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The Response of the Surface Circulation of the Arabian Sea to Monsoonal Forcing Verena Hormann, SIO/UCSD [email protected] Lisa M

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The Response of the Surface Circulation of the Arabian Sea to Monsoonal Forcing Verena Hormann, SIO/UCSD Vhormann@Ucsd.Edu Lisa M The response of the surface circulation of the Arabian Sea to monsoonal forcing Verena Hormann, SIO/UCSD [email protected] Lisa M. Beal, Rick Lumpkin, Gregory R. Foltz, Luca Centurioni DBCP-29 Workshop, 23 Sep. 2013, Paris, France Monsoon circulation of the Indian Ocean § Circulation of the Arabian Sea switches direction annually under the influence of strong monsoon winds § Upwelling wedges associated with eddy structures in the western Arabian Sea connected to changes in the wind field [Vecchi et al., 2004] Schott et al., 2009 Indian monsoon rainfall § Horizontal advection, coastal upwelling, wind-driven mixing, and Ekman pumping all affect sea surface temperature (SST) in the Arabian Sea [Lee et al., 2000] § Beside the winds, SST feeds back on rainfall over India Izumo et al., 2008 Data § Large amounts of data required to resolve the evolution of the flow in a region with no steady circulation § Monthly-mean scatterometer winds (QuickSCAT) § Monthly-mean circulation from a global drifter climatology of absolute near-surface currents [Lumpkin and Jonson, 2013] § Weekly maps of geostrophic flow from a drifter-altimeter synthesis [Lumpkin and Garzoli,2011; Hormann et al., 2012] § Satellite altimeter data from AVISO [Le Traon et al., 1998] Drifter array in the western Arabian Sea (left)and correlation with currents from altimetry (right) Monsoons as static circulations § Seasonally-averaged circulations can show currents and connections that do not actually exist NMC SC § Northeast Monsoon Current appears to SECC feed into southward Somali Current § Northward Somali Current appears to feed into Southwest Monsoon Current § South Equatorial Counter Current appears to be lacking during the southwest monsoon GW Magenta arrows are schematics of identified SC SMC currents after Schott and McCreary [2001]. Beal, Hormann et al., 2013 Monthly winds and curl § Nov. – Mar.: prevailing northeasterly winds and positive curl § May – Sep.: prevailing southwesterly winds and negative curl, with strong positive (upwelling) curl along the Somali, Yemeni, and Omani coasts § Weakest winds in the intermonsoon months of April and October Beal, Hormann et al., 2013 Absolute near-surface currents Jan Feb Mar 30oN 30oN 30oN 20oN 1 m/s 20oN 1 m/s 20oN 1 m/s 10oN 10oN 10oN 0o 0o 0o § Few observations combined 10oS 10oS 10oS 40oE 50oE 60oE 70oE 80oE 40oE 50oE 60oE 70oE 80oE 40oE 50oE 60oE 70oE 80oE with high variance typically Apr May Jun 30oN 30oN 30oN lead to large errors, with 20oN 1 m/s 20oN 1 m/s 20oN 1 m/s 10oN 10oN 10oN maxima in the west and along 0o 0o 0o the equator 10oS 10oS 10oS 40oE 50oE 60oE 70oE 80oE 40oE 50oE 60oE 70oE 80oE 40oE 50oE 60oE 70oE 80oE Jul Aug Sep § Eastward Wyrtki Jets along 30oN 30oN 30oN 20oN 1 m/s 20oN 1 m/s 20oN 1 m/s the equator in Apr./May and 10oN 10oN 10oN Oct./Nov. (>1 m/s) o o o 0 0 0 o o o 10 S o o o o o 10 S o o o o o 10 S o o o o o § Indications of closed Southern 40 E 50 E 60 E 70 E 80 E 40 E 50 E 60 E 70 E 80 E 40 E 50 E 60 E 70 E 80 E Oct Nov Dec 30oN 30oN 30oN Gyre during the southwest 20oN 1 m/s 20oN 1 m/s 20oN 1 m/s monsoon 10oN 10oN 10oN 0o 0o 0o 10oS 10oS 10oS 40oE 50oE 60oE 70oE 80oE 40oE 50oE 60oE 70oE 80oE 40oE 50oE 60oE 70oE 80oE error speed [m/s] 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 >0.5 Beal, Hormann et al., 2013 Geostrophic surface currents § Early appearance of northward Somali Current and Great Whirl in April § Year-round South Equatorial Counter Current, fed by the East African Coastal Current § Southern Gyre appears to be retroflection of the EACC § Monsoon currents