Sea surface temperature anomalies (SSTA) in portions of the extratropics are known to recur from one winter to the next without persisting through the intervening summer. Previous studies identified only a limited number of midlatitude regions where this reemergence occurs. Here we find that most of the global oceans exhibit winter‐to‐winter recurrence of SSTA. Indeed, recurrence of SSTA is the default process in the global ocean. Only regions strongly linked to El Niño do not show signs of SST reemergence. In midlatitudes, the temperature anomalies that recur persist below the shallow mixed layer in summer. However, SST recurrence is also found at some tropical locations and appears to be independent of subsurface ocean heat storage. Reemergence at these locations is linked to the recurrence of atmospheric drivers of SSTA, predominantly the wind‐stress forcing. Our results are supported by different ocean data sets and by state‐of‐the‐art climate model simulations.
[1] Analysis of long time series of current meter data from a mooring at 77°E and the equator during 2003-2007, along with mean sea level anomaly data, throws light on the occurrence of the lower-frequency (24 to 40 day) Yanai waves in the upper water column of the central equatorial Indian Ocean (EIO) during the positive Indian Ocean dipole (IOD) years of 2003, 2004, 2006, and 2007 and its absence during the negative IOD year 2005. This result is in contrast with the earlier studies that observed only the higher-frequency (biweekly period) Yanai wave in this region. We propose a new notion for the generation of the lower-frequency Yanai wave in the upper central EIO owing to the positive IOD phenomenon. The strong meridional current shear created by the northward shifting and strengthening of the westward flowing south equatorial current associated with positive IOD and the eastward flowing southwest monsoon current provides energy for the generation of lower-frequency Yanai waves. Vertical stratification of the water column appears to be responsible for the trapping of the different frequency of Yanai waves, with only the higher-frequency Yanai wave in the region of lower pycnocline. During positive IOD the strongly stratified upper water column responds to the lower-frequency Yanai wave, while the deeper ocean (4000 m) exhibited a longer-period (47 day) oscillation. The expected surface signature of Madden-Julian oscillation seems to be suppressed by strong easterlies during the positive IOD years.
The role of local atmospheric forcing on the ocean mixed layer depth (MLD) over the 36 global oceans is studied using ocean reanalysis data products and a single-column 37 ocean model coupled to an atmospheric general circulation model. The focus of this 38 study is on how the annual mean and the seasonal cycle of the MLD relate to various 39 forcing characteristics in different parts of the world's oceans, and how anomalous 40 variations in the monthly mean MLD relate to anomalous atmospheric forcings. By 41 analysing both ocean reanalysis data and the single-column ocean model, regions with 42 different dominant forcings and different mean and variability characteristics of the 43 MLD can be identified. Many of the global oceans' MLD characteristics appear to be 44 directly linked to the different atmospheric forcing characteristics at different 45 locations. Here, heating and wind-stress are identified as the main drivers; in some, 46 mostly coastal, regions the atmospheric salinity forcing also contributes. The annual 47 mean MLD is more closely related to the annual mean wind-stress and the MLD 48 seasonality is more closely related to the seasonality in heating. The single-column 49 ocean model, however, also points out that the MLD characteristics over most global 50 ocean regions, and in particular in the tropics and subtropics, cannot be maintained by 51 local atmospheric forcings only, but are also a result of ocean dynamics that are not 52 simulated in a single-column ocean model. Thus, lateral ocean dynamics are essential 53 in correctly simulating observed MLD.
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