H51G-01 INVITED
Recent Analyses On The Mid Summer Drought
The Mid Summer Drought (MSD), also known as Canícula, has been a subject of interest in the scientific community in recent years. This phenomenon has been examined since the early work of climatologists in Mesoamerica. The MSD is basically a relative minimum in precipitation in the middle of the summer rainy season. The MSD signal extends from central Mexico all the way to Central America, mainly along the Pacific coast. Magaña et al (1999) has shown that the MSD is part of the annual cycle and not a form of low frequency variability. Among a number of hypotheses to explain this phenomenon, a mechanism as the thermostatic hypothesis was originally proposed by Magaña et al. (1999) However, various new mechanisms have been proposed, such as stationary circulations slowly moving and controlling the mean flow over the region. After a field campaign over the north eastern Pacific and the Caribbean warm pools, Magaña and Caetano (2005) showed that a teleconnection between these warm pools was involved in the dynamics of the MSD, and consequently, a simple column model could not explain the evolution of precipitation over the warm pools. Recently, it was found that easterly wave activity over the Caribbean Sea also showed a bimodal structure coinciding with the MSD. Even tropical cyclones in the region appear to show a bimodal structure in their temporal activity. Considering that easterly waves and tropical cyclones over the Caribbean and Mesoamerica are crucial to determine monthly rainfall the MSD dynamics seems to involve a more complex range of processes than previously thought. The implications of such recent findings in the analysis of MSD will be discussed in this work.
H51G-02 INVITED
Atmospheric dynamics hypotheses for midsummer droughts
This talk will review mechanisms that I hypothesize are responsible for the Central American mid-summer drought. Proximate causes, positive feedbacks, and ultimate causes are carefully distinguished. The proximate cause of dry weather appears to be a change in the configuration of the North Atlantic subtropical high pressure cell. Feedbacks, for example from latent heating anomalies associated with the rainfall shift are apparently strong and positive. Logically, though, these anomalies cannot "explain" their own existence and timing. We suggest that the ultimate causes involve the zonal mean zonal flow. If so, the midsummer drought phenomenon spans all scales from truly global to convective, so predicting its variations from year to year may depend on a challenging array of faraway factors and physical process parameterizations.
H51G-03
The Mid-Summer Drought: large scale forcing and interannual variability.
A mid-summer drought (MSD) is experienced in southern Mexico, Central America, the Caribbean, as well as
North American states bordering the Gulf of Mexico. The timing and magnitude of the drought affects agriculture in
this region, so it is important to understand the mechanisms governing its formation and its variability. This
presentation discusses some potential controlling factors of the MSD. A linear baroclinic model is used to
investigate the influence of large scale diabatic heat anomalies on the local circulation. It is found that the
seasonal progression of the Pacific Inter-Tropical Convergence Zone, which moves northward following warm
sea surface temperature during the early summer, and of the Atlantic (Bermuda) sub-tropical high, which moves
westward, are the most important remote factors that contribute towards the low level easterly flow and
divergence during the MSD. It is also shown that the circulation associated with the MSD precipitation deficit helps
to maintain the deficit by inducing low level anticyclonic flow and easterlies across Central America. We
investigate how the MSD is modified by the Madden-Julian-Oscillation, which has a prominent signature in the
region in summer: it appears that the phasing of the Madden-Julian-Oscillation is important to interannual
modifications of the MSD.
http:iprc.soest.hawaii.edu/~justins/
H51G-04
Spatial Structure of Cloudiness Associated with the Mid-Summer Drought from MODIS and GOES Imagery
The Mid-Summer Drought (MSD), a dry period within the overall rainy season, affects a region extending from
southern Mexico, through parts of Central America, and into parts of northern South America. Studies based on
rainfall data have shown that the MSD has a complex spatial structure. This talk will highlight some aspects of
the local- and regional-scale aspects of the MSD using cloudiness data from both MODIS and GOES imagery.
The MSD onset and demise dates for the past 6 years are identified from both North American Regional
Reanalysis (NARR) winds and special pilot balloon observations made in two gaps in the Central American
cordillera. By comparing the mean cloudiness prior to the MSD and during the MSD we show the spatial structure
and intensity of the MSD from a cloud field perspective. A large decrease in cloudiness is seen on the Pacific
side of the Isthmus of Tehuantepec during the onset of the MSD, while increasing cloudiness occurs to the north.
There is an extremely sharp transition between the cloudiness regimes. We will show that this also agrees with
detailed rainfall climatologies of the region. In general, the MODIS imagery, with 250m resolution, show the very
close relationship between the underlying topography and the geographical variations of the MSD. GOES
imagery, with coarser 10km resolution (but with the full diurnal cycle), has been used to depict the larger-scale
aspects of the cloudiness changes associate with the onset and decay of the drought over different parts of
Central America and the surrounding oceanic regions.
http:www.nssl.noaa.gov/projects/pacs/web/MODIS/
H51G-05
Impacts of the Mid - Summer Drought on Agricultural Activities. Two study cases
Mid - summer drought is a known climatic phenomenon for maize producers in the State of Tlaxcala, Mexico.The forecasts delivered by the Autonomous University of Tlaxcala and the National Autonomous University of Mexico during several years included an estimation of the severity of the "canícula", in order that the producers could plan their agricultural activities. The intensity of the mid - summer drought increases during strong EL Niño events (i. e. 1982 - 1983; 1997 - 1998) , that can be also associated to an increase of sudden frosts in autumn that might affect the maize production. In this paper the perception of farmers in Tlaxcala, their needs for specific forecasts and the possibilities of useful climatic information is presented. Also, using an agricultural simulation model, the possible effects on maize yields of a severe mid- summer drought is analyzed. A second study case was performed in Veracruz, were it has been documented that the intensity of the mid - summer drought decreases during strong El Niño events. Possible impacts of this condition on agricultural activities in the state are also presented.
