Surface observations and subsurface ocean assimilation datasets are examined to contrast two distinct types of El Niñ o-Southern Oscillation (ENSO) in the tropical Pacific: an eastern-Pacific (EP) type and a central-Pacific (CP) type. An analysis method combining empirical orthogonal function (EOF) analysis and linear regression is used to separate these two types. Correlation and composite analyses based on the principal components of the EOF were performed to examine the structure, evolution, and teleconnection of these two ENSO types. The EP type of ENSO is found to have its SST anomaly center located in the eastern equatorial Pacific attached to the coast of South America. This type of ENSO is associated with basinwide thermocline and surface wind variations and shows a strong teleconnection with the tropical Indian Ocean. In contrast, the CP type of ENSO has most of its surface wind, SST, and subsurface anomalies confined in the central Pacific and tends to onset, develop, and decay in situ. This type of ENSO appears less related to the thermocline variations and may be influenced more by atmospheric forcing. It has a stronger teleconnection with the southern Indian Ocean. Phase-reversal signatures can be identified in the anomaly evolutions of the EP-ENSO but not for the CP-ENSO. This implies that the CP-ENSO may occur more as events or epochs than as a cycle. The EP-ENSO has experienced a stronger interdecadal change with the dominant period of its SST anomalies shifted from 2 to 4 yr near 1976/77, while the dominant period for the CP-ENSO stayed near the 2-yr band. The different onset times of these two types of ENSO imply that the difference between the EP and CP types of ENSO could be caused by the timing of the mechanisms that trigger the ENSO events.

Decadal changes of El Niño‐Southern Oscillation (ENSO) persistence barriers in various indices of sea surface temperature (SST) and ocean heat content (OHC) are examined in this study using observations and ocean data assimilation products for the period 1958–2001. It is found that the SST indices in the eastern and central equatorial Pacific exhibit very different decadal barrier variability. The variability is large for the eastern Pacific SST indices (NINO1+2 and NINO3) whose persistence barriers shifted abruptly in 1976/1977 and 1989/1990. In contrast, the central Pacific SST indices (NINO3.4 and NINO4) experienced little decadal barrier variability and have had their persistence barriers fixed in spring in the past four decades. The zonal mean OHC index averaged over the equatorial Pacific shows decadal barrier changes similar to those in the eastern Pacific SST indices and always leads the NINO3 SST barrier by about one season. It is noticed that the SST persistence barrier appeared first in the eastern Pacific before 1976/1977, first in the central Pacific between 1976/1977 and 1989/1990, and almost simultaneous in both the eastern and central Pacific after 1989/1990. These timings coincide with the westward propagating, eastward propagating, and standing pattern of ENSO SST anomalies observed in these three periods. These results suggest that ENSO SST anomalies in the equatorial Pacific can be considered to consist of two different processes: a central Pacific process whose phase transition (such as onset) and barrier always happen in spring, and an eastern Pacific process whose phase transition and barrier change from decade to decade and are influenced by changes in the mean thermocline depth along the equatorial Pacific.

Interannual sea surface temperature (SST) variability in the central equatorial Pacific consists of a component related to eastern Pacific SST variations (called Type-1 SST variability) and a component not related to them (called Type-2 SST variability). Lead–lagged regression and ocean surface-layer temperature balance analyses were performed to contrast their control mechanisms. Type-1 variability is part of the canonical, which is characterized by SST anomalies extending from the South American coast to the central Pacific, is coupled with the Southern Oscillation, and is associated with basinwide subsurface ocean variations. This type of variability is dominated by a major 4–5-yr periodicity and a minor biennial (2–2.5 yr) periodicity. In contrast, Type-2 variability is dominated by a biennial periodicity, is associated with local air–sea interactions, and lacks a basinwide anomaly structure. In addition, Type-2 SST variability exhibits a strong connection to the subtropics of both hemispheres, particularly the Northern Hemisphere. Type-2 SST anomalies appear first in the northeastern subtropical Pacific and later spread toward the central equatorial Pacific, being generated in both regions by anomalous surface heat flux forcing associated with wind anomalies. The SST anomalies undergo rapid intensification in the central equatorial Pacific through ocean advection processes, and eventually decay as a result of surface heat flux damping and zonal advection. The southward spreading of trade wind anomalies within the northeastern subtropics-to-central tropics pathway of Type-2 variability is associated with intensity variations of the subtropical high. Type-2 variability is found to become stronger after 1990, associated with a concurrent increase in the subtropical variability. It is concluded that Type-2 interannual variability represents a subtropical-excited phenomenon that is different from the conventional ENSO Type-1 variability.

Sea surface salinity (SSS) measurements from the Aquarius/SAC‐D satellite during September–December 2011 provide the first satellite observations of the salinity structure of tropical instability waves (TIWs) in the Pacific. The related SSS anomaly has a magnitude of approximately ±0.5 PSU. Different from sea surface temperature (SST) and sea surface height anomaly (SSHA) where TIW‐related propagating signals are stronger a few degrees away from the equator, the SSS signature of TIWs is largest near the equator in the eastern equatorial Pacific where salty South Pacific water meets the fresher Inter‐tropical Convergence Zone water. The dominant westward propagation speed of SSS near the equator is approximately 1 m/s. This is twice as fast as the 0.5 m/s TIW speed widely reported in the literature, typically from SST and SSHA away from the equator. This difference is attributed to the more dominant 17‐day TIWs near the equator that have a 1 m/s dominant phase speed and the stronger 33‐day TIWs away from the equator that have a 0.5 m/s dominant phase speed. The results demonstrate the important value of Aquarius in studying TIWs.

International audienceAt its seasonal peak the Amazon/Orinoco plume covers a region of 10⁶ km² in the western tropical Atlantic with more than 1 m of extra freshwater, creating a near-surface barrier layer (BL) that inhibits mixing and warms the sea surface temperature (SST) to >29°C. Here new sea surface salinity (SSS) observations from the Aquarius/SACD and SMOS satellites help elucidate the ocean response to hurricane Katia, which crossed the plume in early fall, 2011. Its passage left a 1.5 psu high haline wake covering >10⁵ km² (in its impact on density, the equivalent of a 3.5°C cooling) due to mixing of the shallow BL. Destruction of this BL apparently decreased SST cooling in the plume, and thus preserved higher SST and evaporation than outside. Combined with SST, the new satellite SSS data provide a new and better tool to monitor the plume extent and quantify tropical cyclone upper ocean responses with important implications for forecasting