Zonal Advection Research Articles

A Precipitation Seesaw Mode between Northeast Asia and Siberia in Summer Caused by Rossby Waves over the Eurasian Continent

Abstract There is a dominant seesaw pattern in summertime precipitation between northeast Asia and Siberia. Examined here is a mechanism of the seesaw mode, focusing on quasi-stationary Rossby waves propagating on two upper-tropospheric waveguides along the Asian jet and over northern Eurasia. Empirical orthogonal function (EOF) analysis to the low-frequency (10-day low-pass filtered) variation during summer reveals wave-propagation patterns on both of the waveguides. The time evolution of the two composite fields, constructed using the obtained EOF modes for each phase of the seesaw mode, was investigated by the wave-activity flux. In both composite fields, eastward-propagating Rossby waves originating from the North Atlantic–European sector branch off around eastern Europe onto the two waveguides, which become out of phase toward the east because of the difference in longitudinal wavelengths along them. This results in opposite phase anomalies of meridional winds over northeast Asia and Siberia. Budget analyses of vorticity, heat, and water vapor on a day with significant wave patterns along the two waveguides revealed the main balances; the zonal advection of vorticity anomalies around the tropopause was partly balanced by an anomalous stretching effect over these two regions, and the adiabatic heating anomalies in the midtroposphere associated with the stretching effect around the tropopause were balanced with diabatic heating anomalies, explained by condensation heating related to the precipitation anomalies. These events occur frequently in July, when the climatological precipitation is the largest, particularly over northeast Asia.

The Role of the Equatorial Currents on the Evolution of SSTA in the Eastern Pacific during Two Different Kinds of El Niño Events

Based on the Simple Ocean Data Assimilation (SODA) from 1970 to 2001, equatorial currents and their association with the warm water propagation mechanism during two patterns of El Nio events are studied. In this study, the middle-pattern of El Nio (ME) and the eastern-pattern of El Nio (EE) events are defined as anomalous warm water originating first to the west and the east of 120°W, respectively. It is pointed out that the westerly and eastward anomalous currents in the western Pacific are stronger during the ME event than the EE event, which is conducive to the eastward migration of warm water from western Pacific by zonal advection of temperature. In contrast, the weaker westerly and the westward anomalous currents east of the dateline would be unfavorable for the eastward migration of warm water during EE events. More importantly, another propagation mechanism of the warm water is attributed to the anomalous convergence of the surface currents, as well as the anomalous divergence of the subsurface currents, which obstruct the upwelling of colder water from the deep ocean. Meanwhile, the anomalous convergence of the surface currents and the anomalous divergence of the subsurface currents maintain eastward migration, which plays an important role in the eastward migration of the warm water during ME events. Although there is anomalous convergence in the upper ocean and anomalous divergence in the subsurface ocean during EE events, they appear quasi-stationary in the western Pacific. The warm water over the eastern Pacific during EE events is caused by the local anomalous convergence of surface currents and the anomalous divergence of subsurface currents.

