Warm Pool SST Research Articles

Cross-hemispheric SST propagation enhances the predictability of tropical western Pacific climate

The tropical western Pacific (TWP) has profound influences on climate. ENSO is an important source of interannual variability of TWP SST, but extratropical precursors are far less known. Here we show a significant interhemispheric influence from subtropical Southwest Pacific (SWP) on the TWP. Observational analysis suggests that SWP SST in boreal spring has strong coherence with TWP 6 months later. The spring SWP warming signal exhibits a unique interhemispheric propagation embedded in the southerly cross-equatorial flow over the western Pacific. The wind-evaporation-SST feedback initiates and maintains the progression of warm SST anomalies toward the TWP in autumn. The climate model successfully reproduces such an interhemispheric SST propagation. The seasonal evolution of SST variability improves the predictability of the warm pool SST by about 6 months. An SWP SST-based prediction model shows considerable hindcast skill (r = 0.80, p < 0.01), indicating that it is a valuable precursor of the TWP.

Projected sea surface temperature changes in the equatorial Pacific relative to the Warm Pool edge

Projections of equatorial sea surface temperature from CMIP5 climate models are important for understanding possible future changes in marine habitats, local rainfall and climate processes such as El Niño Southern Oscillation. Interpreting the projected changes in the tropical Pacific is complicated by the systematic cold tongue bias and overly westward location of the Warm Pool edge at the equator in coupled models. Here an index based on the maximum zonal salinity gradient is used to differentiate the Warm Pool from the cold tongue in each of 19 CMIP5 models. Warming is then calculated relative to the dynamic edge of the Warm Pool between the second halves of the 20th and 21st Centuries from the RCP8.5 scenario to provide a bias adjusted SST projection.Based on this definition of the edge, while the Warm Pool edge is projected to warm, it is likely to remain within 10° of its present longitude. This is in stark contrast to the large projected eastward displacements of the isotherms that are usually used to define the edge. Adjusting for the edge, warming within the Warm Pool is projected to be fairly uniform with surface water freshening. Projected warming is enhanced over the cold tongue with the net effect of reducing the zonal SST gradient. In contrast, if the warming is calculated without correcting for the edge of the Warm Pool, the warming signature is dominated by the poorer performing models with an overly westward Warm Pool, resulting in enhanced warming across the equatorial Pacific. Bias adjusting realigns the warming signature and reduces the model spread of projected warming. The biased warming signature also introduces spurious meridional and zonal SST gradients. This will potentially alter the behaviour of the atmospheric convergence zones and the dynamics of ENSO which is influenced by the extent of the Warm Pool and zonal SST gradients.

How Many ENSO Flavors Can We Distinguish?*

AbstractIt is now widely recognized that El Niño–Southern Oscillation (ENSO) occurs in more than one form, with the canonical eastern Pacific (EP) and more recently recognized central Pacific (CP) ENSO types receiving the most focus. Given that these various ENSO “flavors” may contribute to climate variability and long-term trends in unique ways, and that ENSO variability is not limited to these two types, this study presents a framework that treats ENSO as a continuum but determines a finite maximum number of statistically distinguishable representative ENSO patterns. A neural network–based cluster analysis called self-organizing map (SOM) analysis paired with a statistical distinguishability test determines nine unique patterns that characterize the September–February tropical Pacific SST anomaly fields for the period from 1950 through 2011. These nine patterns represent the flavors of ENSO, which include EP, CP, and mixed ENSO patterns. Over the 1950–2011 period, the most significant trends reflect changes in La Niña patterns, with a shift in dominance of La Niña–like patterns with weak or negative western Pacific warm pool SST anomalies until the mid-1970s, followed by a dominance of La Niña–like patterns with positive western Pacific warm pool SST anomalies, particularly after the mid-1990s. Both an EP and especially a CP El Niño pattern experienced positive frequency trends, but these trends are indistinguishable from natural variability. Overall, changes in frequency within the ENSO continuum contributed to the pattern of tropical Pacific warming, particularly in the equatorial eastern Pacific and especially in relation to changes of La Niña–like rather than El Niño–like patterns.

The impact of warm pool SST and general circulation on increased temperature over the Tibetan Plateau

In this paper, the possible reason of Tibetan Plateau (TP) temperature increasing was investigated. An increase in T min (minimum temperature) plays a robust role in increased TP temperature, which is strongly related to SST over the warm pool of the western Pacific Ocean, the subtropical westerly jet stream (SWJ), and the tropical easterly upper jet stream (TEJ), and the 200-hPa zonal wind in East Asia. Composite analysis of the effects of SST, SWJ, and TEJ on pre- and post-abrupt changes in T a (annual temperature) and T min over the TP shows remarkable differences in SST, SWJ, and TEJ. A lag correlation between T a/T min, SST, and SWJ/TEJ shows that changes in SST occur ahead of changes in T a /T min by approximately one to three seasons. Partial correlations between T a/T min, SST, and SWJ/TEJ show that the effect of SWJ on T a/T min is more significant than the effect of SST. Furthermore, simulations with a community atmospheric model (CAM3.0) were performed, showing a remarkable increase in T a over the TP when the SST increased by 0.5°C. The main increase in T a and T min in the TP can be attributed to changes in SWJ. A possible mechanism is that changes in SST force the TEJ to weaken, move south, and lead to increased SWJ and movement of SWJ northward. Finally, changes in the intensity and location of the SWJ cause an increase in T a/T min. It appears that TP warming is governed primarily by coherent TEJ and SWJ variations that act as the atmospheric bridges to remote SSTs in warm-pool forcing.

