Taws, Sarah Lilian
Seasonal Re-emergence of Sea Surface Temperature Anomalies in the North Atlantic: An Observational and Ocean Model Study
University of Southampton, Ocean and Earth Science,
The aim of this thesis is to provide a more comprehensive and dynamic understanding of sea surface temperature anomaly (SSTA) re-emergence in the North Atlantic, by (a) re-evaluating the utility of re-emergence for sea surface temperature (SST) variability quantitatively and/or by analysing its temporal variability, and (b) investigating the e?ects of horizontal advection, subduction and mesoscale phenomena on the occurrence and nature of SSTA re-emergence in the ocean. Such analyses may contribute towards an improved representation of SSTA re-emergence in seasonal forecast models, which is currently unsatisfactory. Analyses are undertaken with a new observational product and ocean model hindcasts.
Through analysis of observations, it is demonstrated that SSTA re-emergence links the European winter extremes of 2009/10 and early 2010/11. Reemergence contributes towards the winter-to-winter persistence of a SSTA tripole pattern, associated with a record negative phase of the North Atlantic Oscillation (NAO). Its timing coincides with a shift to record negative NAO values. This suggests an active involvement in the extreme winter weather of early-winter 2010/11. The atmospheric and oceanic conditions of 2009-11 are analogous to previous years in the late-1960s, which suggest similar physical processes are involved in the establishment of a large and signi?cant SSTA re-emergence.
The dynamic influences of subduction and horizontal advection on SSTA re-emergence are investigated through numerical passive tracer experiments at 1? resolution. A dominant and time-dependent influence of horizontal advection upon the degree of re-emergence in the northeast Atlantic is revealed. The time-dependent role is linked to spatial variations in the subtropical-subpolar gyre boundary, caused by gyre-scale changes in the upper ocean velocity field. Subduction is more detrimental to SSTA re-emergence in the Sargasso Sea. Significant interannual
variations in the amount of subduction are apparent.
These variations are primarily driven by interannual changes in lateral induction.
Passive tracer experiments, and the statistical properties of SSTA re-emergence in an eddy-permitting (1/4 ?) ocean model are analysed, to ascertain the effects of mesoscale phenomena on the re-emergence process. Statistical diagnostics of SSTA re-emergence at 1/4 ? resolution demonstrate better correspondence with observations than at 1? resolution. This highlights an important contribution from mesoscale processes to the shape and extent of SSTA re-emergence. These mesoscale influences involve significant reorganisations of the large-scale near-surface circulation, imposing different regional influences of horizontal advection upon the occurrence of re-emergence. This is due to the more realistic simulation of key features of the large-scale ocean circulation at 1/4
? resolution. These results support the growing call for higher resolution ocean models in future simulations of the large-scale mean climate and its variability.
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