Pacific Sea Surface Temperature Variability Research Articles

Assessment of CMIP6 models' skill for tropical Indian Ocean sea surface temperature variability

AbstractThe present study examines the ability of Coupled Model Inter‐comparison Project phase 6 (CMIP6) models in representing the dominant modes of tropical Indian Ocean (TIO) sea surface temperature (SST) variability on the interannual and decadal time scale. Historical simulations from 27 CMIP6 models are assessed against Extended Reconstructed SST over the period of 1854 to 2014. Spectrum analysis reveals that many models reproduce interannual and decadal variability of TIO SST but underestimate the amplitude of variability with some disparity in the periodicity. All models can reproduce the dominant basin‐wide mode of interannual and decadal variability of TIO SST reasonably well. Skill score analysis of TIO SST variability reveals that KACE‐1‐0‐G has highest skill, followed by FGOALS‐f3‐L, EC‐Earth3‐Veg‐LR, ACCESS‐ESM1‐5, CanESM5‐CanOE on the interannual timescale and FGOALS‐f3‐L, CanESM5‐CanOE, KACE‐1‐0‐G and CanESM5, respectively, showed highest skills for decadal variability. It is found that variations in radiation and latent heat flux are primarily responsible for interannual variability in TIO SST, the basin‐wide warming, in the observations. Taylor diagram analysis reveals that all the models exhibit better skill for the radiative flux; however, skill for the latent heat and momentum flux varies from model to model. It is important to note that the models in which the latent heat flux and zonal wind are better represented have produced better TIO SST variability compared to other models. A higher discrepancy in latent heat and zonal momentum flux leads to improper wind‐evaporation‐SST and wind‐circulation‐SST feedback, which in turn restricts the model skill. Besides, model that has realistic central and eastern Pacific SST variability show better skill for TIO SST variability in both interannual and decadal time scales. The present study advocates that better representation of latent heat flux and zonal wind in coupled models is important for the accurate simulation of interannual and decadal variability in TIO SST.

Recent changes in the major modes of Asian summer monsoon rainfall: influence of ENSO-IOD relationship

Indo-Pacific Ocean sea surface temperature (SST) anomalies in the form of Indian Ocean Dipole (IOD) and El Nino-Southern Oscillation (ENSO) have a major role in determining the climate of the tropics and extratropics. The tropical Pacific variability was dominated by canonical ENSO (EOF1) and El Nino Modoki (EOF2) during the 1980–1998 period (period 1), while during the 1999–2017 period (period 2), both the flavors of ENSO contributed to EOF1. This Pacific Ocean variability weakened the ENSO-IOD relationship also. The present study estimates the changes that occurred to the major modes of Asian summer monsoon (ASM) rainfall during these two periods and addresses the role of the recent changes in SST boundary forcing in ASM interannual variability. During period 1, the first mode (EOF1) of ASM rainfall was associated with ENSO and IOD with 32% variability. But after 1998, EOF1 depends only on the ENSO flavors (canonical and Modoki ENSO) and has slightly reduced variability (26%). The second mode was controlled by the decay phase of ENSO during the 1980–1998 period (12%), while the eastern Indian Ocean and central-northeast Pacific SST variability contributed to EOF2 (15%) during period 2. EOF3 is controlled by IOD in both periods. Further analysis indicates that the extended warming pattern from central to eastern Pacific of ENSO during period 2 weakened ENSO-IOD co-occurrence through the modification of tropical circulation. In the absence of IOD, ENSO-monsoon teleconnection becomes stronger during period 2 than in the previous period. Indian summer monsoon rainfall (ISMR) relationship with the developing phase of ENSO became stronger after 1998, and that of the decay phase of ENSO is decreased. The ISMR-ENSO Modoki relationship has a phase shift from summer maxima to winter maxima. Meanwhile, the ISMR-IOD relationship in the absence of ENSO became slightly stronger in the 1999–2017 period. Thus, the Pacific variability in these two periods influenced ASM rainfall variability through the modulation of its relationship with tropical Indo-Pacific SST anomalies.

