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SUMMARY:Quantifying the relative importance of external forcing in determi
 ning SE Pacific Subantarctic mode water properties using density-following
  adjoint experiments. - Ciara Pimm\, University of Liverpool
DTSTART:20230322T140000Z
DTEND:20230322T150000Z
UID:TALK198511@talks.cam.ac.uk
CONTACT:Dr Michael Haigh
DESCRIPTION:Subantarctic mode water (SAMW) is a subsurface water mass whic
 h is formed through surface heat loss. This leads to thick winter mixed la
 yers which are then subducted resulting in low stratification water-masses
 . SAMW formation regions are important for the storage and transport of he
 at and carbon around the globe. In the South Pacific mode water formation 
 region\, central and eastern pools of mode water have been found to have w
 intertime thicknesses that vary strongly interannually and out of phase ac
 ross the basin. This thickness variability is associated with changes in a
 tmospheric forcing\, as evident in peaks in variability in sea level press
 ure between the central and eastern pools. However\, the mechanisms by whi
 ch the external forcing drives this thickness variability have not been qu
 antified. \n \nTo investigate how external forcing affects the properties 
 of SAMW in the central and eastern mode water pools\, a set of adjoint sen
 sitivity experiments are conducted. The traditional approach to adjoint se
 nsitivity experiments in the ECCOv4 state estimate uses a vertical mask th
 at is fixed at all times. Instead\, here the adjoint is developed so that 
 a density following mask is employed\, which more closely reflects how wat
 er masses preferentially spread along density surfaces.    \n \nThe adjoin
 t sensitivity experiments are used to directly quantify the role of local 
 versus remote forcing in driving the variability in regional mode water pr
 operties raised in recent studies. Two separate adjoint sensitivity experi
 ments are completed with horizontal masks in the central and eastern pools
  of mode water in the south Pacific. The objective function used here is t
 he yearly averaged volume over the pool and the density surfaces. The anal
 ysis of the adjoint sensitivities focus on comparing the sensitivities to 
 the north and the south of the Subantarctic Front (SAF) and in winter and 
 summer months. The impact of the sensitivities is provided by multiplying 
 the sensitivities by their respective anomalies and then comparing the cen
 tral and eastern pools. The analysis reveals the dominant control of the t
 hickness variability comes from local and far field surface heat fluxes. I
 t is also found that there are opposing responses to net heat flux to the 
 north and south of the SAF.\n
LOCATION:BAS Seminar Room 1\; https://ukri.zoom.us/j/97506928986
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