A change in the relationship between tropical central Pacific SST variability and the extratropical atmosphere around 1990 (original) (raw)
Related papers
Geophysical Research Letters, 2018
The North Pacific Oscillation (NPO), which is characterized by a north-south dipole-like pattern of sea level pressure (SLP) in the North Pacific, is an atmospheric circulation that is a key to understanding tropical-extratropical interactions in the Pacific. We show that the center of the southern lobe SLP in the NPO during boreal winter (December-January-February) is shifted to the east after the mid-1990s compared to before the mid-1990s. This leads to the change in the relationship between the NPO and El Niño and the Southern Oscillation (ENSO). The NPO is closely associated with the convective forcing in the tropical Pacific during boreal winter before the mid-1990s. After the mid-1990s, in contrast, the simultaneous relationship of the NPO and ENSO during boreal winter becomes weak. However, an eastward shift of the NPO's southern lobe SLP during boreal winter causes a close relationship with the ENSO in the winter of the following year through atmosphere-ocean coupled processes after the mid-1990s. These results indicate that atmospheric circulation in the North Pacific characterized by the NPO becomes more influential in the tropical Pacific with a lagged time after the mid-1990s, likely due to the eastward shift in the NPO's structure. We also briefly discuss which processes cause an eastward shift in the NPO's southern lobe SLP. Plain Language Summary Understanding the North Pacific Oscillation (NPO), which is characterized by a north-south dipole-like pattern of sea level pressure (SLP) in the North Pacific, is a key to understanding tropical-extratropical interactions in the Pacific. This study examines the decadal changes in the NPO's spatial structure and its consequent change in the relationship with El Niño and the Southern Oscillation (ENSO) for 1979-2016. It is found that the center of the southern lobe SLP in the NPO during boreal winter (December-January-February) is shifted to the east after the mid-1990s compared to before the mid-1990s. This leads to the change in the relationship between the NPO and ENSO. An eastward shift of the NPO's southern lobe SLP during boreal winter causes a close relationship with the ENSO in the winter of the following year through atmosphere-ocean coupled processes after the mid-1990s.
Changes in the Tropical Pacific SST Trend from CMIP3 to CMIP5 and Its Implication of ENSO*
2012
This study assesses the changes in the tropical Pacific Ocean sea surface temperature (SST) trend and ENSO amplitude by comparing a historical run of the World Climate Research Programme Coupled Model Intercomparison Project (CMIP) phase-5 multimodel ensemble dataset (CMIP5) and the CMIP phase-3 dataset (CMIP3). The results indicate that the magnitude of the SST trend in the tropical Pacific basin has been significantly reduced from CMIP3 to CMIP5, which may be associated with the overestimation of the response to natural forcing and aerosols by including Earth system models in CMIP5. Moreover, the patterns of tropical warming over the second half of the twentieth century have changed from a La Niñ a-like structure in CMIP3 to an El Niñ o-like structure in CMIP5. Further analysis indicates that such changes in the background state of the tropical Pacific and an increase in the sensitivity of the atmospheric response to the SST changes in the eastern tropical Pacific have influenced the ENSO properties. In particular, the ratio of the SST anomaly variance in the eastern and western tropical Pacific increased from CMIP3 to CMIP5, indicating that a center of action associated with the ENSO amplitude has shifted to the east.
2009
Formation processes of negative (positive) sea surface temperature anomalies (SSTAs) in the subtropical North and South Pacific associated with the ENSO warm (cold) events are examined using reanalysis and in-situ observational datasets. During the premature stage of the ENSO warm events, negative SSTAs appear over the subtropical North Pacific in the February–March period and over the subtropical South Pacific after April, and vice versa in the ENSO cold events. One month prior to the formation of these subtropical negative SSTAs, the negative air humidity anomaly and anomalous downward motion appear at the same location in either the Northern or Southern hemisphere. Associated with these atmospheric anomalies, the strengthened descending branch of local Hadley circulation is observed during the January–February period in the Northern hemisphere and after March in the Southern hemisphere, which coincides with the seasonal transition of the climatological local Hadley circulation fr...
International …, 2010
Several studies have provided observational and numerical evidence that the tropical Pacific and Indian Oceans are influenced by the tropical Atlantic within a one season time scale. The influence of the Atlantic equatorial mode (AEM) in the Pacific El Niño-Southern Oscillation (ENSO) mode is observationally re-examined. The analyses focus on the ENSO-related evolving sea surface temperature (SST) and sea level pressure (SLP) anomalies in the Tropics that follow the occurrences of AEM events and those that are independent of the AEM. The cold (warm) AEM followed by El Niño (La Niña) shows a sequence of maps that might be explained by the mechanism previously outlined on the relationship of the tropical Atlantic and the other tropical Oceans. This mechanism involves an anomalous Atlantic Walker circulation and a Gill-Matsuno-type atmospheric response to anomalous cooling or warming in the tropical Atlantic. The seasonal timing of the relationship studied differs from that of the previous studies. Here, it is noted that the Atlantic SST anomalous conditions are persistent and might be noted 5-6 months before that proposed in previous results. Furthermore, the ENSO extreme conditions are reinforced and maintained by the east-west SST anomalous gradient in the tropical Pacific. Also, the precipitation composites over South America for the ENSO extremes, which are AEM-dependent and AEM-independent cases, are discussed. The AEM-dependent ENSO extremes combine the effects from the tropical Pacific, and equatorial and tropical South Atlantic on the rainfall over South America. The results presented here, to the authors' knowledge, have not been discussed before and might represent a potential for long lead predictability of the climate variations in the tropical Pacific.
