Late monsoon threatens coral refugia in the Andaman Sea

Internal waves (IWs) mitigate thermal stress and provide refugia for corals against increasingly frequent mass bleaching. However, climate events may bring uncertainty regarding the resistance of such refugia. Here, using in situ observation data in the Andaman Sea (AS), we conduct a case study in which a monsoon anomaly associated with El Niño event threatens IW coral refugia. IW cooling in the AS coral reefs is modulated by the thermocline depth variation, which is significantly driven by Kelvin wave signals from the equator. In premonsoon period, distinct variations in IW cooling and surface heating form a time window of quickly-growing cumulative heat exposure. The El Niño induces a typical 2-week delayed summer monsoon, which prolongs the duration of thermal stress growth and brings severe bleaching risk to corals. As global warming increases the frequency of extreme El Niño events, IW coral refugia will face great challenges in the future.

Study on resistance characteristics of submarine near water surface

The submarine is usually affected by free surface and the navigation resistance increases when sailing near the surface. In order to study the specific resistance characteristics of submarine sailing near the surface, the SUBOFF with appendages was taken as the research object, and the calculation model was built based on Star CCM+ fluid simulation software, and the resistance coefficients under different submarine depths and speeds were calculated. Through comparative analysis, the influence of the depth and speed of the submarine on the resistance components was obtained, and the cause of the formation was analyzed. The results show that the influence of the depth of submarine on friction resistance coefficient is small in general. With the increase of the depth of the submarine, the pressure resistance coefficient decreases, and the wave amplitude decreases. The shear wave of Kelvin wave system is more obvious and the effect of scattering is weakened, which is of great significance for the study of submarine concealment. With the increase of speed, friction resistance coefficient decreases, the overall change trend of pressure resistance coefficient is first increased and then decreases. The interference effect between free surface and hull increases first and then decreases at each depth. The wave shape changes and resistance results mutually confirm. The free surface mainly generates waves by interacting with the hull, which affects the resistance characteristics of the submarine. The interference effect is greatly affected by the depth and speed of the submarine.

Linking Equatorial African precipitation to Kelvin wave processes in the CP4-Africa convection-permitting regional climate simulation

Observational studies have shown the link between Convectively Coupled KelvinWaves (CCKWs) and eastward propagating rainfall anomalies. We explore the mechanisms in which CCKWs modulate the propagation of precipitation from west to east over Equatorial Africa. We examine a multi-year state-of-the-art Africa-wide climate simulation from a convection permitting model (CP4A) along with a parameterised global driving-model simulation (G25) and evaluate both against observations (TRMM) and ERA-Interim (ERA-I), with a focus on precipitation and Kelvin wave activity. We show that the two important related processes through which CCKWs influence the propagation of convection and precipitation from west to east across Equatorial Africa are: 1) low-level westerly wind anomalies that lead to increased low-level convergence, and 2) westerly moisture flux anomalies that amplify the lower-to-mid-tropospheric specific humidity. We identify Kelvin wave activity using zonal wind and geopotential height. Using lagged composite analysis, we show that modelled precipitation over Equatorial Africa can capture the eastward propagating precipitation signal that is associated with CCKWs. Composite analysis on strong (high-amplitude) CCKWs shows that both CP4A and G25 capture the connection between the eastward propagating precipitation anomalies and CCKWs. In comparison to TRMM, however, the precipitation signal is weaker in G25, while CP4A has a more realistic signal. Results show that both CP4A and G25 generally simulate the key horizontal structure of CCKWs, with anomalous low-level westerlies in phase with positive precipitation anomalies. These findings suggest that for operational forecasting, it is important to monitor the day-to-day Kelvin wave activity across Equatorial Africa.

On the Effect of Surface Friction and Upward Radiation of Energy on Equatorial Waves

