Underway measurements for 2-day upwelling front experiment during METEOR cruise M93

While being a major source for atmospheric CO2 the Peruvian upwelling region exhibits strong variability in surface fCO2 on short spatial and temporal scales. Understanding the physical processes driving the strong variability is of fundamental importance for constraining the effect of marine emissions from upwelling regions on the global CO2 budget. In this study, a frontal decay on length scales of (10km) was observed off the Peruvian coast following a pronounced decrease in downfrontal wind speed with a time lag of 9 hours. Simultaneously, the sea-to-air flux of CO2 on the inshore (cold) side of the front dropped from up to 80 to 10 mmol/m**2/day, while the offshore (warm) side of the front was constantly outgassing at a rate of 10-20 mmol/m**2/day. Based on repeated ship transects the decay of the front was observed to occur in two phases. The first phase was characterized by a development of coherent surface temperature anomalies which gained in amplitude over 6-9 hours. The second phase was characterized by a disappearance of the surface temperature front within 6 hours. Submesoscale mixed layer instabilities were present but seem too slow to completely remove the temperature gradient in this short time period. Dynamics such as a pressure driven gravity current appear to be a likely mechanism behind the evolution of the front.秘鲁上升流区作为大气CO₂的主要源区之一，其海表二氧化碳逸度分压（fCO₂）在短空间和时间尺度上存在显著变异性。厘清驱动该强变异性的物理过程，对于约束上升流区海洋排放在全球CO₂收支中的影响具有基础性重要意义。本研究中，在秘鲁沿海观测到尺度约10公里的锋面消散现象，该过程滞后于顺锋风速的显著下降达9小时。与此同时，锋面近岸（冷水侧）的CO₂海-气通量从最高80 mmol·m⁻²·d⁻¹降至10 mmol·m⁻²·d⁻¹，而锋面远岸（暖水侧）则始终以10~20 mmol·m⁻²·d⁻¹的速率持续向外脱气。基于重复船舶走航断面（ship transects）观测，锋面消散过程可分为两个阶段：第一阶段以相干海表温度异常的发展为特征，其振幅在6~9小时内逐渐增大；第二阶段表现为海表温度锋面在6小时内完全消散。亚中尺度混合层不稳定性（submesoscale mixed layer instabilities）虽已被观测到，但在该短时间尺度内其发展速率过慢，不足以完全消除温度梯度。诸如压力驱动重力流的动力过程，似乎是该锋面演化背后的潜在机制。