A33B-01 INVITED 13:40h
Moist Teleconnection Mechanisms
Teleconnections have traditionally been studied for the case of dry dynamical response to a given diabatic heat source. Important anomalies often occur within convective zones, for instance, in the observed remote response to El Ni\~no. The reduction of rainfall and teleconnection propagation in deep convective regions pose theoretical challenges because of the role of moist convective feedbacks. During El Ni\~no, large-scale negative precipitation anomalies often occur over equatorial South America and the Atlantic intertropical convergence zone (ITCZ), and in a horseshoe pattern around the central Pacific region of positive precipitation. Analysis of these in an intermediate complexity model is used to propose and review some general principals of teleconnections occurring in deep convective zones, contrasting land and ocean cases. Tropospheric temperature and wind anomalies are spread by wave dynamics modified by interaction with the moist convection zones. The traditional picture of gradual descent balanced by radiative damping misses the most important balances in the moist static energy (MSE) budget. A small ``zoo'' of mechanisms is active in producing strong regional descent and negative precipitation anomalies. Factors common to several mechanisms include the role of convective quasi-equilibrium (QE) in linking low-level moisture anomalies to free tropospheric temperature anomalies in a two-way interaction referred to as {\it QE mediation}. Convective heating feedbacks change the net static stability to a gross moist stability (GMS) $M$. The large cloud-radiative feedback terms may be manipulated to appear mathematically similar to a modified static stability $M_{\rm eff}$. This $M_{\rm eff}$ differs over land versus over ocean at time scales up to many months due to surface energy flux balance. Apparently modest terms in the MSE budget can be acted on by the {\it GMS multiplier effect} which yields substantial precipitation anomalies due to the large ratio of the moisture convergence to the MSE divergence. Advection terms enter in several mechanisms, including advection of mean moisture gradients by anomalous winds ${\bf v}'\cdot\nabla \bar q$ in the Pacific. In eastward teleconnection, advection by mean winds is important to MSE and momentum balances in a {\it Kelvinoid solution}. The opposition of moist wave speed by easterly flow enhances {\it moist wave decay} mechanisms, permitting relatively small damping terms by surface drag and radiative damping to produce the substantial eastward temperature gradients seen in observations and simulations and contributing to precipitation anomalies. The leading mechanism for drought in eastern equatorial South America is the {\it upped-ante mechanism}: teleconnected tropospheric temperature anomalies induce moisture gradients between non-convective zones and convection zones where QE-mediation increases low-level moisture; mean winds advect the anomalous gradient to produce descent anomalies and negative precipitation anomalies. The upped-ante mechanism is also important to Atlantic ITCZ rainfall reductions, especially as SST equilibrates in passive-SST (coupled ocean-mixed layer) experiments. For fixed SST experiments, or while SST is adjusting during passive SST experiments, heat flux to the ocean is lost to the atmosphere and can induce descent and precipitation anomalies by several pathways, referred to as {\it troposphere/SST disequilibrium mechanisms}. These are important in the South Pacific and the Atlantic ITCZ.
A33B-02 13:55h
Mechanisms for Lagged Atmospheric Response to ENSO SST Forcing
The sensitivity and mechanism for the lagged response of tropical tropospheric temperature to El Ni\~no SST forcing are examined using results from the quasi-equilibrium tropical circulation model (QTCM) coupled to a slab mixed-layer ocean model. It is found that the lag and amplitude of tropospheric temperature response depend on mixed-layer ocean depth, ENSO SST forcing period and areal fraction of mixed-layer ocean region. The phase lag is not a monotonic function of mixed-layer depth. It maximizes at moderate MLD and increases with SST forcing period. The amplitude decreases with MLD and increases with SST forcing period. The amplitude and phase lag of tropospheric temperature response seem to be insensitive to the seasonal cycle of SST. While ENSO region convective heating (precipitation) anomalies, are closely tied to SST anomalies, the tropical mean precipitation seems best viewed as a complex by-product of the response rather than as a driver. One useful parameter determining the lag of tropospheric temperature to ENSO SST is the free decay time scale of the coupled system. This parameter combines effects of top-of-atmosphere fluxes, surface flux exchanges, tropics-midlatitude moist static energy transports and mixed-layer ocean heat capacity. It is indicative of the extent to which the lagged response of tropical tropospheric temperature to ENSO SST is a coupled phenomenon.