In theoretical models of tropical dynamics, the effects of both surface friction and upward wave radiation through interaction with the stratosphere are oft-ignored, as they greatly complicate mathematical analysis. In this study, we relax the rigid-lid assumption and impose surface drag, which allows the barotropic mode to be excited in equatorial waves. In particular, a previously developed set of linear, strict quasi-equilibrium tropospheric equations is coupled with a dry, passive stratosphere, and surface drag is added to the troposphere momentum equations. Theoretical and numerical model analysis is performed on the model in the limits of an inviscid surface coupled to a stratosphere, as well as a frictional surface under a rigid-lid. This study confirms and extends previous research that shows the presence of a stratosphere strongly shifts the growth rates of fast propagating equatorial waves to larger scales, reddening the equatorial power spectrum. The growth rates of modes that are slowly propagating and highly interactive with cloud-radiation are shown to be negligibly affected by the presence of a stratosphere. Surface friction in this model framework acts as purely a damping mechanism and couples the baroclinic mode to the barotropic mode, increasing the poleward extent of the equatorial waves. Numerical solutions of the coupled troposphere-stratosphere model with surface friction show that the stratosphere stratification controls the extent of tropospheric trapping of the barotropic mode, and thus the poleward extent of the wave. The superposition of phase-shifted barotropic and first baroclinic modes is also shown to lead to an eastward vertical tilt in the dynamical fields of Kelvin-wave like modes.

Tide characteristics and tidal wave propagation in the Persian Gulf

A 2D hydrodynamic model is employed to study the characteristics of tidal wave propagation in the Persian Gulf (PG). The study indicates that tidal waves propagate from the Arabian Sea and the Gulf of Oman into the PG through the Strait of Hormuz. The numerical model is first validated using the measured water levels and current speeds around the PG and the principal tidal constituents of Admiralty tide tables. Considering the intermediate width of the PG, in comparison to Rossby deformation radius, the tidal wave propagates like a Kelvin wave on the boundaries. Whereas the continental shelf oscillation resonance of the basin is close to the period of diurnal constituents, the results show that the tide is mixed mainly semidiurnal. A series of numerical tests is also developed to study the various effects of geometry and bathymetry of the PG, Coriolis force, and bed friction on tidal wave deformation. Numerical tests reveal that the Coriolis force, combined with the geometry of the gulf, results in generation of different amphidromic systems of diurnal and semidiurnal constituents. The configuration of the bathymetry of the PG, with a shallow zone at the closed end of the basin that extends along its longitudinal axis in the southern half (asymmetrical cross section), results in the deformations of incoming and returning tidal Kelvin waves and consequently the shifts of amphidromic points (APs). The bed friction also results in the movements of the APs from the centerline to the south border of the gulf.

Energetics of Interactions between African Easterly Waves and Convectively Coupled Kelvin Waves

Perturbation kinetic and available energy budgets are used to explore how convectively coupled equatorial Kelvin waves (KWs) impact African easterly wave (AEW) activity. The convective phase of the Kelvin wave increases the African easterly jet’s meridional shear, thus enhancing the barotropic energy conversions, leading to intensification of southern track AEWs perturbation kinetic energy. In contrast, the barotropic energy conversion is reduced in the suppressed phase of KW. Baroclinic energy conversion of the southern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. AEWs in the convective phase of a Kelvin wave have stronger perturbation available potential energy generation by diabatic heating and stronger baroclinic overturning circulations than in the suppressed phase of a Kelvin wave. These differences suggest that southern track AEWs within the convective phase of Kelvin waves have more vigorous convection than in the suppressed phase of Kelvin waves. Barotropic energy conversion of the northern track AEWs is not significantly different between Kelvin waves’ convective and suppressed phases. The convective phase of the Kelvin wave increases the lower-tropospheric meridional temperature gradient north of the African easterly jet, thus enhancing the baroclinic energy conversion, leading to intensification of northern track AEWs perturbation kinetic energy. In contrast, the baroclinic energy conversion is reduced in the suppressed phase of KW. These results provide a physical basis for the modulation of AEWs by Kelvin waves arriving from upstream.

Influence of 2019 strong positive IOD on the upwelling variability along the southern coast of Java

The seas along the Southern Coast of Java, which are parts of the Indian Ocean, are exposed to climate variability conditions that influence the dynamic of oceanographic parameters in these areas. In terms of interannual climate variability, previous studies showed that Indian Ocean Dipole (IOD) variability is more influential than El Niño Southern Oscillation on the upwelling variability along the Southern Coast of Java. This study aimed to determine the effect of strong positive IOD in 2019 on the upwelling along the Southern coast of Java and investigate the possible mechanisms. This study used sea surface temperature data from OISST, wind speed data from the ASCAT satellite, chlorophyll-a data from the Aqua-MODIS, and sea level anomaly data obtained from altimetry satellites. All data were processed using the composite method. The results show enhanced southeast monsoon upwelling during the 2019 strong positive IOD along the Southern Coast of Java as denoted by higher positive (negative) anomaly of chlorophyll-a (SST) from the climatology. Interestingly, the easterly wind speed is lower than the climatology. Since the IOD influences upwelling along the Southern Coast of Java through the propagation of Kelvin wave, our results indicate the enhancing (weakening) upwelling (downwelling) Kelvin wave during the strong positive IOD in 2019 with the propagation speed of about 1.16 m/s. This Kelvin Wave propagation may amplify the coastal upwelling along the Southern Coast of Java.