H51G-06
Origins of the Caribbean Bimodal Rainfall Pattern
The Caribbean region is geographically located along a relative land-free tropical band, where the Tropical Atlantic and Pacific equatorial regions influence the inter-annual variability of its rainy season. Its position in a middle tropical warm pool and the ageostrophic dynamic circulation also plays an important role in that variability. Past studies have ascertained that the Caribbean rainfall season has a bimodal nature, where the initial peak of this season, called the early rainfall season (ERS), begins in May and extends until June, with a brief dry period in July. The second half of the overall rainy season, or late rainfall season (LRS), spans from August to November. During the rainy season, the easterly waves and tropical storms begin to be frequent and the rainfall begins to increase. Intuitively, the rainfall should continue increasing, especially from the beginning of the rainy season until the end of this season. The actual data, however, shows an unexpected rainfall decrease in the month of July. The low rainfall peak during the rainy season defines the Caribbean bimodal behavior. The Caribbean rainfall bimodal structure has been reported in different studies over the Caribbean region. However, very little is known about this summer drought, its origins, and factors influencing it. It has been hypothesized that increases in giant aerosols concentration due to the Saharan Dust across the Caribbean in the summer months may result in precipitation suppression. AIn this paper, a multivariable analysis was carried to determine which climatological variables may correlate with the Caribbean summer drought that included Intertropical Convergence Zone (ITCZ), the North Atlantic Oscillation (NAO) index, the Vertical Wind Shear (VWS) and the aerosol particles (AP) coming from northern Africa. The analysis shows that the ITCZ and the SST are weakly correlated with the Caribbean bimodal precipitation; however, the VWS and aerosol particles revealed an important contribution to rainfall during the summer months. The multiple regression analysis also reveals an independent strong correlation of the VWS. Spectral analysis revealed that the NAO does not have periodicity leaving the VWS and AP as potential controlling variables. Numerical experiments are performed to uncouple the VWS and AP effects in the lower summer rainfall time. The numerical approach uses the regional atmospheric modeling system (RAMS) with a new cloud microphysics module which considers small and giant AP. A control run is carried out for July 2003 which has a VWS closer to the climatology and a second run for July, 2002, where a VWS anomaly is present. These numerical experiments supported the statistical result that the VWS effect uncoupled to the AP influences the rainfall production in July. Two additional numerical experiments are performed in which observed AP are ingested into RAMS. The first of these simulations uses observed Cloud Condensation Nuclei (CCN) and Giant CCN measured in July 2003, and in a fourth run the concentration of AP are increased maintaining the same atmospheric conditions as in the previous run. Results indicate that higher AP concentrations decreases further rainfall in July across the Caribbean. Possible variability of this bimodal trend with climate changes is also explored in the paper.
H51G-07
Caribbean Low-Level Jet, Mid-Summer Drought and Climate Variability
A maximum of easterly zonal wind at 925-hPa in the Caribbean region is called the Caribbean Low-Level Jet (CLLJ). Observations show that the easterly CLLJ varies semi-annually, with two maxima in the summer and winter and two minima in the fall and spring. Associated with the summertime strong CLLJ are a maximum of sea level pressure (SLP), a mid-summer drought in rainfall, and a minimum of tropical cyclogenesis in July in the Caribbean Sea. It is found that both the meridional gradients of sea surface temperature (SST) and SLP show a semi-annual feature, consistent with the semi-annual variation of the CLLJ. The CLLJ anomalies vary with the Caribbean SLP anomalies that are connected to the variation of the North Atlantic Subtropical High (NASH). In association with the cold (warm) Caribbean SST anomalies, the atmosphere shows the high (low) SLP anomalies near the Caribbean region that are consistent with the anomalously strong (weak) easterly CLLJ. The CLLJ is also remotely related to the SST anomalies in the Pacific and Atlantic, reflecting that these SST variations affect the NASH. During the winter, warm (cold) SST anomalies in the tropical Pacific correspond to a weak (strong) easterly CLLJ. However, this relationship is reversed during the summer. This is because the effects of ENSO on the NASH are opposite during the winter and summer. The CLLJ varies in phase with the North Atlantic Oscillation (NAO) since a strong (weak) NASH is associated with a strengthening (weakening) of both the CLLJ and the NAO. The CLLJ is positively correlated with the 925-hPa meridional wind anomalies from the ocean to the United States via the Gulf of Mexico. Thus, the CLLJ and the meridional wind carry moisture from the ocean to the central United States, usually resulting in an opposite (or dipole) rainfall pattern in the tropical North Atlantic Ocean and Caribbean versus the central United States.
H51G-08
Features of the Caribbean Low Level Jet
The Caribbean Low Level Jet (CLLJ) is shown to be a real and dominant climatological feature of the early summer Caribbean climate. It manifests as an intensification in the trades in the western Caribbean basin (70oW- 80oW) with an east-west axis along 15oN. It is confined to heights below 600 mb and has maximum wind speeds approaching 16 m/s near the surface. The study shows that there is variability in the strength and zonal extent of the CLLJ which can be related to zonal SST gradients between the eastern equatorial Pacific and the north tropical and/or equatorial Atlantic. When the gradient is driven by the Pacific (as in an EL Niño event) the CLLJ winds are intensified to the north and in its south-westernmost quadrant. When the winds are driven by the tropical north Atlantic there is uniform wind intensification about the jet axis. The CLLJ is also linked to a precipitation maximum over the near waters and along the Caribbean coast of Central America (up to 16oN) during June and more so July. There is evidence of variation at the northern extent of this wet zone depending on the ocean basin forcing the SST gradient. The CLLJ is also linked to moderate drying in the main Caribbean basin.