Understanding Tropical Cyclone induced changes in upper ocean temperature and salinity

Abstract

Tropical Cyclones (TC) induce changes in both ocean temperature and salinity. This thesis uses available ocean in situ data to detect and quantify temperature and salinity changes for various TCs and determine the robustness of previous works. First, we present the Composite Tropical cyclone Footprint method (CTCFP), a new methodology developed during this thesis to compute the composite TC induced ocean response across multiple TC events. The method is applied to two case studies taken from the 2011 and 2014 TC seasons in the North Atlantic. Results show that the CTCFP mitigates the poor spatial and temporal representation of upper ocean state linked to the scarcity of in situ data within a given TC location. The CTCFP also proves to be a versatile tool amenable also to: 1) study TC induced changes at one TC location; 2) perform analyses of TC induced ocean response on local, regional and global scales; and 3) apply the same method to different data sources. The application of CTCFP to Argo data for TC Katia (2011) over the Amazon plume confirms a cooling of over 2˚C and an INCREASE of salinity up to 1 psu, in line with changes reported by previous studies. Similar effects are found also for TC Gonzalo (2014). However, CTCFP reveals other cases where the surface cooling after the passage of the TC is accompanied by cooling and salinity DECREASE. These results indicate that, whilst thermal TC induced changes seem always negative (cooling), the TC induced changes in salinity can be both positive (increased salinity) or negative (decreased salinity). Next, the CTCFP is applied to Argo temperature and salinity profiles from 2005 to 2015 to examine the composite signatures of TC with time and depth (top 400 m) in the North Atlantic. Results show that TC up to category 2 induce a cooling of -0.5˚C on average and a thermal recovery time between 20-25 days. Our observation-based analyses also confirm that slow moving TCs induce larger surface cooling. On average, TC induce changes in salinity of the order of 0.1 psu salinity increase, but TC induced changes in salinity are less clearly related to TC intensity than thermal changes. Finally, the CTCFP is applied to output from the data-assimilating FOAM system, indicating that while models capture the ocean thermal response to TCs relatively well, the TC induced

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