Annual and seasonal variability of net heat flux in the Northern Indian Ocean

We investigate the spatial and temporal variability of the net ocean–atmosphere heat flux (<i>Q</i>_net) over the Indian Ocean (north of 30 S) for the period 2005–2008, as estimated from remotely sensed observations. Net heat flux data are based on turbulent fluxes from the Institut Français pour la Recherche et l’Exploitation de la MER (IFREMER) and radiative fluxes, both shortwave (SW) and longwave (LW) as produced at the University of Maryland (UMD). Special attention is given to the Arabian Sea (AS) and the Bay of Bengal (BoB). The estimates are evaluated against the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA). Results are also compared to several widely used atmospheric re-analyses products such as the European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis-Interim (ERA-I), the ERA5, the Modern-Era Retrospective Analysis for Research and Applications (MERRA-2), and the Climate Forecast System Reanalysis (CFSR). It is demonstrated that satellite-based estimates of heat flux components are in better agreement with in-situ buoy observations than re-analysis-based products. As such, the satellite products can be instrumental in diagnosing model performance and utilized for addressing research issues related to atmospheric and oceanic dynamics in the Indian Monsoon. It was found that the mean <i>Q</i>_net is higher in the AS than in the BoB, yet the phase of their monthly time series is well correlated with ocean heat gain/loss extremes. We have also investigated the relationship between <i>Q</i>_net and Sea Surface Temperature (SST) (mixed layer temperature proxy) with respect to their consistency. It was found that over most of the open Indian Ocean, at least half of the seasonal SST magnitude is explained by the seasonal <i>Q</i>_net. Given the small (≤1 month) delay between an annual maximum of the seasonal mixed layer heat content rate of change and the seasonal <i>Q</i>_net, it is concluded that <i>Q</i>_net data presented in this paper are generally consistent with independently observed SST.

本研究针对2005—2008年南纬30°以北印度洋海域的海洋-大气净热通量（net ocean–atmosphere heat flux, Q_net）时空变异性展开探究，该通量由遥感观测数据估算得到。所用净热通量数据基于法国海洋开发研究院（Institut Français pour la Recherche et l’Exploitation de la MER, IFREMER）提供的湍流通量，以及马里兰大学（University of Maryland, UMD）生成的短波（shortwave, SW）、长波（longwave, LW）辐射通量。研究重点关注阿拉伯海（Arabian Sea, AS）与孟加拉湾（Bay of Bengal, BoB），并采用非亚澳季风监测与预测锚定阵列（Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction, RAMA）的原位观测数据对估算结果进行验证，同时将其与多款广泛使用的大气再分析产品开展对比，包括欧洲中期天气预报中心（European Centre for Medium-Range Weather Forecasts, ECMWF）再分析-Interim（ERA-I）、ERA5、现代近世研究与应用回顾分析（Modern-Era Retrospective Analysis for Research and Applications, MERRA-2）以及气候预报系统再分析（Climate Forecast System Reanalysis, CFSR）。结果表明，相较于基于再分析的产品，基于卫星的热通量分量估算结果与原位浮标观测数据的吻合度更高。据此，该卫星产品可用于诊断模式性能，并为解决与印度季风相关的大气、海洋动力学研究问题提供支撑。研究发现，阿拉伯海的平均Q_net高于孟加拉湾，但二者的月时间序列相位与海洋热增益/损失极值均呈现良好相关性。此外，本研究还探究了Q_net与海表温度（Sea Surface Temperature, SST，混合层温度代理指标）之间的一致性关系。结果显示，在绝大多数开阔印度洋海域，季节性SST的变化幅度至少有一半可由季节性Q_net解释。由于季节性混合层热含量变化率的年峰值与季节性Q_net的年峰值之间的滞后时间极短（≤1个月），因此可认为本文所呈现的Q_net数据与独立观测得到的SST整体上具有一致性。