대기-해양 접합 모델에서 모사한 ENSO의 특징

새롭게 개발한 대기-해양 접합모형(CGCM)에서 모사한 열대 태평양 해수면 온도(SST)의 경년 변동성의 특징을 조사하였다. 모형은 SST의 경년변동과 평균분포, 계절변동, 강수량, 그리고 해양 내부 구조를 관측과 유사하게 모사하였다. 모델은 서태평양 경계 부근에서 관측에 비해 큰 SST 경년변동을 보였는데 이 원인으로 적도 태평양의 SST 동서 변화율을 제시하였다. 즉 관측에 비해 강한 SST 경도는 zonal advection feedback을 강화시켜 SST 아노말리의 서진(westward propagation)과 서태평양의 경년 변동성 증가를 가져왔다. 간단한 two-strip 모델을 이용한 민감도 실험 결과는 이를 뒷받침하였다. 동서 평균한 수온약층의 깊이와 NINO3 SST 인덱스의 분석 결과는 모델의 경년 변동이 충전-방전 진동자에 의한 것임을 나타냈다. 그리고 지연 회귀분석 결과를 이용하여 엘니뇨가 발달하기 전에 barrier layer thickness(BLT) 증가에 의한 열 축적 과정이 서태평양에서 먼저 일어난다는 것을 제시하였다. This paper explored the characteristics of the interannual sea surface temperature (SST) variability in the equatorial Pacific by analyzing the simulated data from a newly coupled general circulation model (CGCM). The CGCM simulates well the realistic ENSO variability as well as the mean climatologies including SST, seasonal cycle, precipitation, and subsurface structures. It is argued that the zonal gradient of SST in the equatorial Pacific is responsible for the over-energetic SST variability near the equatorial western boundary in the model. This variability could also be related to the strong westward propagation of SST anomalies which resulted from the enhanced the zonal advection feedback. The simple two-strip model supports this by sensitivity tests. Analysis of the relationship between zonal mean thermocline depth and NINO3 SST index suggested that the ENSO variability is controlled by the recharge-discharge oscillator of the model. The lead-lag regression result reveals that heat buildup process in the western equatorial Pacific associated with the increase of the barrier layer thickness (BLT) is a precedent condition for El <TEX>$Ni\widetilde{n}o$</TEX> to develop.

Rectification of ENSO Variability by Interdecadal Changes in the Equatorial Background Mean State in a CGCM Simulation

Abstract The link between the changes in equatorial background stratification and El Niño–Southern Oscillation (ENSO) modulation is investigated using a simulation from a 260-yr-long coupled general circulation model (CGCM). The work focuses on the role of nonlinearities associated with equatorial wave dynamics. As a first step, the low-frequency change in mean stratification is diagnosed and documented from the shallow-water parameters derived from a vertical mode decomposition of the CGCM. The parameters vary differently according to the baroclinic mode order, which may explain why a flattening thermocline does not necessarily lead to reduced ENSO activity. Estimations of baroclinic mode contributions to zonal current anomalies indicate that the decadal variability projects differently for the baroclinic modes as compared to the interannual variability. In particular, the high-order modes associated with decadal variability have a more pronounced signature in the western Pacific, whereas that associated with interannual variability (i.e., ENSO) shows more energy in the eastern Pacific. In the light of the results of the CGCM vertical mode decomposition, an intermediate coupled model (ICM) is used to test whether the nonlinearities associated with the changes in the baroclinic mode energy distribution can lead to coherent ENSO modulation. The results indicate that rectification of the interannual variability (ENSO time scales) by the interdecadal variability associated with changes in the oceanic mean states takes place in the ICM. The rectified effect results mostly in an increased variability and skewness of the zonal advection, which tends to produce a zonal seesaw of the sea surface temperature anomaly. A tropical mechanism for producing ENSO modulation is then proposed that reconciles both the rectified effect resulting from nonlinearities associated with equatorial wave dynamics and the tropical decadal mode of thermocline depth arising from Ekman-pumping anomalies located in the central South Pacific.

Snow cover variability in Poland in relation to the macro‐ and mesoscale atmospheric circulation in the twentieth century

