Observations of the sub-inertial, near-surface East India Coastal Current

- First description of the near-surface EICC based on HF radar and ADCP data from the shelf.
- The EICC is shallow, with a strong vertical shear off southeast India during October-December.
- Upward phase propagation, evident in the ADCP data, does not always reach the surface owing to this stratification barrier.
- The EICC, which is a large-scale boundary current forced by basin-scale winds, leaves its imprint on the nearshore current.
- This result implies that large-domain models are needed even for simulating the nearshore current.

We present surface current measurements made using two pairs of HF (high-frequency) radars deployed on the east coast of India. The radar data, used in conjunction with data from acoustic Doppler current profiler (ADCP) measurements on the shelf and slope off the Indian east coast, confirm that the East India Coastal Current (EICC) flows poleward as a deep current during February-March. During the summer monsoon, when the EICC flows poleward, and October-December, when the EICC flows equatorward, the current is shallow (<40m deep), except towards the northern end of the coast. Data from Argo floats confirm a shallow mixed layer that leads to a strong vertical shear off southeast India during October-December. A consequence of the strong stratification is that the upward propagation of phase evident in the ADCP data does not always extend to the surface. Even within the seasons, however, the poleward and equatorward flows show variability at periods of the order of 20-45 days, implying that the EICC direction is the same over the top ∼100 m for short durations. The high spatial resolution of the HF radar data brings out features at scales shorter than those resolved by the altimeter and the high temporal resolution captures short bursts that are not captured in satellite-derived estimates of surface currents. The radar data show that the EICC, which is a boundary current, leaves a strong imprint on the current at the coast. Since the EICC is known to be affected significantly by remote forcing, this correlation between the boundary and nearshore current implies the need to use large-domain models even for simulating the nearshore current. Comparison with a simulation by a state-of-the-art Ocean General Circulation Model, run at a resolution of 0.1°×0.1°, shows that the model is able to simulate only the low-frequency variability.