Relative Contributions of Atmospheric, Oceanic, and Coupled Processes to North Pacific and North Atlantic Variability

AbstractPatterns of sea surface temperature (SST) variability over the northern oceans arise from a combination of atmospheric, oceanic, and coupled processes. Here we use a novel methodology and a suite of observations to quantify the processes contributing to the dominant patterns of interannual SST variability over these regions. We decompose the upper ocean heat content tendency associated with such dominant patterns into contributions from different heat fluxes: (a) atmospherically driven, (b) surface feedbacks, and (c) oceanic. We find that in the subtropics, cloud radiative flux, turbulent heat flux, and residual oceanic processes each contributes substantially to North Pacific SST variability, whereas turbulent heat flux primarily induces North Atlantic SST variability. Cloud radiative fluxes therefore provide a major source of interannual SST variability in the North Pacific but not in the North Atlantic. In midlatitudes, SST fluctuations over the northern oceans are driven by the combination of turbulent and oceanic heat fluxes.

Pacific Meridional Mode‐Western North Pacific Tropical Cyclone Linkage Explained by Tropical Pacific Quasi‐Decadal Variability

AbstractPrevious studies argued that the Pacific Meridional Mode (PMM) impacts tropical cyclone (TC) genesis variability over the southeastern part of the western North Pacific (SE‐WNP). Here, we find that the statistical relationship between PMM and SE‐WNP TC genesis frequency is dominated by their co‐variability on decadal timescales. The decadal component of the PMM exhibits very similar temporal and spatial features to quasi‐decadal tropical Pacific sea surface temperature (SST) variability. The latter can affect SE‐WNP TC activity via changes in both zonal vertical wind shear and low‐level vorticity. In contrast, the interannual component of the PMM exhibits no statistically significant correlation with SE‐WNP TC genesis. Furthermore, observations show that both interannual and decadal variability of SE‐WNP TC activity are well correlated with the commonly used Niño3.4 El Niño‐Southern Oscillation index. Thus, equatorial Pacific SST variability is the dominant source of SE‐WNP TC activity predictability on different timescales.

Contributions of atmospheric and oceanic feedbacks to subtropical northeastern sea surface temperature variability

Previous studies show that the dominant mode of variability in the Northeastern subtropical Pacific and Atlantic are analogous. Most attention has been given to the wind-evaporation-sea surface temperature (WES) feedback, but more recent studies suggest that clouds and ocean play a role. Here, it is shown that, while the mode of variability is similar, the quantitative role of clouds and ocean are different. Using Community Earth System Model, version 1.2, cloud feedbacks and interactive ocean dynamics are disabled separately to diagnose the relative contributions of each to sea surface temperature (SST) variability in subtropical northeastern ocean basins. Results from four experiments show that the relative contributions from WES and cloud radiative feedback depend on the role of the ocean. Positive cloud radiative feedback is evident in both basins but has less impact on SST variance in the Atlantic than in the Pacific. The reason for this is that ocean processes strongly damp SST anomalies in the Pacific and weakly enhance SST anomalies in the Atlantic. When cloud feedbacks are disabled, ocean processes become a larger driver of SST variability in the Atlantic. In line with previous studies, the Northeast Pacific SST variability may be understood as a white-noise-forced linear stochastic system with positive feedback from cloud and damping by latent heat flux and ocean processes, while Atlantic SST is driven partially by variations in ocean circulation and requires vertical mixing for rendition. Between these two regions, different ocean dynamics lead to different roles for atmospheric feedbacks but still result in similar patterns of SST variability.

Variability and Trends of the Alaska Gyre From Argo and Satellite Altimetry

AbstractThe interannual variability and trends of the Alaska Gyre and Gulf of Alaska (GOA) circulation are examined using meridional geostrophic transport from Argo temperature and salinity (2004–2017) and altimetric sea surface height (1993–2017). More than half of the top 1,500 m meridional transport variability in the Alaska Gyre is accounted for by a statistical mode strongly correlated with the Pacific Decadal Oscillation (PDO) index, consistent with the PDO exerting a major influence on North Pacific sea surface temperature variability. During a positive phase of the PDO, the zero‐transport streamline separating the subtropical from the Alaska Gyre is shifted to the south from its mean position, while more transport is diverted northward, associated with a stronger and larger Alaska Gyre. Additionally, over the 25‐year altimetric record there is a linear, increasing trend in strength of the Alaska Gyre (but not in areal extent), accompanied by an increasing trend for the incoming North Pacific Current. The effect of the PDO transport mode on GOA circulation is weak. Temperature and salinity volume averaged for the GOA covary with the PDO index, with warmer and fresher waters during a positive phase. Despite correlated anomalies for temperature, salinity, and northward transport into the GOA, however, geostrophic advection from the south contributes only minimally to the interannual variations of water properties in the GOA. An exception was the marine heat wave of 2013/2014 and its aftermath when temperature advection from the south played a more appreciable role for warming and subsequent cooling of the GOA.

