Sub-seasonal variability of the ITCZ over the East Pacific Ocean in reanalysis and climate models: The role of Wind-Evaporation-Sea Surface Temperature (WES) feedback
Date
2024-05
Authors
Ganguly, Indrani
Major Professor
Advisor
Gonzalez, Alex O
Gutowski, William
Gallus Jr., William A.
Hornbuckle, Brian K.
Wanamaker, Alan
Committee Member
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Altmetrics
Abstract
This dissertation encompasses research on one of the lesser-understood aspects of the Intertropical Convergence Zone (ITCZ) over the East Pacific Ocean: its sub-seasonal (daily to monthly) variability. The ITCZ was first documented in the 1600s by mariners and scientists such as physicist Edmund Halley. Since then, the ITCZ's seasonal latitudinal shift following the insolation seasonal cycle at a one to three-month lag has been well known. Only recently, however, has it been found that over the East Pacific, the ITCZ oscillates between five different configurations relative to the equator, or ITCZ ``states", on sub-seasonal time scales during boreal spring. Of these five ITCZ states, northern ITCZ (ITCZ in the Northern Hemisphere), double ITCZ (one ITCZ on both sides of the equator simultaneously), and southern ITCZ (ITCZ in the Southern Hemisphere) occur most frequently. Air-sea interaction involving dynamic and thermodynamic factors like sea surface temperatures, surface fluxes, etc. is one of the most important factors influencing the variability of the ITCZ over the East Pacific Ocean.
Furthermore, despite the recent advances in global climate modeling, the East Pacific ITCZ has been plagued for nearly three decades by the overestimation of precipitation in the Southern Hemisphere and underestimation of precipitation in the Northern Hemisphere during the boreal spring season.
The first study investigates the air-sea interactions in ECMWF’s ERA5 reanalysis and observations associated with the spatial variability in the eastern Pacific's northern ITCZ state and combined double ITCZ state and southern ITCZ state classified using infrared satellite data. The sea surface temperature (SST) anomalies and surface latent heat flux anomalies distribution during the ITCZ events in the Northern and Southern Hemispheres conform to the classic wind-evaporation-SST (WES) positive feedback mechanism. However, there is an alternate mechanism that is less commonly discussed in the literature that considers the impact of SST anomalies on vertical stratification and momentum mixing, resulting in a negative WES feedback. The findings indicate that the ``fingerprints" of both the positive and negative WES feedbacks mechanisms compete in the surface layer (\(\sim \)10m) for northern and combined double ITCZ and southern ITCZ events. Furthermore, the spatial variability displays mirror images for these opposite ITCZ events over the East Pacific during boreal spring.
The second study in this dissertation uses the insights gained from the first study and advances the understanding of the atmospheric aspects of positive and negative WES feedback mechanisms to NCAR's Community Atmosphere Model 6 (CAM6). A new algorithm is devised to classify the ITCZ states based on the precipitation field, which is a more common field in model output than infrared satellite imagery. The fingerprints of positive and negative WES feedbacks during the three most common ITCZ states (northern, double, and southern) are compared between ECMWF's ERA5 reanalysis and two different CAM6 simulations, one with monthly SST forcing and the other with daily SST forcing. It is found that the WES feedback fingerprints are simulated with sufficient accuracy in both the monthly SST forced and daily SST forced CAM6 simulations for northern and southern ITCZ states, but they are considerably different during double ITCZ states.
It is also found that the ITCZ states created from daily SST-forced CAM6 simulation precipitation are more accurate than the ITCZ states created from monthly SST-forced CAM6 simulation precipitation output when compared to the ITCZ states created using ERA5 reanalysis data.
The results of both these studies show that diagnostics such as the fingerprints of the opposing WES feedback mechanisms, or the ITCZ states created from precipitation field can be applied to atmospheric general circulation models as well as coupled general circulation models to better understand and improve ITCZ biases.
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dissertation