AbstractThe main aim of the study is to investigate the atmospheric circulation impact on snow cover duration (SCD) and maximum seasonal depth of snow cover (MDS) in Poland in the twentieth century. The most important is to answer four questions: (1) which of the two, macro‐ or mesoscale atmospheric circulation, dominates in influencing the snow cover variability and changes in Poland area? (2) which of the two, meridional (S–N/N–S) or zonal (W–E/E–W) advection, dominates in influencing the snow cover variability and changes? (3) are the atmospheric circulation–snow cover relationships stable with time or did they change in circulation epochs? (4) does any strong relation exist between the snow cover in Poland and the atmospheric circulation enabling the possibility to forecast the nival conditions for the next winter season?The principal results of the study are as follows: (1) the macroscale circulation connected to atmospheric patterns over the Atlantic Ocean impacts stronger on the SCD variability in Poland than the circulation patterns located directly over the Poland area; (2) during the period 1966/67–1995/96 the zonal circulation dominated over the meridional one in influencing the SCD variability and change (the exceptions were areas with the most intensive foehn effect); (3) the decreasing trend of the dependence strength of the SCD on the meridional circulation with respect to the increasing tendency for zonal circulation‐SCD impact was observed in the twentieth century; (4) the meridional circulation dominated over the zonal one in influencing the SCD till the 1930s of the twentieth century only (5) the snow cover in Poland is positively correlated with the North Atlantic Oscillation (NAO) in October prior to the winter season.The instability of the snow cover dependence on the atmospheric circulation in the twentieth century corresponds roughly to the circulation epochs and periods of intensity changes of the zonal/meridional advection over Europe. Copyright © 2007 Royal Meteorological Society

Interaction between Near-Annual and ENSO Modes in a CGCM Simulation: Role of the Equatorial Background Mean State

Abstract A 260-yr-long coupled general circulation model (CGCM) simulation is used to investigate the interaction between ENSO mode and near-annual variability and its sensitivity to the equatorial background mean stratification and seasonal cycles. Although the thermocline mean vertical structure of the model favors the high-order baroclinic modes that are associated with the slow time scales of the coupled variability, the simulated ENSO oscillates at a dominant quasi-biennial frequency. Biases of the climatological velocity field are favorable to the dominance of the zonal advective feedback over the thermocline feedback, the model exhibiting an overenergetic westward seasonal zonal current in the central-western equatorial Pacific, and an upwelling rate that is about half the observations. This sets the conditions for the enhancement of a near-annual mode that is observed to oscillate at an 8-month period in the model. Using an intermediate coupled model of the tropical Pacific where the climatological fields are prescribed to the ones derived from the CGCM, it is demonstrated that the quasi-biennial ENSO variability simulated by the CGCM is mostly due to the biases in the climatological currents of the CGCM. These biases favor the dominance of the fast “zonal advective feedback” over the slow “thermocline feedback” in the coupled system and enhance a fast coupled basin mode. This fast mode differs from the theoretical Pacific Ocean basin mode in that, besides mean temperature advection by the zonal current anomalies, it is also driven by anomalous temperature advection by the total current. Results suggest that the near-annual mode destabilizes the ENSO mode to produce overenergetic quasi-biennial oscillations in the model. It also contributes to the ENSO asymmetry and the cold bias of the CGCM mean state by nonlinear accumulation of temperature zonal advection, which works toward the cold in the western Pacific more than the warm in the east. It is suggested that the model equilibrium results from the interaction between the ENSO mode, the near-annual mode, and the mean state.

A coupled‐stability index for ENSO

Based on the simple framework of the recharge oscillator for El Niño‐Southern Oscillation (ENSO), this paper describes the formulation of a coupled stability index for the large‐scale tropical ocean‐atmosphere interactions in the equatorial Pacific region. This index, referred to as the Bjerknes (BJ) stability index, takes its negative contributions from the mean upwelling and thermal damping and its positive contributions from the thermocline, the zonal advection, and the Ekman feedbacks. The validity of the BJ stability index formula is tested through a detailed eigen‐analysis of an intermediate‐coupled model, which shows that the BJ stability index captures the dependence of growth rate of the leading coupled ENSO‐like mode on various climate conditions. The general formula of the BJ stability index may be useful for assessing the coupled stability of ENSO and its sensitivity to changes in tropical climate conditions.

