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.

Zonal asymmetry of Southern Ocean air-sea carbon fluxes

<p>The Southern Ocean modulates the climate system by exchanging heat and carbon dioxide (CO<sub>2</sub>) between the atmosphere and deep ocean. While this region plays an outsized role in the global oceanic anthropogenic carbon uptake, CO<sub>2</sub> is released into the atmosphere across large swaths of the Antarctic Circumpolar Current (ACC). Southern Ocean outgassing has long been attributed to remineralized carbon from upwelled deep water, but the precise mechanisms by which this water reaches the surface are not well known. Using data from a novel array of autonomous biogeochemical profiling floats, we estimate Southern Ocean air-sea CO<sub>2</sub> fluxes at unprecedented spatial resolution and determine the pathways that transfer carbon from the ocean interior into the mixed layer where air-sea exchange occurs. Float-based flux estimates suggest that carbon outgassing occurs predominantly in the Indo-Pacific sector of the ACC due to variations in the mean surface ocean partial pressure of CO<sub>2</sub> (<em>p</em>CO<sub>2</sub>). Within the Polar Frontal Zone and Antarctic Southern Zone of the ACC, the annual mean <em>p</em>CO<sub>2</sub> difference between the Indo-Pacific and Atlantic is 40.1 ± 12.9 μatm and 17.9 ± 12.4 μatm, respectively. We show that this zonal asymmetry in surface <em>p</em>CO<sub>2</sub> and consequently air-sea carbon fluxes stems from regional variability in the mixed-layer entrainment of carbon-rich deep water. These results suggest that long-term trends of the Southern Ocean carbon sink inferred from sparse shipboard data may depend on the fraction of measurements from each basin in a given year. Furthermore, sampling these different air-sea flux regimes is necessary to monitor future changes in oceanic carbon release and uptake.</p>

Assessment of the zonal asymmetry trend in Antarctic total ozonecolumn using TOMS measurements and CCMVal-2 models

In the paper the seasonal trends in the zonal asymmetry in the quasi-stationary wave pattern of total ozone column (TOC) at southern polar latitudes have been investigated. We evaluated and compared seasonal trends in the zonal TOC asymmetry from modern era satellite measurements using the Total Ozone Mapping Spectrometer data and the second Chemistry Climate Model Validation (CCMVal-2) assessment. The model longitude phase shifts in asymmetry are in general consistent with the eastward phase shifts observed in historical period 1979–2005, however, there are underestimated values in individual seasons. Future trends in zonal asymmetry from the eleven CCMVal-2 models up to 2100 are presented. They demonstrate the appearance of reverse (westward) future phase shifts, mainly in austral summer. The results are in agreement with previous study and highlight that the general eastward/westward phase shift is caused by both greenhouse gases changes and ozone depletion/recovery. The greenhouse gases change drives a basic long-term eastward shift, which is enhanced (decelerates or reverses) in austral spring and summer by ozone depletion (recovery). The trends in the TOC asymmetry are forced by a general strengthening of the stratospheric zonal flow, which is interacting with the asymmetry of the Antarctic continent to displace the quasi-stationary wave-1 pattern and thus influences the TOC distribution. The results will be useful in prediction of seasonal anomalies in ozone hole and long-term changes in the local TOC trends, in ultraviolet radiation influence on the Southern Ocean biological productivity and in regional surface climate affected by the zonally asymmetric ozone hole.

Midlatitude Fronts and Variability in the Southern Hemisphere Tropical Width

This study examines the relationship between midlatitude synoptic activity and variations in the width of the tropics in the Southern Hemisphere for the period 1979–2016. The edge of the tropical belt is defined here in terms of the latitude of the subtropical ridge (STR) of sea level pressure, and eddy activity in the midlatitudes is characterized by the behavior of atmospheric fronts. It is shown that the location and intensity of the STR are significantly correlated with the number of cold fronts between 20° and 40°S and that these relationships exhibit seasonal and zonal asymmetry. The link between the STR and the number of fronts is analyzed in five sectors of the Southern Hemisphere to reveal regional differences in their behavior and relationship with the southern annular mode. Some earlier studies on the widening of the tropics suggest that such changes may be caused by a shift in the location of midlatitude eddies. Our analysis explores the connection between these on a synoptic time scale. It shows that the variability of the width of the tropics is indeed strongly influenced by changes in the midlatitude synoptic activity, and that changes in synoptic activity lead those in the edge of the tropical belt by approximately one day.