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A multi-season convection permitting regional climate simulation of the Maritime Continent using the Met Office Unified Model with 2.2-km grid spacing is presented and evaluated. The simulations pioneer the use of atmosphere-ocean coupling with the multi-column K profile parametrisation (KPP) mixed layer ocean model in atmospheric convection permitting climate simulations. Comparisons are made against a convection parametrised simulation in which it is nested, and which in turn derives boundary conditions from ERA5 reanalysis. This paper describes the configuration, performance of the mean state and variability of the two simulations compared against observational datasets. The models both have minor sea surface temperature (SST) and wet precipitation biases. The diurnal cycle, representation of equatorial waves and relationship between SST and precipitation are all improved in the convection permitting model compared to the convection parametrised model. The MJO is present in both models with a faster than observed propagation speed. However, it is unclear whether fidelity of the MJO simulation is inherent to the model or whether it predominantly arises from the forcing at the boundaries. [ABSTRACT FROM AUTHOR]

In 2018, Typhoon Mangkhut (locally known as Typhoon Ompong) triggered thousands of landslides in the area of Itogon (Philippines). A landslide inventory of 1101 landslides over a 570 km² area is used to study the geomorphological characteristics and land cover more prone to landsliding as well as the rainfall and soil moisture conditions that led to widespread failure. Landslides mostly occurred in slopes covered by wooded grassland in clayey materials, predominantly facing East-Southeast. The analysis of both satellite rainfall (GPM IMERG) and soil moisture (SMAP-L4) finds that, in addition to rainfall from the typhoon, soil water content plays an important role in the triggering mechanism. Rainfall associated with Typhoon Mangkhut is compared with 33 high intensity rainfall events that did not trigger regional landslide events in 2018 and with previously published rainfall thresholds. Results show that: (a) it was one of the most intense rainfall events in the year but not the highest, and (b) despite satellite data tending to underestimate intense rainfall, previous published regional and global thresholds are to be too low to discriminate between landslide triggering and non-triggering rainfall events. This work highlights the potential of satellite products for hazard assessment and early warning in areas of high landslide activity where ground-based data is scarce [ABSTRACT FROM AUTHOR]

Equatorward-propagating wave trains in the upper troposphere are observed to be associated with deep convection over the eastern tropical Pacific on the submonthly timescale during northern winter. The convection occurs in the regions of ascent and reduced static stability ahead of cyclonic anomalies in the wave train. In this study an atmospheric primitive equation model is used to examine the roles of the dry wave dynamics and the diabatic heating associated with the convection. Many features of a dry integration initialized with a localized wave train in the African-Asian jet on a three-dimensional climatological basic state quantitatively agree with the observations, including the zonal wavenumber 6-7 scale of the waves, the time period of approximately 12 days, and the cross-equatorial Rossby wave propagation over the eastern Pacific. There is ascent and reduced static stability ahead of the cyclonic anomalies, consistent with the interpretation that the waves force the convection. The spatial scale of the waves appears to be set by the basic state; baroclinic growth upstream in the Asian jet favors waves with zonal wavenumber 6. On reaching the Pacific sector, lower-wavenumber components of the wave train are not refracted so strongly equatorward, while higher-wavenumber components are advected quickly along the Pacific jet before they can propagate equatorward. Once over the Pacific, the wave train approximately obeys barotropic Rossby wave dynamics. The observed lower-tropospheric anomalies include an equatorial Rossby wave that propagates westward from the region of cross-equatorial wave propagation and tropical convection. However, this equatorial Rossby wave is not forced directly by the dry equatorward-propagating wave train but appears in a separate integration as a forced response to the observed diabatic heating associated with the tropical convection.