A numerical case study of the passage of a cold surge across Taiwan

A numerical study of a cold surge that occurred from 19–23 December 2001 was conducted to better understand the cold surge characteristics over the Taiwan area. The National Centers for Environmental Prediction (NCEP) nested Mesoscale Spectral Model (MSM) was used for this study. Simulation results demonstrated that the nested NCEP MSM captured salient features of the selected cold surge case. We demonstrated that the local minimum center of the time change of virtual potential temperature (dVPT) can serve as the location of severe weather of the cold surge for all Taiwan regions. Furthermore, thermodynamic equation analyses revealed that the leading edge of the cold surge was maintained primarily by meridian thermal advection, while diabatic heating, vertical and zonal thermal advections were less important.

A model study of the seasonal mixed layer heat budget in the equatorial Atlantic

In the present study, the physical processes that control the seasonal cycle of sea surface temperature in the tropical Atlantic Ocean are investigated. A high‐resolution ocean general circulation model is used to diagnose the various contributions to the mixed layer heat budget. The simulation reproduces the main features of the circulation and thermal structure of the tropical Atlantic. A close examination of the mixed layer heat budget is then undertaken. At a first order, the mixed layer temperature balance in the equatorial band results from cooling by vertical processes and heating by atmospheric heat fluxes and eddies (mainly tropical instability waves). Cooling by subsurface processes is the strongest in June–August, when easterlies are strong, with a second maximum in December. Heating by the atmosphere is maximum in February–March and September–October, whereas eddies are most active in boreal summer. Unlike previous observational studies, horizontal advection by low‐frequency currents plays here only a minor role in the heat budget. Off equator, the sea surface temperature variability is mainly governed by atmospheric forcing all year long, except in the northeastern part of the basin where strong eddies generated at the location of the thermal front significantly contribute to the heat budget in boreal summer. Finally, comparisons with previously published heat budgets calculated from observations show good qualitative agreement, except that subsurface processes dominate the cooling over zonal advection in the present study.

The circulation of the eastern tropical Pacific: A review

During the 1950s and 1960s, an extensive field study and interpretive effort was made by researchers, primarily at the Scripps Institution of Oceanography, to sample and understand the physical oceanography of the eastern tropical Pacific. That work was inspired by the valuable fisheries of the region, the recent discovery of the equatorial undercurrent, and the growing realization of the importance of the El Niño phenomenon. Here we review what was learned in that effort, and integrate those findings with work published since then as well as additional diagnoses based on modern data sets. Unlike the central Pacific, where the winds are nearly zonal and the ocean properties and circulation are nearly independent of longitude, the eastern tropical Pacific is distinguished by wind forcing that is strongly influenced by the topography of the American continent. Its circulation is characterized by short zonal scales, permanent eddies and significant off-equatorial upwelling. Notably, the Costa Rica Dome and a thermocline bowl to its northwest are due to winds blowing through gaps in the Central American cordillera, which imprint their signatures on the ocean through linear Sverdrup dynamics. Strong annual modulation of the gap winds and the meridional oscillation of the Intertropical Convergence Zone generates a Rossby wave, superimposed on the direct forcing, that results in a southwestward-propagating annual thermocline signal accounting for major features of observed thermocline depth variations, including that of the Costa Rica Dome, the Tehuantepec bowl, and the ridge–trough system of the North Equatorial Countercurrent (NECC). Interannual variability of sea surface temperature (SST) and altimetric sea surface height signals suggests that the strengthening of the NECC observed in the central Pacific during El Niño events continues all the way to the coast, warming SST (by zonal advection) in a wider meridional band than the equatorially trapped thermocline anomalies, and pumping equatorial water poleward along the coast. The South Equatorial Current originates as a combination of equatorial upwelling, mixing and advection from the NECC, and Peru coastal upwelling, but its sources and their variability remain unresolved. Similarly, while much of the Equatorial Undercurrent flows southeast into the Peru Undercurrent and supplies the coastal upwelling, a quantitative assessment is lacking. We are still unable to put together the eastern interconnections among the long zonal currents of the central Pacific.

