A ubiquitous feature of the winds over the North Indian Ocean (NIO), which are dominated by monsoons, is the occurrence of variability with the annual period. It is equally pervasive in the ocean's wind-driven circulation. Here we report observations from the shelf off the east coast of India where this periodicity is absent even though local alongshore wind stress has it prominently, and so does the East India Coastal Current (EICC) that flows along the slope off the shelf only about 40 km away. Our observations are based on a high-frequency coastal radar (HF-R) installed at approximately 11.7°N on the east coast of India. It provided surface currents up to 200 km offshore. We use hourly data from two years, January 2017 to December 2018, to compare the alongshore current over the depth contour 50 m (taken to represent the shelf current, Sh-C) with that over the depth contour 1700 m (taken to represent the slope current, Sl-C). Wavelet analysis shows that Sh-C did not have the annual cycle and had periods primarily less than about 50 days. In contrast, Sl-C, i.e., the EICC, shows the annual period prominently and other lower periods from days to months. The two time-series when low-passed with a 100-day filter are uncorrelated. Theoretical models (Brink (2006), for example) attribute the absence of long periods on the shelf to finite friction on the shelf. It prompts longer-period shelf-wave modes to be weak near the coastline and stronger in deeper waters, making the shelf a high pass filter. Most marine processes (including biogeochemistry and fishery) in the NIO have been assumed to have an annual cycle due to a monsoon driven annual cycle in large-scale physical processes. Our observations show that this need not be the case on the shelf. Hence, a re-evaluation of existing ideas on shelf processes is needed.

北印度洋（NIO）的风无处不在，以季风为主，是随年变化的。在海洋的风循环中同样普遍。在这里，我们报告了从印度东海岸的陆架观察到的情况，尽管当地沿海风应力非常明显，但这种周期性却不存在，沿着该陆架的斜坡流动的东印度沿海海流（EICC）也只有大约40公里远。我们的观察结果基于安装在印度东海岸大约11.7°N的高频沿海雷达（HF-R）。它提供了近海200 km的地表水流。我们使用2017年1月至2018年12月这两年的小时数据来比较深度等高线50 m上的沿岸电流（用来表示架子电流，Sh-C），其深度轮廓为1700 m（用于表示斜率电流Sl-C）。小波分析表明，Sh-C没有年度周期，周期主要少于约50天。相反，S1-C，即EICC，则突出显示了年度期间，而其他较低的时期则显示了从几天到几个月的时间。两个时间序列在通过100天过滤器时低通时是不相关的。理论模型（例如，Brink（2006））将货架上没有长时间存在归因于货架上的有限摩擦。它促使较长时期的架子波模式在海岸线附近变弱，而在较深的水域中变强，使架子成为高通滤波器。由于季风驱动的大规模物理过程中的年度周期，NIO中的大多数海洋过程（包括生物地球化学和渔业）被假定为具有年度周期。我们的观察表明，架子上不一定是这种情况。因此，需要对现有工艺的货架方法进行重新评估。