Global zonal mean subsurface temperature anomaly

'''DEFINITION'''

The 2019 anomaly of zonal mean subsurface temperature relative to the period 1993-2014 at each grid point (in depth and latitude) is evaluated to obtain a vertical section depth-latitude plot of subsurface temperature anomaly, expressed in °C. Due to the lack of in situ data prior to 2003, the CORAOA anomaly was computed relative to the 2003-2014 period. It has been shifted back to the 1993-2014 period using the ARMOR3D product which uses also in situ data but no model. A multi-product approach is used, meaning that the anomaly is first computed for 6 different temperature estimates. The standard deviation between the six anomalies provides a qualitative estimate of the robustness of the indicator.

'''CONTEXT'''

Large-scale temperature variations in the upper layers are mainly related to the heat exchange with the atmosphere and surrounding oceanic regions, while the deeper ocean temperature in the main thermocline and below varies due to many dynamical forcing mechanisms (Bindoff et al., 2019). Together with ocean acidification and deoxygenation (IPCC, 2019), ocean warming can lead to dramatic changes in ecosystem assemblages, biodiversity, population extinctions, coral bleaching and infectious disease, change in behavior (including reproduction), as well as redistribution of habitat (e.g. Gattuso et al., 2015, Molinos et al., 2016, Ramirez et al., 2017). Ocean warming also intensifies tropical cyclones (Hoegh-Guldberg et al., 2018; Trenberth et al., 2018).

'''CMEMS KEY FINDINGS'''

The meridional section (zonal average) shows pronounced anomalies in 2019 in the top hundred meters. The significance of the anomalies (as expressed by a signal exceeding twice the noise, Mulet & al, 2018) is generally limited to the top 200 meters. The warming is not uniform in longitude, and the major warming concerns the central Pacific and the Western Indian near 100m. The cooling between 100m and 400m corresponds to a branch of the North Equatorial Current from Hawaii to the Philippines. The warming of the Pacific and Atlantic near 40°S is significant down to 1000 meters. The warming of the Atlantic can be seen down to 1500m between 30°N and 40°N. The 2019 anomaly exceeds the cumulative trend over 1993-2019 in the northern hemisphere and in the circumpolar (see Global cumulative trend of zonal mean subsurface temperature, “GLOBAL_OMI_TEMPSAL_Tyz_trend”).