The impacts of Persian Gulf water and ocean-atmosphere interactions on tropical cyclone intensification in the Arabian Sea

During the last two decades, the number of tropical cyclone (TC) events in the Arabian Sea has increased dramatically. These events have led to severe human and economic damage in Oman, Iran and Pakistan. Within this context, Gonu, Phet and Shaheen were the Arabian Sea's most destructive TCs on record, leading to a total of 6.07 billion USD in damages and 159 fatalities. Previous studies have mainly focused on atmospheric, sea surface temperature (SST) and anthropogenic impacts of TC generation and intensification. By contrast, oceanographic currents, Persian Gulf water outflow and the role of ocean-atmospheric interactions on the distribution of outflow water into the Arabian Sea and their impacts on TC intensification, are poorly understood. In order to address this issue, we use historical TC records, satellite data, atmospheric and reanalyzed oceanographic data to shed new light on the relationship between large-scale atmospheric forcing and ocean currents on TC intensification in the Arabian Sea. The results demonstrate that pre-monsoon TCs mainly occurred during co-existing La Niña, cold Indian Ocean Basin Model (IOBM) and anomalous northern hemisphere circulations over the Persian Gulf. By contrast, post-monsoon TCs were generally generated during warming acceleration period. Poleward movement of the monsoon belt provided the required humidity and energy for TC generation and increased upwelling events. Water salinity and temperature have increased in the north and northwestern parts of the Arabian Sea following rising upwelling events and a decrease in Persian Gulf outflow water depth. Rapid TC intensification has increased noticeably since 2007 and >72 % of cyclones have reached category 3 or more. We find that the rate of SST rise in the Arabian Sea is higher than the other parts of the northern Indian Ocean since 1998. SST and salinity in the Arabian Sea have been controlled by Persian Gulf outflow water and oceanographic currents. TC intensity is controlled by warm and saline (>36.6 PSU) water distribution patterns, mediated by eddy and jet currents. Rapid intensification of pre-monsoon TCs occurred by tracking to the north and northwest, with most landfalls occurring during this period. Post-monsoon TCs generally affect the center and the southwest of the Arabian Sea. The risk of intensive TCs manifests an increasing trend since 2007, therefore education programs via international platforms such as the International Ocean Institute (IOI) and UNESCO are required for the countries most at risk.

Persistent meanders and eddies lead to quasi-steady Lagrangian transport patterns in a weak western boundary current

The Brazil Current (BC) is a weak western boundary current flowing along the Southwestern Atlantic Ocean. It is frequently described as a flow with intense mesoscale activity and relatively low volume transport between 5.0 and 10.0 Sv. We use a 13-year eddy-resolving primitive-equation simulation to show that the presence of persistent meanders and eddies leads to characteristic quasi-steady Lagrangian transport patterns, aptly extracted through climatological Lagrangian Coherent Structures (cLCS). The cLCS position the surface expression of the BC core along the 2000-m isobath, in excellent visual agreement with high resolution satellite sea-surface temperature and the model Eulerian mean velocity. The cLCS deformation pattern also responds to zonally persistent cross-shelf SSH transition from positive (high) values near coastline to low (negative) values between 200- and 2000-m and back to positive (high) offshore from the 2000-m isobath. Zonally-paired cyclonic and anticyclonic structures are embedded in this transition, also causing the cLCS to deform into chevrons. An efficient transport barrier is identified close to the 200-m isobath confirmed by limited inshore movement of drogued drifters and accurately indicated by an along slope maxima of climatological strength of attraction. We also show that the persistent cyclonic and anticyclonic structures may induce localized cross-shelf transport. Regions of low climatological strength of attraction coincide with large shelves and with stagnant synthetic trajectories. We also show that cLCS accurately depict trajectories initiated at the location of Chevron’s spill (November 2011) as compared to synthetic and satellite-tracked trajectories, and the outline of the oil from that accident. There is also an agreement between the large-scale oil slicks reaching the Brazilian beaches (from August 2019 to February 2020) and the strength of climatological attraction at the coast. Our work also clarifies the influence of persistent mesoscale structures on the regional circulation. The identification and quantitative description of climatological Lagrangian coherent structures is expected to improve the effectiveness of future emergency response to oil spills, contingency planning, rescue operations, larval and fish connectivity assessment, drifter launch strategies, waste pollutant and marine debris dispersion and destination.

Short-Term Variation of the Surface Flow Pattern South of Lombok Strait Observed from the Himawari-8 Sea Surface Temperature

Spatial and temporal information on oceanic flow is fundamental to oceanography and crucial for marine-related social activities. This study attempts to describe the short-term surface flow variation in the area south of the Lombok Strait in the northern summer using the hourly Himawari-8 sea surface temperature (SST). Although the uncertainty of this temperature is relatively high (about 0.6 ∘ C), it could be used to discuss the flow variation with high spatial resolution because sufficient SST differences are found between the areas north and south of the strait. The maximum cross-correlation (MCC) method is used to estimate the surface velocity. The Himawari-8 SST clearly shows Flores Sea water intruding into the Indian Ocean with the high-SST water forming a warm thermal plume on a tidal cycle. This thermal plume flows southward at a speed of about 2 m / s . The Himawari-8 SST indicates a southward flow from the Lombok Strait to the Indian Ocean, which blocks the South Java Current flowing eastward along the southern coast of Nusa Tenggara. Although the satellite data is limited to the surface, we found it useful for understanding the spatial and temporal variations in the surface flow field.