Extreme IOD induced tropical Indian Ocean warming in 2020

The tropical Indian Ocean (TIO) basin-wide warming occurred in 2020, following an extreme positive Indian Ocean Dipole (IOD) event instead of an El Niño event, which is the first record since the 1960s. The extreme 2019 IOD induced the oceanic downwelling Rossby waves and thermocline warming in the southwest TIO, leading to sea surface warming via thermocline-SST feedback during late 2019 to early 2020. The southwest TIO warming triggered equatorially antisymmetric SST, precipitation, and surface wind patterns from spring to early summer. Subsequently, the cross-equatorial “C-shaped” wind anomaly, with northeasterly–northwesterly wind anomaly north–south of the equator, led to basin-wide warming through wind-evaporation-SST feedback in summer. This study reveals the important role of air–sea coupling processes associated with the independent and extreme IOD in the TIO basin-warming mode, which allows us to rethink the dynamic connections between the Indo-Pacific climate modes.

Mechanism for Southward Shift of Zonal Wind Anomalies During the Mature Phase of ENSO

The cause of southward shift of anomalous zonal wind in the central equatorial Pacific (CEP) during ENSO mature winter was investigated through observational analyses and numerical model experiments. Based on an antisymmetric zonal momentum budget diagnosis using daily ERA-Interim data, a two-step physical mechanism is proposed. The first step involves advection of the zonal wind anomaly by the climatological mean meridional wind. The second step involves the development of an antisymmetric mode in the CEP, which promotes a positive contribution to the observed zonal wind tendency by the pressure gradient and Coriolis forces. Two positive feedbacks are responsible for the growth of the antisymmetric mode. The first involves the moisture–convection–circulation feedback, and the second involves the wind–evaporation–SST feedback. General circulation model experiments further demonstrated that the boreal winter background state is critical in generating the southward shift, and a northward shift of the zonal wind anomaly is found when the same SST anomaly is specified in boreal summer background state.

Contributions of Internal Variability and External Forcing to the Recent Trends in the Southeastern Pacific and Peru–Chile Upwelling System

In a warming world context, sea surface temperature (SST) off central-south Peru, northern Chile, and farther offshore increases at a slower rate than the global average since several decades (i.e., cools, relative to the global average). This tendency is synchronous with an interdecadal Pacific oscillation (IPO) negative trend since ~1980, which has a cooling signature in the southeastern Pacific. Here, we use a large ensemble of historical coupled model simulations to investigate the relative roles of internal variability (and in particular the IPO) and external forcing in driving this relative regional cooling, and the associated mechanisms. The ensemble mean reproduces the relative cooling, in response to an externally forced southerly wind anomaly, which strengthens the upwelling off Chile in recent decades. This southerly wind anomaly results from the poleward expansion of the Southern Hemisphere Hadley cell. Attribution experiments reveal that this poleward expansion and the resulting enhanced upwelling mostly occur in response to increasing greenhouse gases and stratospheric ozone depletion since ~1980. An oceanic heat budget confirms that the wind-forced upwelling enhancement dominates the relative cooling near the coast. In contrast, a wind-forced deepening of the mixed layer drives the offshore cooling. While internal variability contributes to the spread of tendencies, the ensemble-mean relative cooling in the southeastern Pacific is consistent with observations and occurs irrespectively of the IPO phase, hence, indicating the preeminent role of external forcing.

Unprecedented reduction and quick recovery of the South Indian Ocean heat content and sea level in 2014–2018

Following the onset of the strong 2014–2016 El Niño, a decade-long increase of the basin-wide sea level and heat content in the subtropical southern Indian Ocean (SIO) in 2004–2013 ended with an unprecedented drop, which quickly recovered during the weak 2017–2018 La Niña. Here, we show that the 2014–2016 El Niño contributed to the observed cooling through an unusual combination of both the reduced heat advection from the Pacific (dominant in the eastern SIO) and the basin-wide cyclonic wind anomaly that led to shoaling of isotherms (dominant in the western SIO). The ensuing recovery was mainly forced by an anticyclonic wind anomaly associated with stronger trade winds that caused deepening of isotherms and upper-ocean warming, effectively suppressing the 2014–2016 cooling signal propagating from the eastern boundary. The results presented here highlight the complexity of the SIO heat content variability driven by remote and local forcing.

Wind Feedback Mediated by Sea Ice in the Nordic Seas

Air–sea interactions play a critical role in the climate system. This study investigates wind-induced changes in the ocean surface temperature and sea ice cover feeding back onto the atmospheric circulation. This interaction was modeled in the Nordic seas, using a partial coupling method to constrain the ocean with prescribed wind forcing in an otherwise fully coupled Earth system model. This enabled the evaluation of not only the direct oceanic, but also the indirect atmospheric response to idealized forcing scenarios of perturbed winds over the Nordic seas. The results show that an anticyclonic wind anomaly forcing leads to significant surface cooling in the Greenland Sea mostly due to anomalous drift of sea ice. During winter, the cooling reduces the net surface heat flux to the atmosphere and increases sea level pressure. The pressure gradients result in anomalous geostrophic southerly winds, which locally are comparable both in direction and in velocity to the prescribed forcing anomalies, suggesting a positive feedback.