ENSO variability and the eastern tropical Pacific: A review

El Niño-Southern Oscillation (ENSO) encompasses variability in both the eastern and western tropical Pacific. During the warm phase of ENSO, the eastern tropical Pacific is characterized by equatorial positive sea surface temperature (SST) and negative sea level pressure (SLP) anomalies, while the western tropical Pacific is marked by off-equatorial negative SST and positive SLP anomalies. Corresponding to this distribution are equatorial westerly wind anomalies in the central Pacific and equatorial easterly wind anomalies in the far western Pacific. Occurrence of ENSO has been explained as either a self-sustained, naturally oscillatory mode of the coupled ocean–atmosphere system or a stable mode triggered by stochastic forcing. Whatever the case, ENSO involves the positive ocean–atmosphere feedback hypothesized by Bjerknes. After an El Niño reaches its mature phase, negative feedbacks are required to terminate growth of the mature El Niño anomalies in the central and eastern Pacific. Four requisite negative feedbacks have been proposed: reflected Kelvin waves at the ocean western boundary, a discharge process due to Sverdrup transport, western Pacific wind-forced Kelvin waves, and anomalous zonal advections. These negative feedbacks may work together for terminating El Niño, with their relative importance being time-dependent. ENSO variability is most pronounced along the equator and the coast of Ecuador and Peru. However, the eastern tropical Pacific also includes a warm pool north of the equator where important variability occurs. Seasonally, ocean advection seems to play an important role for SST variations of the eastern Pacific warm pool. Interannual variability in the eastern Pacific warm pool may be largely due to a direct oceanic connection with the ENSO variability at the equator. Variations in temperature, stratification, insolation, and productivity associated with ENSO have implications for phytoplankton productivity and for fish, birds, and other organisms in the region. Long-term changes in ENSO variability may be occurring and are briefly discussed. This paper is part of a comprehensive review of the oceanography of the eastern tropical Pacific.

A Modeling Study of the Impact of Tropical Instability Waves on the Heat Budget of the Eastern Equatorial Pacific

Abstract A numerical simulation is used to investigate the mixed layer heat balance of the tropical Pacific Ocean including the equatorial cold tongue and the region of vortices associated with tropical instability waves (TIWs). The study is motivated by a need to quantify the effects that TIWs have on the climatological heat budget of the cold tongue mixed layer; there has been some discrepancy between observations indicating very large equatorward heat transport by TIWs and models that disagree on the full three-dimensional budget. Validation of the model reveals that the TIW-induced circulation patterns are realistic but may have amplitudes about 15% weaker than those in the observations. The SST budget within tropical instabilities is first examined in a frame of reference moving with the associated tropical instability vortices (TIVs). Zonal advection of temperature anomalies and meridional advection of temperature by current anomalies dominate horizontal advection. These effects strongly heat the cold cusps and slightly cool the downwelling areas located at the leading edge of the vortices. Cooling by vertical mixing is structured at the vortex scale and almost compensates for horizontal advective heating in the cold cusps. In contrast to some previous studies, TIW-induced vertical advection is found to be negligible in the SST budget. Cooling by this term is only significant below the mixed layer. The effect of TIWs on the climatological heat budget is then investigated for the region bounded by 2°S–6°N, 160°–90°W, where instabilities are most active. TIW-induced horizontal advection leads to a warming of 0.84°C month−1, which is of the same order as the 0.77°C month−1 warming effect of atmospheric fluxes, while the mean currents and vertical mixing cool the upper ocean by −0.59°C month−1 and −1.06°C month−1, respectively. The cooling effect of TIW-induced vertical advection is also negligible in the long-term surface layer heat budget and only becomes significant below the mixed layer. The results above, and in particular the absence of cancellation between horizontal and vertical TIW-induced eddy advection, are robust in three other sensitivity experiments involving different mixing parameterizations and increased vertical resolution.

