals that the local anomalous wind stress curl associated with tropical Pacific forcing is responsible for generating the sea level dipole pattern via oceanic Rossby wave adjustment on decadal time scale. This study highlights that the local ocean–atmosphere dynamical adjustment is critical for the decadal salinity variability in the SEIO.

The Role of Background Meridional Moisture Gradient on the Propagation of the MJO over the Maritime Continent

This study investigates the role of the background meridional moisture gradient (MMG) on the propagation of the Madden–Julian Oscillation (MJO) across the Maritime Continent (MC) region. It is found that the interannual variability of the seasonal mean MMG over the southern MC area is associated with the meridional expansion and contraction of the moist area in the vicinity of the MC. Sea surface temperature anomalies associated with relatively high and low seasonal mean MMG exhibit patterns that resemble those of the El Niño–Southern Oscillation. By contrasting the years with anomalously low and high MMG, we show that MJO propagation through the MC is enhanced (suppressed) in years with higher (lower) seasonal mean MMG, though the effect is less robust when MMG anomalies are weak. Column-integrated moisture budget analysis further shows that sufficiently large MMG anomalies affects MJO activity by modulating the meridional advection of the mean moisture via MJO wind anomalies. Our results suggest that the background moisture distribution has a strong control over the propagation characteristics of the MJO in the MC region.

From Mixing to the Large Scale Circulation: How the Inverse Cascade Is Involved in the Formation of the Subsurface Currents in the Gulf of Guinea.

<p>We analyse the results from a numerical model at high resolution. We focus on the formation and maintenance of subsurface equatorial currents in the Gulf of Guinea and we base our analysis on the evolution of potential vorticity (PV). We highlight the link between submesoscale processes (involving mixing, friction and filamentation), mesoscale vortices and the mean currents in the area. In the simulation, eastward currents, the South and North Equatorial Undercurrents (SEUC and NEUC respectively) and the Guinea Undercurrent (GUC), are shown to be linked to the westward currents located equatorward. We show that east of 20<sup>◦</sup>W, both westward and eastward currents are associated with the spreading of PV tongues by mesoscale vortices. The Equatorial Undercurrent (EUC) brings salty waters into the Gulf of Guinea. Mixing diffuses the salty anomaly downward. Meridional advection, mixing and friction are involved in the formation of fluid parcel swith PV anomalies in the lower part and below the pycnocline, north and south of the EUC, in the Gulf of Guinea. These parcels gradually merge and vertically align, forming nonlinear anticyclonic vortices that propagate westward, spreading and horizontally mixing their PV content by stirring filamentation and diffusion, up to 20<sup>◦</sup>W. When averaged over time, this creates regions of nearly homogeneous PV within zonal bands between 1.5<sup>◦</sup> and 5<sup>◦</sup>S or N. This mean PV field is associated with westward and eastward zonal jets flanking the EUC with the homogeneous PV tongues corresponding to the westward currents, and the strong PV gradient regions at their edges corresponding to the eastward currents. Mesoscale vortices strongly modulate the mean fields explaining the high spatial and temporal variability of the jets.</p>

Deep Eastern Boundary Currents: idealized models and dynamics

Concentrated poleward flows along eastern boundaries between two and four kilometers depth in the southeast Pacific, Atlantic, and Indian Oceans have been observed, and appear in data-assimilation products and regional model simulations at sufficiently high horizontal resolution, but their dynamics are still not well understood. We study the local dynamics of these Deep Eastern Boundary Currents (DEBCs) using idealized GCM simulations and use a conceptual vorticity model for the DEBCs, to gain additional insights into the dynamics. Over most of the zonal width of the DEBCs, the vorticity balance is between meridional advection of planetary vorticity and vortex stretching, which is an interior-like vorticity balance. Over a thinner layer very close to the eastern boundary, a balance between vorticity tendencies due to friction and stretching that rapidly decay away from the boundary is found. Over the part of the DEBC that is governed by an interior-like vorticity balance, vertical stretching is driven by both the topography and temperature diffusion, while in the thinner boundary layer, it is driven instead by parameterized horizontal temperature mixing. The topographic driving acts via a cross-isobath flow that leads to stretching and thus to vorticity forcing for the concentrated DEBCs.