Solar Influences on Stratospheric Circulation Patterns

<p>The impact of the solar cycle on the NH winter stratospheric circulation is analyzed using<br>simulations of a Model of an idealized Moist Atmosphere (MiMA). By comparing solar minimum<br>periods to solar maximum periods, the solar impact on the stratosphere is evaluated: Solar<br>maximum periods are accompanied by warming of the tropics that extends into the midlatitudes<br>due to an altered Brewer Dobson Circulation. This warming of the subtropics and the altered<br>Brewer Dobson Circulation leads to an increase in zonal wind in midlatitudes, which is then<br>followed by a decrease in E-P flux convergence near the winter pole which extends the enhanced<br>westerlies to subpolar latitudes.<br>We use the transformed Eulerian mean framework to reveal the processes that lead to the<br>formation of this sub-polar zonal wind anomaly and its downward propagation from the top of the<br>stratosphere to the tropopause.</p>

The Western Tibetan Vortex as an emergent feature of near-surface temperature variation?

<p>The Western Tibetan Vortex (WTV) was identified through research efforts to understand the causal mechanisms responsible for the ‘Karakoram Anomaly’. The WTV has been shown to be an important anomalous circulation system influencing near surface climate over the Tibetan Plateau (TP). Existing researches have characterised the dynamical characteristics and thermodynamic behaviours of the WTV in detail. Scientific consensus has not yet been established. However, regarding the physical mechanisms which produce the WTV itself, a recent argument has asserted that the WTV is the set of wind field anomalies resulting from changes in 2m near-surface air temperatures (T<sub>2m</sub>) over the western TP. This argument can spur constructive discussion for improving our understanding on the WTV. This paper examines whether a putative thermal-generating machanism for the WTV can explain the established defining features of the WTV. In particular we evaluate if warmer (colder) T<sub>2m</sub> over the western TP is sufficient to drive downward (upward) wind anomaly in the overlying air column. Detailed consideration is given to whether the supposedly thermally induced vortex does indeed have the expected baroclinic structure – i.e. cyclonic (anti-cyclonic) wind anomaly at the mid-lower (mid-higher) troposphere – rather than a quasi-barotropic structure – i.e. cyclonic or anti-cyclonic wind anomaly at both the mid-lower and mid-higher troposphere –  as the research first identifying the WTV reported. This work thus seeks to determine the ‘direction of causality’ of whether  the WTV drives T<sub>2m</sub> over the western TP or the thermal forcing of the western TP’s T<sub>2m</sub> is the mechanism generating the WTV. This work utilises ERA5 meteorological reanalysis data to assess how the WTV may impact the western TP’s T<sub>2m</sub> through modulating the cloud cover and hence net surface radiation. These analyses complement previously published evaluation of the prosoposed adiabatic heating mechimism through which the WTV impacts the mid-lower tropospheric and near surface air temperarure. It is important to note that further evaluations of the skill of the newly released ERA5 dataset in representing the atmospheric conditions accurately over the western TP are still needed.</p>

Modulation of ENSO on Fast and Slow MJO Modes during Boreal Winter

This study investigates modulation of El Niño–Southern Oscillation (ENSO) on the Madden–Julian oscillation (MJO) propagation during boreal winter. Results show that the spatiotemporal evolution of MJO manifests as a fast equatorially symmetric propagation from the Indian Ocean to the equatorial western Pacific (EWP) during El Niño, whereas the MJO during La Niña is very slow and tends to frequently “detour” via the southern Maritime Continent (MC). The westward group velocity of the MJO is also more significant during El Niño. Based on the dynamics-oriented diagnostics, it is found that, during El Niño, the much stronger leading suppressed convection over the EWP excites a significant front Walker cell, which further triggers a larger Kelvin wave easterly wind anomaly and premoistening and heating effects to the east. However, the equatorial Rossby wave to the west tends to decouple with the MJO convection. Both effects can result in fast MJO propagation. The opposite holds during La Niña. A column-integrated moisture budget analysis reveals that the sea surface temperature anomaly driving both the eastward and equatorward gradients of the low-frequency moisture anomaly during El Niño, as opposed to the westward and poleward gradients during La Niña, induces moist advection over the equatorial eastern MC–EWP region due to the intraseasonal wind anomaly and therefore enhances the zonal asymmetry of the moisture tendency, supporting fast propagation. The role of nonlinear advection by synoptic-scale Kelvin waves is also nonnegligible in distinguishing fast and slow MJO modes. This study emphasizes the crucial roles of dynamical wave feedback and moisture–convection feedback in modulating the MJO propagation by ENSO.