Se­crets of South­ern Ocean probed

Business Day - - INNOVATION - TA­MAR KAHN kahnt@bdfm.co.za

The (ex­panse of wa­ter) … ab­sorbs about half of the man-made car­bon diox­ide taken up by the oceans

THEY can with­stand se­vere storms, freez­ing tem­per­a­tures and waves 10m tall. SA’s intrepid sea robots have since last month been hard at work in the South­ern Ocean col­lect­ing and re­lay­ing data to sci­en­tists snug and warm in their of­fices on land.

The sea robots are part of an am­bi­tious year-long experiment that sci­en­tists hope will demon­strate the im­por­tance of myr­iad small yet pow­er­ful events tak­ing place in this vast and hos­tile tract of wa­ter.

The South­ern Ocean, which sur­rounds Antarc­tica, plays a dis­pro­por­tion­ately im­por­tant role in the earth sys­tem. It con­nects the world’s oceans with cur­rents that trans­fer heat and car­bon diox­ide from the at­mos­phere to the deep ocean, and sup­ports the oceans’ pro­duc­tiv­ity with the nu­tri­ents it sup­plies.

A big ques­tion con­fronting sci­en­tists is how cli­mate change will af­fect the South­ern Ocean’s ca­pac­ity to ful­fil these cru­cial tasks, and whether these changes will re­sult in feed­back that ac­cel­er­ates or damp­ens the pace of cli­mate change.

Hu­man ac­tiv­ity puts about 10 gi­ga­tons of car­bon diox­ide into the at­mos­phere each year, about half of which is ab­sorbed by ter­res­trial and ocean reser­voirs. The South­ern Ocean is one of the world’s most im­por­tant car­bon sinks, as it ab­sorbs about half of the man-made car­bon diox­ide taken up by the oceans.

Sci­en­tists have his­tor­i­cally stud­ied the South­ern Ocean with in­ter­mit­tent mea­sure­ments taken on board “ships of op­por­tu­nity” trav­el­ling to and from Antarc­tica mainly in the sum­mer months.

But these snap­shots have cap­tured a mere frac­tion of the ac­tiv­ity in its icy wa­ters, says Coun­cil for Sci­en­tific and In­dus­trial Re­search (CSIR) oceanog­ra­pher Sandy Thoma­lla.

SCI­EN­TISTS are now turn­ing their at­ten­tion to the de­tailed dy­nam­ics at play, scru­ti­n­is­ing changes tak­ing place over short time spans — within a few days — and over ar­eas less than 30km wide in the hope of de­vel­op­ing a bet­ter un­der­stand­ing of the South­ern Ocean.

“Only once you have ac­cu­rately char­ac­terised the sys­tem can you pre­dict how it will re­spond to… cli­mate change,” says Dr Thoma­lla, whose team is try­ing to get a bet­ter grasp of the South­ern Ocean’s year-long sea­sonal cy­cle and its sen­si­tiv­ity to pa­ram­e­ters such as tem­per­a­ture change and storm fre­quency. An­tic­i­pat­ing this will en­able them to bet­ter pre­dict changes over a much longer time.

Last month the po­lar re­search ves­sel SA Agul­has II de­liv­ered two pairs of sea robots to sites at 42 and 45 de­grees south, in the heart of the tem­pes­tu­ous “roar­ing for­ties”, which in­ter­ests sci­en­tists be­cause it has a par­tic­u­larly high up­take of car­bon diox­ide.

The midwinter de­liv­ery of the sea robots marked the start of the CSIR’s year-long South­ern Ocean Sea­sonal Cy­cle Experiment III. This ded­i­cated science voy­age was funded by the Depart­ment of Science and Tech­nol­ogy and the Na­tional Re­search Foun­da­tion.

Each pair of robots in­cludes a yel­low surf­board-like wave glider that moves in a 25km² oc­tagon, tak­ing con­tin­u­ous mea­sure­ments of the weather and the flux of ocean-ato­m­o­sphere car­bon diox­ide; and a buoy­ancy glider that trav­els up and down the wa­ter col­umn to 1,000m be­low the sur­face four to six times a day col­lect­ing data on wa­ter den­sity, tem­per­a­ture and phy­to­plank­ton.

The phy­to­plank­ton is not mea­sured di­rectly, but cal­cu­lated from mea­sure­ments of flu­o­res­cence (which in­di­cates how much chloro­phyll is present) and the wa­ter’s light-scat­ter­ing prop­er­ties which shows how much par­tic­u­late car­bon is present.

Each glider re­lays the data it col­lects via satel­lite to CSIR sci­en­tists in Cape Town.

Sci­en­tists think the sea­sonal cy­cle of the South­ern Ocean is im­por­tant, be­cause it links phys­i­cal ad­just­ments as­so­ci­ated with cli­mate change to ecosys­tem re­sponses in pro­duc­tiv­ity, plant di­ver­sity and ul­ti­mately to car­bon ex­port, says Dr Thoma­lla. She is in­ter­ested in the spring bloom of phy­to­plank­ton, when mi­cro­scopic plants at the base of the Antarc­tic food web start to grow.

THESE plants play a cru­cial role in the Earth’s car­bon cy­cle, be­cause they re­move car­bon diox­ide from sea­wa­ter when they pho­to­syn­the­sise, en­abling the ocean to ab­sorb ad­di­tional car­bon diox­ide from the at­mos­phere. When these tiny plants die, they sink with their car­bon stores to the ocean floor, where they re­main for cen­turies.

“We’re try­ing to work out how the bloom sus­tains it­self for so long into the sum­mer. We think storms and ed­dies play a crit­i­cal role in sup­ply­ing nu­tri­ents to the sur­face wa­ters late in the sea­son,” Dr Thoma­lla says, ex­plain­ing that sci­en­tists have ob­served a host of miniblooms in the ed­dies whirling off the ma­jor cur­rents, in ad­di­tion to the main spring bloom. “This is why we think its so im­por­tant to re­solve ev­ery­thing on a fine scale.”

When SA Aghu­las II makes her sum­mer voy­age to SA’s Antarc­tic base in Novem­ber, she will stop at both sites to re­trieve the robots. The wave glid­ers will be re­fur­bished and the buoy­ancy glid­ers re­placed so they can con­tinue col­lect­ing data un­til the ves­sel re­turns to Cape Town in late Fe­bru­ary. On her voy­age south, a set of bio-op­tic floats will be de­ployed at each site, and left to float with the Antarc­tic cir­cum­po­lar cur­rent: they will take mea­sure­ments of car­bon, chloro­phyll, tem­per­a­ture, salin­ity, and dis­solved oxy­gen un­til au­tumn, pos­si­bly longer — it all de­pends on whether they are float­ing near the ves­sels’ route when she trav­els to Mar­ion Is­land in April, says Dr Thoma­lla.

Pic­ture: SUPPLIED

Dr Sandy Thoma­lla of the CSIR and her team are try­ing to get a bet­ter grasp of the South­ern Ocean’s year-long sea­sonal cy­cle.

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