BoBBLE: Bay of Bengal Boundary Layer Experiment

南亚夏季风（6月至9月）为超过10亿人口提供了80%的年降雨量，其中许多人依靠季风降雨维持自给农业和淡水。不仅要准确预报季节降水，而且要准确预报夏季降水的变化。分季节的“活跃”和“中断”阶段可能持续数周，导致南亚广大地区发生洪涝和干旱。海气相互作用是理解和预测季风行为的关键。整个夏天，印度东部孟加拉湾的海洋表面温度都非常温暖（28摄氏度以上）。孟加拉湾的蒸发为在孟加拉湾上空形成的季风洼地提供了水分和能量，并为印度带来了大量降雨。目前还不清楚墨西哥湾是如何在这些系统失去能量的情况下保持温暖的。众所周知，整个海湾的海洋温度和盐度变化会导致海湾和周围土地的降雨变化，但目前尚不清楚这些变化是如何产生的，也不清楚它们是如何维持的。南海湾的东西变化尤其如此，这是本项目的重点。虽然海气相互作用对季风很重要，但天气预报只能用大气模型来完成。通过在模式中加入良好的海气相互作用，有可能改善季风预报。孟加拉湾边界层实验（BoBBLE）建议在季风形成期间（6月中旬至7月中旬）对南部海湾进行观测活动。BoBBLE将部署两艘船、六架海洋滑翔机和八艘浮筒，收集前所未有的海洋和海气通量观测数据。这些船将占据海湾西南部和东南部的位置，并追踪这些位置之间的东西和南北路径，测量海洋温度、盐度和洋流。每艘船将配备两台滑翔机（自动水下航行器），两艘船之间还有两台滑翔机，每两小时下潜500米，测量温度、盐度和洋流。海气通量和海洋温度的日变化可能影响天气系统的发展。一种新的滑翔机结构将允许计算热量和盐的水平传输。浮标（自动潜水器）将被部署在海湾，每5天测量2000米以下的海面。在BoBBLE结束后，他们将留在海湾，加强观测网络。船只和滑翔机也将测量海洋叶绿素，它吸收阳光并改变海洋近表面温度，影响海气相互作用。BoBBLE的科学家将分析这些观测结果以及常规数据集，以了解海湾条件的演变以及它们如何影响大气。将特别强调对现有海气通量数据集的不确定性进行估计，方法是对照所有现有的观测资料对这些数据集进行验证。性能最好的数据集将用于改进对海气交换及其在日至十年时间尺度上的变化的估计，计算印度洋和海湾的热量和淡水通量的预算，并验证模式的模拟结果。一系列的模型模拟将揭示海湾的条件是如何维持的，以及海气相互作用是如何影响季风的。利用海洋模式进行模拟，并根据BoBBLE观测结果进行验证，将分离出海气通量（包括日循环）、叶绿素和水平输送在每次天气系统通过后维持和补充海洋结构中的作用。用仅大气和大气-海洋耦合模式对BoBBLE期的回顾性预报将展示海气相互作用如何影响季风降雨预测。多年代际模拟将评估海气相互作用和耦合模式系统误差如何影响日-季季风变率。

The South Asian summer monsoon (June-September) provides 80% of the annual rainfall for over one billion people, many of whom depend on monsoon rains for subsistence agriculture and freshwater. It is critical to forecast accurately not only the seasonal rainfall, but also rainfall variations within the summer. Sub-seasonal "active" and "break" phases can last weeks, resulting in floods and droughts across broad areas of South Asia.Air-sea interactions are key to understanding and predicting monsoon behaviour. Ocean surface temperatures in the Bay of Bengal, east of India, remain very warm (above 28 C) throughout the summer. Evaporation from the Bay provides moisture and energy to monsoon depressions that form over the Bay and bring substantial rain to India. It is not understood how the Bay remains warm despite losing energy to these systems. Ocean temperature and salinity variations across the Bay are known to drive changes in rainfall over the Bay and surrounding land, but it is not clear how these arise or how they are maintained. This is particularly true for east-west variations in the southern Bay, a focus of this project. Although air-sea interactions are important to the monsoon, weather predictions are made with models of only the atmosphere. There is potential to improve monsoon forecasts by including well-represented air-sea interactions in models.The Bay of Bengal Boundary Layer Experiment (BoBBLE) proposes an observational campaign for the southern Bay, during the established monsoon (mid-June to mid-July). BoBBLE will deploy two ships, six ocean gliders and eight floats to collect an unprecedented range of oceanic and air-sea flux observations. The ships will occupy locations in the southwest and southeast Bay, as well as tracing east-west and north-south paths between those locations, measuring ocean temperature, salinity and currents. Two gliders (automated underwater vehicles) will accompany each ship, with two others between the ships, diving to 500 metres every 2 hours to measure temperature, salinity and currents. Diurnal variations in air-sea fluxes and ocean temperatures may affect the development of weather systems. A novel configuration of the gliders will allow computations of horizontal transports of heat and salt. The floats (automated submersibles) will be deployed in the Bay to measure the ocean to 2000 metres every 5 days. They will remain in the Bay after BoBBLE, enhancing the observing network. Ships and gliders will also measure ocean chlorophyll, which absorb sunlight and alter near-surface ocean temperature, influencing air-sea interactions.BoBBLE scientists will analyse these observations, along with routine datasets, to understand the evolution of conditions in the Bay and how they influence the atmosphere. Particular emphasis will be placed on estimating the uncertainty in existing datasets of air-sea fluxes by validating them against all available observations. The best-performing datasets will be used to improve estimates of air-sea exchanges and their variability on daily to decadal timescales, to calculate budgets of heat and freshwater fluxes in the Indian Ocean and the Bay, and to validate model simulations.A hierarchy of model simulations will reveal how conditions in the Bay are maintained and how air-sea interactions influence the monsoon. Simulations with an ocean model, forced by and validated against BoBBLE observations, will isolate the roles of air-sea fluxes (including the diurnal cycle), chlorophyll and horizontal transports in maintaining and recharging ocean structure after each weather system passage. Retrospective forecasts of the BoBBLE period with atmosphere-only and atmosphere-ocean coupled models will demonstrate how air-sea interactions influence monsoon rainfall predictions. Multi-decadal simulations will evaluate how air-sea interactions and coupled-model systematic errors influence daily-to-seasonal monsoon variability.