On Internal Tides Driving Residual Currents and Upwelling on an Island

Tides oscillate the world’s oceans, and because warmer lighter water sits on cooler dense water, wave energy travels along this interface. This process, called internal tides, creates waves which propagate in the ocean interior and are quite common in the ocean. When these waves encounter a coastline or island they function similarly to waves on a beach: they can turn, change shape, break, or reflect. These changes to the waves can create currents, often quite strong, and pull deeper cooler water into the shallow coastal areas. We performed a modeling study to determine which factors are most important in creating strong currents from these internal tides across a range of possible conditions on islands worldwide. The results show that the internal tides create complex currents and bring cooler dense water into the shallow regions under most conditions. The factor that creates potentially the best conditions for organisms living on the ocean floor is gradual ocean floor slopes (compared to steeper). The results for this condition had strong currents, increased deep ocean waters, and cooler temperatures in the shallow coastal areas which may buffer effects from future ocean warming.

潮汐振荡着全球的海洋，而由于温度更高、密度更低的水体浮于温度更低、密度更高的水体之上，波浪能量会沿这一界面传播。这一过程被称为内潮（internal tides），其所生成的波浪会在海洋内部传播，是海洋中十分常见的现象。当这类波浪遭遇海岸线或岛屿时，其行为与海岸浅滩上的波浪类似：可以发生转向、形态改变、破碎或反射。这类波浪的变化会催生通常强度颇高的海流，并将深层的低温高密度水体抽吸至近岸浅水区。本研究开展了模拟研究，旨在明确在全球各岛屿的多种潜在环境条件下，哪些因素对内潮催生强海流的过程影响最为关键。研究结果显示，在多数环境条件下，内潮会催生复杂的海流，并将低温高密度的深层水体输送至浅水区。其中，对海底栖息生物而言最有利的条件为平缓的海底坡度（相较于陡峭坡度）：在该条件下的模拟结果显示，近岸浅水区会形成强海流、涌入更多深层海水且水温更低，这或许能够缓解未来海洋变暖所带来的影响。