Subsurface Cooling in the Tropical Pacific Under a Warming Climate

Abstract

The present study investigates the subsurface temperature trend in the tropical Pacific in historical and SSP2‐4.5 simulation by 14 models from Coupled Model Intercomparison Project Phase 6. Robust cooling trends exist between 100 and 200 m in the off‐equatorial region (2°–8°N/S) of both the North and South Pacific during 1950–2100. Both spiciness variation and thermocline heave are important in driving this off‐equatorial subsurface cooling at a fixed level. The cooling spiciness trends in the off‐equatorial region originates from the eastern subtropical Pacific along isopycnal streamlines. Besides, the contribution of the thermocline heave to the cooling is governed by the large wind stress curl trend. Furthermore, the maximum negative spiciness trends occur in the eastern subtropics of both the North and South Pacific. The consistently poleward migration of outcropping lines makes cooler subducting water at higher latitudes directly flow into the subtropical subsurface, which induces the negative spiciness trends here. The generation of the negative spiciness trends is also partly attributed to the anomalous advection process on distinct isopycnal surfaces. These two processes both contribute to the generation of spiciness variation in both the North and South Pacific. The generation region is partly covered by the outcropping region, thus, the strong vertical mixing within the mixed layer may weaken the effect of the subduction and advection. This study mainly highlights the significance of the spiciness variation in the projection of the subsurface temperature trend on a longer time scale under global warming.

Vertical structures of marine heatwaves

“…The dominant processes of ocean temperature change differ across the regions. In the subtropical oceans, the downward displacement of isopycnals (heaving) due to wind-driven warm water convergence dominates the subsurface warming 56 ; while in the tropical oceans, the upward displacement of isopycnals caused by wind-driven warm water divergence (heaving) and cooling trends on isopycnals propagating along subtropical gyres from mid-high latitudes resulting from migration of the outcropping lines (spiciness) combine to cause the subsurface cooling 62 . For different response times, the deep ocean (>700 m) temperature has continued to warm, while the upper ocean (<300 m) temperature appears to have stabilized during the global warming hiatus 60 , 61 .…”

Location and Intensity Changes of the North Equatorial Countercurrent Tied to ITCZ Under Global Warming

“…It is found both the location and intensity of ITCZ convergence are closely related to the intensity of NECC in spring (Figures 4a and 4b), with correlation coefficients of 0.55 and 0.87, respectively. Previous studies only focusing on the annual‐mean change suggest the role of ITCZ intensity change (Ju et al., 2022; Luo & Rothstein, 2011). Here, we found that the location changes are also important for the NECC intensity changes in spring.…”

“…This phenomenon has been identified in both precipitation and atmospheric circulation fields. However, how the intensity and location of NECC would change under global warming receives much less attention but deserves investigation considering their close connection (Hu et al., 2015; Ju et al., 2022; Luo & Rothstein, 2011). Luo and Rothstein (2011) found there is a weakening of NECC under global warming, which is suggested to be related to the weakening of tropical atmospheric circulation as constrained by the atmospheric energetics (Held & Soden, 2006; Vecchi & Soden, 2007).…”

“…(2015) based on CMIP5 and Ju et al. (2022) based on CMIP6 models. Although the weakening of NECC under global warming is consistent among different generations of climate models, whether the location and seasonality of NECC have changed is still largely unknown.…”