Isotopic composition of precipitation during strong El Niño–Southern Oscillation events in the Southeast Region of Brazil

AbstractEquatorial Pacific sea surface temperature variations interact with processes of atmospheric circulation, creating conditions for the occurrence of El Niño–Southern Oscillation (ENSO). ENSO events represent the most important interannual phenomena affecting climate patterns worldwide and causing significant socio‐economic impacts. In the Brazilian territory, ENSO leads to an increase in drought episodes in the north‐eastern region and an increase in precipitation in the southern region, whereas the effects over the south‐east region are yet not well understood. The main goal of this study is to compare variations of isotopic composition in precipitation across the south‐east portion of the Brazilian territory during two very strong ENSO events: 1997–1998 (ENSO 1) and 2014–2016 (ENSO 2). Daily isotopic records, available from the Global Network of Isotopes in Precipitation database for ENSO 1, and samples collected during ENSO 2 were used to compare the influence of both events on the isotopic composition of precipitation. Seasonal variations indicated more depleted precipitation during the wet seasons (δ18O = −5.4 ± 4.0‰) and enriched precipitation during the dry seasons (δ18O = −2.8 ± 2.3‰). Observed rainfall variations were associated with atmospheric large‐scale processes and moisture transport from the Amazon region, whereas extreme values (enriched or depleted) appear to be associated with particular convective and stratiform precipitation events. Overall, more depleted isotopic composition of precipitation (δ18O = −4.60‰) and higher d‐excess (up to +15‰) were observed during the dry season of ENSO 1 when compared with ENSO 2 dry season (δ18O‰ = −2.80‰, d‐excess lower than +14‰). The latter is explained by greater atmospheric moisture content, particularly associated with recycling of transpiration fluxes from the Amazon region, during dry season of ENSO 1. No significant differences for δ18O and δ2H were observed during the wet season; however, d‐excess from ENSO 2 was greater than ENSO 1, due to the slightly greater atmospheric moisture content and very strong upward motion observed. Our findings highlight the opportunity that environmental isotopes offer towards understanding hydrometeorological processes, particularly, the evolution of extreme climatic events of global resonance such as ENSO.

Ocean Impacts on Australian Interannual to Decadal Precipitation Variability

In Australia, successful seasonal predictions of wet and dry conditions are achieved by utilizing the remote impact of sea surface temperature (SST) variability in tropical oceans, particularly the Pacific Ocean, on the seasonal timescale. Beyond seasonal timescales, however, it is still unclear which processes and oceans contribute to interannual-to-decadal wet/dry conditions in Australia. This research examines the interannual-to-decadal relationship between global SST anomalies (SSTAs) and Australian wet/dry variability by analyzing observational data and global climate model experiments conducted with the NCAR Community Earth System Model (CESM) and the Model for Interdisciplinary Research on Climate (MIROC). A 10-member ensemble simulation suite for 1960–2015 (CESM) and 1950–2010 (MIROC) is conducted by assimilating the observed three-dimensional ocean temperature and salinity anomalies into fully coupled global climate models. In both observational analyses and ocean assimilation experiments, the most dominant annual mean precipitation variability shows a clear relationship with SSTAs in the tropical Pacific and the Atlantic. Our partial ocean assimilation experiment, in which the ocean component of the CESM and MIROC are assimilated by the observed ocean temperature and salinity anomalies in the equatorial Pacific only, shows that the tropical Pacific SST variability is the main driver of Australian precipitation variability on the interannual-to-decadal timescales. However, our additional partial ocean assimilation experiment, in which the climate models incorporate the observed anomalies solely in the Atlantic ocean, demonstrates that the Atlantic Ocean can also affect Australian precipitation variability on the interannual-to-decadal timescale through changes in tropical Pacific SSTAs and the modulation of the global Walker circulation. Our results suggest that about a half of Australian interannual-to-decadal precipitation variability originates from the Atlantic Ocean.

