Interactions of South Pacific Tropical Cyclones Pam and Winston with Underlying Warm Ocean Features

The ocean's response to strong tropical cyclones Pam and Winston is examined using satellite fields derived from the South Pacific Ocean Climatology (SPOC) and in situ measurements. SPOC, developed to estimate ocean heat content (OHC) variability, is a blend of temperature and salinity fields from the World Ocean Atlas 2001 and Generalized Digital Environmental Model v.2.1. Using a 2.5 layer model approach together with SPOC and satellite-derived sea surface height anomaly and sea surface temperature (SST) fields, daily depths of the 20°C and 26°C isotherms and mixed layer, and OHC are investigated before, during, and after the storms' passage.

Pam and Winston were important weather systems given their peak strengths in the South Pacific and potential interactions with the underlying warm ocean. During the austral summer months, SST in the South Pacific can reach over 30°C and warm waters greater than 26°C can extend below 150m. Subsequent to cyclone passage, the ocean's thermal response can last for several weeks due in part to the combination of upwelling along the track and the spreading of the 3-dimensional near-inertial wave wake.

In 2015, Cyclone Pam became one of the most intense storms in the South Pacific, reaching sustained winds at 145 knots and a central pressure of 896 hPa as the storm moved through Vanuatu. Pam induced upwelling – cooling the surface by more than 3°C and reducing the upper OHC by more than 80 kJ cm^-2 when it stalled east of the Solomon Islands. In the following year, Cyclone Winston rapidly intensified twice to Severe Tropical Cyclone status (Australian Intensity Scale) after encountering very warm waters and little atmospheric shear, both east and west of Fiji along a looping track. Winston's second intensification peaked as a Category 5 storm with 160 knots and central pressure of 915 hPa. Cooling along Winston's track lowered the SST more than 4°C and reduced the upper OHC more than 65 kJ cm^-2.