not connected to western boundary circulation Beal, Hormann et al., 2013 Dominant modes of variance § Strongest Ekman currents are normal to the Somali coast in boreal summer § Geostrophic mode dominated by northward flow along the western boundary, the Great Whirl and South Equatorial Counter Current Leading modes correspond to largely annual variability Beal, Hormann et al., 2013 South Equatorial Counter Current <0 − 5°S>, <50 − 80°E> 1 mean: 0.26 m/s 0.5 § Eastward SECC obscured [m/s] g 0 by strong Ekman currents U −0.5 during the southwest 1994 1996 1998 2000 2002 2004 2006 2008 2010 monsoon <50 − 80°E> Eq. mean: 2.6 °S 1°S § SECC shifted to the 2°S south/north when it is 3°S 4°S weakest/strongest 5°S 1994 1996 1998 2000 2002 2004 2006 2008 2010 § Dominant semi-annual SECC variability tied to Ekman: 5 - 10°S, Ekman pumping over the 50 - 80°E tropical gyre Beal, Hormann et al., 2013 Early initiation of western boundary flow § Onset of northward Somali Current and Great Whirl one or two months before wind curl forcing § Linear theory predicts 1-month lag [e.g., Lighthill, 1969] § Early appearance of northward SC and GW coincident with arrival of annual Rossby wave [Brandt et al., 2002] Beal and Donohue, 2013 Northern Indian Ocean waveguide § Westward Rossby wave from wind curl region off the Somali coast, continuing as coastal then equatorial Kelvin waves to the east coast § Kelvin wave around the rim of the Bay of B e n g a l , r a d i a t i n g Rossby waves back into the Arabian Sea § time scale for wave processes: ~300 days [Shetye, 1998] Rao et al., 2008 Evolution along 8°N <7 − 9°N> Dec Dec Dec § Annual Rossby wave Nov Nov Nov radiates off India in Oct Oct Oct Oct./Nov., reaching Sep Sep Sep the west coast in April Aug Aug Aug § Offshore gradient of Jul Jul Jul Jun Jun Jun SSH anomalies changes May May May sign and coastal Apr Apr Apr current switches from Mar Mar Mar south- to northward Feb Feb Feb § Strong local wind curl Jan Jan Jan 50°E 60°E 70°E 80°E 50°E 60°E 70°E 80°E 50°E 60°E 70°E 80°E obliterates Rossby −6 −3 0 3 6 24 26 28 30 −20 −10 0 10 20 Curl [10−7 N/m3] SST [°C] SSH [cm] waves in Jun. - Aug. SSH: sea surface height Beal, Hormann et al., 2013 Flow off the Arabian Peninsula Summer Monsoon 25oN 100 § Strongest offshore flow between 0.5 m/s 90 10 ‑ 18°N during the 20oN southwest monsoon 80 § Alongshore winds and double monsoon- 15oN 70 jet maximum cause ADT [cm] strong Ekman 60 currents (vectors) 10oN and geostrophic flow 50 (implicit via ADT) at the mouth of the 5oN 40 45oE 50oE 55oE 60oE 65oE Gulf of Aden ADT: absolute dynamic topography Beal, Hormann et al., 2013 Conclusions § Drifter and satellite data can resolve the seasonal circulation of the Arabian Sea § SECC flows eastward year-round, fed by the EACC; semi‑annual SECC variability governed by Ekman pumping over the tropical gyre § EACC overshoots the equator during the southwest monsoon and Southern Gyre appears to be EACC retroflection § Broad northward and offshore flow at the mouth of the Gulf of Aden coincident with alongshore winds and a switch in sign of the wind curl along the monsoon-jet axis § Northward Somali Current and Great Whirl appear one or two months before the monsoon onset, driven by planetary waves initiated during the previous southwest monsoon; oceanic mechanism through which one monsoon may precondition the next? Perspectives § Frequent piracy largely prevented in-situ observations in the Arabian Sea over the past 10 or 15 years, hampering the advancement of ocean-atmosphere research § Lack of observations can be remedied by recent advances in technology of autonomous instruments § Future investigations of phenomena in the Arabian Sea, like the annually reversing Somali Current system, will require optimal deployment locations Drifters deployed in the western Indian Ocean during the Semester-at-Sea cruise in mid-March 2013 .
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