monsoon-modulated ring generation in the eastern gulf of aden

david m. fratantoniwoods hole oceanographic institution

acknowledgements: amy bower, bill johns, hartmut peters deb glickson, heather furey

summer

the western indian ocean is governed by the monsoons

düing (1970)

ncep reanalysis: fratantoni et al. (2006)

wind speed is bold

summer winter

schott and mccreary (2001)

the western indian ocean is governed by the monsoons

the western indian ocean is dominated by eddies

see al safaani et al. (JGR 2007)

the western indian ocean is dominated by eddies and pirates

United Nations Institute for Training and Research Operational Satellite Applications Programme (UNOSAT)

red sea outflow experiment (redsox)

amy bower & dave fratantoni (whoi)bill johns and hartmut peters (miami)

rafos floats: eddies fill the gulf of aden

satellite ocean color and altimetry

aviso ssh

seawifs chlorophyll

things to notice:

‣ blue colors indicate relatively lifeless oceanic water

‣ a variety of eddies translate into the gulf of aden from the east

‣ there is a general pattern of alternately rotating eddies in the gulf

‣ ring formation appears to occur in oct/nov and (less clearly) in apr/may

time-series of ocean color and altimetry

ring characteristics from frontal tracings and altimetry

diameter 250 km

radius of max velocity 80-100 km

azimuthal speed 30-50+ cm/s

translation speed 5-8 cm/s

observed lifetime 5 months

core watermass origin 5° S

ring-eddy pairs

H1: ring generation occurs during episodes of maximal flow through socotra passage

what do we know?

discrete translating rings composed of near-equatorial indian ocean water are generated in the eastern gulf of aden in boreal autumn and spring

H2: maximal flow coincides with the fully-established monsoon circulation

need: measurements of socotra passage flow

schott et al. (1997): ICM-7 moorings 1995-1996

extended time-series of socotra passage flow

H1: YES ring generation occurs during episodes of maximal flow through socotra passage

17 years of socotra passage flow variability: time-series

17 years of socotra passage flow variability: annual cycle

socotra passage velocity anomaly is bold

max flow

17 years of socotra passage flow variability: annual cycle

socotra passage velocity anomaly is bold

monsoon transitions

max flowmax wind

H2: NO maximal flow through socotra passage coincides with the monsoon transitions

conclusions

next: new field measurements? numerical experiments?

discrete translating rings composed of indian ocean water are generated in the eastern gulf of aden in boreal autumn and spring

maximal flow coincides with the transitions between monsoons

ring generation occurs during episodes of maximal flow through socotra passage

rings with near-equatorial watermass properties appear to form as the “steady” monsoon circulation regimes collapse