Understanding ENSO Regime Behavior upon an Increase in the Warm-Pool Temperature Using a Simple ENSO Model

Abstract The regime behavior of the low-order El Niño–Southern Oscillation (ENSO) model, according to an increase in the radiative–convective equilibrium sea surface temperature (SST; Tr), is studied to provide a possible explanation for the observed increase in ENSO irregularity characterized by decadal modulation. During recent decades, a clear increasing trend of the warm-pool SST has been observed. In this study, the increase in the warm-pool maximum SST is interpreted as an increase in Tr following previous studies. A bifurcation analysis with Tr as a control parameter is conducted to reveal that the degree of ENSO irregularity in the model is effectively controlled by the equilibrium states of the model. At a critical value of Tr, bifurcation analysis reveals that period-doubling bifurcation occurs and an amplitude-modulated ENSO emerges. At this point, a subcycle appears within the preexisting ENSO cycle, which initiates decadal modulation of ENSO. As Tr increases further, nested oscillations are successively generated, illustrating clear decadal modulation of ENSO. The qualitative regime changes revealed in this study are supported by the observation of regime shifts in the 1970s. With increasing Tr, the mean zonal SST gradient increases, and the model adjusts toward a “La Niña–like” mean state. Further constraint with shoaling of the mean thermocline depth and increasing stratification causes ENSO to exhibit stronger amplitude modulation. Furthermore, the timing of the period-doubling bifurcation advances with these two effects.

The Effect of Subtropical Cooling on the Amplitude of ENSO: A Numerical Study

The effect of an enhanced subtropical surface cooling on El Nino-Southern Oscillation (ENSO) through the ''ocean tunnel'' is investigated using a coupled model. Here, the term ''ocean tunnel'' refers to the water pathway that connects the equatorial upwelling water to the subtropical/extratropical surface water. The subtropical cooling is introduced through a reduction of the radiative-convective equilibrium SST (SSTp) in that region. The SSTp for the equatorial region is kept fixed. It is found that an enhanced cooling in the subtropics results in a regime with stronger ENSO. This is because an enhanced subtropical cooling reduces the temperature of the water feeding the equatorial undercurrent through the ocean tunnel. The resulting larger difference between the warm-pool SST and the temperature of the equatorial thermocline water—the source water for the equatorial upwelling—tends to increase the equatorial zonal SST contrast between the western and the eastern Pacific. In response to this destabilizing forcing to the coupled equatorial ocean-atmosphere, a stronger ENSO develops. ENSO is found to regulate the time-mean difference between the warm-pool SST and the temperature of the equatorial undercurrent. The findings provide further support for the ''heat pump'' hypothesis for ENSO, which states that ENSO is an instability driven by the meridional differential heating over the Pacific Ocean and that ENSO regulates the long-term stability of the coupled equatorial Pacific climate. The results also substantiate the notion that surface variability from higher latitudes may influence equatorial SST variability through the ocean tunnel.

Corrected TOGA COARE Sounding Humidity Data: Impact on Diagnosed Properties of Convection and Climate over the Warm Pool

Abstract This study reports on the humidity corrections in the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean–Atmosphere Response Experiment (COARE) upper-air sounding dataset and their impact on diagnosed properties of convection and climate over the warm pool. During COARE, sounding data were collected from 29 sites with Vaisala-manufactured systems and 13 sites with VIZ-manufactured systems. A recent publication has documented the characteristics of the humidity errors at the Vaisala sites and a procedure to correct them. This study extends that work by describing the nature of the VIZ humidity errors and their correction scheme. The corrections, which are largest in lower-tropospheric levels, generally increase the moisture in the Vaisala sondes and decrease it in the VIZ sondes. Use of the corrected humidity data gives a much different perspective on the characteristics of convection during COARE. For example, application of a simple cloud model shows that the peak in convective mass flux shifts from about 8°N with the uncorrected data to just south of the equator with corrected data, which agrees better with the diagnosed vertical motion and observed rainfall. Also, with uncorrected data the difference in mean convective available potential energy (CAPE) between Vaisala and VIZ sites is over 700 J kg−1; with the correction, both CAPEs are around ∼1300 J kg−1, which is consistent with a generally uniform warm pool SST field. These results suggest that the intensity and location of convection would differ significantly in model simulations with humidity-corrected data, and that the difficulties which the reanalysis products had in reproducing the observed rainfall during COARE may be due to the sonde humidity biases. The humidity-corrected data appear to have a beneficial impact on budget-derived estimates of rainfall and radiative heating rate, such that revised estimates show better agreement with those from independent sources.