Disentangling Global Warming, Multidecadal Variability, and El Niño in Pacific Temperatures

AbstractA key challenge in climate science is to separate observed temperature changes into components due to internal variability and responses to external forcing. Extended integrations of forced and unforced climate models are often used for this purpose. Here we demonstrate a novel method to separate modes of internal variability from global warming based on differences in time scale and spatial pattern, without relying on climate models. We identify uncorrelated components of Pacific sea surface temperature variability due to global warming, the Pacific Decadal Oscillation (PDO), and the El Niño–Southern Oscillation (ENSO). Our results give statistical representations of PDO and ENSO that are consistent with their being separate processes, operating on different time scales, but are otherwise consistent with canonical definitions. We isolate the multidecadal variability of the PDO and find that it is confined to midlatitudes; tropical sea surface temperatures and their teleconnections mix in higher‐frequency variability. This implies that midlatitude PDO anomalies are more persistent than previously thought.

Linear and nonlinear winter atmospheric responses to extreme phases of low frequency Pacific sea surface temperature variability

This study examines Northern Hemisphere winter (DJFM) atmospheric responses to opposite strong phases of interdecadal (low frequency, LF) Pacific sea surface temperature (SST) forcing, which resembles El Niño-Southern Oscillation (ENSO) on a longer time scale, in observations and GFDL and CAM4 model simulations. Over the Pacific–North America (PNA) sector, linear observed responses of 500-hPa height (Z500) anomalies resemble the PNA teleconnection pattern, but show a PNA-like nonlinear response because of a westward Z500 shift in the negative (LF−) relative to the positive LF (LF+) phase. Significant extratropical linear responses include a North Atlantic Oscillation (NAO)-like Z500 anomaly, a dipole-like Z500 anomaly over northern Eurasia associated with warming over mid-high latitude Eurasia, and a Southern Annular anomaly pattern associated with warming in southern land areas. Significant nonlinear Z500 responses also include a NAO-like anomaly pattern. Models forced by LF+ and LF− SST anomalies reproduce many aspects of observed linear and nonlinear responses over the Pacific–North America sector, and linear responses over southern land, but not in the North Atlantic–European sector and Eurasia. Both models simulate PNA-like linear responses in the North Pacific–North America region similar to observed, but show larger PNA-like LF+ responses, resulting in a PNA nonlinear response. The nonlinear PNA responses result from both nonlinear western tropical Pacific rainfall changes and extratropical transient eddy feedbacks. With LF tropical Pacific forcing only (LFTP+ and LFTP−, climatological SST elsewhere), CAM4 simulates a significant NAO response to LFTP−, including a linear negative and nonlinear positive NAO response.

Synchronous multi-decadal climate variability of the whole Pacific areas revealed in tree rings since 1567

Oceanic and atmospheric patterns play a crucial role in modulating climate variability from interannual to multi-decadal timescales by causing large-scale co-varying climate changes. The brevity of the existing instrumental records hinders the ability to recognize climate patterns before the industrial era, which can be alleviated using proxies. Unfortunately, proxy based reconstructions of oceanic and atmospheric modes of the past millennia often have modest agreements with each other before the instrumental period, raising questions about the robustness of the reconstructions. To ensure the stability of climate signals in proxy data through time, we first identified tree-ring datasets from distant regions containing coherent variations in Asia and North America, and then interpreted their climate information. We found that the multi-decadal covarying climate patterns of the middle and high latitudinal regions around the northern Pacific Ocean agreed quite well with the climate reconstructions of the tropical and southern Pacific areas. This indicates a synchronous variability at the multi-decadal timescale of the past 430 years for the entire Pacific Ocean. This pattern is closely linked to the dominant mode of the Pacific sea surface temperature (SST) after removing the warming trend. This Pacific multi-decadal SST variability resembles the Interdecadal Pacific Oscillation.