Journal of Climate, 2008
The extratropical response to El Niño in late fall departs considerably from the canonical El Niño signal. Observational analysis suggests that this response is modulated by anomalous forcing in the tropical west Pacific (TWP), so that a strong fall El Niño teleconnection is more likely when warm SST conditions and/or enhanced convection prevail in the TWP. While these TWP SST anomalies may arise from noise and/or long-term variability, they may also be generated by differences between El Niño events, through variations in the tropical "atmospheric bridge." This bridge typically drives subsidence west of the date line and enhanced trade winds over the far TWP, which cool the ocean. In late fall, however, some relatively weaker and/or more eastward-shifted El Niño events produce a correspondingly weakened and displaced tropical bridge, which results in no surface cooling and enhanced convection in the TWP. Because the North Pacific circulation is very sensitive to forcing from the TWP at this time of year, the final outcome is a strong extratropical El Niño teleconnection.
On the Relationship between the North Pacific Climate Variability and the Central Pacific El Niño
This study examined connections between the North Pacific climate variability and occurrence of the central Pacific (CP) El Niño for the period from 1950 to 2012. A composite analysis indicated that the relationship between the North Pacific sea surface temperature (SST), along with its overlying atmospheric circulation, and the CP El Niño during the developing and mature phases was changed when the occurrence frequency of the CP El Niño significantly increased after 1990. Empirical orthogonal function (EOF) and singular value decomposition (SVD) analyses of variability in the tropical Pacific and its relationship to the North Pacific show that the North Pacific anomalous SST and the atmospheric variability are more closely associated with the occurrence of the CP El Niño after 1990 than before 1990. There were noticeable differences in terms of the atmospheric variability conditions over the North Pacific, such as the North Pacific Oscillation (NPO)-like atmospheric variability during the spring and its associated SST anomalies during the following winter before 1990 and after 1990. In addition, combined EOF analysis also indicated that the NPO-like atmospheric circulation becomes more effective at playing a role in initiating El Niño after 1990. Consequently, such a change might have been associated with the frequent occurrence of the CP El Niño after 1990.
2013
1] The distinct impact of tropical Indian Ocean (IO) and western Pacific (WP) sea surface temperatures (SSTs) after the El Niño winter has been investigated in relation to the summer North Pacific high (NPH) and western North Pacific subtropical high (WNPSH). After the El Niño winter, warming of the IO leads to a summer eastern Pacific (EP) SST anomaly distinct from the cooling of WP; EP cooling occurs in the extreme IO warming case and EP warming in the WP cooling case. Both the warming of the IO and cooling of the WP are responsible for the development of the WNPSH, whereas the summer EP cooling induces an enhanced NPH, especially if it coexists with IO warming. The IO warming triggers an abrupt termination of the El Niño event by causing the easterly anomaly in the WP, which leads to the coexistence of IO warming and EP cooling during the boreal summer. The tropical EP cooling may change the North Pacific SST anomalies via the atmospheric bridge and consequently strengthen the extratropical NPH. The experimental results, which have been obtained from the use of atmospheric general circulation model, support the distinct roles of EP cooling on the NPH and of IO warming and WP cooling on the WNPSH. This finding suggests that the combined effect of IO warming and EP cooling generates a coupled pattern of NPH and WNPSH.
Interdecadal variability and climate change in the eastern tropical Pacific: A review
Progress in Oceanography, 2006
In this paper, we review interdecadal climatic variability in the eastern tropical Pacific Ocean. This variability dominates the climatic fluctuations in the North Pacific on scales between ENSO and the centennial trend and is commonly referred to as the Pacific Decadal Oscillation or PDO. We include a historical overview and a summary of observational work that describes the surface, tropospheric and subsurface signatures of this variability. Descriptions of interdecadal variability are incomplete at best, mostly due to limitations in the observational record. We emphasize that the well-known "ENSO-like" sea surface temperature (SST) pattern describing the PDO may not be an accurate representation. In the eastern tropical Pacific, the SST maxima are displaced north and south of the equator with larger amplitudes in the northern branch near the coast of North America, which has significant implications for the tropospheric driven circulations.
Weakened Interannual Variability in the Tropical Pacific Ocean Since 2000
2012
An interdecadal shift in the variability and mean state of the tropical Pacific Ocean is investigated within the context of changes in El Niñ o-Southern Oscillation (ENSO). Compared with 1979-99, the interannual variability in the tropical Pacific was significantly weaker in 2000-11, and this shift can be seen by coherent changes in both the tropical atmosphere and ocean. For example, the equatorial thermocline tilt became steeper during 2000-11, which was consistent with positive (negative) sea surface temperature anomalies, increased (decreased) precipitation, and enhanced (suppressed) convection in the western (central and eastern) tropical Pacific, which reflected an intensification of the Walker circulation.