Seasonal Variability of Aragonite Saturation State in the North Pacific Ocean Predicted by Multiple Linear Regression

Abstract:

Seasonal variation of aragonite saturation state (Ω_arag) in the North Pacific Ocean (NPO) was investigated, using multiple linear regression (MLR) models produced from the PACIFICA (Pacific Ocean interior carbon) dataset. Data within depth ranges of 50–1200m were used to derive MLR models, and three parameters (potential temperature, nitrate, and apparent oxygen utilization (AOU)) were chosen as predictor variables because these parameters are associated with vertical mixing, DIC (dissolved inorganic carbon) removal and release which all affect Ω_arag in water column directly or indirectly.

The PACIFICA dataset was divided into 5° × 5° grids, and a MLR model was produced in each grid, giving total 145 independent MLR models over the NPO. Mean RMSE (root mean square error) and r² (coefficient of determination) of all derived MLR models were approximately 0.09 and 0.96, respectively. Then the obtained MLR coefficients for each of predictor variables and an intercept were interpolated over the study area, thereby making possible to allocate MLR coefficients to data-sparse ocean regions. Predictability from the interpolated coefficients was evaluated using Hawaiian time-series data, and as a result mean residual between measured and predicted Ω_arag values was approximately 0.08, which is less than the mean RMSE of our MLR models.

The interpolated MLR coefficients were combined with seasonal climatology of World Ocean Atlas 2013 (1° × 1°) to produce seasonal Ω_arag distributions over various depths. Large seasonal variability in Ω_arag was manifested in the mid-latitude Western NPO (24–40°N, 130–180°E) and low-latitude Eastern NPO (0–12°N, 115–150°W). In the Western NPO, seasonal fluctuations of water column stratification appeared to be responsible for the seasonal variation in Ω_arag (~ 0.5 at 50 m) because it closely followed temperature variations in a layer of 0–75 m. In contrast, remineralization of organic matter was the main cause for the seasonal variation in Ω_arag (> 0.5) found at depths between 50 and 100 m in the Eastern NPO. In fact, the predicted seasonal variation in Ω_arag in this region was successfully reproduced from DIC production expected by change in AOU.