The Sensitive Area for Targeting Observations of Paired Mesoscale Eddies Associated With Sea Surface Height Anomaly Forecasts

For paired mesoscale eddies, the most sensitive initial errors in relation to sea surface height anomaly (SSHA) forecasts are investigated by utilizing the conditional nonlinear optimal perturbation (CNOP) method in a two-layer quasigeostrophic model and then the sensitive areas are identified accordingly. For counter-rotating eddies, the CNOP initial errors primarily occur within the eddies themselves, especially in areas characterized by clear high-to low-velocity gradients, accompanied by shear structures; while for co-rotating eddies, besides sharing the feature of the former, their CNOP initial errors also concentrate between the two eddies and are obliquely tangential to their boundaries with a positive and negative shear structure. The utility of the CNOP initial errors in determining the sensitive areas for target observation is assessed through observing system simulation experiments (OSSEs). The OSSEs indicate that giving priority to the implementation of target observations in the sensitive areas identified by large CNOP initial errors, whether within eddies or between two co-rotating eddies, particularly with a specific array guided by the shear structure of CNOP initial errors, leads to a notable improvement in SSHA forecasting. Finally, the effectiveness of the target observations, especially within the sensitive area situated between two co-rotating eddies, is interpreted from the perspective of barotropic instability. The results of this study offer valuable scientific insights into the targeted observation of paired mesoscale eddies. These findings have the potential to provide important guidance for initializating paired mesoscale eddies, ultimately contributing to improvements in SSHA forecast accuracy. Sea surface height anomaly (SSHA) is a critical variable for describing ocean surface dynamics, and improving the accuracy of SSHA predictions is of utmost importance. Given that the initialization of mesoscale eddies plays a pivotal role in SSHA forecasts, and these eddies often appear in pairs in the world's oceans, we investigate the impact of paired mesoscale eddies on the sensitivity of SSHA forecasting. Our findings indicate that, for paired mesoscale eddies, in contrast to individual eddies, the sensitive areas associated with SSHA forecasting are not only confined to the eddies themselves, specifically the area within eddies characterized by a high-to low-velocity gradient, but also located between two eddies, especially in the case of co-rotating eddies. Targeting observations in these sensitive areas of paired mesoscale eddies, especially with a specific array, can significantly enhance the accuracy of SSHA forecasts. This improvement can be explained by the mechanism of barotropic instability. This research holds the potential to make a substantial contribution to the initialization of paired mesoscale eddies, consequently enhancing the precision of SSHA forecasts. The sensitive areas for targeting observations of paired eddies are revealed through the advanced method of conditional nonlinear optimal perturbation Conducting targeted observations with a specific array can significantly enhance the accuracy of sea surface height anomaly forecasting The specific array for targeted observations is interpreted as a result of the barotropic instability mechanism