From Mixing to the Large Scale Circulation: How the Inverse Cascade Is Involved in the Formation of the Subsurface Currents in the Gulf of Guinea

In this paper, we analyse the results from a numerical model at high resolution. We focus on the formation and maintenance of subsurface equatorial currents in the Gulf of Guinea and we base our analysis on the evolution of potential vorticity (PV). We highlight the link between submesoscale processes (involving mixing, friction and filamentation), mesoscale vortices and the mean currents in the area. In the simulation, eastward currents, the South and North Equatorial Undercurrents (SEUC and NEUC respectively) and the Guinea Undercurrent (GUC), are shown to be linked to the westward currents located equatorward. We show that east of 20° W, both westward and eastward currents are associated with the spreading of PV tongues by mesoscale vortices. The Equatorial Undercurrent (EUC) brings salty waters into the Gulf of Guinea. Mixing diffuses the salty anomaly downward. Meridional advection, mixing and friction are involved in the formation of fluid parcels with PV anomalies in the lower part and below the pycnocline, north and south of the EUC, in the Gulf of Guinea. These parcels gradually merge and vertically align, forming nonlinear anticyclonic vortices that propagate westward, spreading and horizontally mixing their PV content by stirring filamentation and diffusion, up to 20° W. When averaged over time, this creates regions of nearly homogeneous PV within zonal bands between 1.5° and 5° S or N. This mean PV field is associated with westward and eastward zonal jets flanking the EUC with the homogeneous PV tongues corresponding to the westward currents, and the strong PV gradient regions at their edges corresponding to the eastward currents. Mesoscale vortices strongly modulate the mean fields explaining the high spatial and temporal variability of the jets.

Assessment of responses of North Atlantic winter SST to the NAO in 13 CMIP5 models on the interannual scale

This study evaluates the response of winter-averaged sea surface temperature (SST) to the winter North Atlantic Oscillation (NAO) simulated by 13 CMIP5 Earth System Models in the North Atlantic (NA) (0–65° N) on the interannual scale. Only 7 models can reproduce an observed tripolar pattern of the response of SST anomalies to the NAO, and most of the models cannot generate the observed impact of variations of the turbulent heat flux on the response of SST anomalies to the NAO. In the subpolar NA (45–65° N) where the influences of sensible/latent heat fluxes on SST are obvious, most of the models simulate a positive response of SST to the turbulent heat flux in the large area of this region, which is opposite to the observations, and probably generate the incorrect positive response of SST to the NAO in some models. In the subtropical NA (25–45° N), the observations show a significant influence of latent heat flux (LHF) on SST, but the overestimated oceanic role in the interaction of the LHF and SST in most CMIP5 models results in an incorrect positive response of SST anomalies to the LHF anomalies in a large area of the subtropical NA. Besides the turbulent heat flux, the meridional advection is also important to the change of the SST in the NA. The analysis of the simulated and observed results shows that NAO-driven meridional advection can cause the increase/decrease of SST during the positive phase of the NAO in the subtropical/subpolar NA. This is probably one of the main causes why the models can simulate the realistic positive response of SST anomalies to the NAO in the subtropical NA, but the strength of the positive response is relatively weak.

Physical drivers of the nitrate seasonal variability in the Atlantic cold tongue

Ocean color observations show semiannual variations in chlorophyll in the Atlantic cold tongue with a main bloom in boreal summer and a secondary bloom in December. In this study, ocean color and in situ measurements and a coupled physical–biogeochemical model are used to investigate the processes that drive this variability. Results show that the main phytoplankton bloom in July–August is driven by a strong vertical supply of nitrate in May–July, and the secondary bloom in December is driven by a shorter and moderate supply in November. The upper ocean nitrate balance is analyzed and shows that vertical advection controls the nitrate input in the equatorial euphotic layer and that vertical diffusion and meridional advection are key in extending and shaping the bloom off Equator. Below the mixed layer, observations and modeling show that the Equatorial Undercurrent brings low-nitrate water (relative to off-equatorial surrounding waters) but still rich enough to enhance the cold tongue productivity. Our results also give insights into the influence of intraseasonal processes in these exchanges. The submonthly meridional advection significantly contributes to the nitrate decrease below the mixed layer.

Physical drivers of the nitrate seasonal variability in the Atlantic cold tongue

Ocean color observations show semiannual variations of chlorophyll in the Atlantic cold tongue with a main bloom in boreal summer and a secondary bloom in December. In this study, ocean color and in situ measurements, and a coupled physical-biogeochemical model are used to investigate the processes that drive this variability. Results show that the main phytoplankton bloom in July-August is driven by a strong vertical supply of nitrate in May-July and the secondary bloom in December is driven by a shorter and moderate supply in November. The upper ocean nitrate balance is analyzed and shows that vertical advection controls the nitrate input in the equatorial euphotic layer and that vertical diffusion and meridional advection are key in extending and shaping the bloom off equator. Horizontal advection mostly acts to bring nitrate low water below the mixed layer. Our results also give insights on the influence of intraseasonal processes in these exchanges. Observations and model show that the Equatorial Undercurrent brings low-nitrate water (relatively to off-equatorial surrounding waters) but still rich enough to enhance the cold tongue productivity.