A33B-03 INVITED 14:10h
Aspects of the Remote Influence of ENSO in the Tropical Atlantic Region
The influence of El Nino-Southern Oscillation (ENSO) on surface pressure and sea surface temperature variability in the Tropical Atlantic region is well known from observational studies. However, the exact mechanisms responsible for this influence are not yet fully understood. In this study, we focus on the processes that control surface fluxes in the tropical Atlantic and their relative contribution to the remote influence. Conventional wisdom holds that anomalies in surface latent heat flux play the dominant role. We assess the role of latent heating, as well as that of surface radiative fluxes, including the effect of cloudiness, using general circulation model (GCM) experiments. We analyze two suites of experiments, one using the NCAR atmospheric GCM and another using the NASA Seasonal-to-Interannual Prediction Project (NSIPP) atmospheric GCM. In our analysis, we also consider the role of land surface hydrology in mediating the remote influence of ENSO in the Tropical Atlantic region.
A33B-04 14:25h
The Oceanic Aspects of the Remote Influence of ENSO on TAV
An anomaly coupled ocean-atmosphere model, consisting of the NCAR CCM3 and a Zebiak-Cane type of reduced gravity ocean model (RGO), is employed to investigate tropical Atlantic response to ENSO remote forcing. A suite of numerical experiments were carried out to assess the relative importance of various oceanic processes contributing to the Atlantic SST response. The result shows that ocean dynamics are required to produce a realistic simulation of the coupled system. It is further shown that the Atlantic response to ENSO can be divided into two stages. Stage one consists of basin-wide SST anomaly which lags ENSO by about a season and peaks in the boreal spring. This response can be largely explained by the ocean mixed layer response to changes in surface heat fluxes induced by ENSO, and is less affected by ocean dynamics. Stage two consists of a cross-equatorial SST anomaly which develops in the late spring and early summer. This response is affected strongly by ocean dynamics. The experiments show that entrainment dynamics are particularly important for maintaining the correct SST structure. Without entrainment the simulated equatorial SST anomaly tends to have the opposite sign of the observed SST. In constrast, the horizontal heat advection appears to be less important and mainly acts to broaden the structure of the SST anomaly.
A33B-05 14:40h
The ENSO Signature in Land Surface Photosynthetic Activity
Seasonal climate prediction in the tropics is still based almost entirely on observation and forecasting of the slowly varying ocean state. By comparison, the land surface state has received rather little attention, even though it has response times of weeks to months and can exert similar magnitudes of atmospheric forcing as the oceans. Here, we present 6 years of a global homogeneous satellite FAPAR product describing the fraction of photosynthetically active radiation absorbed by vegetation. Time series are analysed pixel by pixel at 0.5 by 0.5 degree resolution for significant lagged correlations with Nino-3 SST anomalies. We find essentially the same patterns as with gridded monthly climate observations derived from station data, albeit with far more detail. Further, there appears to be a response time of FAPAR against precipitation of 3-5 months. A biosphere model driven with the same climate data reveals a similar response time for biosphere-atmosphere net CO$_2$ fluxes. Such a response time carries the potential of improving seasonal climate predictions. We conclude that global FAPAR observations from satellites represent a source of information that could be used to study ENSO teleconnections on land, and to improve forecasts through assimilation into coupled biosphere-atmosphere models.
A33B-06 14:55h
The Relationship Between ENSO and Typhoon Activity
The influence of the El Ni\~no - Southern Oscillation (ENSO) on tropical cyclone intensity in the Western North Pacific basin is examined in interannual and shorter time scales. An index of TC activity [accumulated cyclone energy (ACE)] is used, which is shown to be positively correlated with ENSO indices. There is a tendency in El Ni\~no years towards tropical cyclones which are both more intense and longer-lived than in La Ni\~na years. The relationship is consistent over the annual cycle of tropical cyclone activity, and ACE during the peak season (northern summer and fall) is correlated approximately as strongly with ENSO indices up to six months later (northern winter) as simultaneously. The lifetime and intensity effects both contribute significantly to the ENSO signal in ACE, though the lifetime effect appears to be slightly more important. The influence of western north Pacific (WNP) tropical cyclones (TCs)on their large-scale environment is also examined by lag-regressing various large-scale climate variables on ACE on a weekly time scale. At all leads and lags out to several months, persistent, slowly evolving signals indicative of the El Nino - Southern Oscillation (ENSO) phenomenon are seen in all the variables, reflecting the known seasonal relationship of TCs in the WNP to ENSO. Superimposed on this are more rapidly evolving signals, at leads and lags of one or two weeks, directly associated with the TCs themselves. In the same region, lagging ACE by a week or two and so presumably reflecting the influence of TCs on the local environment, signals are found which might be expected to negatively influence the environment for later cyclogenesis, and thus to play a role in regulating the total annual number of TCs in the basin, which has very little interannual variability. On the same short time scale, an increase in equatorial SST near and east of the date line is seen, presumably associated with equatorial surface westerly anomalies that are also found. This, combined with the correlation between ACE and ENSO indices on the seasonal time scale, suggests the possibility that TCs may play an active role in ENSO dynamics.