A Possible Effect of an Increase in the Warm-Pool SST on the Magnitude of El Niño Warming

El Niño warming corresponds to an eastward extension of the western Pacific warm pool; one thus naturally wonders whether an increase in the warm pool SST will result in stronger El Niños. This question, though elementary, has not drawn much attention. The observation that the two strongest El Niños in the instrumental record occurred during the last two decades, when the warm pool SST was anomalously high, however, has added some urgency to answering this question. Here observational and numerical results that support a positive answer to this question are shown. The observational results come from an analysis of the heat balance of the tropical Pacific over the period 1980–99. The analysis confirms that El Niño acts as a major mechanism by which the tropical Pacific transports heat poleward—the poleward heat transport is achieved episodically, and those episodes correspond well with the occurrence of El Niños. Moreover, the analysis shows that El Niño is a regulator of the heat content in the western Pacific: the higher the heat content, the stronger the subsequent El Niño warming, which transports more heat poleward, and results in a larger drop in the heat content in the western Pacific. These empirical results suggest that a higher warm-pool SST may result in stronger El Niño events. Specifically, raising the tropical maximum SST through an increase in the radiative heating across the equatorial Pacific initially increases the zonal SST contrast. A stronger zonal SST contrast then strengthens the surface winds and helps to store more heat in the subsurface ocean. Because of the stronger winds and the resulting steeper tilt of the equatorial thermocline, the coupled system is potentially unstable and is poised to release its energy through a stronger El Niño warming. A stronger El Niño then pushes the accumulated heat poleward and prevents heat buildup in the western Pacific, and thereby stabilizes the coupled system. Numerical experiments with a coupled model in which the ocean component is a primitive equation model (the NCAR Pacific basin model), and therefore explicitly calculates the heat budget of the entire equatorial upper ocean, support this suggestion. The numerical experiments further suggest that in the presence of El Niños, the time-mean zonal SST contrast may not be sensitive to increases in the surface heating because the resulting stronger El Niños cool the western Pacific and warm the eastern Pacific.

Low-Frequency Controls on the Thresholds of Sea Surface Temperature over the Western Tropical Pacific

Abstract In this study, the Florida State University Coupled Global Spectral Model (FSUCGSM) is utilized to examine the possible regulation of the warm pool SST and its contributors. The model is run for 1 yr to obtain the residue-free time evolution of the warm pool SST. The results are verified against NCEP SST analysis for the period of the model integration. The best agreement was seen over the western equatorial Pacific. The initial analysis of the model output has suggested that the warm pool SST is derived mainly by three important types of oscillations, namely, semiannual, 10–25-, and 30–60-day oscillations. Further examination using Butterworth bandpass filter and EOF analysis has revealed that the tendency of solar radiation is the primary cause of the high-frequency oscillations (20–25 and 30–60 day) and secondary cause for the low-frequency oscillations. Moreover, the evaporative cooling is found to be the primary cause of the low-frequency oscillations and secondary cause for the high-frequen...

Warm Pool SST Variability in Relation to the Surface Energy Balance

The warm tropical oceans underlie the most convective regions on earth and are a critical component of the earth’s climate, yet there are differing opinions on the processes that control warm pool SST. The Indo‐Pacific warm pool is characterized by large-scale variations in SST approaching 308C on intraseasonal timescales. In this study, surface heat flux anomalies associated with composite warm episodes over three spatial scales in both the Pacific and Indian Ocean basins are examined. The current study benefits from the recently available National Centers for Environmental Prediction‐National Center for Atmospheric Research reanalysis dataset that enables the examination of variability in surface evaporation with moderate confidence. Solar flux estimates from the reanalysis are somewhat less reliable than evaporation estimates, however, and two techniques that infer surface shortwave radiation from satellite retrievals of cloud properties are considered. Error in all measurements is quantified. Both shortwave and evaporative flux variability play significant roles in modifying the temperature of the warm pool, though the relative importance of individual flux anomalies depends on SST tendency and geographical location. There also exist differences in the relative heating roles of the flux anomalies among episodes within a fixed location, though in instances the resolved differences are less than likely flux estimation error. Differences also exist between the ocean basins. A more pronounced annual cycle exists in the eastern Indian Ocean, and SST there is less sensitive to surface thermal forcing. Finally, the analysis offers evidence that SST is not regulated by a simple atmospheric thermodynamic response to the surface. Instead, the relationship between warm pool variability and large-scale dynamical features of the Tropics (e.g., intraseasonal oscillation and the seasonal monsoon) is demonstrated. The conclusions are shown to be robust to spatial scale and are consistent with a recent analysis of Tropical Oceans and Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment observations.