Convective couplings with equatorial Rossby waves and equatorial Kelvin waves. Part I: Coupled wave structures.

This study investigates precipitation amounts and apparent heat sources, which are coupled with equatorial Kelvin waves and equatorial Rossby waves, using TRMM PR level 2 data products. The synoptic structures of wave disturbances are also studied using the ERA5 reanalysis dataset. We define the wave phase of equatorial waves based on FFT filtered brightness temperature and conduct composite analyses. Rossby waves show a vertically upright structure and their upright vortices induce large amplitude column water vapor (CWV) anomalies. Precipitation activity is almost in phase with CWV, and thus is consistent with a moisture mode. Kelvin waves, on the other hand, indicate a nearly quadrature phase relationship between temperature and vertical velocity, like gravity wave structure. Specific humidity develops from near the surface to middle troposphere as the Kelvin wave progresses. A clear negative CWV anomaly also does not exist despite the existence of negative precipitation anomalies. Convective activity corresponds well with its tilting structure of moisture and modulates the phase relationship between temperature and vertical motion. For both wave cases, apparent heat sources can amplify available potential energy despite of the difference of coupling mechanisms of these two waves; precipitation is driven by CWV fluctuation for the Rossby wave case, and by buoyancy-based fluctuations for the Kelvin wave case. These can be an observational evidence of actual coupling processes that is comparable to previous idealized studies.

Response of western North Pacific anomalous anticyclone in the summer of decaying El Niño to global warming: Diverse projections based on CMIP6 and CMIP5 models

The anomalous anticyclone over the western North Pacific (WNPAC) is a key atmospheric bridge through which El Niño-Southern Oscillation (ENSO) affects East Asian climate. In this study, the response of the anomalous WNPAC to global warming under the high-emission scenario is investigated based on 40 models from CMIP6 and 30 models from CMIP5. Despite low inter-model consensus, the multi-model median (MMM) of CMIP6 models projects an enhanced anomalous WNPAC but the MMM of CMIP5 models projects a weakened anomalous WNPAC, both of which reach about 0.5 standard deviation of the decadal internal variability derived from the pre-industrial control experiment. As consistently projected by CMIP6 and CMIP5 models, a same magnitude of sea surface temperature anomaly (SSTA) over the tropical Indian Ocean (TIO) stimulates a weaker anomalous WNPAC under a warmer climate, and this mechanism is responsible for the weakened anomalous WNPAC based on the CMIP5-MMM. However, the above mechanism is overwhelmed by another mechanism related to the changes in tropical SSTA based on the CMIP6-MMM. As a result of the enhanced warm SSTA over the TIO and the eastward shift of the warm SSTA over the equatorial Pacific during the decaying El Niño, the warm Kelvin wave emanating from the TIO is enhanced along with the stronger zonal SSTA gradient based on the CMIP6-MMM, enhancing the anomalous WNPAC. The diverse changes in the zonal SSTA gradient between the TIO and the equatorial western Pacific also explain the inter-model diversity of the changes in anomalous WNPAC.

Mini review: Characteristics of upwelling in several coastal areas in the world

This paper provides an overview of the upwelling processes and the controlling factors as an effort to reveal the characteristics and the intensities of upwelling events in several parts of the world. This review aimed at identifying the characteristics of upwelling and how the global climate controlling this physic phenomenon such as ENSO, IOD and Kelvin Wave. Several places in the Southeast Asia experienced high intensities of upwelling when El Nino events such as in South of Java, East Coast of Malaysia Peninsula and in Vietnam Coastal area. The results of this review found that area with the most intensive and productive upwelling in the world is South American waters and Banguela Upwelling System (BUS) in the African Coast. However, several other areas also show intensive and high productivity of upwelling, such as off the southern coast of Java Island in Indonesia and the Banda Sea and its surroundings. It is found that upwelling with stronger intensity can result in increase of mortality of certain organisms such as scallops. Additionally, increase of nutrients in a waters is often accompanied by an increase in several species of toxic algae that are harmful to the local fishery system (harmful algae blooms, HABs).