A Reconstruction of Subtropical Western North Pacific SST Variability Back to 1578, Based on a Porites Coral Sr/Ca Record from the Northern Ryukyus, Japan

AbstractWe present a seasonal reconstruction of sea surface temperature (SST) from 1578 to 2008, based on a Porites coral Sr/Ca record from the northern Ryukyus, within the Kuroshio southern recirculation gyre. Interannual SST anomalies are generally ~0.5°C, making Sr/Ca‐derived SST reconstructions a challenging task. Replicate measurements along adjacent coral growth axes, enabled by the laser ablation inductively coupled plasma mass spectrometry technique used here, give evidence of rather large uncertainties. Nonetheless, derived winter SST anomalies are significantly correlated with the Western Pacific atmospheric pattern which has a dominant influence on winter temperature in East Asia. Annual mean SSTs show interdecadal variations, notably cold intervals between 1670 and 1700 during the Maunder Minimum (MM) and between 1766 and 1788 characterized by a negative phase of the North Atlantic Oscillation. Cold summers in 1783 and 1784 coincide with the long‐lasting Laki eruption that had a profound impact on the Northern Hemisphere climate, including the severe “Tenmei” famine in Japan. The decades between 1855 and 1900 are significantly cooler than the first half of the twentieth century, while those between 1700 and 1765, following the MM, are warmer than average. SST variability in the Ryukyus is only marginally influenced by the Pacific Decadal Oscillation, so that external forcing remains the main driver of low‐frequency temperature changes. However, the close connection between the Kuroshio extension (KE) and its recirculation gyre suggests that decadal SST anomalies associated with the KE front also impact the Ryukyus, and there is a possible additional role for feedback of the Kuroshio‐Oyashio variability to the large‐scale atmosphere at decadal timescale.

Variations in North Pacific sea surface temperature caused by Arctic stratospheric ozone anomalies

Recently, observations and simulations have shown that Arctic stratospheric ozone (ASO) variations affect the middle–high latitude tropospheric climate in the Northern Hemisphere. In particular, a connection from the ASO to El Niño–Southern Oscillation (ENSO) has been reported. However, no detailed study has been made of a key process in the connection, the influence of ASO on the North Pacific sea surface temperature (SST) and its underlying mechanism. Using observations, reanalysis and simulations, it is found that the ASO changes in March have the strongest connection with North Pacific SST variations in April. This implies a leading effect of ASO on North Pacific SST. The stratospheric circulation anomalies caused by March ASO changes can rapidly extend to the lower troposphere in the region 60°–90°N, 180°–120°W. Nevertheless, a theoretical analysis indicates that circulation anomalies from the region 60°–90°N, 180°–120°W in the lower troposphere would take about a month to propagate horizontally to the North Pacific middle latitudes (30°–60°N, 180°–120°W).

A strengthened impact of November Arctic oscillation on subsequent tropical Pacific sea surface temperature variation since the late-1970s

The connection between November Arctic Oscillation (AO) and the following spring-summer tropical Pacific sea surface temperature (SST) experienced a pronounced enhancement around the late-1970s. The connection was weak before, but strong after the late-1970s. The present study investigates the plausible reasons for the connection change. After the late-1970s, significant anomalous cyclone appears over subtropical North Pacific and pronounced anomalous westerly winds occur over tropical western Pacific in the positive phase of November AO. As such, November AO could exert a significant influence on the subsequent spring-summer tropical Pacific SST variation. By contrast, before the late-1970s, anomalous cyclone and associated anomalous westerly winds to its south are weak and shift northward. Thereby, the connection between November AO and tropical Pacific SST is weak. Further analyses suggest that the interdecadal change in the anomalous cyclone over the subtropical North Pacific may be related to the change in the November AO’s Pacific component and the strength of feedback of synoptic-scale eddy to mean flow. Specifically, after the late-1970s, November AO’s Pacific component displays a zonally elongated structure, which is favorable for the formation of easterly wind anomalies over the mid-latitude North Pacific and related wave-mean flow interaction. In addition, feedback of synoptic eddy to mean flow is enhanced after the late-1970s, which is partly related to the intensification of the North Pacific storm track. This may also contribute to enhancement of the anomalous cyclone over the subtropical Pacific.