A33B-07 INVITED 15:10h
Tropical Teleconnection Sensitivities to Characteristics of El Niño
Examination of all major El Niño events of the past century and the associated monsoon rainfall anomalies over India suggests different impacts. This empirical evidence challenges the conventional wisdom that El Niño events in the tropical Pacific Ocean lead to a monolithic deficit in Indian summer monsoon rainfall. For example, during 1997, the strongest El Niño event on record, Indian monsoon rainfall was inexplicably high. Yet in 2002, a relatively weaker El Niño event, India experienced one of the worst droughts in recent history. To understand this variability, we explore tropical-wide teleconnection sensitivities to the strength and location of warm sea surface temperature (SST) anomalies in tropical Pacific. Can it be shown, that SST anomalies shifted towards the tropical far Eastern Pacific, (a flavor seen in 1997) alters the subsidence limb of the Walker circulation in a manner to circumscribe its reach over the Indian subcontinent? Likewise, can it be shown that SST anomalies near the dateline are more effective in suppressing Indian monsoon rainfall, as seen in 2002? We test these hypotheses using atmospheric general circulation model (AGCM) experiments forced with idealized tropical Pacific warmings. Implications for tropical monsoon prediction are drawn, including requirements for coupled model forecasts of the SST variations.
A33B-08 INVITED 15:25h
Role of ENSO-induced Indian Ocean SST on the Asian Monsoons
Recent studies point to dynamic air-sea interaction over the eastern equatorial Indian Ocean during July-November and over the southwest Indian Ocean during December-May, leading to considerable local SST anomalies. The effect of these SST anomalies in conjunction with ENSO-related SST anomalies on the Indian Summer Monsoon and East Asian Winter Monsoon are examined. An Atmospheric General Circulation Model (AGCM) is used to elucidate the relative importance of local and remote forcing, and model solutions were sought for experiments where SST anomalies are inserted in the (i) tropical Indo-Pacifc, (ii) tropical Pacific and (iii) tropical Indian Ocean. A 10-member ensemble simulation is carried out for each of the forcing scenarios. Diagnostics from observations reveal that after the 1976-77 climate shift in the Pacific despite stronger El Niño events the Indian Summer Monsoon (ISM) during July-August was above normal. Based on 1950-1975 (PRE76), and 1977-2001 (POST76) El Niño composites we note that the major differences between the two epochs, in terms of observed SST during July-August, are the presence of cold SST anomalies over the Indo-Pacific warm pool and the intensity of warm SST anomalies in the central Pacific in POST76. Model solutions demonstrate that low-level anticyclonic circulation anomalies that develop as a Rossby wave response to these convective anomalies increase the south-westerlies over the northern Indian Ocean to favor a stronger ISM. During boreal winter, model solutions show that precipitation variations over the southwest Indian Ocean are tied to local SST anomalies and insensitive to changes in model initial conditions. Changes in the Indian-Ocean Walker Circulation suppress precipitation over the tropical west Pacific - Maritime Continent, contributing to the development of a low-level anticyclone over the Philippine and South China Seas. This anticyclone increases precipitation along the East Asian Winter monsoon front from December to May. The anomalous subsidence over the Maritime Continent in conjunction with persistent SST anomalies and precipitation over the southern Indian Ocean in spring prevent the north-northwestward migration of a deep moist layer, causing a significant delay in the ISM onset in June by 6-7 days. Our results indicate that Indian Ocean SST anomalies that develop in response to El Niño and/or due to internal air-sea interaction processes need to be considered for a complete understanding of regional climate variability, particularly around the Indian Ocean rim.