Propagating coupled modes between the tropical Indo-Pacific ocean heat content and SST anomalies in the interannual timescale

This study uses a time-mode extended singular value decomposition (TESVD) analysis to identify the propagating coupled modes between the upper ocean heat content (UOHC) and sea surface temperature (SST) anomalies in the tropical Indo-Pacific. Four dominant TESVD modes stand out in the interannual timescale. The TESVD1 and TESVD2 depict the thermocline-SST feedback process during ENSO's persistence period and its turnabout, respectively. With an intrinsic 42-month periodicity, both modes together form an UOHCA pattern propagating counterclockwise around the circuit of equatorial-to-northern tropical Pacific followed by the SSTA propagating eastward in the equatorial waveguide. The leading role of UOHCA is mainly associated with El Niño occurrence. In contrast, the TESVD3 depicts the onset stage of ENSO that has the westward propagating SSTA lead the UOHCA in the cold tongue region. Suppressed equatorial upwelling directly through the positive feedback between the warmer SSTs and anomalous wind convergence from the northwestern Pacific and Caribbean Sea is another factor to trigger El Niño over the NINO3 region, in addition to the thermocline-SST feedback. The TESVD4 depicts the low-frequency evolution of SSTA when ENSO signal is absent. It has the westward migrating SSTA lead the UOHCA 4-month in the equatorial East Pacific. However, the pace of coupled UOHCA is nearly stagnant elsewhere due to the lack of thermocline adjustment mechanism when the zonal wind anomalies largely disappear over the equatorial Indian and West Pacific Oceans. In the Indian Ocean, while the ENSO-related SSTA evolution mainly displays an eastward basin-scale change in the first two TESVD modes, SSTA pattern depicted by the TESVD3 (TESVD4) resembles the double (single) polarity of the Indian Ocean dipole (IOD) event. It is mainly in its east pole region of southeastern Indian Ocean that the SSTA evolution tends to decouple with the underlying UOHCA.