The thermohaline modes of the global ocean

The first 2000 m of the global thermohaline structure of the ocean are statistically decomposed into vertical modes, using a multivariate Functional Principal Component Analysis (FPCA). This method is applied on the Monthly Isopycnal & Mixed-layer Ocean Climatology (MIMOC). The first three modes account for 92% of the joint temperature and salinity (T-S) variance, which yields a surprisingly good reduction of dimensionality. The first mode (69% of the variance) is related to the thermocline depth and delineates the subtropical gyres. The second mode (18%) is mostly driven by salinity and displays the asymmetry between the North Pacific and Atlantic basins and the salty circumpolar deep waters in the Southern Ocean. The third mode (5%) identifies the low and high salinity intermediate waters, covarying with the freshwater inputs of the upper ocean. The representation of the ocean in the space defined by the first three modes offers a simple visualisation of the global thermohaline structure that strikingly emphasizes the role of the Southern Ocean in linking and distributing water masses to the other basins. The vertical thermohaline modes offer a convenient framework for model and observation data comparison. This is illustrated by projecting the repeated Pacific section P16 together with profiles from the Global Array of Profiling Floats (ARGO) on the modes defined with the climatology MIMOC. These thermohaline modes have a potential for water mass identification and robust analysis of heat and salt content.