Using High-Resolution Reanalysis Data to Explore Localized Western North America Hydroclimate Relationships with ENSO

Many studies have used observational data to explore associations between El Niño–Southern Oscillation (ENSO) and western North America (NA) hydroclimate at regional spatial scales. However, relationships between tropical Pacific sea surface temperature (SST) variability and western NA hydroclimate at local scales using reanalysis data are less understood. Here, the current understanding of relationships between large-scale tropical Pacific SST variability and western NA hydroclimate is extended to localized headwaters. To accomplish this, high-resolution reanalysis data (i.e., monthly mean surface precipitation rate, 2-m temperature, 850-mb specific humidity, and 500-mb omega) were used for gridpoint correlation analyses with Niño-3.4 SST and El Niño Modoki indices. Reanalysis time series data were provided by the National Centers for Environmental Prediction North American Regional Reanalysis (NARR) product. To validate the accuracy of NARR surface data, observational Livneh precipitation and temperature data were used. Resulting correlations between tropical Pacific indices and NARR surface precipitation and 2-m temperature are consistent with previous research both spatially and temporally, indicating that the strongest correlations occur primarily over southwestern NA during the winter (DJF). The results herein demonstrate the potential of high-resolution reanalysis data to reveal distinct correlations over topographically complex watersheds in the U.S. Intermountain West (IMW) over the recent record, 1979–2015. The use of the high-resolution NARR product as a viable option to explore western NA hydroclimate is demonstrated here.

Distinct mechanisms of Korean surface temperature variability during early and late summer

AbstractSeveral climate factors were identified that affect the surface air temperature (SAT) variations in Korea during summer (June‐July‐August). Korean summer SAT variation exhibits remarkable differences between early summer (June) and late summer (July and August). On one hand, the significant warming trend during early summer is primarily influenced by the global‐scale trend that is manifested in East Asia. The residual variability, obtained by removing warming trend from total SAT, represents Korean SAT variability independent of the global‐scale trend. This residual variability is closely related to the meridional dipole‐like air temperature structure between Korea and northeastern China, which is largely controlled by the atmospheric circulations over East Asia. However, this atmospheric structure does not originate from the remote oceanic forcing such as sea surface temperature (SST) variability over Pacific. During late summer, on the other hand, the Korean SAT is dominantly regulated by the atmospheric variability, which is closely related to the Pacific SST variability, while the contribution of global warming signal is insignificant. The SST anomalies in the central to eastern tropical Pacific lead to a dipole‐like atmospheric circulation from the tropics to East Asia, which modulates SAT in Korea. These results imply that the Korean SAT variability during early and late summer has different sources. That is, both the global‐scale trend and atmospheric variability over the East Asia should be considered in monitoring Korean SAT during early summer, whereas the SST variability in the central to eastern tropical Pacific needs more attention during late summer.

What Caused the Global Surface Warming Hiatus of 1998–2013?

Research into the mechanisms for the global warming slowdown or “hiatus” of 1998–2013 is reviewed here. Observational and modeling studies identify tropical Pacific sea surface temperature variability as a major pacemaker of global mean surface temperature (GMST) change, as corroborated by the GMST increase following a major El Nino event. Specifically, the decadal cooling of the tropical Pacific contributes to the recent global warming hiatus. This tropical Pacific pacemaker effect appears larger for decadal than interannual variability, but the decadal effect remains to be quantified from observations. Our critical review of the literature reveals that the internal and radiatively forced GMST changes are distinct in pattern, energetics, mechanism, and predictability. In contrast to greenhouse gas-induced warming that is spatially uniform in sign and driven by energy perturbations, internal variability in GMST is an order of magnitude smaller than spatial variations, for which ocean-atmosphere interaction is of first-order importance while planetary energetics is not. In fact, decadal variability in GMST is poorly correlated with net radiation at the top of the atmosphere, highlighting the need to distinguish internal and forced GMST change in planetary energy budget. While the planetary energy budget can now be closed observationally over multi-decadal periods, the recent hiatus highlights the need and challenges to measure and quantify decadal changes in both global ocean heat uptake (e.g., for the effect of radiative forcing on the hiatus) and heat redistribution in the ocean. Hiatus research has led to a wide recognition of the importance of internal variability for GMST trends over a decade and longer. The strengthened connection between the climate variability and change communities is an important legacy of hiatus research.

