Progress in understanding of Indian Ocean circulation, variability, air–sea exchange, and impacts on biogeochemistry

Over the past decade, our understanding of the Indian Ocean has advanced through concerted efforts toward measuring the ocean circulation and air–sea exchanges, detecting changes in water masses, and linking physical processes to ecologically important variables. New circulation pathways and mechanisms have been discovered that control atmospheric and oceanic mean state and variability. This review brings together new understanding of the ocean–atmosphere system in the Indian Ocean since the last comprehensive review, describing the Indian Ocean circulation patterns, air–sea interactions, and climate variability. Coordinated international focus on the Indian Ocean has motivated the application of new technologies to deliver higher-resolution observations and models of Indian Ocean processes. As a result we are discovering the importance of small-scale processes in setting the large-scale gradients and circulation, interactions between physical and biogeochemical processes, interactions between boundary currents and the interior, and interactions between the surface and the deep ocean. A newly discovered regional climate mode in the southeast Indian Ocean, the Ningaloo Niño, has instigated more regional air–sea coupling and marine heatwave research in the global oceans. In the last decade, we have seen rapid warming of the Indian Ocean overlaid with extremes in the form of marine heatwaves. These events have motivated studies that have delivered new insight into the variability in ocean heat content and exchanges in the Indian Ocean and have highlighted the critical role of the Indian Ocean as a clearing house for anthropogenic heat. This synthesis paper reviews the advances in these areas in the last decade.

Divergent trajectories of ocean warming and acidification

The ocean provides a major sink for anthropogenic heat and carbon. This sink results in ocean changes through the dual stressors of warming and acidification which can negatively impact the health of the marine ecosystem. Projecting the ocean’s future uptake is essential to understand and adapt to further climate change and its impact on the ocean. Historical ocean uptake of heat and CO2 are tightly correlated, but here we show the trajectories diverge over the 21st century. This divergence occurs regionally, increasing over time, resulting from the unique combination of physical and chemical drivers. We explored this relationship using a high-resolution ocean model and a ‘business as usual’ CO2 emission pathway, and demonstrate that the regional variability in the carbon-to-heat uptake ratios is more pronounced than for the subsequent carbon-to-heat storage (change in inventory) ratios, with a range of a factor of 30 (6) in heat-to-carbon uptake (storage) ratios among the defined regions. The regional differences in heat and carbon trajectories result in coherent regional patterns for sea surface warming and acidification by the end of this century. Relative to the mean global change (MGC) at the sea surface of 2.55°C warming and a decrease of 0.32 in pH, the North Pacific will exceed the MGC for both warming and acidification, the Southern Ocean for acidification only, and the tropics and midlatitude northern hemisphere will exceed MGC only for warming. Regionally, mapping the ocean warming and acidification informs where the marine environment will experience larger changes in one or both. Globally, the projected ocean uptake of anthropogenic heat and carbon informs the degree to which the ocean can continue to serve as a sink for both into the future.

Revising the definition of anthropogenic heat flux from buildings: role of human activities and building storage heat flux

Buildings are a major source of anthropogenic heat emissions, impacting energy use and human health in cities. The difference between building energy consumption and building anthropogenic heat emission magnitudes and time lag and are poorly quantified. Energy consumption (QEC) is a widely used proxy for the anthropogenic heat flux from buildings (QF,B). Here we revisit the latter’s definition. If QF,B is the heat emission to the outdoor environment from human activities within buildings, we can derive it from the changes in energy balance fluxes between occupied and unoccupied buildings. Our derivation shows the difference between QEC and QF,B is attributable to a change in the storage heat flux induced by human activities (∆So-uo) (i.e., QF,B = QEC − ∆So-uo). Using building energy simulations (EnergyPlus) we calculate the energy balance fluxes for a simplified isolated building (obtaining QF,B, QEC, ∆So-uo) with different occupancy states. The non-negligible differences in diurnal patterns between QF,B and QEC caused by thermal storage (e.g. hourly QF,B to QEC ratios vary between −2.72 and 5.13 within a year in Beijing, China). Negative QF,B can occur as human activities can reduce heat emission from building but are associated with a large storage heat flux. Building operations (e.g., open windows, use of HVAC system) modify the QF,B by affecting not only QEC but also the ∆So-uo diurnal profile. Air temperature and solar radiation are critical meteorological factors explaining day-to-day variability of QF,B. Our new approach could be used to provide data for future parameterisations of both anthropogenic heat flux and storage heat fluxes from buildings. It is evident that storage heat fluxes in cities may also be impacted by occupant behaviour.

Urban Effect on Sea-Breeze-Initiated Rainfall: A Case Study for Seoul Metropolitan Area

We examined the sea-breeze-initiated rainfall in the Seoul Metropolitan area (SMA) on 6 July 2017 using the weather research and forecasting (WRF) model. The model captures the arrival of the sea breeze front (SBF), the development of afternoon rainfall in the SMA, and the location of the sea-breeze-initiated maximum rainfall in the northeastern SMA reasonably well but overestimates the subsequent rainfall. We conducted sensitivity tests to better understand the urban effect on the sea-breeze-initiated rainfall event. Through factor separation analysis, we first examined the explicit role of sea and urban effect on sea-breeze-initiated rainfall. The results show that the interaction of sea and urban effects cause rainfall in the northwest and northeast of the SMA, indicating that both urban heat island circulation (UHIC) and sea breeze play an important role in the study case’s rainfall. We further examined the relative role of urban roughness and anthropogenic heat on the sea-breeze-initiated rainfall through factor separation analysis. Both anthropogenic heat and urban roughness play a role in increasing precipitation in the northeastern area of the SMA, with a larger contribution of anthropogenic heat than urban roughness. The relationship between low-level convergence at the SBF and urban factors is discussed.

Enhanced surface urban heat islands due to divergent urban-rural greening trends

Satellite observations show that the surface urban heat island intensity (SUHII) has been increasing over the last two decades. This is often accompanied by an increased urban-rural contrast of vegetation greenness. However, the contribution of uneven vegetation trends in urban and rural areas to the trend of SUHII is unclear, due to the confounding effects of climate change and changes in man-made amenities and anthropogenic heat sources. Here we use a data-model fusion approach to quantify such contributions during the peak growing season. We show that the LAIdif (the urban-rural difference of leaf area index) is increasing (P<0.05) in 189 of the selected 228 global megacities. The increasing trend of LAIdif from 2000 to 2019 accounts for about one quarter of the trend in satellite-derived SUHII, and the impact is particularly evident in places with rapid urbanization and rural cropland intensification. The marginal sensitivity of SUHII to LAIdif is the strongest in hot-humid megacities surrounded by croplands and in hot-dry megacities surrounded by mixed woody and herbaceous vegetation. Our study highlights the role of long-term vegetation trends in modulating the trends of urban-rural temperature differences.