Near-Annual SST Variability in the Equatorial Pacific in a Coupled General Circulation Model

Abstract Equatorial Pacific sea surface temperature (SST) anomalies in the Center for Ocean–Land–Atmosphere Studies (COLA) interactive ensemble coupled general circulation model show near-annual variability as well as biennial El Niño–Southern Oscillation (ENSO) variability. There are two types of near-annual modes: a westward propagating mode and a stationary mode. For the westward propagating near-annual mode, warm SST anomalies are generated in the eastern equatorial Pacific in boreal spring and propagate westward in boreal summer. Consistent westward propagation is seen in precipitation, surface wind, and ocean current. For the stationary near-annual mode, warm SST anomalies develop near the date line in boreal winter and decay locally in boreal spring. Westward propagation of warm SST anomalies also appears in the developing year of the biennial ENSO mode. However, warm SST anomalies for the westward propagating near-annual mode occur about two months earlier than those for the biennial ENSO mode and are quickly replaced by cold SST anomalies, whereas warm SST anomalies for the biennial ENSO mode only experience moderate weakening. Anomalous zonal advection contributes to the generation and westward propagation of warm SST anomalies for both the westward propagating near-annual mode and the biennial ENSO mode. However, the role of mean upwelling is markedly different. The mean upwelling term contributes to the generation of warm SST anomalies for the biennial ENSO mode, but is mainly a damping term for the westward propagating near-annual mode. The development of warm SST anomalies for the stationary near-annual mode is partially due to anomalous zonal advection and upwelling, similar to the amplification of warm SST anomalies in the equatorial central Pacific for the biennial ENSO mode. The mean upwelling term is negative in the eastern equatorial Pacific for the stationary near-annual mode, which is opposite to the ENSO mode. The development of cold SST anomalies in the aftermath of warm SST anomalies for the westward propagating near-annual mode is coupled to large easterly wind anomalies, which occur between the warm and cold SST anomalies. The easterly anomalies contribute to the cold SST anomalies through anomalous zonal, meridional, and vertical advection and surface evaporation. The cold SST anomalies, in turn, enhance the easterly anomalies through a Rossby-wave-type response. The above processes are most effective during boreal spring when the mean near-surface-layer ocean temperature gradient is the largest. It is suggested that the westward propagating near-annual mode is related to air–sea interaction processes that are limited to the near-surface layers.

The Forcing of the Pacific Decadal Oscillation*

Abstract The Pacific decadal oscillation (PDO), defined as the leading empirical orthogonal function of North Pacific sea surface temperature anomalies, is a widely used index for decadal variability. It is shown that the PDO can be recovered from a reconstruction of North Pacific sea surface temperature anomalies based on a first-order autoregressive model and forcing by variability of the Aleutian low, El Niño–Southern Oscillation (ENSO), and oceanic zonal advection anomalies in the Kuroshio–Oyashio Extension. The latter results from oceanic Rossby waves that are forced by North Pacific Ekman pumping. The SST response patterns to these processes are not orthogonal, and they determine the spatial characteristics of the PDO. The importance of the different forcing processes is frequency dependent. At interannual time scales, forcing from ENSO and the Aleutian low determines the response in equal parts. At decadal time scales, zonal advection in the Kuroshio–Oyashio Extension, ENSO, and anomalies of the Aleutian low each account for similar amounts of the PDO variance. These results support the hypothesis that the PDO is not a dynamical mode, but arises from the superposition of sea surface temperature fluctuations with different dynamical origins.

A heat budget study on the mechanism of SST variations in the Indian ocean dipole regions

