( ( ( ( ( ( The ( deep ( oceanic currents ) ) ) ( -LRB- ( ( not shown ) -RRB- ) ) ) ( are ( ( caused primarily ) ( by ( water ( density differences ) ) ) ) ) ) and ) ( ( in ( general return ) ) ( ( the ( ( -LRB- ( ( now coolder ) -RRB- ) ) water ) ) ( back ( towards ( the tropics ) ) ) ) ) ) . )
(ROOT (S (S (NP (NP (DT The) (JJ deep) (JJ oceanic) (NNS currents)) (PRN (-LRB- -LRB-) (VP (RB not) (VBN shown)) (-RRB- -RRB-))) (VP (VBP are) (VP (VBN caused) (ADVP (RB primarily)) (PP (IN by) (NP (NN water) (NN density) (NNS differences)))))) (CC and) (S (PP (IN in) (NP (JJ general) (NN return))) (NP (DT the) (PRN (-LRB- -LRB-) (NP (RB now) (NN coolder)) (-RRB- -RRB-)) (NN water)) (VP (VBP back) (PP (IN towards) (NP (DT the) (NNS tropics))))) (. .)))
The deep oceanic currents (not shown) are caused primarily by water density differences and  in general return the (now coolder) water back towards the tropics.
(ROOT (S (NP (JJ Deep) (NNS currents)) (VP (VBP are) (VP (VBN caused) (PP (IN by) (NP (NP (NNS differences)) (PP (IN in) (NP (NN density))))) (PP (IN at) (NP (NP (DT the) (NN top) (CC and) (NN bottom)) (PP (IN of) (NP (DT the) (NN ocean))))))) (. .)))