CYCLONE MITCHELL - AN EXPLANATION.

 

The following is a closer look at cyclone Mitchell and its path tracking down the coast of Northwest Australia - but not coming onshore as did the Bala Bala cyclone of cyclone Ilsa.

Note the subtle difference in subsequent surface current patterns post cyclone

https://koombanarevisited.blogspot.com/2023/04/cyclone-ilsa.html

https://koombanarevisited.blogspot.com/2019/05/analysis-of-wreckage.html

For an A.I. (GROK) explanation:

1. Background on the Cyclones in Question

• Cyclone Ilsa (April 2023): A Category 5 system that made direct landfall between Port Hedland and Wallal Downs (Pilbara region), with a diameter of about 100 miles. It was similar in size and behavior to the 1912 Balla Balla cyclone, which also targeted the area south of Port Hedland. Bedout Island recorded extreme winds (218 km/h sustained), but impacts were minimal further afield in places like Port Hedland and Broome.
• Cyclone Mitchell (February 2026): Based on recent reports, this was a Category 3 system at peak that tracked down the Northwest coast, affecting the Pilbara with heavy rain (100+ mm in spots) and gusts up to 165 km/h, but it did not make landfall in the Pilbara/Bedout Island vicinity. Instead, it weakened to Category 1-2 and crossed the coast further south in the Gascoyne region (near Shark Bay, east of Carnarvon). This aligns with your description of it "tracking down the coast... but not coming onshore" in the relevant Northwest/Pilbara area, making it a contrast to landfalling systems like Ilsa.

The key oceanographic factor here is how cyclones influence surface currents through wind stress, Ekman transport (where winds drive water movement at a 90-degree angle to the wind direction), and interactions with coastal topography. Landfalling cyclones often cause more localized, intense disruptions due to their proximity to shore, while offshore ones tend to have broader but less abrupt effects.
2. Post-Cyclone Current Patterns for Landfalling Systems (e.g., Ilsa and Balla Balla)
Your detailed daily observations for Ilsa illustrate a characteristic pattern for cyclones that hit the Pilbara coast directly:

• Pre-cyclone (e.g., 13 April): Predominantly westward-trending currents in the broader Bedout Island area, which is typical for the region's baseline circulation influenced by the Leeuwin Current (flowing southward along the WA coast) and trade winds.
• During landfall (14 April): Distinct changes occur due to the cyclone's intense winds and pressure gradients. Currents become disrupted, often with swirling or variable directions as the storm's rotation interacts with the shelf waters.
• Immediate post-cyclone (1-2 days after, e.g., 15-16 April): A temporary "counter" shift to northeastward-trending currents near Bedout Island. This reversal is likely driven by:
  • Residual storm winds pushing water onshore (storm surge) and then offshore rebound.
  • Ekman upwelling or downwelling, where the cyclone's counterclockwise rotation (in the Southern Hemisphere) induces divergent flows.
  • Coastal trapping effects, where the landfall amplifies local eddies or gyres.
• Subsequent recovery (3+ days after, e.g., 17-29 April): Currents re-establish a generally westward trend, persisting for up to a fortnight or more. This supports your flotsam drift theory for the Koombana wreck, where wreckage (e.g., stateroom doors, lifebelts, panels) distributed westward from the oil patch aligns with this pattern. The westward flow stabilizes as the cyclone's influence dissipates and normal oceanic drivers (tides, winds, and the Indonesian Throughflow) resume dominance.

In summary, landfalling cyclones like Ilsa introduce a brief but notable disruption—an initial northeastward anomaly—before settling into a prolonged westward pattern. This was evident even though impacts were localized, with little felt in Port Hedland or Broome.
3. Post-Cyclone Current Patterns for Offshore-Tracking Systems (e.g., Mitchell)
Since Mitchell stayed offshore relative to the Pilbara/Bedout Island area (tracking parallel without direct landfall there), the patterns differ subtly but importantly:

• Pre-cyclone: Similar to Ilsa—generally westward currents as the baseline.
• During passage: Some disruption occurs from the cyclone's outer bands, including increased wind-driven mixing and potential for minor eddies. However, without direct coastal interaction, changes are less localized and intense. Currents might strengthen westward or develop slight southward components due to the cyclone's broader wind field, but without the sharp reversals seen in landfalling cases.
• Immediate post-cyclone (1-2 days after): Likely no significant northeastward reversal. Instead, currents remain predominantly westward or experience only mild variations (e.g., temporary slowing or minor looping). This is because:
  • The lack of landfall reduces storm surge and coastal rebound effects.
  • Winds are more uniform and offshore-directed, reinforcing rather than opposing the baseline flow.
  • Less upwelling or eddy formation near the shelf edge, as the cyclone's core doesn't interact as closely with shallower coastal waters.
• Subsequent period (3+ days after): Westward trends persist steadily, similar to Ilsa's recovery phase, but without the initial "counter" anomaly. Reports post-Mitchell indicate ongoing heavy rain and winds in the Pilbara, but ocean conditions normalized quickly, with no noted major current shifts in the Bedout area (based on BOM forecasts and general cyclone aftermath data).

This steadier pattern aligns with cyclones that "hug" the coast without hitting it, where the influence on surface currents is more diffuse and less prone to temporary reversals.
4. The Subtle Difference and Implications

• Key Contrast: The "subtle difference" you noted likely refers to the presence vs. absence of an initial counter-current shift. Landfalling cyclones (Ilsa/Balla Balla) cause a short-lived northeastward anomaly (1-2 days) due to intense coastal dynamics, while offshore ones (Mitchell) maintain more consistent westward flows throughout the post-cyclone period. This difference is "subtle" because the long-term outcome (westward persistence for ~2 weeks) is similar, but the immediate aftermath varies, affecting things like flotsam dispersal or oil slick movement.
• Why It Matters for Koombana Wreck Search: For the 1912 Balla Balla cyclone (landfalling like Ilsa), the post-cyclone westward currents support your theory that wreckage originated near Captain Upjohn's oil patch (27-28 nautical miles NE of Bedout) and drifted west, aligning with finds like the awning spar and planks. An offshore cyclone like Mitchell might not produce the same initial "push" for rising debris, potentially leading to different distribution patterns. This reinforces using landfalling analogs like Ilsa for modeling the 1912 event.
• Supporting Evidence: Your BOM link (for NWWA SST/Currents) shows real-time maps that can confirm current states, but historical comparisons (e.g., via archives or models like HYCOM) often highlight these wind-driven shifts. General studies on Australian cyclones (e.g., from CSIRO or BOM reports) note that landfall intensifies local current anomalies, while offshore tracks cause milder, broader changes.

http://www.bom.gov.au/oceanography/forecasts/idyoc300.shtml?region=NWWA&forecast=SSTCur