By using a new heat budget equation that is closely related to the sea surface temperature (SST) and a dataset from an ocean general circulation model (MOM2) with 10-a integration (1987–1996), the relative importance of various processes determining SST variations in two regions of the Indian Ocean is compared. These regions are defined by the Indian Ocean Dipole Index and will be referred to hereafter as the eastern (0°–10°S, 90°–110°E) and western regions (10°S–10°N, 50°–70°E), respectively. It is shown that in each region there is a falling of SST in boreal summer and a rising in most months of other seasons, but the phases are quite different. In the eastern region, maximum cooling rate occurs in July, whereas in the western region it occurs in June with much larger magnitude. Maximum heating rate occurs in November in the eastern region, but in March in the western one. The western region exhibits another peak of increasing rate of SST in October, indicating a typical half-year period. Net surface heat flux and entrainment show roughly the same phases as the time-varying term, but the former has much larger contribution in most of a year, whereas the latter is important in the boreal summer. Horizontal advection, however, shows completely different seasonal variations as compared with any other terms in the heat budget equation. In the eastern region, it has a maximum in June/November and a minimum in March/September, manifesting a half-year period; in the western region, it reaches the maximum in August and the minimum in November. Further investigation of the horizontal advection indicates that the zonal advection has almost the opposite sign to the meridional advection. In the eastern region, the zonal advection is negative with a peak in August, whereas the meridional one is positive with two peaks in June and October. In the western region, the zonal advection is negative from March to November with two peaks in June and November, whereas the meridional one is positive with one peak in July. Different phases can be clearly seen between the two regions for each component of the horizontal advection. A detailed analysis of the data of 1994, a year identified when the Indian Ocean dipole event happened, indicates that the horizontal advection plays a dominant role in the remarkable cooling of the eastern region, in which zonal and meridional advections have the same sign of anomaly. However, in the western region in 1994 no any specialty was shown as compared with other years, for the SST anomaly is not positive in large part of this region. All these imply that the eastern and western regions may be related in a quite complex way and have many differences in dynamics. Further study is needed.

A Study of the Impact of Off-Equatorial Warm Pool SST Anomalies on ENSO Cycles

Abstract Concurrent with most large El Niño events, cold sea surface temperature (SST) anomalies are observed over the western Pacific warm pool region (WPWP). Observational evidence that SST anomalies that form in the off-equatorial western Pacific during El Niño–Southern Oscillation (ENSO) cycles are forced by subsurface ocean processes equatorward of 12°N and air–sea fluxes poleward of 12°N is presented. It is demonstrated that diurnal mixing in the ocean equatorward of 12°N plays a significant role in bringing subsurface temperature anomalies to the sea surface during an El Niño event. The role of SST anomalies equatorward of 12°N in ENSO cycles is tested in the Zebiak–Cane coupled model, modified to allow for the impact of subsurface temperatures on SSTs. This coupled model successfully simulates cold SST anomalies in the off-equatorial northwestern Pacific that are observed to occur during the warm phase of ENSO and the atmospheric response to these anomalies, which is composed of both westerlies in the central Pacific and easterlies in the far western equatorial Pacific. It is found that there is little net change in the zonal mean wind stress at the equator, suggesting that the westerlies cancel the impact of the easterlies on the basin-scale tilt of the equatorial zonal mean thermocline depth. The anomalous westerly winds in the central equatorial Pacific are found to increase the amplitude of an El Niño event directly by increasing anomalous warm zonal advection and reducing upwelling. Moreover, the off-equatorial anticyclonic wind stress associated with the cold SST anomalies during the warm phase of ENSO tends to reduce the discharge of the equatorial heat content. Thus, the coupled processes over the western Pacific warm pool can serve as a positive feedback to amplify ENSO cycles.

Relative roles of the equatorial upper ocean zonal current and thermocline in determining the timescale of the tropical climate system

In this study, it is demonstrated that the amplitude of the equatorial upper-ocean zonal current anomaly induced by the fast-varying wind forcing (shorter than a year) is much greater than that induced by the slowly varying wind forcing (longer than 2 year), and the center of maximum zonal current anomaly shifts from the central Pacific to the western Pacific with an increase in the timescale of wind forcing. As a result, the zonal advective feedback (the zonal advection of mean sea surface temperature by anomalous current) in a slowly varying climate system becomes weaker and barely induces a low-frequency mode such as El Nino-Southern Oscillation. On the other hand, both amplitude and zonal location of the maximum thermocline anomaly are little changed by the change in the timescale of wind forcing – confined at the strong equatorial upwelling region of the eastern Pacific. Accordingly, the thermocline feedback (the vertical advection of anomalous subsurface temperature by the mean upwelling) is more favorable to generate a low-frequency mode.