The effects of remote SST forcings on ENSO dynamics, variability and diversity

Air-sea interactions with remote regions in the tropical Indian and Atlantic, and extra-tropical oceans can influence ENSO features in the tropical Pacific. In this study these effects are explored by using an AGCM coupled with a Slab Ocean and a simple recharge oscillator ENSO model through switched on/off air-sea interaction in respective ocean area. It is shown that the decoupling in different remote regions has different impacts on ENSO dynamics, variability and diversity. The most interesting result is that the air-sea interactions with remote tropical oceans provide a delayed negative feedback to ENSO similar to that of the tropical Pacific Ocean internal wave dynamics. This is caused by the ENSO teleconnections: they lead to a delayed remote warming and cooling, which in turn feedbacks to ENSO effectively giving a delayed negative feedback. The model simulations suggest that this remote delayed feedback may contribute about 40% to the total delayed negative feedback of ENSO. Thus a central element of ENSO dynamics is partly due to interactions with other tropical ocean basins by atmospheric teleconnections. Furthermore, all remote regions effectively provide stochastic forcings for the ENSO variability and therefore increase the ENSO variability. The influence from the remote regions also causes different patterns of sea surface temperature (SST) variability in the tropical Pacific, contributing to the diversity of the ENSO mode. In particular the extra-tropical Pacific regions force SST variability that is different from the equatorial ENSO mode of variability. The influence that the remote regions have on the ENSO dynamics and variability is significantly altered by the interaction between the equatorial recharge oscillator dynamics and the simple thermodynamic slab ocean processes.

The El Ni\xf1o effect on Ethiopian summer rainfall

While El Niño is known to cause failure of Kiremt (boreal summer) rainfall in Ethiopia, the mechanisms are not fully understood. Here we use the ECHAM5 Atmospheric General Circulation Model to investigate the physical link between Pacific sea surface temperature (SST) anomalies and Kiremt rainfall. We compare ECHAM5 simulations forced with reconstructed SST data, to gauge-based rainfall observations and atmospheric reanalysis for the time period of 1961–2009. We perform composite analysis and sensitivity experiments driven only with equatorial Pacific SST anomalies. Our results show warm SST anomalies in the equatorial Pacific drive a corresponding large-scale circulation anomaly with subsidence over Ethiopia in dry Kiremt seasons. Horizontal wind fields show a slow-down of the whole Indian monsoon system with a weaker Tropical Easterly Jet and a weaker East African Low-Level Jet in these summers. These changes can be seen as an anomalous circulation cell over northern Africa with westerlies at 100–200 hPa and easterlies below 500 hPa. Surface easterlies might reduce the moisture inflow from the Atlantic and Congo basin into Ethiopia. This and the general subsidence over the region could explain the reduction in Kiremt rainfall. Our results suggest up to 50% of the Kiremt rainfall anomalies is driven by equatorial Pacific SST variability.

North Pacific decadal variability: insights from a biennial ENSO environment

This study examines the mechanisms of the Pacific decadal oscillation (PDO) in the NASA GEOS-5 general circulation model (GCM). Similar to several other state-of-the-art GCMs, the El Nino-Southern Oscillation (ENSO) simulated by the GEOS-5 has a strong biennial periodicity. Since this is a model bias that precludes a strong role of ENSO, it provides a unique environment to assess the other leading mechanisms of North Pacific decadal variability. Despite the biennial ENSO periodicity, the model simulates a realistic PDO pattern in the North Pacific that is resolved as the first empirical orthogonal function (EOF) of winter mean sea surface temperature (SST). The spectrum of the PDO indicates no preferred periodicity. The SST anomalies associated with the PDO, particularly its basin wide structure, are primarily forced by the Aleutian low through Ekman transport. The slow geostrophic transport in association with the meridional adjustment of the subtropical gyre is limited to a narrow region in the Kuroshio–Oyashio extension, north of 40°N. The atmosphere’s response to the PDO, while weak, projects onto the North Pacific Oscillation (NPO), a meridional dipole in sea level pressure. Both the lack of preferred periodicity and the weak atmospheric response indicate an air-sea coupled oscillation is an unlikely mechanism in this model. In agreement with recent studies, the NPO is correlated with the North Pacific Gyre Oscillation, which is another leading EOF of North Pacific SST variability. The results emphasize the role of atmospheric variability in the North Pacific SST modes, thereby bringing into question the potential for their predictability.