The structure of the 33-h Kelvin wave, a normal mode of the atmosphere, is examined in 6-hourly station and NCEP-NCAR reanalysis data. Cross-spectral analysis of 6 yr (1993-98) of tropical station pressure data shows a peak in coherence in a narrow frequency band centered near 0.74 cycles per day, corresponding to a period of approximately 33 h. The phase angles are consistent with an eastward-propagating zonal-wavenumber-1 structure, implying an equatorial phase speed of approximately 340 m [s.sup.-1]. The global structure of the mode is revealed by empirical orthogonal function and regression analysis of 31 yr (1968-98) of reanalysis data. The horizontal structure shows a zonal-wavenumber-1 equatorial Kelvin wave with an equatorial trapping scale of approximately 34 [degrees] lat. The vertical structure has zero phase change. The amplitude of the wave is approximately constant in the troposphere with an equatorial geopotential height perturbation of 0.9 m, and then increases exponentially with height in the stratosphere. Cross-spectral analysis between the station and reanalysis data shows that the results from the two datasets are consistent. No evidence can be found for forcing of the wave by deep tropical convection, which is is examined using a twice-daily outgoing longwave radiation dataset.

'Nonlinear Kelvin wave-CISK modes' are critically assessed as a possible mechanism for the Madden-Julian Oscillation (MJO) with a global spectral transform model and a one-dimensional analog. Convection is parameterized using a simple 'positive-only CISK' scheme, where tropospheric diabatic heating is proportional to the low-level convergence, and is set to zero in regions of low-level divergence. Although the modes have many properties that are consistent with the MJO, they also have a serious drawback. The growth rate of unstable modes depends crucially on the width of the heating region, which is resolution dependent. The 'CISK catastrophe' has not been averted, and the heating region collapses to the smallest localized scale that the model can support. This scale is larger than the model resolution, as measured by both the gridpoint scale and the inverse wavenumber or half-zonal-wavelength of the highest wavenumber basis function, and is associated with the appearance of negative Gibbs fringes, which are then cut off by the positive-only CISK parameterization.

The Bay of Bengal (BoB) generally exhibits surface oligotrophy, due to nutrient limitation induced by strong salinity stratification. Nevertheless, there are hot spots of biological activity in the BoB where the monsoonal forcings are strong enough to break the stratification; one such region being the southern BoB, east of Sri Lanka. A recent field program conducted during the summer monsoon of 2016, as a part of the Bay of Bengal Boundary Layer Experiment (BoBBLE), provides a unique high-resolution dataset of the vertical distribution of chlorophyll in the southern BoB using ocean gliders along with shipboard CTD measurements. Observations were carried out for a duration of 12-20 days during a suppressed phase of the Boreal Summer Intraseasonal Oscillation (BSISO), along a longitudinal transect at 8° N, extending from 85.3-89° E, covering the dynamically active regions of the Sri Lanka Dome (SLD) and the Southwest Monsoon Current (SMC). Mixing and upwelling induced by the monsoonal wind forcing enhanced chlorophyll concentrations (0.3-0.7 mg m-3) in the surface layers. Observations reveal the presence of prominent deep chlorophyll maxima (DCM; 0.3-1.2 mg m-3) at intermediate depths (20-50 m), generally below the mixed layer and above the thermocline, signifying the contribution of subsurface productivity on the biological carbon cycling in the BoB. The shape of chlorophyll profiles varied in different dynamical regimes indicating that the mechanisms determining the vertical distribution of chlorophyll are intricate; upwelling favoured sharp and intense DCM, whereas mixing resulted in diffuse and weaker DCM. Within the SLD, open ocean Ekman pumping and the doming of thermocline favoured a substantial increase in chlorophyll concentration. Farther east, the thermocline was deeper and moderate surface blooms were triggered by intermittent mixing events. Stabilising surface freshening events and barrier layer formation were often observed to inhibit the surface blooms. The pathway of SMC intrusion was marked by a distinct band of chlorophyll, indicating the advective effect of biologically rich Arabian Sea waters. The region of monsoon current exhibits the strongest DCM as well as the highest column-integrated chlorophyll. Observations suggest that the persistence of DCM in the southern BoB is promoted by surface oligotrophy, which reduces the self-shading effect of phytoplankton and shallow mixed layers, which prevent the vertical redistribution of subsurface phytoplankton. Results from a coupled physical-ecosystem model substantiate the dominant role of mixed layer processes associated with the monsoon in controlling the nutrient distribution and biological productivity in the southern BoB. The present study provides new insights into the vertical distribution of chlorophyll in the BoB, which is not captured in satellite measurements, emphasizing the need for extensive in situ sampling and ecosystem model-based efforts for a better understanding of the monsoonal bio-physical interactions and the potential climatic feedbacks. [ABSTRACT FROM AUTHOR]