Quasi-Lagrangian Large Eddy Simulations of Cross-Equatorial Flow in the East Pacific Atmospheric Boundary Layer

Using a large eddy simulation (LES), the atmospheric boundary layer (ABL) is numerically modeled along 958W from 88 St o 48N during boreal autumn, and compared to observations from the East Pacific Investigation of Climate Processes in the Coupled Ocean‐Atmosphere System (EPIC) 2001. Since the local ABL winds are predominantly southerly in this season, a ‘‘quasi-Lagrangian’’ forcing is used in which the ABL air column is forced as if it were advecting northward with the mean September‐October 2001 meridional wind across the equatorial cold tongue and the rapidly warming SSTs to the north. Pressure gradients and large-scale zonal advective tendencies are prescribed as a function of latitude. Where possible, observations from the EPIC 2001 experiment are used for forcing and for comparison with model results. The ABL’s modeled vertical structure accords with the conceptual model of Wallace et al. and agrees well with observations. Surface stability accounts for the minimum in surface wind over the equatorial cold tongue and the maximum over the warm water to the north. Stability of the lower ABL over the cold tongue allows a jet to accelerate at about 500-m height, relatively uncoupled to the frictional surface layer. Vertical mixing over the warm water to the north distributes this momentum to the surface. Additional simulations were performed to explore the modeled ABL’s sensitivity to pressure gradients, zonal advection, free-tropospheric humidity, and initial conditions. The model ABL was robust: changing the forcings resulted in little change in the modeled structure. The strongest sensitivity was of stratocumulus clouds over the cold tongue to cloud-top radiative cooling. Once formed at the southern edge of the cold tongue, modeled stratocumulus clouds demonstrate a remarkable ability to maintain themselves over the cold tongue in the absence of surface fluxes by radiative cooling at their tops. The persistence of thin stratocumulus clouds in this Lagrangian model suggests that horizontal advection of condensate might be an important process in determining cloudiness over the cold tongue.

Temperature Advection: Internal versus External Processes

Abstract Local advection of temperature is the inner product of vector velocity and spatial gradient of temperature. This product is often integrated spatially to infer temperature advection over a region. However, the contribution along an individual direction can be dominated by internal processes that redistribute heat within the domain but do not control the heat content of the domain. A new formulation of temperature advection is introduced to elucidate external heat source and sink that control the spatially averaged temperature. It is expressed as the advection of interfacial temperature relative to the spatially averaged temperature of the domain by inflow normal to the interface. It gives a total advection of temperature that is identical to the spatial integration of local temperature advection, yet the contributions along individual directions depict external processes. The differences between the two formulations are illustrated by analyzing zonal advection of near-surface temperature in the e...

A Near-Annual Pacific Ocean Basin Mode

Some fairly regular and nearly annual variability in the equatorial Pacific after the major 1997/98 El Nino event is studied. Sea level, sea surface temperature (SST), and surface wind anomalies of this variability are tied together in a way similar to the slow cycles of El Nino-Southern Oscillations (ENSO). Despite a slightly longer-than-annual time scale, similar variability was also found in the datasets prior to 1997/98. This fast- coupled mode is superimposed on the slow 3-5-yr ENSO cycles in the tropical Pacific. It contributed to the occurrences of some minor El Nino and La Nina events. Zonal currents associated with the equatorial waves play a dominating role in generating SST anomalies of this fast mode through anomalous zonal advection. It is suggested that this fast mode may be best understood as a coupled Pacific Ocean basin (POB) mode. The existence of this fast mode, which appears independent of the slow ENSO mode, has important implications in understanding and predicting the tropical Pacific SST anomaly.